Transforming growth factor-β and breast cancer: Transforming growth factor-β/SMAD signaling defects and cancer
© Current Science Ltd 2000
Received: 20 December 1999
Accepted: 26 January 2000
Published: 1 April 2000
Transforming growth factor-β (TGF-β) is a tumor suppressor, the function of which is compromised in many types of human cancer, including breast cancer. The tumor suppressive effects of TGF-β are caused by potent inhibition of cell proliferation due to cell cycle arrest in the G1 phase. Such antiproliferative responses are mediated by a signaling system that includes two types of cell surface receptors and intracellular signal transducers, the SMAD proteins. Different molecular mechanisms can lead to loss of antiproliferative TGF-β responses in tumor cells, including mutations in components of the signaling system and inhibition of the SMAD signaling pathway by aberrant activities of various regulatory molecules. Some of these mechanisms will be discussed, with emphasis on their potential involvement in breast tumorigenesis.
The transforming growth factor-β (TGF-β) family of polypeptide growth factors regulates cellular processes, including cell division, differentiation, motility, adhesion, and death, in virtually all tissues ( and references therein). TGF-β is an important regulator of normal mammary gland development and function, as well as of the development and progression of breast tumors. TGF-β potently inhibits cell cycle progression of epithelial cells, including those of the lobules and ducts of the mammary gland, and it thereby controls epithelial cell proliferation and regression during mammary gland development, and during and after lactation in the adult gland .
In breast cancer, TGF-β has been suggested to play a dual role . It acts as a tumor suppressor in early stages of the disease when it inhibits the outgrowth of carcinomas in situ via its antiproliferative functions. This has been demonstrated in transgenic mouse models, in which over expression of TGF-β1 (one isoform of TGF-β) is targeted to the mammary gland, and tumor formation is induced by concomitant over-expression of TGF-β and administration of a chemical carcinogen . In later stages of the disease, TGF-β is believed to promote tumor progression, in part by enhancing tumor cell motility and invasiveness [5,6] and the capacity to form metastases [6,7,8]. Tumor promoting functions of TGF-β correlate with increased secretion of TGF-β by the cancer cells during tumor progression .
This apparent switch of the role of TGF-β in the regulation of tumorigenesis is reflected in changes of tumor cell responsiveness. Similar to other types of carcinomas, many malignant breast carcinoma cells have lost most or all sensitivity to TGF-β-induced growth inhibition, while tumor cells derived from early stages of the disease are usually inhibited . This loss of antiproliferative responsiveness thereby predisposes to or causes cancer progression.
TGF-β induces growth inhibition by arresting cells in the G1 phase of the cell cycle, leading in some circumstances to terminal differentiation or induction of apoptosis . G1 arrest is achieved by several mechanisms that may act in a complimentary fashion in the same cell, and which include the transcriptional upregulation of the cyclin-dependent kinase inhibitors p15Ink4B and p21cip1/waf1, the downregulation of the cyclin-dependent kinase activating phosphatase Cdc25A, and the downregulation of the proto-oncogene c-Myc. Together, these events lead to a hypophosphorylated, activated state of the pRB tumor suppressor protein, and thereby to the arrest of the cell cycle in G1.
The antiproliferative response to TGF-β depends on a signaling pathway that is initiated by the ligand-activated TGF-β receptor complex on the cell surface and is transduced into the nucleus by signaling mediators, the SMAD proteins [1,11] (Fig. 1). In this pathway, TGF-β binds to a specific pair of type I and type II receptor serine/threonine kinases, leading to the transphosphorylation and activation of the type I receptor (TβR-I) by the type II receptor (TβR-II). Activated TβR-I then phosphorylates a specific subset of SMAD proteins, Smad2 and Smad3, which subsequently translocate into the nucleus. On their way to the nucleus, these receptor-activated SMAD proteins associate with the related Smad4 protein. Once in the nucleus, SMAD proteins form functional transcription complexes in association with DNA binding factors, coactivators, or corepressors .
Resistance to TGF-β-induced growth inhibition in breast carcinoma cells may be caused by a number of mechanisms, some of which are just starting to become clear. In this review, I will focus on molecular events that directly interfere with the TGF-β/SMAD signaling pathway.
Inactivating mutations in components of the TGF-beta/SMAD signaling pathway
Disruption of the TGF-β/SMAD signaling pathway by mutation has been observed in several types of human cancer. TβR-II is inactivated by mutation in a majority of colon and gastric cancers with microsatellite instability [13**,14,15] and, to a smaller percentage, also in microsatellite stable colon cancers . In comparison, mutations in TβR-II are relatively rare in cancers of the pancreas, liver, and pituitary gland, and in myelodisplastic syndrome or endometrial cancers with microsatellite instability [14,17,18,19,20]. Mutations or structural alterations in TβR-II have not been found in primary human breast carcinomas [20,21*,22,23] or breast carcinoma cell lines . Inactivation of TβR-II by mutation therefore appears to be specifically selected for in gastrointestinal cancers .
Homozygous deletions of TβR-I were found in a small percentage of pancreatic and biliary adenocarcinomas , and a large deletion mutation was detected in TβR-I in one case of anaplastic large cell lymphoma . Furthermore, a polymorphism resulting in a deletion of three residues from a nine alanine stretch has been observed in some colorectal and cervical carcinomas, and homozygous carriers of this polymorphism may be at enhanced risk for cancer development [28,29]. In breast cancer, a point mutation (S387Y) in TβR-I, which diminishes its signaling capacity, has been reported in 2 of 31 primary carcinomas (6%) and 5 of 12 lymph node metastases (42%), indicating that this mutation may represent an important event in the progression of breast cancer to malignancy [21*]. This view has been challenged, however, by another study in which no mutation at this site was detected in 20 cases of breast cancer metastases [30*].
Inactivating mutations in SMAD genes are found in a number of human cancers, with the highest frequency in pancreatic and colon carcinomas. Smad4/DPC4 was originally isolated as a tumor suppressor gene on chromosome 18q21 that is deleted or mutated in nearly half of all human pancreatic carcinomas [31**], and much less frequently in other cancers [32,33,34,35,36,37]. Smad2, which is also located on 18q21, is mutated in a small number of colon, head and neck, and lung carcinomas [38**,39,40,41,42], but appears to be unaffected in other types of carcinomas as well as leukemias and lymphomas [34,35,43,44,45,46,47,48,49,50]. In breast cancer, inactivating mutations in Smad2 have not been reported and they are rare in Smad4 [37,51]. Inactivating mutations in Smad3 have not been observed in any of a large number of tumors, including those from gastrointestinal, breast, lung, ovarian, and pancreatic cancers [39,43,44,48,52,53,54].
Why inactivating mutations in TβR-II and Smad4 are involved in significant proportions of gastrointestinal and pancreatic cancers, respectively, but are rare in breast (and other) cancers is currently unclear. It suggests, however, that activities of these molecules might be required for the development and/or progression of breast cancer, and therefore mutational inactivation would not confer a selective advantage.
Reduced expression of TGF-beta/SMAD signaling components
In a limited number of cases, reduced expression of TβR-I has been suggested as the cause for loss of TGF-β-mediated growth inhibition in pancreatic or colon carcinoma cell lines [59,60,61] or cells from patients with chronic lymphocytic leukemia . However, normal TβR-I expression was observed in the vast majority of primary epithelial cell cultures derived from malignant breast tissue .
In a panel of both estrogen-responsive and estrogen-insensitive breast carcinoma cell lines, Smad2, Smad3, Smad4, and TβR-I were all expressed at the mRNA level, with the exception of one cell line (MDA-MB-468) with a homozygous deletion of Smad4 . TβR-II mRNA could not be detected in two of the cell lines tested (ZR-75-1 and T47D).
Taken together, these results suggest that limited or no presence of functional Smad4, TβR-I or TβR-II, due to mutation or aberrant expression, may contribute to loss of TGF-β growth inhibition in a small percentage of breast carcinomas. Aberrations in functional expression of Smad2 and Smad3 are unlikely to play a significant role. Alternative mechanisms to abolish growth inhibitory responses to TGF-β must therefore evolve in most breast carcinomas.
Inhibition of the TGF-beta/SMAD pathway by Ras/MAP-kinase signaling
TGF-β can override the proliferative effects of EGF and other Ras-activating mitogens in normal epithelial cells. However, cells harboring oncogenic Ras mutations often show a loss of TGF-β antiproliferative responses. Oncogenic Ras can achieve inhibition of TGF-β signaling in mammary epithelial cells by negatively regulating Smad2 and Smad3; that is, by inhibiting their TGF-β-induced nuclear accumulation and transcriptional activity [64**]. Acting via Erk MAP kinases, Ras causes phosphorylation of Smad2 and Smad3 at specific sites in the central portions of the proteins. These sites are separate from the carboxy terminal sites targeted by TβR-I. Mutation of these MAP kinase sites in Smad3 yields a Ras-resistant form that can partially rescue the growth inhibitory response to TGF-β in Ras-transformed cells [64**,65]. EGF, which activates MAP-kinase transiently, induces a less extensive phosphorylation and cytoplasmic retention of Smad2 and Smad3 [64**]. These results have suggested a mechanism for counterbalanced regulation of Smad2 and Smad3 by TGF-β and Ras signals in normal cells, and for the inhibition of antiproliferative TGF-β functions by hyperactive Ras in cancer cells. In addition to such direct effects on SMAD signaling, Ras can also interfere with TGF-β functions at other levels; for example, by upregulating the activities of G1 phase cyclin-dependent kinases .
Even though oncogenic Ras mutations are relatively rare in breast cancers (found in about 5% of all cases ), about one-third of all breast cancers display overexpression or amplification of the HER-2/Neu receptor tyrosine kinase [68,69,70]. In addition, the related EGF receptor is also overexpressed in a significant number of cases . Stimulation of the Ras/MAP-kinase pathway is a major component of the proliferative signals by these receptors. Amplification and overexpression of HER-2/Neu and EGF-R have been correlated with aggressive tumor phenotype and poor clinical prognosis [69,72]. It is plausible, therefore, that one function of oncogenic Ras mutations and elevated HER-2/Neu or EGF-R activity in breast carcinomas is to impede the growth inhibitory function of TGF-β via phosphorylation of SMAD proteins by Erk MAP-kinase. Consistent with such a model, an inverse correlation between the presence of oncogenic Ras mutations and the ability of TGF-β to inhibit cell proliferation has been observed in a panel of carcinoma cell lines, derived in this case from colon cancers [64**].
Apparently conflicting results with this model are presented in a study which proposes that EGF and HGF signal positively through the Smad2 protein in certain cell lines, and that Smad2 thereby functions as a common effector of receptor tyrosine kinase and receptor serine/threonine kinase signaling . The observed effects are suggested to be mediated by phosphorylation of Smad2 at sites that are also phosphorylated in response to TGF-β but are different from the carboxy terminal sites targeted by TβR-I. A clear evaluation of these results will have to await the mapping and mutational analysis of the involved phosphorylation sites. Interactions between TGF-β signaling and various MAP-kinase pathways are currently the subject of intense investigation, fueled by observations that factors of the TGF-β family and receptor tyrosine kinase activating factors synergize with or antagonize each other's actions during developmental processes, and that under certain conditions, TGF-β factors elevate MAP-kinase activities in cultured cells .
An interesting example of a tumor cell line that features more than one mechanism of inhibition of TGF-β/SMAD signaling has recently been described . The colon carcinoma cell line SW480.7 lacks expression of a functional Smad4 protein and also harbors an activating mutation in the Ki-Ras oncogene. As expected, this cell line does not show antiproliferative responses to TGF-β. Furthermore, exogenous expression of Smad4 does not rescue these responses . Only the concomitant expression of Smad4 and a Ras-resistant Smad3 mutant protein restored antiproliferative TGF-β responses, indicating that both lack of Smad4 and inhibition of Smad3 through oncogenic Ras signaling contribute to the repression of TGF-β/SMAD signaling in this cell line . It is likely that these two mechanisms have evolved at different stages of tumor development, with the Ki-Ras mutation being an earlier event than the Smad4 mutation .
Altered expression of TGF-beta/SMAD inhibitory molecules
Several molecules have been identified that can interfere with SMAD signaling by competitive protein-protein interactions either in the cytoplasm or the nucleus. Two members of the SMAD family, Smad6 and Smad7, inhibit the formation of transcriptionally active SMAD complexes by ligand-induced association with the receptor complex [76**,77,78**] or Smad4 [79**]. Association with the receptor complex prevents the interaction between activated TβR-I and its SMAD substrates, thereby blocking the TGF-β-induced phosphorylation and activation of Smad2 and Smad3. One physiological function of these inhibitory SMAD proteins might be to provide a negative feedback regulation of TGF-β family signaling [78**,80,81]. Northern blot and in situ hybridization analyses indicated that Smad6 and Smad7 are overexpressed in pancreatic cancer tissues and cell lines, suggesting that elevated expression of inhibitory SMAD proteins may contribute to the loss of growth inhibitory TGF-β responses in pancreatic carcinomas [82,83*]. In other studies, however, no increased expression of Smad7 in a panel of pancreatic carcinoma cell lines  and no clear differences in Smad6/7 immunostaining of epithelial cells from normal and tumor tissues of the colon were observed . There have been no reports as yet concerning expression levels of Smad6 and Smad7 in breast tissues.
Very recently, a novel mechanism of interference with certain TGF-β responses emerged. Several SMAD-interacting proteins were identified that serve as transcriptional corepressors in the TGF-β response [85**,86,87**,88,89**,90]. Two of these, the related proto-oncoproteins c-Ski and c-SnoN, associate with Smad2, Smad3, or Smad4 in the nucleus to repress the ability of TGF-β-activated SMAD complexes to activate transcription [85**,86,87**,88,89**]. Both c-Ski and c-SnoN achieve this repression by recruitment of the transcriptional corepressors N-CoR and mSin3 , which in turn associate with histone deacetylases, thereby leading to the formation of a repressive SMAD complex on the target promoter. Similar to a model proposed for another SMAD corepressor, TGIF [90**], the recruitment of c-Ski and c-SnoN is likely to compete with binding of the transcriptional coactivators p300 or CBP that possess histone acetyl transferase activity. The acetylation state of core histones plays a critical role in transcriptional regulation . Thus, the balance between SMAD corepressors and coactivators might be important in determining the transcriptional activities of nuclear SMAD complexes. Overexpression of c-Ski is thought to be sufficient for oncogenic activation , and elevated expression of c-Ski was detected in several tumor cell lines derived from neuroblastoma, melanoma and prostate cancer [94,95]. A thorough analysis of expression levels of c-Ski or c-SnoN in normal and cancerous breast tissue has not been reported, but the recent identification of their involvement in TGF-β signaling warrants closer investigation of this issue. Furthermore, a recent study suggested that c-Ski expression is upregulated in response to estrogen signaling in epithelial cells of the uterus , raising the possibility that c-Ski might also be involved in the interactions between estrogen and TGF-β in breast cancer.
A nuclear zinc-finger protein, Evi-1 is involved in leukemic transformation of hematopoietic cells subsequent to chromosomal translocations that lead to expression of an AML1/Evi-1 fusion product under the control of the AML1 promoter . The biological functions of Evi-1 are not well defined, but one way of contributing to a transformed phenotype may be through its ability to inhibit TGF-β/SMAD signaling [98**]. Evi-1 is proposed to associate specifically with the Smad3 protein, thereby preventing DNA binding and transcriptional activity of Smad3 containing complexes. Evi-1 or the AML1/Evi-1 fusion protein can suppress TGF-β-induced growth inhibition when expressed in lung epithelial or myeloid cells [98**,99]. It has been suggested that Evi-1 might also be involved in solid tumors since overexpression has been observed in ovarian cancer samples  and Evi-1 causes transformation when exogenously expressed in fibroblasts [101,102]. Aberrant expression of Evi-1 in breast cancer, however, has not been reported to date.
Another recent addition to the growing list of molecules that can regulate TGF-β/SMAD signaling by direct association with one of the signaling components is the pseudoreceptor BAMBI [103**]. BAMBI was isolated by an expression screen for molecules involved in BMP4 (a member of the TGF-β family) signaling. BAMBI is related to the TGF-β family type I receptors but lacks an intracellular kinase domain. Its intracellular domain, however, contains the homodimerization interface that allows BAMBI to form complexes with typeI receptors, thereby preventing the formation of functional type I receptor homodimers [103**]. Compared with nonmetastatic melanoma cell lines and normal tissues, the expression of BAMBI in certain metastatic melanoma cell lines is strongly reduced . It will be important to confirm this observation in primary melanoma tissue and in other types of tumors, such as breast cancer, in which a loss of BAMBI may enhance the tumorigenic activity of TGF-β.
Interference with TGF-beta/SMAD responses downstream of SMAD proteins
The antiproliferative response of tumor cells to TGF-β can also be affected by aberrant expression or inactivation of cell cycle regulators that function downstream of or independently of the TGF-β/SMAD pathway. Such alterations are frequently found in human breast tumors and they include increased expression of CyclinD1, CyclinE, MDM2, or c-Myc, decreased expression of p16INK4A or p27Kip1, and mutations in pRB or p53 . For more detailed discussion of these alterations, see [68,105].
A number of different molecular mechanisms have been described that might contribute to the development of resistance to the growth inhibitory effects of TGF-β in breast carcinoma cells. More work needs to be carried out to determine the relative significance of these various mechanisms for the etiology of breast cancer. Each individual case of breast cancer might feature a specific combination of these mechanisms. The nature of this combination is likely to influence the course of the disease by determining the extent of resistance to TGF-β-induced growth inhibition at different stages and by the manifestation of other TGF-β responses that may contribute to the invasive and metastatic potential of the tumor cells. For example, while inactivating mutations in TβR-II should abolish all TGF-β responses, inhibition of SMAD signaling by various mechanisms will block SMAD-dependent responses, such as growth inhibition, to varying extents and will allow potential SMAD-independent TGF-β effects to remain intact. The continued elucidation of TGF-β signaling and its molecular and functional interactions with oncogenic events involved in breast cancer has the potential to lead to novel ways of treating the disease; for example, by selectively restoring tumor suppressive TGF-β functions and/or inhibiting tumor promoting TGF-β functions.
The author is grateful to Dr Joan Massagué and Dr Inna Timokhina for critical comments on the manuscript.
- Massagué J: TGFβ signal transduction. Annu Rev Biochem. 1998, 67: 753-791. 10.1146/annurev.biochem.67.1.753.PubMedView ArticleGoogle Scholar
- Arteaga CL, Dugger TC, Hurd SD: The multifunctional role of transforming growth factor (TGF)-βs on mammary epithelial cell biology. Breast Cancer Res Treat. 1996, 38: 49-56.PubMedView ArticleGoogle Scholar
- Reiss M, Barcellos-Hoff MH: Transforming growth factor-β in breast cancer: a working hypothesis. Breast Cancer Res Treat. 1997, 45: 81-95. 10.1023/A:1005865812918.PubMedView ArticleGoogle Scholar
- Pierce DFJ, Gorska AE, Chytil A, et al: Mammary tumor suppression by transforming growth factor beta 1 transgene expression. Proc Natl Acad Sci USA. 1995, 92: 4254-4258.PubMedPubMed CentralView ArticleGoogle Scholar
- Oft M, Peli J, Rudaz C, Schwarz H, Beug H, Reichmann E: TGF-β1 and Ha-Ras collaborate in modulating the phenotypic plasticity and invasiveness of epithelial tumor cells. Genes Dev. 1996, 10: 2462-2477.PubMedView ArticleGoogle Scholar
- Welch DR, Fabra A, Nakajima M: Transforming growth factor β stimulates mammary adenocarcinoma cell invasion and metastatic potential. Proc Natl Acad Sci USA. 1990, 87: 7678-7682.PubMedPubMed CentralView ArticleGoogle Scholar
- Yin JJ, Selander K, Chirgwin JM, et al: TGF-β signaling blockade inhibits PTHrP secretion by breast cancer cells and bone metastases development. J Clin Invest. 1999, 103: 197-206.PubMedPubMed CentralView ArticleGoogle Scholar
- Oft M, Heider KH, Beug H: TGFβ signaling is necessary for carcinoma cell invasiveness and metastasis. Curr Biol. 1998, 8: 1243-1252. 10.1016/S0960-9822(07)00533-7.PubMedView ArticleGoogle Scholar
- Fynan TM, Reiss M: Resistance to inhibition of cell growth by transforming growth factor-β and its role in oncogenesis. Crit Rev Oncog. 1993, 4: 493-540.PubMedGoogle Scholar
- Alexandrow MG, Moses HL: Transforming growth factor β and cell cycle regulation. Cancer Res. 1995, 55: 1452-1457.PubMedGoogle Scholar
- Kretzschmar M, Massagué J: SMADs: mediators and regulators of TGFβ family signalling. Curr Opin Genet Dev. 1998, 8: 103-111. 10.1016/S0959-437X(98)80069-5.PubMedView ArticleGoogle Scholar
- Derynck R, Zhang Y, Feng XH: Smads: transcriptional activators of TGF-β responses. Cell. 1998, 95: 737-740.PubMedView ArticleGoogle Scholar
- Markowitz S, Wang J, Myeroff L, et al: Inactivation of the type II TGF-β receptor in colon cancer cells with microsatellite instability. Science. 1995, 268: 1336-1338. This study identifies mutations clustered within small repeated sequences in the type II TGF-β receptor gene in colon cancers with DNA repair defects, thereby linking DNA repair defects with the escape from TGF-β-mediated cell growth control in this type of cancer.PubMedView ArticleGoogle Scholar
- Myeroff LL, Parsons R, Kim S-J, et al: A transforming growth factor β receptor type II gene mutation common in colon and gastric but rare in endometrial cancers with microsatellite instability. Cancer Res. 1995, 55: 5545-5547.PubMedGoogle Scholar
- Parsons R, Myeroff LL, Liu B, et al: Microsatellite instability and mutations of the transforming growth factor β type II receptor gene in colorectal cancer. Cancer Res. 1995, 55: 5548-5550.PubMedGoogle Scholar
- Grady WM, Myeroff LL, Swinler SE, et al: Mutational inactivation of transforming growth factor β receptor type II in microsatellite stable colon cancers. Cancer Res. 1999, 59: 320-324.PubMedGoogle Scholar
- D'Abronzo FH, Swearingen B, Klibanski A, Alexander JM: Mutational analysis of activin/transforming growth factor-β type I and type II receptor kinases in human pituitary tumors. J Clin Endocrinol Metab. 1999, 84: 1716-1721. 10.1210/jc.84.5.1716.PubMedView ArticleGoogle Scholar
- Kaneko H, Horiike S, Taniwaki M, Misawa S: Microsatellite instability is an early genetic event in myelodysplastic syndrome but is infrequent and not associated with TGF-β receptor type II gene mutation. Leukemia. 1996, 10: 1696-1699.PubMedGoogle Scholar
- Venkatasubbarao K, Ahmed MM, Swiderski C, et al: Novel mutations in the polyadenine tract of the transforming growth factor β type II receptor gene are found in a subpopulation of human pancreatic adenocarcinomas. Genes Chromosomes Cancer. 1998, 22: 138-144. 10.1002/(SICI)1098-2264(199806)22:2<138::AID-GCC8>3.0.CO;2-Y.PubMedView ArticleGoogle Scholar
- Vincent F, Hagiwara K, Ke Y, Stoner GD, Demetrick DJ, Bennett WP: Mutation analysis of the transforming growth factor β type II receptor in sporadic human cancers of the pancreas, liver, and breast. Biochem Biophys Res Commun. 1996, 223: 561-564. 10.1006/bbrc.1996.0934.PubMedView ArticleGoogle Scholar
- Chen T, Carter D, Garrigue-Antar L, Reiss M: Transforming growth factor β type I receptor kinase mutant associated with metastatic breast cancer. Cancer Res. 1998, 58: 4805-4810. This is the first report of mutations in TβR-I in human cancer. The predominance of the mutation in lymph node metastases suggests an importance for breast cancer progression.PubMedGoogle Scholar
- Takenoshita S, Mogi A, Tani M, et al: Absence of mutations in the analysis of coding sequences of the entire transforming growth factor-β type II receptor gene in sporadic human breast cancers. Oncol Rep. 1998, 5: 367-371.PubMedGoogle Scholar
- Tomita S, Deguchi S, Miyaguni T, Muto Y, Tamamoto T, Toda T: Analyses of microsatellite instability and the transforming growth factor-β receptor type II gene mutation in sporadic breast cancer and their correlation with clinicopathological features. Breast Cancer Res Treat. 1999, 53: 33-39. 10.1023/A:1006167210269.PubMedView ArticleGoogle Scholar
- Vincent F, Nagashima M, Takenoshita S, et al: Mutation analysis of the transforming growth factor-β type II receptor in human cell lines resistant to growth inhibition by transforming growth factor-β. Oncogene. 1997, 15: 117-122. 10.1038/sj.onc.1201166.PubMedView ArticleGoogle Scholar
- Markowitz SD, Roberts AB: Tumor suppressor activity of the TGF-β pathway in human cancers. Cytokine Growth Factor Rev. 1996, 7: 93-102. 10.1016/1359-6101(96)00001-9.PubMedView ArticleGoogle Scholar
- Goggins M, Shekher M, Turnacioglu K, Yeo CJ, Hruban RH, Kern SE: Genetic alterations of the transforming growth factor β receptor genes in pancreatic and biliary adenocarcinomas. Cancer Res. 1998, 58: 5329-5332.PubMedGoogle Scholar
- Schiemann WP, Pfeifer WM, Levi E, Kadin ME, Lodish HF: A deletion in the gene for transforming growth factor β type I receptor abolishes growth regulation by transforming growth factor β in a cutaneous T-cell lymphoma. Blood. 1999, 94: 2854-2861.PubMedGoogle Scholar
- Chen T, de Vries EG, Hollema H, et al: Structural alterations of transforming growth factor-β receptor genes in human cervical carcinoma. Int J Cancer. 1999, 82: 43-51. 10.1002/(SICI)1097-0215(19990702)82:1<43::AID-IJC9>3.0.CO;2-0.PubMedView ArticleGoogle Scholar
- Pasche B, Kolachana P, Nafa K, et al: TβR-I(6A) is a candidate tumor susceptibility allele. Cancer Res. 1999, 59: 5678-5682.PubMedGoogle Scholar
- Anbazhagan R, Bornman DM, Johnston JC, Westra WH, Gabrielson E: The S387Y mutations of the transforming growth factor-β receptor type I gene is uncommon in metastases of breast cancer and other common types of adenocarcinoma. Cancer Res. 1999, 59: 3363-3364. This study failed to identify the mutation reported in [21•] in breast cancer metastases and other forms of adenocarcinomas. The importance of such a mutation for breast or other cancers is therefore questioned.PubMedGoogle Scholar
- Hahn SA, Schutte M, Hoque ATMS, et al: DPC4, a candidate tumor suppressor gene at human chromosome 18q21.1. Science. 1996, 271: 350-353. This study identifies Smad4/DPC4 as a candidate tumor suppressor that is inactivated in a large percentage of pancreatic carcinomas, and possibly also in other human cancers.PubMedView ArticleGoogle Scholar
- Barrett MT, Schutte M, Kern SE, Reid BJ: Allelic loss and mutational analysis of the DPC4 gene in esophageal adenocarcinoma. Cancer Res. 1996, 56: 4351-4353.PubMedGoogle Scholar
- Kim SK, Fan Y, Papadimitrakopoulou V, et al: DPC4, a candidate tumor suppressor gene, is altered infrequently in head and neck squamous cell carcinoma. Cancer Res. 1996, 56: 2519-2521.PubMedGoogle Scholar
- Kawate S, Takenoshita S, Ohwada S, et al: Mutation analysis of transforming growth factor β type II receptor, Smad2, and Smad4 in hepatocellular carcinoma. Int J Oncol. 1999, 14: 127-131.PubMedGoogle Scholar
- Kong XT, Choi SH, Inoue A, et al: Expression and mutational analysis of the DCC, DPC4, and MADR2/JV18-1 genes in neuroblastoma. Cancer Res. 1997, 57: 3772-3778.PubMedGoogle Scholar
- Nagatake M, Takagi Y, Osada H, et al: Somatic in vivo alterations of the DPC4 gene at 18q21 in human lung cancers. Cancer Res. 1996, 56: 2718-2720.PubMedGoogle Scholar
- Schutte M, Hruban RH, Hedrik L, et al: DPC4 gene in various tumor types. Cancer Res. 1996, 56: 2527-2530.PubMedGoogle Scholar
- Eppert K, Scherer SW, Ozcelik H, et al: MADR2 maps to 18q21 and encodes a TGFβ-regulated MAD-related protein that is functionally mutated in colorectal carcinoma. Cell. 1996, 86: 543-552. 10.1016/S0092-8674(00)80128-2. This study identifies Smad2 as a candidate tumor suppressor that is inactivated in some colorectal carcinomas and is regulated specifically by TGF-β signaling. Its inactivation is likely to contribute to loss of TGF-β-mediated growth inhibition in affected cells.PubMedView ArticleGoogle Scholar
- Miyaki M, Iijima T, Konishi M, et al: Higher frequency of Smad4 gene mutation in human colorectal cancer with distant metastasis. Oncogene. 1999, 18: 3098-3103. 10.1038/sj.onc.1202642.PubMedView ArticleGoogle Scholar
- Riggins GJ, Thiagalingam S, Rozenblum E, et al: Mad-related genes in the human. Nat Genet. 1996, 13: 347-349.PubMedView ArticleGoogle Scholar
- Takagi Y, Koumura H, Futamura M, et al: Somatic alterations of the SMAD-2 gene in human colorectal cancers. Br J Cancer. 1998, 78: 1152-1155.PubMedPubMed CentralView ArticleGoogle Scholar
- Uchida K, Nagatake M, Osada H, et al: Somatic in vivo alterations of the JV18-1 gene at 18q21 in human lung cancers. Cancer Res. 1996, 56: 5583-5585.PubMedGoogle Scholar
- Bevan S, Woodford-Richens K, Rozen P, et al: Screening SMAD1, SMAD2, SMAD3, and SMAD5 for germline mutations in juvenile polyposis syndrome. Gut. 1999, 45: 406-408.PubMedPubMed CentralView ArticleGoogle Scholar
- Jonson T, Gorunova L, Dawiskiba S, et al: Molecular analyses of the 15q and 18q SMAD genes in pancreatic cancer. Genes Chromosomes Cancer. 1999, 24: 62-71. 10.1002/(SICI)1098-2264(199901)24:1<62::AID-GCC9>3.0.CO;2-4.PubMedView ArticleGoogle Scholar
- Ikezoe T, Takeuchi S, Kamioka M, et al: Analysis of the Smad2 gene in hematological malignancies. Leukemia. 1998, 12: 94-95. 10.1038/sj.leu.2400888.PubMedView ArticleGoogle Scholar
- Latil A, Pesche S, Valeri A, Fournier G, Cussenot O, Lidereau R: Expression and mutational analysis of the MADR2/Smad2 gene in human prostate cancer. Prostate. 1999, 40: 225-231. 10.1002/(SICI)1097-0045(19990901)40:4<225::AID-PROS3>3.0.CO;2-3.PubMedView ArticleGoogle Scholar
- Maesawa C, Tamura G, Nishizuka S, et al: MAD-related genes on 18q21.1, Smad2 and Smad4, are altered infrequently in esophageal squamous cell carcinoma. Jpn J Cancer Res. 1997, 88: 340-343.PubMedView ArticleGoogle Scholar
- Roth S, Sistonen P, Salovaara R, et al: SMAD genes in juvenile polyposis. Genes Chromosomes Cancer. 1999, 26: 54-61. 10.1002/(SICI)1098-2264(199909)26:1<54::AID-GCC8>3.0.CO;2-D.PubMedView ArticleGoogle Scholar
- Shitara Y, Yokozaki H, Yasui W, et al: No mutations of the Smad2 gene in human sporadic gastric carcinomas. Jpn J Clin Oncol. 1999, 29: 3-7. 10.1093/jjco/29.1.3.PubMedView ArticleGoogle Scholar
- Wieser R, Gruber B, Rieder H, Fonatsch C: Mutational analysis of the tumor suppressor Smad2 in acute lymphoid and myeloid leukemia. Leukemia. 1998, 12: 1114-1118. 10.1038/sj.leu.2401073.PubMedView ArticleGoogle Scholar
- Yokota T, Matsumoto S, Yoshimoto M, et al: Mapping of a breast cancer tumor suppressor gene locus to a 4-cM interval on chromosome 18q21. Jpn J Cancer Res. 1997, 88: 959-964.PubMedView ArticleGoogle Scholar
- Arai T, Akiyama Y, Okabe S, Ando M, Endo M, Yuasa Y: Genomic structure of the human Smad3 gene and its infrequent alterations in colorectal cancers. Cancer Lett. 1998, 122: 157-163. 10.1016/S0304-3835(97)00384-4.PubMedView ArticleGoogle Scholar
- Riggins GJ, Kinzler KW, Vogelstein B, Thiagalingam S: Frequency of Smad mutations in human cancers. Cancer Res. 1997, 57: 2578-2580.PubMedGoogle Scholar
- Wang D, Kanuma T, Takama F, et al: Mutation analysis of the smad3 gene in human ovarian cancers. Int J Oncol. 1999, 15: 949-953. 10.1021/bp990088o.PubMedGoogle Scholar
- Kalkhoven E, Roelen BA, de Winter JP, et al: Resistance to transforming growth factor β and activin due to reduced receptor expression in human breast tumor cell lines. Cell Growth Diff. 1995, 6: 1151-1161. This study demonstrates that in some breast cancer cells, loss of TGF-β growth inhibition is due to reduced expression of TβR-II. Exogenous expression of TβR-II can restore TGF-β sensitivity.PubMedGoogle Scholar
- Ko Y, Banerji SS, Liu Y, et al: Expression of transforming growth factor-β receptor type II and tumorigenicity in human breast adenocarcinoma MCF-7 cells. J Cell Physiol. 1998, 176: 424-434. 10.1002/(SICI)1097-4652(199808)176:2<424::AID-JCP21>3.0.CO;2-1.PubMedView ArticleGoogle Scholar
- Sun L, Wu G, Willson JK, et al: Expression of transforming growth factor β type II receptor leads to reduced malignancy in human breast cancer MCF-7 cells. J Biol Chem. 1994, 269: 26449-26455. This study demonstrates that in a breast cancer cell line, loss of TGFβ growth inhibition is due to reduced expression of TβR-II. Exogenous expression of TβR-II can restore TGFβ sensitivity and reduce malignancy of the cells.PubMedGoogle Scholar
- Chakravarthy D, Green AR, Green VL, Kerin MJ, Speirs V: Expression and secretion of TGF-β isoforms and expression of TGF-β-receptors I,II and III in normal and neoplastic human breast. Int J Oncol. 1999, 15: 187-194.PubMedGoogle Scholar
- Baldwin RL, Friess H, Yokoyama M, et al: Attenuated ALK5 receptor expression in human pancreatic cancer: correlation with resistance to growth inhibition. Int J Cancer. 1996, 67: 283-288. 10.1002/(SICI)1097-0215(19960717)67:2<283::AID-IJC21>3.0.CO;2-B.PubMedView ArticleGoogle Scholar
- Wagner M, Kleeff J, Lopez ME, Bockman I, Massaque J, Korc M: Transfection of the type I TGF-β receptor restores TGF-β responsiveness in pancreatic cancer. Int J Cancer. 1998, 78: 255-260. 10.1002/(SICI)1097-0215(19981005)78:2<255::AID-IJC21>3.0.CO;2-8.PubMedView ArticleGoogle Scholar
- Wang J, Han W, Zborowska E, et al: Reduced expression of transforming growth factor β type I receptor contributes to the malignancy of human colon carcinoma cells. J Biol Chem. 1996, 271: 17366-17371. 10.1074/jbc.271.29.17366.PubMedView ArticleGoogle Scholar
- DeCoteau JF, Knaus PI, Yankelev H, et al: Loss of functional cell surface transforming growth factor β (TGF-β) type 1 receptor correlates with insensitivity to TGF-β in chronic lymphocytic leukemia. Proc Natl Acad Sci USA. 1997, 94: 5877-5881. 10.1073/pnas.94.11.5877.PubMedPubMed CentralView ArticleGoogle Scholar
- Pouliot F, Labrie C: Expression profile of agonistic Smads in human breast cancer cells: absence of regulation by estrogens. Int J Cancer. 1999, 81: 98-103. 10.1002/(SICI)1097-0215(19990331)81:1<98::AID-IJC17>3.0.CO;2-9.PubMedView ArticleGoogle Scholar
- Kretzschmar M, Doody J, Timokhina I, Massagué J: A mechanism of repression of TGFβ/Smad signaling by oncogenic Ras. Genes Dev. 1999, 13: 804-816. This study describes a novel mechanism of regulation of TGF-β/SMAD signaling via phosphorylation of SMAD proteins by Erk MAP-kinase. This mechanism contributes to loss of TGF-β growth inhibition in Ras-transformed epithelial cells, and may be of importance in cancer cells with elevated Ras and Erk activities.PubMedPubMed CentralView ArticleGoogle Scholar
- Calonge MJ, Massague J: Smad4/DPC4 silencing and hyperactive ras jointly disrupt transforming growth factor-β antiproliferative responses in colon cancer cells. J Biol Chem. 1999, 274: 33637-33643. 10.1074/jbc.274.47.33637.PubMedView ArticleGoogle Scholar
- Marshall C: How do small GTPase signal transduction pathways regulate cell cycle entry?. Curr Opin Cell Biol. 1999, 11: 732-736. 10.1016/S0955-0674(99)00044-7.PubMedView ArticleGoogle Scholar
- Rochlitz CF, Scott GK, Dodson JM, et al: Incidence of activating ras oncogene mutations associated with primary and metastatic human breast cancer. Cancer Res. 1989, 49: 357-360.PubMedGoogle Scholar
- Dillon DA, Howe CL, Bosari S, Costa J: The molecular biology of breast cancer: accelerating clinical applications. Crit Rev Oncog. 1998, 9: 125-140.PubMedView ArticleGoogle Scholar
- Press MF, Bernstein L, Thomas PA, et al: HER-2/neu gene amplification characterized by fluorescence in situ hybridization: poor prognosis in node-negative breast carcinomas. J Clin Oncol. 1997, 15: 2894-2904.PubMedGoogle Scholar
- Slamon DJ, Clark GM, Wong SG, Levin WJ, Ullrich A, McGuire WL: Human breast cancer: correlation of relapse and survival with amplification of the HER-2/neu oncogene. Science. 1987, 235: 177-182.PubMedView ArticleGoogle Scholar
- Rajkumar T, Gullick WJ: The type I growth factor receptors in human breast cancer. Breast Cancer Res Treat. 1994, 29: 3-9.PubMedView ArticleGoogle Scholar
- Seshadri R, Firgaira FA, Horsfall DJ, McCaul K, Setlur V, Kitchen P: Clinical significance of HER-2/neu oncogene amplification in primary breast cancer. The South Australian Breast Cancer Study Group. J Clin Oncol. 1993, 11: 1936-1942.PubMedGoogle Scholar
- de Caestecker MP, Parks WT, Frank CJ, et al: Smad2 transduces common signals from receptor serine-threonine and tyrosine kinases. Genes Dev. 1998, 12: 1587-1592.PubMedPubMed CentralView ArticleGoogle Scholar
- Piek E, Heldin CH, Ten Dijke P: Specificity, diversity, and regulation in TGF-β superfamily signaling. FASEB J. 1999, 13: 2105-2124.PubMedGoogle Scholar
- Kinzler KW, Vogelstein B: Lessons from hereditary colorectal cancer. Cell. 1996, 87: 159-170.PubMedView ArticleGoogle Scholar
- Hayashi H, Abdollah S, Qiu Y, et al: The MAD-related protein Smad7 associates with the TGFβ receptor and functions as an antagonist of TGFβ signaling. Cell. 1997, 89: 1165-1173. 10.1016/S0092-8674(00)80303-7. This study identifies a novel mechanism of regulation of TGF-β family signaling that involves a new class of SMAD proteins, the inhibitory SMAD proteins. Inhibition is mediated by competition between the inhibitory SMAD and receptor-regulated SMAD proteins for interaction with the receptors.PubMedView ArticleGoogle Scholar
- Imamura T, Takase M, Nishihara A, et al: Smad6 inhibits signalling by the TGF-β superfamily. Nature. 1997, 389: 622-626. 10.1038/39355.PubMedView ArticleGoogle Scholar
- Nakao A, Afrakhte M, Morén A, et al: Identification of Smad7, a TGFβ-inducible antagonist of TGF-β signaling. Nature. 1997, 389: 631-635. 10.1038/39369.PubMedView ArticleGoogle Scholar
- Hata A, Lagna G, Massagué J, Hemmati-Brivanlou A: Smad6 inhibits BMP/Smad1 signaling by specifically competing with the Smad4 tumor suppressor. Genes Dev. 1998, 12: 186-197. This study identifies a novel mechanism of regulation of TGF-β family signaling that involves a new class of SMAD proteins, the inhibitory SMAD proteins. Inhibition is mediated by competition between the inhibitory SMAD and Smad4 for association with receptor-activated SMAD proteins.PubMedPubMed CentralView ArticleGoogle Scholar
- Afrakhte M, Moren A, Jossan S, et al: Induction of inhibitory Smad6 and Smad7 mRNA by TGF-β family members. Biochem Biophys Res Commun. 1998, 249: 505-511. 10.1006/bbrc.1998.9170.PubMedView ArticleGoogle Scholar
- Takase M, Imamura T, Sampath TK, et al: Induction of Smad6 mRNA by bone morphogenetic proteins. Biochem Biophys Res Commun. 1998, 244: 26-29. 10.1006/bbrc.1998.8200.PubMedView ArticleGoogle Scholar
- Kleeff J, Maruyama H, Friess H, Buchler MW, Falb D, Korc M: Smad6 suppresses TGF-β-induced growth inhibition in COLO-357 pancreatic cancer cells and is overexpressed in pancreatic cancer. Biochem Biophys Res Commun. 1999, 255: 268-273. 10.1006/bbrc.1999.0171.PubMedView ArticleGoogle Scholar
- Kleeff J, Ishiwata T, Maruyama H, et al: The TGF-β signaling inhibitor Smad7 enhances tumorigenicity in pancreatic cancer. Oncogene. 1999, 18: 5363-5372. 10.1038/sj.onc.1202909. This study provides evidence that elevated expression of inhibitory SMAD proteins may play a role in human cancer.PubMedView ArticleGoogle Scholar
- Korchynskyi O, Landstrom M, Stoika R, et al: Expression of Smad proteins in human colorectal cancer. Int J Cancer. 1999, 82: 197-202. 10.1002/(SICI)1097-0215(19990719)82:2<197::AID-IJC8>3.0.CO;2-V.PubMedView ArticleGoogle Scholar
- Akiyoshi S, Inoue H, Hanai J-i, et al: c-Ski acts as a transcriptional Co-repressor in transforming growth factor-β signaling through interaction with Smads. J Biol Chem. 1999, 274: 35269-35277. 10.1074/jbc.274.49.35269. This study identifies the oncoprotein c-Ski as a transcriptional corepressor for SMAD proteins that can inhibit TGF-β signaling when overexpressed. This suggests that inhibition of TGF-β signaling might be an important component of the oncogenic properties of c-Ski.PubMedView ArticleGoogle Scholar
- Luo K, Stroschein SL, Wang W, et al: The Ski oncoprotein interacts with the Smad proteins to repress TGFβ signaling. Genes Dev. 1999, 13: 2196-2206. 10.1101/gad.13.17.2196.PubMedPubMed CentralView ArticleGoogle Scholar
- Stroschein SL, Wang W, Zhou S, Zhou Q, Luo K: Negative feedback regulation of TGF-β signaling by the SnoN oncoprotein. Science. 1999, 286: 771-774. 10.1126/science.286.5440.771. This study identifies the oncoprotein c-SnoN as a transcriptional corepressor for SMAD proteins that maintains TGF-β-responsive genes in a repressed state in the absence of TGF-β signaling and is involved in negative feedback regulation subsequent to TGF-β stimulation.PubMedView ArticleGoogle Scholar
- Sun Y, Liu X, Ng-Eaton E, Lodish HF, Weinberg RA: SnoN and ski protooncoproteins are rapidly degraded in response to transforming growth factor β signaling. Proc Natl Acad Sci USA. 1999, 96: 12442-12447. 10.1073/pnas.96.22.12442.PubMedPubMed CentralView ArticleGoogle Scholar
- Sun Y, Liu X, Eaton EN, Lane WS, Lodish HF, Weinberg RA: Interaction of the Ski oncoprotein with Smad3 regulates TGF-β signaling. Mol Cell. 1999, 4: 499-509.PubMedView ArticleGoogle Scholar
- Wotton D, Lo RS, Lee S, Massague J: A Smad transcriptional corepressor. Cell. 1999, 97: 29-39. This study describes a novel mechanism of regulation of nuclear SMAD activity that involves the interaction of SMAD proteins with a transcriptional corepressor, TGIF. TGIF competes with association of SMAD proteins with the coactivators p300/CBP, presumably resulting in opposing effects on histone acetylation.PubMedView ArticleGoogle Scholar
- Nomura T, Khan MM, Kaul SC, et al: Ski is a component of the histone deacetylase complex required for transcriptional repression by Mad and thyroid hormone receptor. Genes Dev. 1999, 13: 412-423.PubMedPubMed CentralView ArticleGoogle Scholar
- Kouzarides T: Histone acetylases and deacetylases in cell proliferation. Curr Opin Genet Dev. 1999, 9: 40-48. 10.1016/S0959-437X(99)80006-9.PubMedView ArticleGoogle Scholar
- Colmenares C, Sutrave P, Hughes SH, Stavnezer E: Activation of the c-ski oncogene by overexpression. J Virol. 1991, 65: 4929-4935.PubMedPubMed CentralGoogle Scholar
- Fumagalli S, Doneda L, Nomura N, Larizza L: Expression of the c-ski proto-oncogene in human melanoma cell lines. Melanoma Res. 1993, 3: 23-27.PubMedView ArticleGoogle Scholar
- Nomura N, Sasamoto S, Ishii S, Date T, Matsui M, Ishizaki R: Isolation of human cDNA clones of ski and the ski-related gene, sno. Nucleic Acids Res. 1989, 17: 5489-5500.PubMedPubMed CentralView ArticleGoogle Scholar
- Yamanouchi K, Soeta C, Harada R, Naito K, Tojo H: Endometrial expression of cellular protooncogene c-ski and its regulation by estradiol-17β. FEBS Lett. 1999, 449: 273-276. 10.1016/S0014-5793(99)00424-X.PubMedView ArticleGoogle Scholar
- Nucifora G: The EVI1 gene in myeloid leukemia. Leukemia. 1997, 11: 2022-2031. 10.1038/sj.leu.2400880.PubMedView ArticleGoogle Scholar
- Kurokawa M, Mitani K, Irie K, et al: The oncoprotein Evi-1 represses TGF-β signalling by inhibiting Smad3. Nature. 1998, 394: 92-96. 10.1038/27945. This study identifies the oncoprotein Evi-1 as a repressor of growth inhibitory TGF-β signaling. Repression is achieved by direct interaction between Evi-1 and Smad3, resulting in the inhibition of Smad3 transcriptional activity.PubMedView ArticleGoogle Scholar
- Kurokawa M, Mitani K, Imai Y, Ogawa S, Yazaki Y, Hirai H: The t(3;21) fusion product, AML1/Evi-1, interacts with Smad3 and blocks transforming growth factor-β-mediated growth inhibition of myeloid cells. Blood. 1998, 92: 4003-4012.PubMedGoogle Scholar
- Brooks DJ, Woodward S, Thompson FH, et al: Expression of the zinc finger gene EVI-1 in ovarian and other cancers. Br J Cancer. 1996, 74: 1518-1525.PubMedPubMed CentralView ArticleGoogle Scholar
- Kurokawa M, Ogawa S, Tanaka T, et al: The AML1/Evi-1 fusion protein in the t(3;21) translocation exhibits transforming activity on Rat1 fibroblasts with dependence on the Evi-1 sequence. Oncogene. 1995, 11: 833-840.PubMedGoogle Scholar
- Kilbey A, Stephens V, Bartholomew C: Loss of cell cycle control by deregulation of cyclin-dependent kinase 2 kinase activity in Evi-1 transformed fibroblasts. Cell Growth Diff. 1999, 10: 601-610.PubMedGoogle Scholar
- Onichtchouk D, Chen YG, Dosch R, et al: Silencing of TGF-β signalling by the pseudoreceptor BAMBI. Nature. 1999, 401: 480-485. 10.1038/46794. This study describes a novel transmembrane protein, BAMBI, that is similar to type I TGF-β receptors but lacks a kinase domain. BAMBI can inhibit TGF-β family signaling by preventing the formation of signaling competent receptor complexes.PubMedView ArticleGoogle Scholar
- Degen WG, Weterman MA, van Groningen JJ, et al: Expression of nma, a novel gene, inversely correlates with the metastatic potential of human melanoma cell lines and xenografts. Int J Cancer. 1996, 65: 460-465. 10.1002/(SICI)1097-0215(19960208)65:4<460::AID-IJC12>3.0.CO;2-E.PubMedView ArticleGoogle Scholar
- Donovan J, Slingerland J: Transforming growth factor-β and breast cancer: cell cycle arrest by transforming growth factor-β and its disruption in cancer. Breast Cancer Res. 2000, 2: 116-124. 10.1186/bcr43.PubMedPubMed CentralView ArticleGoogle Scholar