The characterization of structural, growth factor and signaling molecules that facilitate microenvironmental change in the normal and malignant breast was a theme that cut across many sessions at the meeting. At the level of tissue structure, Dale Laird (University of Western Ontario, London, Ontario, Canada) has demonstrated that gap junctional subtypes may play a role in the functional segregation of the myoepithelial and luminal epithelial compartments in the normal mammary gland and that the loss of these structures plays a role in breast tumor progression. In an interesting twist, however, Laird's latest data indicate that forced expression of gap junctional connexin proteins in aggressive tumor lines does not lead to functional communication, at least in two-dimensional monolayer culture. Instead, the connexins were targeted to the lysosomal pathway for degradation in these cells. Despite this novel and potentially significant in vitro finding, connexin expression did decrease the tumorigenicity of these cells in vivo, which led Laird to speculate that the tumor microenvironment may facilitate transitory channel formation that could affect cell–cell adhesion.
Decreased adhesion between cells probably facilitates migration at the invasive front of malignant tumors. One microenvironmental factor that can initiate such a decrease in cell–cell adhesion, at least transiently, is hepatocyte growth factor (HGF). HGF is normally produced in the mammary stroma and acts in a paracrine fashion to stimulate epithelial ductal morphogenesis by activating the c-Met tyrosine kinase receptor. Interestingly, Bruce Elliott (Queens University, Kingston, Ontario, Canada) has shown that HGF can act as an autocrine factor at the invasive front in mammary tumors and has presented new data implicating c-src tyrosine kinase signaling in this process. Specifically, it appears that c-src activity, which is commonly upregulated in breast tumors, impinges on STAT-3 signaling to increase HGF expression. The chronic upregulation of c-src activity in breast tumors has a strong epigenetic component. For example, work from Donald Fujita (University of Calgary, Calgary, Alberta, Canada) indicates that the protein tyrosine phosphatase 1B upregulates c-src activity by dephosphorylating the negative regulatory C-terminal Tyr530 residue of the kinase molecule.
Morag Park's group (McGill University, Montreal, Quebec, Canada) has identified a number of signaling events that contribute to the morphogenic responses elicited by the HGF-mediated activation of the c-Met receptor. At this meeting, Hanane Khoury (from Park's group) presented data demonstrating that a constitutively active form of the ErbB2 receptor tyrosine kinase disrupts cell–cell junctions and initiates an invasive program by coopting pathways that are also utilized by c-Met. Specifically, the oncogenic form of ErbB2 acts via the Gab1 docking protein to initiate a sustained activation of Erk signaling that is necessary for cell junction disruption. Interestingly, this does not occur when the regulatable wild-type ErbB2 receptor is transiently activated. These data go a long way towards reconciling the apparently paradoxical finding that ErbB2 signaling plays critical roles in both differentiative alveolar morphogenesis and neoplastic progression in the breast. Coupling findings such as these together with the combinatorial signaling pathway data being generated by laboratories such as that of Bill Muller (McMaster University, Hamilton, Ontario, Canada) will contribute greatly to the development of the second generation of rational chemotherapeutic agents that is being accelerated by the significant, but still limited, success of the upstream ErbB receptor modulators.
Finally, Evelyn Voura, who is working in the laboratory of Rama Khokha (Ontario Cancer Institute, Toronto, Ontario, Canada), used an elegant in vitro tissue reconstitution approach to show that focal proteolysis initiated by the action of microenvironmentally regulated matrix metalloproteases facilitates the migration of breast tumor cells across endothelial cell membranes. This transendothelial migration is a critical step in tumor extravasation. Specifically targeting this process may therefore lead to the development of another class of rational therapeutic tools that could be used in combination anti-proliferative and anti-migratory therapies to halt the progression of invasive carcinomas attempting to leave the primary tumor – the ultimate architectural disruption that signals the onset of metastasis.