From bone to breast and back - the bone cytokine RANKL and breast cancer

Receptor activator of nuclear factor-κB ligand (RANKL) plays a pivotal role in regulating bone homeostasis. Osteoporosis and malignant bone disease secondary to breast cancer are characterized by enhanced RANKL production and increased bone turnover. Thus, denosumab, a monoclonal antibody to RANKL, has been developed and is now approved for various bone loss conditions. Recent results indicate that RANKL may also promote the development and osseous migration of breast cancer.

Breast cancer and osteoporotic fractures aff ect one in eight and one in three women, respectively, during lifetime, ranking these two diseases high on the women's health agenda. In recent years, screening programs have been established and effi cient therapies for the treatment of both diseases have become available. Hormone replace ment therapy is eff ective against bone loss in postmenopausal osteoporosis, but is associated with increased incidence of breast cancer. By contrast, some therapies against osteoporosis may protect against breast cancer, including raloxifene [1] and bisphosphonates [2,3].
Th ere may be even closer links between these two disorders at the molecular level, involving the receptor activator of nuclear factor-κB ligand (RANKL). Its major role in human physiology is to control the diff erentiation and activation of osteoclasts, the bone cells specialized to break down bone [4]. Osteoclasts initiate a sequence of events whereby bone of poor quality is removed and replaced by new bone. Th is remodeling process constantly repairs the skeleton. Th e delicate balance between resorption and formation of bone can be severely impaired by declining sex steroid hormones, as in menopause, or during adjuvant therapy for breast or prostate cancer [5]. At the cellular level, up-regulation of RANKL promotes osteoclast diff erentiation and activity, induces excessive bone resorption, and leads to osteoporotic fractures. Th is common theme applies to several important bone loss disorders, including postmenopausal osteo porosis and osteoporosis associated with aromatase inhibitor and androgen-ablative therapy, osteolytic metastases, myeloma bone disease, and giant cell tumors of the bone [5]. Based on its fundamental role in skeletal homeostasis, RANKL has become a therapeutic target in the treatment of bone disorders, and a monoclonal antibody against RANKL, denosumab, has been approved for the treatment of postmenopausal osteoporosis [6].
Since its discovery, the RANKL/RANK pathway has been implicated in the modulation of several extraskeletal processes, including development of the immune system, vascular biology, skin homeostasis, central control of body temperature, and breast development. For instance, mice that were defi cient of RANK, the receptor of RANKL, did not develop a lactating mammary gland, and their off spring died as a result of starvation [7]. Th e dual role of RANKL/RANK in releasing calcium and other minerals from bone and permitting lactation indicates an evolutionary key role in calcium transfer across generations. Whether the RANKL/RANK pathway also controls epithelial cell growth in breast cancer has long remained unclear.
Recently, two groups have demonstrated that RANKL may be crucial for breast cancer development. Both groups employed a carcinogen-induced murine mammary tumor model supplemented with progesterone, which leads to the development of hormone receptor-positive mammary tumors [8,9]. Upon repeated administration of the carcinogen DMBA (7,12-dimethylbenz[α]anthra cene), along with the progestin medroxy progesterone acetate, mice developed mammary tumors that were associated with increased RANKL expression in the progesterone receptor-positive epithelial compo nent [8,9] and expansion of the CD24-positive/highly CD49-positive cell fraction, a breast cancer stem cell population [8]. RANK-transgenic mice that overexpress the receptor of

Abstract
Receptor activator of nuclear factor-κB ligand (RANKL) plays a pivotal role in regulating bone homeostasis. Osteoporosis and malignant bone disease secondary to breast cancer are characterized by enhanced RANKL production and increased bone turnover. Thus, denosumab, a monoclonal antibody to RANKL, has been developed and is now approved for various bone loss conditions. Recent results indicate that RANKL may also promote the development and osseous migration of breast cancer. Full list of author information is available at the end of the article RANKL had an accelerated incidence of breast cancer after multiparity or medroxyprogesterone acetate treatment [9], whereas interruption of RANKL/RANK signaling attenuated progestin-driven breast cancer. Th e latter was achieved by two diff erent approaches, the use of RANK Δmam mice, in which RANKL has no mammary receptor for its action [8], or application of RANK-Fc protein, which neutralises the eff ects of RANKL [9].
Taken together, the two studies clearly show that RANKL is involved in the development of carcinogeninduced mammary tumorigenesis in mice in the setting of progesterone treatment, and provided proof-ofconcept that blockade of RANKL may attenuate this process. Th ese fi ndings complement an earlier study from Josef Penninger's group where RANKL enhanced breast cancer cell migration into bone and spurred the development of bone metastases [10]. However, these fi ndings are applicable only to the hormone receptorpositive variant of breast cancer, and not to hormone receptor-negative breast cancer, the more aggressive subtype. Th e fi rst translational hurdle to be taken includes a careful characterization of the RANKL/RANK signaling pathway in the development of human breast cancer. If this is confi rmed, one could envision that the RANK status may be determined from human breast cancer specimens, already a routine procedure for the estrogen receptor, the progesterone receptor, and HER2, the receptor for epidermal growth factor. Obviously, one could hypothesize that RANKL blockade with denosumab [6] may be used to prevent breast cancer. A long-term follow-up of the breast cancer incidence in women who had received denosumab for 3 years in the FREEDOM trial [6] may corroborate this hypothesis, and recent results from a study comparing denosumab with zoledronic acid in women with advanced breast cancer indicate superiority of denosumab in preventing skeletalrelated events [11]. A prospective study is currently assessing the eff ects of denosumab in women with early breast cancer (NCT01077154). Th is will reveal whether this promising news from basic research can live up to the great expectations.

Competing interests
LCH has received honoraria for lectures and serving in advisory boards from Merck, Novartis, Nycomed, and Amgen, and has received a research grant from Novartis. TDR has received unrestricted travel grants from Novartis and Amgen. CH has nothing to declare.