The prevalence of obesity has been steadily increasing over the past few decades in several developed and developing countries, with resultant hazardous health implications. Substantial epidemiological evidence has shown that excessive adiposity strongly influences risk, prognosis, and progression of various malignancies, including breast cancer. Indeed, it is now well recognized that obesity is a complex physiologic state associated with multiple molecular changes capable of modulating the behavior of breast tumor cells as well of the surrounding microenvironment. Particularly, insulin resistance, hyperactivation of insulin-like growth factor pathways, and increased levels of estrogen due to aromatization by the adipose tissue, inflammatory cytokines, and adipokines contribute to breast cancerogenesis. Among adipokines, leptin, whose circulating levels increase proportionally to total adipose tissue mass, has been identified as a key member of the molecular network in obesity. This review summarizes the current knowledge on the epidemiological link existing between obesity and breast cancer and outlines the molecular mechanisms underlying this connection. The multifaceted role of the obesity adipokine leptin in this respect is also discussed.

Tumor phenotype is a result of the complex interactions between malignant cells and sorrounding stroma. However, the mechanisms by which cancer cells and fibroblasts, the most abundant and active part of the tumor stroma, interact remain to be elucidated. The K303R mutation of estrogen-receptor (ERα) was identified in 50% of invasive breast tumors and associated with poorer survival outcomes. Here, we show that human cancer-associated fibroblasts (CAFs) stimulated proliferation and migration of wild-type (WT) ERα stably transfected breast cancer cells and to a higher extent in cells expressing the K303R ERα hyperactive receptor. We identified, for the first time, leptin, a known cytokine involved in breast cancer development, as a determinant for CAFs tumor-promoting activities in both WT and K303R ERα-expressing cells. Indeed, we found an increase in leptin receptor isoforms expression, and in its signalling activation in K303R-expressing cells compared to WT ERα clones. These data correlated well with the amplified effects of leptin on cell growth, motility and invasiveness in mutant cells. Mutant expression generated a leptin hypersensitive phenotype also in vivo. Lastly, K303R ERα cell-secreted factors stimulated CAFs proliferation and migration and their ability to secrete leptin. We demonstrated that the epidermal growth factor is the paracrine factor by which breast cancer cells affect CAFs phenotype. Thus, our work uncovers a bidirectional cross-talk between breast cancer cells and ‘educated’ CAFs, which leads via leptin signaling to increased tumor progression. The blockade of these intercellular communications might represent an effective strategy for molecular targeted therapies in breast cancer.

Adipose tissue is a metabolic and endocrine organ that secretes bioactive molecules called adipocytokines. Among these, adiponectin has a crucial role in obesity-associated breast cancer. The key molecule of adiponectin signaling is AMPK, which is mainly activated by liver kinase B1 (LKB1). Here, we demonstrated that estrogen receptor-α (ERα)/LKB1 interaction may negatively interfere with the LKB1 capability to phosphorylate AMPK and inhibit its downstream signaling TSC2/mTOR/p70S6k. In adiponectin-treated MCF-7 cells, AMPK signaling was not working, resulting in its downstream target acetyl-CoA carboxylase (ACC) being still active. In contrast, in MDA-MB-231 cells, AMPK and ACC phosphorylation was enhanced by adiponectin, inhibiting lipogenesis and cell growth. Upon adiponectin, ERα signaling switched the energy balance of breast cancer cells toward a lipogenic phenotype. Therefore, adiponectin played an inhibitory role on ERα-negative cell growth and progression in vitro and in vivo. In contrast, low adiponectin levels, similar to those circulating in obese patients, acted on ERα-positive cells as a growth factor, stimulating proliferation. The latter effect was blunted in vivo by high adiponectin concentration. All this may have translational relevance, addressing how the handling of adiponectin, as a therapeutic tool in breast cancer treatment, needs to be carefully considered in ERα-positive obese patients, where circulating levels of this adipocytokine are relatively low. In other words, in ERα-positive breast cancer obese patients, higher adiponectin doses should be administered with respect to ERα-negative breast cancer, also opportunely combined with antiestrogen therapy. -Mauro, L., Naimo, G. D., Gelsomino, L., Malivindi, R., Bruno, L., Pellegrino, M., Tarallo, R., Memoli, D., Weisz, A., Panno, M. L., Andò, S. Uncoupling effects of estrogen receptor α on LKB1/AMPK interaction upon adiponectin exposure in breast cancer.

It is now generally accepted that estrogen receptor (ESR1) mutations occur frequently in metastatic breast cancers, however we do not yet know how to best treat these patients. We have modeled the three most frequent hormone binding ESR1 (HBD-ESR1) mutations (Y537N, Y537S, and D538G) using stable lentiviral transduction in human breast cancer cell lines. Effects on growth were examined in response to hormonal and targeted agents, and mutation-specific changes were studied using microarray and western blot analysis. We determined that the HBD-ESR1 mutations alter anti-proliferative effects to tamoxifen (Tam), due to cell-intrinsic changes in activation of the insulin-like growth factor receptor (IGF1R) signaling pathway and levels of PIK3R1/PIK3R3. The selective estrogen receptor degrader, fulvestrant, significantly reduced the anchorage-independent growth of ESR1 mutant-expressing cells, while combination treatments with the mTOR inhibitor everolimus, or an inhibitor blocking IGF1R and the insulin receptor significantly enhanced anti-proliferative responses. Using digital drop (dd) PCR we identified mutations at high frequencies ranging from 12% for Y537N, 5% for Y537S, and 2% for D538G in archived primary breast tumors from women treated with adjuvant mono-tamoxifen therapy. The HBD-ESR1 mutations were not associated with recurrence-free or overall survival in response in this patient cohort, and suggest that knowledge of other cell-intrinsic factors in combination with ESR1 mutation status will be needed determine anti-proliferative responses to Tam.