Uncoupling effects of ERalpha on LKB1/AMPK signaling induced by adiponectin in breast cancer cells

Obesity is increasingly recognized as an oncogenic factor. Adipose tissue is an active metabolic and endocrine organ that secretes many adipocytokines, which act as key mediators in several obesity-associated diseases. Among these adipocytokines, adiponectin has been proposed as having a key role in the pathogenesis of cardiovascular disease and type 2 diabetes along with other diseases such as obesity-associated malignancies, including breast cancer. An inverse correlation is reported between obesity and adiponectin, for which low levels of adiponectin represent a risk factor for mammary cancer. It is emerging how the role of adiponectin on breast tumorigenesis seems to be dependent on cell phenotypes. In this concern, our in vitro and in vivo studies demonstrated that adiponectin, at relatively low concentrations, repressed growth in ERα-negative breast cancer cells whereas increased proliferation in ERα-positive cells. Adiponectin interacts with specific receptors (AdipoR1 and AdipoR2) and exerts its effects, including regulation of cell survival, apoptosis and metastasis, via a plethora of signaling pathways. The key molecule of adiponectin action is AMP-activated protein kinase (AMPK), which is mainly activated by liver kinase B1 (LKB1). Here, we investigated the effect of adiponectin on LKB1/AMPK signaling in ERα-negative (MDA-MB-231) and positive (MCF-7) breast cancer cells. Our results indicate that, in MCF-7 cells, adiponectin was unable to phosphorylate AMPK, while, in MDA-MB-231 cells, we observed an increased AMPK activation, as evidenced by the enhanced phosphorylative status and kinase activity. Concomitantly, we observed a significant activation of downstream effectors, such as mTOR and P70S6K only in ERα-positive cells, resulting in the stimulatory effect of adiponectin on proliferation of MCF-7 cells. The crucial role of ERα in modulating the opposite response to adiponectin, as above reported, rises from the evidence that when we knocked down ERα in MCF-7 cells, by siRNA, AMPK phosphorylation was restored, resulting in the inactivation of mTOR and its substrate P70S6K. Then, we investigated the molecular mechanism through which ERα was able to interfere with AMPK activation. It was extremely intriguing to observe how upon adiponectin exposure LKB1 appeared localized in the nucleus with ERα, as revealed by co-immunoprecipitation and ligation proximity assay. This addresses how, in such circumstance, LKB1 may be recruited in the nucleus impairing its capability to couple with AMPK in the cytosol. This may explain why adiponectin at low concentrations, as observed in obese women, plays a dichotomic effect on breast cancer growth depending on ERα expression.