Evidence that Adiponectin Fuels LKB1/ERα Interaction in Breast Cancer Cells

Abstract

BACKGROUNDObesity is characterized by a chronic and excessive growth of adipose tissue and it is increasingly recognized as an oncogenic factor. Due to its endocrine function, adipose tissue secretes several bioactive molecules known as adipocytokines. Among these, adiponectin plays a key role in the pathogenesis of different obesity‐associated diseases, including malignancies, such as breast cancer.The main pathway through which adiponectin exerts its effects is LKB1/AMPK/mTOR signaling. It has been reported that LKB1 plays an essential role in mammary carcinogenesis, regulating apoptosis, proliferation, angiogenesis, energy metabolism and cell polarity. Moreover, low levels of LKB1 have been associated with a more aggressive tumor phenotype and correlate with a poor prognosis. In contrast, LKB1 overexpression induces inhibition of tumor growth, angiogenesis and metastases. In the recent years, our research group has identified ERα as an important regulator of adiponectin action in breast cancer, clarifying that the role of this cytokine seems to be dependent on cell phenotypes.Here, we provided novel insights about the effects of adiponectin on the functional interaction between ERα and LKB1 in ERα‐positive (MCF‐7 and ZR75) and negative (MDA‐MB‐231) breast cancer cells.METHODSRNA levels were assessed by RT‐PCR. Protein expressions were evaluated by immunoblotting in the presence or absence of ERα siRNA and antiestrogen ICI 182, 780. To study promoter activity, transient transfection assays were done. The recruitment of ERα on LKB1 promoter was evaluated by ChIP and DAPA assays. Immunofluorescence assays were performed to evaluate LKB1 localization and function.RESULTSWe demonstrated that adiponectin increased mRNA and protein levels of LKB1 in ERα‐positive breast cancer cells MCF‐7 and ZR75, whereas no significant changes were observed in ERα‐negative MDA‐MB‐231 cells. This up‐regulatory effect was due to the direct action of adiponectin on the promoter region of LKB1 gene. For instance, in MCF‐7 cells, transiently transfected with plasmids containing the full length or a series of 5′ deleted segments of human LKB1 promoter, we identified a sequence (−1626 to −1598) containing Estrogen Responsive Elements (ERE). ChIP and DAPA assays evidenced an increased association of ERα on ERE‐containing sequence proving the recruitment of ERα on LKB1 promoter in adiponectin‐treated cells. Noteworthy, in the presence of ERα siRNA and the pure antiestrogen ICI 182, 780 the upregulation of LKB1 expression was no longer noticeable. It is extremely intriguing to remark how the increasing intracellular pool of LKB1 upon adiponectin was mostly sequestered in the nuclear compartment as coactivator of ERα, which in turn uncoupled it from AMPK interaction, generally leading to the inhibition of cellular energy expenditure.CONCLUSIONSOur results demonstrated how ERα may impact LKB1/AMPK functional interaction upon adiponectin exposure and in such way may negatively interfere in switching cellular energy balance.Support or Funding InformationAssociazione Italiana Ricerca sul Cancro (AIRC) Grants IG‐18602; Progetti di Ricerca di Interesse Nazionale‐Ministero Istruzione Università e Ricerca Grant 2015B7M39T_001This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.