STAT6 silencing up-regulates cholesterol synthesis via miR-197/FOXJ2 axis and induces ER stress-mediated apoptosis in lung cancer cells

MiRNAs and transcription factors have emerged as important regulators for gene expression and are known to regulate various biological processes, including cell proliferation, differentiation and apoptosis. Previously, using genome-wide expression profiling studies, we have shown an inverse relationship of STAT6 and cholesterol biosynthesis and also identified FOXJ2 binding sites in the upstream region of 3 key genes (HMGCR, HMGCS1 and IDI1) of the cholesterol synthesis pathway. Our previous study also provided clues toward the anti-apoptotic role played by STAT6. For better understanding of the cellular response and underlying signaling pathways activated by STAT6 silencing, we examined the changes in miRNome profile after the siRNA-mediated silencing of STAT6 gene in NCI-H460 cells using LNA-based miRNA microarray. Our analysis showed significant downregulation of miRNAs, let-7b and miR-197, out of which miR-197 was predicted to target FOXJ2. We here show that miR-197 not only negatively regulates FOXJ2 expression through direct binding to its respective binding site in its 3'UTR but also alters total cholesterol levels by regulating genes associated with cholesterol biosynthesis pathway. We further demonstrated that STAT6 silencing elicited ER stress-mediated apoptosis in NCI-H460 cells through C/EBP homologous protein (CHOP) induction, alteration of BH3 only proteins expression and ROS production. The apoptosis induced by STAT6 downregulation was partially reversed by NAC, the ROS scavenger. Based on the above findings, we suggest that ER stress plays a major role in STAT6-induced apoptosis.