G alpha(12) gep oncogene inhibits FOXO1 in hepatocellular carcinoma as a consequence of miR-135b and miR-194 dysregulation

Abstract

The high mortality rate of hepatocellular carcinoma (HCC) is associated with its fast-growing malignancy. In tumor microenvironments, certain GPCRs are coupled to G alpha(12) for signal transduction. Given the role of forkhead box O1 (FOXO1) in the inhibition of various tumors, this study investigated whether increase of G alpha(12) in HCC causes FOXO1 repression, and if so, whether this event occurs through microRNA dysregulation. Overexpression of an active mutant of G alpha(12) (G alpha(12)QL) decreased FOXO1 levels, whereas knockdown of G alpha(12) had the opposite effect. Of the microRNAs targeting FOXO1, miR-135b levels were markedly increased by G alpha(12) signaling, which led to FOXO1 repression as shown by the experiments using mimic, antisense oligonucleotide or siRNA. G alpha(12)QL increased the primary form of miR-135b by activating JunB (or c-Jun)/AP-1. Consistently, knockdown of JunB (or c-Jun) decreased miR-135b levels, thereby increasing FOXO1. Moreover, G alpha(12) QL induced MDM2, the deficiency of which facilitated FOXO1 accumulation. In addition, G alpha(12)QL repressed miR-194 cluster gene products (194/192/215), which contributed to MDM2-mediated FOXO1 repression. In functional assays, G alpha(12)QL facilitated tumor cell growth with alterations in cell cycle-associated protein levels, which was antagonized by enforced expression of FOXO1. In human HCCs, FOXO1 levels were decreased as compared with the surrounding liver tissue. Moreover, decrease of FOXO1 or miR-194 was statistically significant between stages T1 and T2, whereas increase of miR-135b discriminated tumor stage T3a versus T1/T2. In conclusion, G alpha(12) gep oncogene inhibits FOXO1, which may result from the inhibition of FOXO1 de novo synthesis by miR-135b in conjunction with MDM2-mediated destabilization of FOXO1. (C) 2014 Elsevier Inc. All rights reserved.