Forkhead Box Protein A2 (FOXA2) Protein Stability and Activity Are Regulated by Sumoylation

The forkhead box protein A2 (FOXA2) is an important regulator of glucose and lipid metabolism and organismal energy balance. Little is known about how FOXA2 protein expression and activity are regulated by post-translational modifications. We have identified that FOXA2 is post-translationally modified by covalent attachment of a small ubiquitin related modifier-1 (SUMO-1) and mapped the sumoylation site to the amino acid lysine 6 (K6). Preventing sumoylation by mutating the SUMO acceptor K6 to arginine resulted in downregulation of FOXA2 protein but not RNA expression in INS-1E insulinoma cells. K6R mutation also downregulated FOXA2 protein levels in HepG2 hepatocellular carcinoma cells, HCT116 colon cancer cells and LNCaP and DU145 prostate cancer cells. Further, interfering with FOXA2 sumoylation through siRNA mediated knockdown of UBC9, an essential SUMO E2 conjugase, resulted in downregulation of FOXA2 protein levels. Stability of sumoylation deficient FOXA2K6R mutant protein was restored when SUMO-1 was fused in-frame. FOXA2 sumoylation and FOXA2 protein levels were increased by PIAS1 SUMO ligase but not a SUMO ligase activity deficient PIAS1 mutant. Although expressed at lower levels, sumoylation deficient FOXA2K6R mutant protein was detectable in the nucleus indicating that FOXA2 nuclear localization is independent of sumoylation. Sumoylation increased the transcriptional activity of FOXA2 on Pdx-1 area I enhancer. Together, our results show that sumoylation regulates FOXA2 protein expression and activity.