GSK3β phosphorylation of the KLF6 tumor suppressor promotes its transactivation of p21

Abstract

KLF6, a ubiquitously expressed Krüppel-like transcription factor, is frequently inactivated in human cancer and has significant roles in cellular proliferation, apoptosis, differentiation and development. A key mechanism of KLF6-mediated growth suppression is through p53-independent transactivation of p21. Several cancer-derived KLF6 mutants lead to the loss of p21-mediated growth suppression through an unknown mechanism. Because several colorectal cancer and hepatocellular carcinoma-derived KLF6 mutations affect a glycogen synthase kinase 3β (GSK3β) phosphorylation consensus site, we investigated the role of GSK3β in the regulation of KLF6 function. Based on transient transfection, GSK3β augments the transactivation of a p21 promoter luciferase by KLF6. Reciprocal co-immunoprecipitation of hemagglutinin (HA)-GSK3β and Flag-KLF6 validated the interaction between these two proteins. KLF6 phosphorylation is augmented in the presence of GSK3β based on in vitro and in vivo 32P incorporation assays. Site-directed mutagenesis of the candidate phosphorylation sites to alanines (‘KLF6-4A’ phosphomutant) eliminated a higher molecular weight phosphorylated isoform of KLF6 based on western blot. GSK3β augmented the transactivation by wild-type KLF6, but not KLF6-4A, towards the p21 promoter, and increased p21 protein. Functionally, GSK3β enhanced KLF6-mediated growth suppression, which was abrogated by the KLF6-4A phosphomutant. These data establish that GSK3β directly phosphorylates KLF6, which augments its induction of p21 and resultant growth suppression. This interaction may account for the growth-promoting effects of cancer-derived KLF6 mutants that lack tumor suppressor activity.

Post-transcriptional activation of PPAR alpha by KLF6 in hepatic steatosis

“…Apoptosis and cellular stress, which are key features of NAFLD, are also closely linked to p53 activity, and thus p53 is an increasing focus in NAFLD research [27]. On a regulatory level, glycogen synthase kinase (GSK3) activation promotes hepatocyte lipoapoptosis, a key feature of NAFLD, and we previously demonstrated that GSK3β directly phosphorylates KLF6, and thus increases its growth suppressor properties in vitro [28, 29]. PPARα, which we identify here as a KLF6-regulated protein, is critical for the development of hepatic steatosis associated with hepatitis C infection, a major risk factor for hepatocellular carcinoma (HCC) [30].…”

KCTD11 inhibits growth and metastasis of hepatocellular carcinoma through activating Hippo signaling

“…Interestingly, MST1 could negatively regulated Akt activity and thereby promote GSK3β [25, 26], which could activate p21 [27]. So, the repression of p21 after KCTD11 knock-down in Huh7 could be a result of up-regulated MST1/GSK3β.…”

Rosiglitazone Role in the Expression of KLF6 Caco-2 Colon Cancer Cells

“…KLF6 plays a key role in cell cycle regulation, differentiation and apoptosis (Lang et al;Bieker;Mallipattu et al, 2015). Furthermore, it has been shown that functional inactivation of KLF6 in various types of cancers such as prostate, ovary and colon, is caused by somatic mutations, loss of heterozygosity (LOH) and silencing by hypermethylation of its promoter Cho et al, 2006;Sangodkar et al, 2009).…”

“…KLF6 has great importance in the control of various processes as cell growth, cell differentiation, apoptosis and angiogenesis (Gehrau et al, 2011;Lang et al, 2013). In relation to its function, it is known that KLF6 is a tumor suppressor, also inactivation either by loss of heterozygosity (LOH), somatic mutation or methylation of its promoter, leads to the development of various types of cancers such as prostate, colon, gliomas and hepatocellular carcinoma (Di Feo et al, 2009).…”