Induction of Beige Adipocyte Markers in 3T3-L1 Cells by Clk1 and PKCbetaII Inhibitors

Excessive dietary intake of fat results in its storage in white adipose tissue (WAT), whereas energy expenditure by lipid oxidation occurs in brown adipose tissue. A third type of fat is beige, and it is observed in WAT depots. Certain WAT can undergo beiging. Numerous factors have roles in beiging. However, little is known about the signaling pathways inducing pre-adipocytes to become beige adipocytes. Known participants are activators of PPARgamma, PGC1alpha, and PRDM16 that induce UCP1-containing mitochondria upon cold exposure. We found a signaling pathway that regulates alternative pre-mRNA splicing is also involved in beiging when it is inhibited. Clk1 kinase phosphorylates proteins with serine and arginine rich domains. Clk1 inhibition by 50 nM TG003 during differentiation of 3T3-L1 adipocytes (post day 3) results in beige-like adipocytes demonstrating increased PGC1alpha and UCP1 mRNA and proteins. TG003 inhibits Clk1, 2, and 4. We demonstrate using siRNA for Clk1, 2, and 4, that Clk1 inhibition increased UCP1, PGC1alpha and PPARgamma mRNA whereas Clk1 and 4 siRNA did not. TG003-treated adipocytes contained fewer lipid droplets and more mitochondria which resulted in significant proton leak. In addition to TG003, CG53353, a PKCbetaII inhibitor, also increased mitochondria number and decreased lipid droplet size. PKCbetaII is an mRNA splice variant regulated by Clk1. Others previously showed that TG003 treatment of mice on high fat diets (HFD) resulted in weight loss, and PKCbeta(I+II) knockout mice did not gain weight on HFD. Using bioinformatics, we determined that PGC1alpha was a substrate for Clk1 and PKCbetaII. PGC1alpha was shown to be phosphorylated by Clk2 previously. Hence, we believe that inhibition of Clk1 and PKCbetaII phosphorylation of PGC1alpha results in upregulation of UCP1 expression in adipocytes. This newly identified pathway supports a role for Clk1 and PKCbetaII as potential targets for inhibition to treat weight loss in obesity. Disclosure A. Patel: None. T. Dobbins: None. S. Kong: None. N.A. Patel: None. D.R. Cooper: None. Funding U.S. Department of Veterans Affairs (5|01BX003689)