High glucose increases phosphocofilin via phosphorylation of LIM kinase due to Rho/Rho kinase activation in cultured pig proximal tubular epithelial cells

In proximal tubular epithelial cells (PTECs), depolymerization of actin by cofilin plays a crucial role in maintaining polarity and function. Cofilin is inactivated when phosphorylated by p-Lin-11/Isl-1/Mec-3 kinase (LIMK) to give p-cofilin. LIMK is phosphorylated by phosphorylated p21-activated kinase (PAK), a downstream signal of phosphoinositide 3-kinase (PI3K), or by Rho kinase (ROCK), and is dephosphorylated by slingshot (SSH). However, in PTECs the signaling pathways regulating phosphorylation and dephosphorylation of cofilin, and the influence of high glucose (HG) on these pathways remain to be elucidated. Here, we show that HG in cultured porcine PTECs (LLC-PK1) increases p-cofilin and p-LIMK1 beyond 6 h and that the simultaneous presence of phlorizin reverses the increase. HG did not influence the levels of PI3K-p85, downstream signals to SSH1 and p-PAK1, and mRNA of cofilin, LIMK1 and SSH1. On the other hand, wortmannin and LY294002 markedly increased p-cofilin and p-LIMK1 without influencing on the level of SSH1 protein. HG-activated RhoA and ROCK2 beyond 3 h, and phlorizin attenuated this activation. GF109203X inhibited HG-induced increase in membranous RhoA and ROCK2, and phorbol ester increased these proteins. Y27632 (a ROCK inhibitor) reversed HG-induced increases of p-cofilin and p-LIMK1. We conclude that HG increases p-cofilin by phosphorylating LIMK1 through activation of Rho/Rho kinase, probably due to diacylglycerol-sensitive PKC activation resulting from increased glucose influx. HG did not alter PI3K or its downstream signals, even though PI3K has a physiological role in maintaining the cofilin level by activating SSH1.