Figure 1. Cellular signaling pathway for insulin regulation of alternative exon usage in cells. Insulin binds to its surface receptor and initiates the cascade via a tyrosine kinase receptor that activates/binds the insulin receptor substrate (IRS) to further activate phosphoinositide kinase (PI3K) which then activates Akt kinase. This is the pivotal step in splicing, as Akt can phosphorylate splicing (SR) proteins or the RS domain of Clk (CDC2‐like) kinases to activate them; these can then phosphorylate the SR proteins. The result is an alternative use of exons or 5′ splice sites, depending on whether RS domain phosphorylation is required for the repression or activation of a factor. IR, insulin receptor; PDK, protein‐dependent kinase; P, phosphate group.

Figure 2. Akt2 regulates Clk/Sty‐mediated SR protein phosphorylation in insulin signaling. L6 myotubes were transfected as indicated with the empty vector or constructs with Akt2 and Clk1 for 36 h, serum‐starved for 6 h, and treated with the Clk‐specific inhibitor, TG003 (10 µM) or TG009, its control compound, LY294002 (10 µM), or dimethylsulfoxide (DMSO) for 60 min prior to insulin addition (10 nM, 30 min), lysed, and analyzed for SR protein phosphorylation with mAb104 in addition to evaluation of the expression of the transfected genes. The graph is the ratio of phospho‐SR/SR protein obtained from densitometric scans of the blot. Reproduced with permission from Ref. [ 28]; © 2009,.