Ceramide-activated protein phosphatase involvement in insulin resistance via Akt, serine/arginine-rich protein 40, and ribonucleic acid splicing in L6 skeletal muscle cells.

Elevated TNFα levels are associated with insulin resistance, but the molecular mechanisms linking cytokine signaling to impaired insulin function remain elusive. We previously demonstrated a role for Akt in insulin regulation of protein kinase CβII alternative splicing through phosphorylation of serine/arginine-rich protein 40, a required mechanism for insulin-stimulated glucose uptake. We hypothesized that TNFα attenuated insulin signaling by dephosphorylating Akt and its targets via ceramide-activated protein phosphatase. Western blot analysis of L6 cell lysates demonstrated impaired insulin-stimulated phosphorylation of Akt, serine/arginine-rich protein 40, and glycogen synthase kinase 3β in response to TNFα and the short chain C6 ceramide analog. TNFα increased serine/threonine phosphatase activity of protein phosphatase 1 (PP1) in response to C6, but not insulin, suggesting a ceramide-specific effect. Myriocin, an inhibitor of de novo ceramide synthesis, blocked stimulation of the PP1 activity. Ceram...