| Abstract | The importance of AMPK and Protein Kinase C (PKC) as effectors of metformin (Met) action on glucose uptake (GU) in skeletal muscle cells was investigated. GU in L6 myotubes was stimulated by 2-fold following 16h of Met treatment and acutely enhanced by insulin in an additive fashion. Insulin-stimulated GU was sensitive to PI3K inhibition, whereas that induced by Met was not. Met and its related biguanide, phenformin, stimulated AMPK activation/phosphorylation to a level comparable to that induced by the AMPK activator, AICAR. However, the increase in GU elicited by AICAR was significantly lower than that induced by either biguanide. Expression of a constitutively active AMPK mimicked the effects of AICAR on GU, whereas a dominant-interfering AMPK or shRNA silencing of AMPK prevented AICAR-stimulated GU and Met-induced AMPK-signaling, but only repressed biguanide-stimulated GU by ~20%. Consistent with this, analysis of GU in muscle cells from α1-/-/α2-/- AMPK-deficient mice revealed a significant retention of Met-stimulated GU, being reduced by ~35% compared with that of wild type cells. Atypical PKCs (aPKCs) have been implicated in Met-stimulated GU and, in line with this, Met and phenformin induced activation/phosphorylation of aPKC in L6 myotubes. However, whilst cellular depletion of aPKC (>90%) led to loss in biguanide-induced aPKC phosphorylation it had no effect on Met-stimulated GU, whereas inhibitors targeting novel/conventional PKCs caused a significant reduction in biguanide-induced GU. Our findings indicate that whilst Met activates AMPK, a significant component of Met-stimulated GU in muscle cells is mediated via an AMPK-independent mechanism that involves novel/conventional PKCs. |
