Metformin enhances insulin signalling in insulin‐dependent and ‐independent pathways in insulin resistant muscle cells

Abstract

1 Metformin lowers blood glucose levels in type 2 diabetic patients. To evaluate the insulin sensitizing action of metformin on skeletal muscle cells, we have used C2C12 skeletal muscle cells di erentiated in chronic presence or absence of insulin. 2 Metformin was added during the last 24 h of di erentiation of the C2C12 myotubes. Insulinstimulated tyrosine phosphorylation of insulin receptor (IR) and insulin receptor substrate-1 (IRS-1) was determined. 3 Chronic insulin treatment resulted in 60 and 40% reduction in insulin-stimulated tyrosine phosphorylation of IR and IRS-1, respectively. Treatment with metformin was able to increase the tyrosine phosphorylation of IR and IRS-1 by 100 and 90% respectively. 4 Chronic insulin treatment drastically reduced (45%) insulin-stimulated phosphatidyl inositol 3-kinase (PI 3-kinase) activity. Metformin treatment restored PI 3-kinase activity in insulin-resistant myotubes. 5 Insulin-stimulated glucose uptake was impaired in chronically insulin-treated myotubes. Metformin increased basal glucose uptake to signi®cant levels (P50.05), but metformin did not increase insulin-stimulated glucose transport. 6 All the three mitogen-activated protein kinases (MAPK) were activated by insulin in sensitive myotubes. The activation of p38 MAPK was impaired in resistant myotubes, while ERK and JNK were una ected. Treatment with metformin enhanced the basal activation levels of p38 in both sensitive and resistant myotubes, but insulin did not further stimulate p38 activation in metformin treated cells. 7 Treatment of cells with p38 inhibitor, SB203580, blocked insulin-and metformin-stimulated glucose uptake as well as p38 activation. 8 Since the e ect of metformin on glucose uptake corresponded to p38 MAPK activation, this suggests the potential role p38 in glucose uptake. 9 These data demonstrate the direct insulin sensitizing action of metformin on skeletal muscle cells.

Resveratrol stimulates glucose transport in C2C12 myotubes by activating AMP-activated protein kinase

“…Second, AMPK activation by t-RVT definitely potentiated the insulin-induced Akt activation ( Figure 4B), leading to improvement of insulin sensitivity. Quite similar to our results, a couple of recent studies demonstrated that -lipoic acid as well as metformin has been reported to improve insulin sensitivity by activating AMPK (Kumar and Dey, 2002;Lee et al, 2005). Despite many efforts, the mechanism by which AMPK increase insulin sensitivity remains almost unknown.…”

The anti-leishmanial drug miltefosine causes insulin resistance in skeletal muscle cells in vitro

“…2a). Inhibition of insulin-stimulated glucose uptake in skeletal muscle contributes to the development of insulin resistance [6,8]. Insulin signalling is initiated by the recruitment of intracellular molecules to the activated IR-β due to its tyrosine phosphorylation, which activates a number of cellular responses [6,8].…”

“…2b). IRS-1 is a key regulator, downstream from IR-β in the insulin-signalling cascade, and is phosphorylated on multiple tyrosine residues [6,8]. When L6E9 cells were pretreated with miltefosine, insulin-stimulated tyrosine phosphorylation of IRS-1 (1.92-fold) was inhibited down to the basal level (Fig.…”

“…Differentiated myotubes were serumstarved for 6 h, treated with or without miltefosine and then stimulated with or without 100 nmol/l insulin for 10 min. Cell lysis, immunoprecepitation and western immunobloting These methods were performed as described previously [8].…”

“…Glucose uptake assay The 2-deoxyglucose (2-DOG) uptake assay was performed as described previously [8]. Data are presented as picomoles per milligram per minute.…”