Background: Skeletal muscles are important target tissues for treatment of type 2 diabetes, as they represent a main site of insulin stimulated glucose uptake in the postprandial state. When examining the signalling pathways related to insulin resistance, we can turn our focus on salicylic acid. Its derivatives not only have anti-inflammatory properties, but also beneficial effects on insulin and metabolism of glucose also because of activation of AMP-activated protein kinase.
Aim: Sulfasalazine is a prodrug from 5-aminosalicylic acid and sulfapyridine. Some clinical reports and animal studies suggested that sulfasalazine could have beneficial effect on concentration of glucose in plasma and prevent the complications of diabetes. Our aim was to determine whether sulfasalazine modulates insulin signalling via Akt kinase.
Hypotheses: 1) Sulfasalazine promotes basal and insulin- or PDGF-stimulated phosphorylation of Akt. 2) Sulfasalazine promotes basal and insulin- or PDGF-stimulated phosphorylation of AS160. 3) Sulfasalazine does not alter insulin- or PDGF-stimulated phosphorylation of ERK1/2.
Methods: We examined the effects of sulfasalazine by using in vitro model of L6 rat skeletal muscle cells. Activation of signalling pathways was assessed by measuring phosphorylation of kinase Akt (Ser473), AS160 (Ser588), ERK1/2 (Thr202/Tyr204), 4E-BP1 (Thr37/Thr46), acetyl-CoA carboxylase (Ser79), using polyacrylamide gel electrophoresis, western blotting and detection of specific proteins with chemiluminescent imaging method.
Results: Sulfasalazine inhibited the insulin-stimulated phosphorylation of Akt, but it did not alter basal or PDGF-stimulated phosphorylation. Sulfasalazine inhibited the phosphorylation of AS160, also in the presence of insulin or PDGF. These results do not support our first and second hypothesis. Sulfasalazine inhibited PDGF-stimulated phosphorylation of ERK1/2, what partially support the third hypothesis.
Conclusions: We found out that sulfasalazine inhibits insulin- and PDGF-stimulated phosphorylation of AS160. Even though sulfasalazine should act as an activator of AMP-activated protein kinase, our results suggest that there is no translocation of GLUT4 on the surface of muscle cells, because phosphorylation of AS160 is the key event in the insulin signalling pathway leading to increased glucose uptake. Additional research should be done to uncover molecular mechanisms by which sulfasalazine suppresses phosphorylation of AS160.