Insulin‐stimulated Rac1‐GTP binding is not impaired by palmitate treatment in L6 myotubes. Issue 24 (27th December 2018)

• Record Type: Journal Article
• Authors: Stierwalt, Harrison D.; Ehrlicher, Sarah E.; Bergman, Bryan C.; Robinson, Matthew M.; Newsom, Sean A.
• Is Part Of: Physiological reports. Volume 6:Issue 24(2018)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2018-12-27
• Subjects: Insulin signaling -- PAK -- Rac1 -- skeletal muscle; Physiology -- Periodicals; 571
• DOI: 10.14814/phy2.13956
• Languages: English
• ISSNs: 2051-817X

Abstract

Abstract: Ras‐related C3 botulinum toxin substrate 1 (Rac1) is required for normal insulin‐stimulated glucose transport in skeletal muscle and evidence indicates Rac1 may be negatively regulated by lipids. We investigated if insulin‐stimulated activation of Rac1 (i.e., Rac1‐GTP binding) is impaired by accumulation of diacylglycerols (DAG) and ceramides in cultured muscle cells. Treating L6 myotubes with 100 nmol/L insulin resulted in increased Rac1‐GTP binding that was rapid (occurring within 2 min), relatively modest (+38 ± 19% vs. basal, P < 0.001), and short‐lived, returning to near‐basal levels within 15 min of continuous treatment. Incubating L6 myotubes overnight in 500 μ mol/L palmitate increased the accumulation of DAG and ceramides ( P < 0.05 vs. no fatty acid control). Despite significant accumulation of lipids, insulin‐stimulated Rac1‐GTP binding was not impaired during palmitate treatment ( P = 0.39 vs. no fatty acid control). Nevertheless, phosphorylation of Rac1 effector protein p21‐activated kinase (PAK) was attenuated in response to palmitate treatment ( P = 0.02 vs. no fatty acid control). Palmitate treatment also increased inhibitory phosphorylation of insulin receptor substrate‐1 and attenuated insulin‐stimulated phosphorylation of Akt at both Thr308 and Ser473 (all P < 0.05 vs. no fatty acid control). Such signaling impairments resulted in near complete inhibition of insulin‐stimulated translocation of glucose transporter protein 4 (GLUT4; P = 0.10 vs. basal during palmitate treatment). In summary, our finding suggests that Rac1 may not undergo negative regulation by DAG or ceramides. We instead provide evidence that attenuated PAK phosphorylation and impaired GLUT4 translocation during palmitate‐induced insulin resistance can occur independent of defects in insulin‐stimulated Rac1‐GTP binding. Abstract : The purpose of this study was to investigate if insulin‐stimulated activation of Rac1 (i.e., Rac1‐GTP binding) is impaired by the accumulation of diacylglycerols and ceramides in cultured muscle cells. In L6 myotubes, palmitate‐induced lipid accumulation did not result in impairments to insulin‐stimulated Rac1‐GTP binding; however, activation of downstream Rac1 effector PAK and Akt activation were attenuated following palmitate treatment. In summary, our finding suggest that Rac1 may not undergo negative regulation by DAG or ceramides.