[Effects of transcription factor sterol regulatory element binding protein-1c in palmitate acid-induced L6 cells insulin resistance and its mechanism].
OBJECTIVE
Sterol regulatory element binding protein-1c (SREBP-1c) is a master regulator of fatty acid synthase and controls lipogenesis. And insulin receptor substrate-1 (IRS-1) is a key insulin signaling mediator in skeletal muscle. The present study was conducted to explore the mechanism of SREBP-1c in the regulation of IRS-1 in skeletal muscle cells and elucidate the role of SREBP-1c in high fat-induced skeletal muscle insulin resistance.


METHODS
L6 cells differentiated into myotubes in differentiation medium with 2%FBS. An in vitro insulin resistant model in L6 myotubes was established by 500 µmol/L of palmitate acid (PA). SREBP-1c, p-IRS-1(Tyr608/612), IRS-1, p-AKT (Ser473) and AKT were detected by Western blot after incubating L6 myobutes with 500 µmol/L of PA for 0.5, 1, 3, 6, 12, 18 or 24 h.SREBP-1c, FAS and molecules related to insulin signaling pathway were detected by Western blot when L6 myotubes over-expressed SREBP-1c or after a treatment of liver X receptor (LXR) agonist (TO901317, 5 µmol/L). The regulatory effects of transcription factor SREBP-1c on promoter region of IRS-1 were assessed by dual-luciferase reporter assay.


RESULTS
SREBP-1c protein expression increased significantly after 1-hour exposure to PA. The protein levels of p-IRS-1(Tyr608/612),IRS-1 and p-AKt (Ser473) decreased significantly after a 6-hour incubation of PA. However AKT protein levels were unaffected. The protein expressions of SREBP-1c and FAS were up-regulated by LXR agonist treatment versus controls. By contrast, LXR agonist treatment led to decreased expressions of IRS-1, p-IRS-1(Tyr608/612) and p-AKt (Ser473)/AKT proteins versus controls. The expressions of related proteins were similar to the observations made with LXR agonist intervention. The results of dual-luciferase reporter assay indicated that IRS-1 promoter activity was repressed significantly by SREBP-1c over-expression or TO901317 treatment whereas the dominant negative form of SREBP-1c (a mutant of Tyr320Ala lacking the ability of binding DNA) had no effect.


CONCLUSIONS
SREBP-1c may suppress IRS-1 expression and the subsequent insulin signaling pathway. And it plays a key role in PA-induced insulin resistance of skeletal muscle.
Department of Endocrinology, Wuxi People's hospital of Nanjing Medical University; Department of Endocrinology, Drum Tower Clinical Medical College of Nanjing Medical University Wuxi 214001, China.
Wenjun
W
Wu
Yan
Y
Bi
Yinyan
Y
Tangsun
Wenwen
W
Yin
Yingying
Y
Chen
Department of Endocrinology,Drum Tower Clinical Medical College,Nanjing Medical University, Nanjing 210008, China. Email: zhudalong@nju.edu.cn.
Dalong
D
Zhu
D002460:Cell Line; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D007333:Insulin Resistance; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; D010168:Palmitates; D011401:Promoter Regions, Genetic; D015398:Signal Transduction; D051780:Sterol Regulatory Element Binding Protein 1; D015854:Up-Regulation
D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; D010168:Palmitates; D051780:Sterol Regulatory Element Binding Protein 1