Sam68 and ERKs regulate leptin-induced expression of OB-Rb mRNA in C2C12 myotubes.
Acute leptin treatment significantly increases the mRNA of the long isoform of leptin receptor (OB-Rb) in C2C12 myotubes after as little as 30min, without affecting that of the short isoform (OB-Ra). The Sam68 STAR protein has been implicated in leptin signal transduction as an adaptor molecule useful to recruit other signalling proteins. We found that leptin increased Sam68 tyrosine-phosphorylation so decreasing its poly(U)-binding capacity. RT-PCR analysis of the mRNA bound to immunoprecipitated Sam68 showed that Sam68 associated with OB-Rb but not OB-Ra mRNA in control and leptin-treated C2C12 cells. The siRNA-mediated silencing of Sam68 reduced its levels by 89% and abolished the leptin-mediated increase in OB-Rb mRNA. Leptin activates ERKs which in turn might phosphorylate Sam68 modifying its influence on mRNA. We did not observe any changes in Sam68 Ser/Thr phosphorylation but using the specific MEK1 inhibitor PD-98059 showed that leptin-mediated ERK activation is essential for leptin's effect on OB-Rb mRNA expression. Thus it appears that leptin has a positive short-term effect on the regulation of OB-Rb mRNA in C2C12 cells, involving both Sam68 and ERKs. These results might suggest that leptin signal acutely favours its own sensitivity.
Maroni
P
P
Istituto Ortopedico Galeazzi, IRCCS, Milan, Italy.
Citterio
L
L
Piccoletti
R
R
Bendinelli
P
P
D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D000818:Animals; D048049:Extracellular Signal-Regulated MAP Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects*; D006801:Humans; D020738:Leptin / Q000494:pharmacology*; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000187:drug effects* / Q000201:enzymology*; D019000:Phosphotyrosine / Q000378:metabolism; D011072:Poly U / Q000378:metabolism; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000378:metabolism*; D054411:Receptors, Leptin / Q000235:genetics* / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; C513253:Khdrbs1 protein, mouse; D020738:Leptin; D020033:Protein Isoforms; D047428:Protein Kinase Inhibitors; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D054411:Receptors, Leptin; D019000:Phosphotyrosine; D011072:Poly U; D048049:Extracellular Signal-Regulated MAP Kinases