Growth hormone-dependent differentiation of 3T3-F442A preadipocytes requires Janus kinase signal transducer and activator of transcription but not mitogen-activated protein kinase or p70 S6 kinase signaling

The signals mediating growth hormone (GH)-dependent differentiation of 3T3- F442A preadipocytes under serum-free conditions have been studied. GH primi ng of cells was required before the induction of terminal differentiation b y a combination of epidermal growth factor, tri-iodothyronine, and insulin. Cellular depletion of Janus kinase-2 (JAK-2) using antisense oligodeoxynuc leotides (ODNs) prevented GH-stimulated JAK-2 and signal transducer and act ivator of transcription (STAT)-5 tyrosine phosphorylation and severely atte nuated the ability of GH to promote differentiation. Although p42(MAPK)/p44 (MAPK) mitogen-activated protein kinases were activated during GH priming, treatment of cells with PD 098059, which prevented activation of these kina ses, did not block GH priming. However, antisense ODN-mediated depletion of mitogen-activated protein kinases from the cells showed that their express ion was necessary for terminal differentiation. Similarly, although p70(s6k ) was activated during GH priming, pretreatment of cells with rapamycin, wh ich prevented the activation of p70(s6k), had no effect on GH priming. Howe ver, rapamycin did partially block epidermal growth factor, tri-iodothyroni ne, and insulin-stimulated terminal differentiation. By contrast, cellular depletion of STAT-B with antisense ODNs completely abolished the ability of GH to promote differentiation. These results indicate that JAK-2, acting s pecifically via STAT-5, is necessary for GH-dependent differentiation of 3T 3-F442A preadipocytes. Activation of p42(MAPK)/p44(MAPK) and p70(s6k) is no t essential for the promotion of differentiation by GH, although these sign als are required for GH-independent terminal differentiation.