Identification and expression analysis of leptin-regulated immediate earlyresponse and late target genes

Using PC12 cells as an in vitro model system, we have identified a series o f transcripts induced through activation of the leptin receptor. On the bas is of kinetic studies, two distinct gene sets could be discerned: signal tr ansducer and activator of transciption-3 (STAT-3), suppressor of cytokine s ignalling-3 (SOCS-3), MT-II (metallothionein-II), the serine/threonine kina se fibroblast-growth-factor-inducible kinase (Fnk) and modulator recognitio n factor (MRF-1), which are immediate early response genes, and pancreatiti s-associated protein I (PAP I), squalene epoxidase, uridine diphosphate glu curonosyltransferase and annexin VIII, which are late induced target genes. At late time points a strong co-stimulation with beta-nerve growth factor or with the adenylate cyclase activator forskolin was observed. To assess t he validity of the PC12-cell model system, we examined the effect of leptin administration on the gene transcription of STAT-3, MT-II, Fnk and PAP I i n vivo. Leptin treatment of leptin-deficient ob/ob mice increased the STAT- 3, SOCS-3, MT-II and Fnk mRNA, and MT-I protein levels in liver, whereas, i n jejunum, expression of PAP I mRNA was down-regulated. Furthermore, admini stration of leptin to starved wild-type mice enhanced the expression of MT- II and Fnk mRNA in liver, but decreased MT-II and PAP I mRNA expression in jejunum. These findings may help to explain the obese phenotype observed in some colonies of MT-I- and MT-II-null mice and/or the observation that lep tin protects against tumour-necrosis-factor toxicity in vivo.