Leptin and tumor necrosis factor-alpha induce the tyrosine phosphorylation of signal transducer and activator of transcription proteins in the hypothalamus of normal rats in vivo.

Tumor necrosis factor-alpha (TNFalpha) reduces food intake and participates in the regulation of energy homeostasis. However, TNFalpha signaling in the brain and the potential interaction with leptin have not been investigated to date. Here we studied the tyrosine phosphorylation of STAT (signal transducer and activator of transcription) proteins in the hypothalamus of normal rats after iv injection of recombinant murine leptin or TNFalpha or coinjection of both cytokines. Immunoblot analysis of hypothalamic lysates with a phospho-specific STAT3 antibody showed a 6- to 7-fold stimulation of STAT3 tyrosine phosphorylation in response to both leptin and TNFalpha. Importantly, when coinjecting both cytokines, a remarkable synergistic activation (24-fold increase in STAT3 phosphorylation) could be detected. No other STAT proteins (STAT1, STAT5) were activated by leptin, whereas TNFalpha injection resulted in a dose-dependent phosphorylation of hypothalamic STAT5. In contrast to its action in the brain, leptin was unable to produce STAT3 phosphorylation in the liver, either alone or in combination with TNFalpha. These data show that TNFalpha, independently of leptin, activates hypothalamic STAT signaling pathways and enhances leptin action at the level of STAT3. We therefore suggest that TNFalpha may represent a modulator of leptin action in the hypothalamus.