Activation of Downstream Signals by the Long Form of the Leptin Receptor

TLDR

Leptin signals energy status by activating the long leptin receptor, which triggers Jak2 kinase and downstream phosphotyrosine-dependent pathways. The study investigates how specific intracellular tyrosine residues of the leptin receptor mediate downstream signaling pathways, using a chimeric erythropoietin receptor system. Using this chimeric receptor, the authors mapped phosphorylation of Tyr985 and Tyr1138 and their downstream effectors SHP‑2, GRB‑2, STAT3, ERK, c‑fos, and SOCS3. Tyr985 phosphorylation recruits SHP‑2 to drive ERK activation and c‑fos expression, whereas Tyr1138 phosphorylation activates STAT3 to induce SOCS3, showing that the two tyrosines mediate distinct signaling cascades.

Abstract

The adipocyte-derived hormone leptin signals the status of body energy stores by activating the long form of the leptin receptor (LRb). Activation of LRb results in the activation of the associated Jak2 tyrosine kinase and the transmission of downstream phosphotyrosine-dependent signals. We have investigated the signaling function of mutant LRb intracellular domains under the control of the extracellular erythropoietin (Epo) receptor. By using this system, we confirm that two tyrosine residues in the intracellular domain of murine LRb become phosphorylated to mediate LRb signaling; Tyr⁹⁸⁵ controls the tyrosine phosphorylation of SHP-2, and Tyr¹¹³⁸ controls STAT3 activation. We furthermore investigated the mechanisms by which LRb controls downstream ERK activation and c-<i>fos</i> and SOCS3 message accumulation. Tyr⁹⁸⁵-mediated recruitment of SHP-2 does not alter tyrosine phosphorylation of Jak2 or STAT3 but results in GRB-2 binding to tyrosine-phosphorylated SHP-2 and is required for the majority of ERK activation during LRb signaling. Tyr⁹⁸⁵ and ERK activation similarly mediate c-<i>fos</i> mRNA accumulation. In contrast, SOCS3 mRNA accumulation requires Tyr¹¹³⁸-mediated STAT3 activation. Thus, the two LRb tyrosine residues that are phosphorylated during receptor activation mediate distinct signaling pathways as follows: SHP-2 binding to Tyr⁹⁸⁵ positively regulates the ERK → c-<i>fos</i>pathway, and STAT3 binding to Tyr¹¹³⁸ mediates the inhibitory SOCS3 pathway.