Divergent roles of SHP-2 in ERK activation by leptin receptors

The protein tyrosine phosphatase SHP-2 has been proposed to serve as a regu lator of leptin signaling, but its specific roles are not fully examined. T o directly investigate the role of SHP-2, we employed dominant negative str ategies in transfected cells. We show that a catalytically inactive mutant of SHP-2 blocks leptin-stimulated ERK phosphorylation by the long leptin re ceptor, ObRb. SHP-2, lacking two C-terminal tyrosine residues, partially in hibits ERK phosphorylation. We find similar effects of the SHP-2 mutants af ter examining stimulation of an ERK-dependent egr-1 promoter-construct by l eptin. We also demonstrate ERK phosphorylation and egr-1 mRNA expression in the hypothalamus by leptin. Analysis of signaling by ObRb lacking intracel lular tyrosine residues or by the short leptin receptor, ObRa, enabled us t o conclude that two pathways are critical for ERK activation. One pathway d oes not require the intracellular domain of ObRb, whereas the other pathway requires tyrosine residue 985 of ObRb. The phosphatase activity of SHP-2 i s required for both pathways, whereas activation of ERK via Tyr-985 of ObRb also requires tyrosine phosphorylation of SHP-2. SHP-2 is thus a positive regulator of ERK by leptin receptors, and both the adaptor function and the phosphatase activity of SHP-2 are critical for this regulation.