Activation of STAT1 and STAT3 by leptin and IFNγ. RINm5F cells stably expressing LEPRb were stimulated with the indicated concentrations of IFNγ or leptin for 15 min. Tyrosyl phosphorylation of STAT1 and STAT3 was determined by Western blot analysis of total cellular lysates with activation state-specific antibodies. The two bands reacting with the pTyr(705)-STAT1 antibody represent the splice variants STAT1α (91kD) and STAT1β (84 kD). The lower band detected by the STAT3 antibodies is probably the splice variant STAT3β. Total STAT3 immunoreactivity is shown as a control.