Overexpression of Cdk5 (wildtype, WT-Cdk5) in SH-SY5Y cells increased leptin-induced pSTAT3 production at 1, 3, and 6 h, whereas DN-Cdk5 (dominant negative without kinase activity) suppressed it at all time points. This was seen for both Y705 and S727, and did not affect the housekeeping gene β-actin. By contrast, WT-Cdk5 reduced SOCS-3 at 1 and 3 h but increased it at 6 h.