Obesity is an established risk factor for endometrial cancer.Leptin,a secreted protein of the ob gene by white adipose tissue,plays an important role in the regulation of food intake and energy consumption in the brain and acts as a potential growth stimulator in normal and neoplastic cancer cells.However,a direct role for leptin in endometrial cancer has not been demonstrated.In the present study,the effect of leptin on the proliferation of Ishikawa endometrial cancer cells was investigated as well as the possible mechanism(s) underlying this action in endometrial cancers which express both short and long isoforms of leptin receptors.The expression of leptin receptor (ObRb) in Ishikawa cells was detected by RT-PCR and Western blotting.The cells after serum starvation,were treated by leptin with various concentrations (0,10,50,100,150 ng/mL) for different durations (6,12,24 h).The effect of leptin treatment on cell proliferation was examined by MTT assay.Meanwhile,inhibitory effect of Janus tyrosine kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) inhibitor AG490 or extracellular signal-regulatedkinase 1/2 (ERK1/2) inhibitor PD98059 on the proliferation of Ishikawa cells induced by leptin was also studied.Ishikawa cells were treated with 100 ng/mL leptin for various periods (0,20,40,60 min),and the levels of STAT3 phosphorylation and ERK1/2 phosphorylation were examined by Western blotting.The results showed that leptin induced the phosphorylation of STAT3and the activation of ERK 1/2 in a time-and dose-dependent manner in the Ishikawa endometrial cancer cells.Blocking STAT3 phosphorylation with the inhibitor AG490,or blocking ERK1/2 activation by the specific ERK1/2 kinase inhibitor,PD98059,abolished leptin-induced proliferation of Ishikawa cells.In addition,leptin was found to potently induce the invasion of endometrial cancer cells in a Matrigel invasion assay.Leptin-stimulated invasion was effectively blocked by pharmacological inhibitors of STAT3 (AG490) and ERK1/2 kinase (PD98059).These results suggested that leptin promotes endometrial cancer growth and invasiveness by activating STAT3 and ERK1/2 signaling pathways and therefore blocking its action at the receptor level can be a rational therapeutic strategy.