Background and aims: Obesity is a major risk factor for the development of colon cancer. Secretion of the hormone leptin from adipocytes is increased in obesity, and serum levels are proportional to body fat mass. Serum leptin levels are an independent risk factor for colon cancer. Leptin receptors are expressed in normal, premalignant and malignant colonic epithelia. We have investigated the effects of leptin on proliferation and apoptosis of colonic cancer cells and the early signalling events involved.
Methods: Proliferation of HT-29 colon cancer cells in response to leptin was assessed by 3-[4, 5-dimethylthiazol-2-y-l]-2, 5-diphenyltetrazolium bromide (MTT) assay, and apoptosis was quantified by enzyme-linked immunosorbent assay (ELISA) for intracellular nucleosomes. Signalling pathways involved were determined by using specific inhibitors, quantification of phosphorylated active intermediates and ELISA of active nuclear-translocated transcription factors.
Results: Leptin stimulated HT-29 cell proliferation and inhibited both serum-starvation and celecoxib-induced apoptosis. The proliferative and anti-apoptotic effects of leptin were abolished by inhibition of JAK2 with AG490, phosphatidylinositol 3'-kinase (PI3 kinase) with LY294002 and c-Jun NH(2)-terminal kinase (JNK) with SP600125. Stimulation of HT-29 cells with leptin increased phosphorylation of JAK2, Akt and JNK. Activation of JAK2 was upstream of PI3 kinase/Akt but not of JNK. Activation of JAK2 was followed by activation and nuclear translocation of STAT3 and JNK activation led to increased activator protein 1 (AP-1) transcriptional activity.
Conclusions: Leptin stimulates proliferation and inhibits apoptosis in human colon cancer cells and may be an important factor in the increased incidence of colon cancer in obesity. This effect involves JAK2, PI3 kinase and JNK and activation of the oncogenic transcription factors signal transducer and activator of transcription (STAT)3 and AP-1.