TNF alpha and leptin inhibit basal and glucose-stimulated insulin secretion and gene transcription in the HIT-T15 pancreatic cells

BACKGROUND: Tumor necrosis factor alpha (TNF alpha), a cytokine produced at inflammatory sites and in adipose tissue, is known primarily for its detri mental effects on insulin action. There is evidence to suggest that TNF alp ha may also influence beta -cell function. Leptin is another adipose tissue -derived hormone that might also act on beta -cells. OBJECTIVE: We explored the independent and combined effects of TNF alpha an d leptin upon basal and glucose-stimulated insulin transcription and secret ion in the HIT-T15 pancreatic beta cell line. METHODS: Cells were cultured for 40 h in the presence of near-normal basal (7 mM) or high (16.7 mM) glucose and treated with either TNF alpha (1, 10 a nd 50 ng/ml) or leptin (10, 50 and 100 ng/ml) or both together. Insulin con centrations were measured by radioimmunoassay. Insulin m RNA levels were ev aluated by a semi-quantitative reverse transcription-polymerase chain react ion (RT-PCR) method, after normalization with beta -actin mRNA. RESULTS: TNF alpha significantly suppressed basal and glucose-stimulated in sulin secretion and proinsulin mRNA transcription in a dose-dependent manne r, an effect that was more powerful in the presence of high glucose. Leptin also inhibited dose-dependent insulin mRNA and protein at both glucose con centrations, but did not appear to further potentiate the suppressive effec ts of TNF alpha CONCLUSION: TNF alpha suppresses both basal and glucose-stimulated insulin transcription and secretion in HIT-T15 cells, an effect that is enhanced si gnificantly by high glucose. Leptin also independently inhibits basal and g lucose-stimulated insulin secretion and transcription but does not modify T NF alpha effects. These effects might contribute to the abnormalities of gl ucose metabolism that characterize conditions of increased TNF alpha and/or leptin production.