Variable regulation by insulin of insulin gene expression in HIT-T15 cells

Glucose and insulin are generally considered to express opposite effects on insulin synthesis and secretion from pancreatic islets. For the most part this generalization has arisen from short-term experiments. Our studies focused on the chronic, long-term effects of variable insulin concentrations on insulin gene expression and secretion in cultures of HIT-T15 cells. From passage 70, HIT cells were split and passed weekly for 25 weeks in media containing either (A) 11.1 mmol/l glucose with no insulin added; (B) 11.1 mmol/l glucose with insulin added to maintain a level of approximately 4,000 μU/ml; (C) 0.8 mmol/l glucose with no insulin added; (D) 0.8 mmol/l glucose with insulin added to maintain a level of approximately 4,000 μU/ml; and (E) 0.8 mmol/l glucose with progressively less insulin added over time to mimic the gradual decrease in media insulin levels found in condition A. Our data indicate that during chronic passing of HIT cells, addition of exogenous insulin led to preserved levels of insulin mRNA, insulin content and insulin secretion in cells cultured in media containing 11.1 mmol/l glucose concentration. However, in media containing 0.8 mmol/l glucose concentration, addition of insulin diminished the levels of insulin mRNA, insulin content and insulin secretion. Nonetheless, in all cases exogenously added insulin sustained greater levels of insulin mRNA, insulin content and insulin secretion than the instance wherein media containing a high concentration of glucose only was used. These results indicate that short-term experiments assessing the effects of insulin on beta-cell function do not necessarily apply to chronic, more long-term experiments in which insulin can have variable time- and concentration-dependent effects on insulin gene expression and secretion.