Long-term regulation of lipolysis and hormone-sensitive lipase by insulin and glucose.
Adipocyte glucose transport can be impaired by prolonged hyperglycemic conditions. However, at the whole body level, lipolysis is quantitatively a more important function of adipocytes than glucose uptake. We have therefore investigated the effect of prolonged high glucose and insulin on adipocyte lipolysis in basal conditions or with maximal concentrations of adenosine deaminase (ADA), dibutyryl cyclic-AMP (dbcAMP), or isoproterenol (ISO). Neither insulin nor glucose alone affected basal or maximally stimulated lipolysis. However, insulin plus glucose increased the rate of ADA-, dbcAMP-, and ISO-stimulated lipolysis by 40-65%, and the effect was maximal by 8 h. When insulin was kept constant, the half-maximally effective concentration (EC50) of glucose was approximately 2.5 mmol/l. We also demonstrated that the effect is not glutamine-dependent and does not induce insulin resistance of lipolysis. Because the effect of insulin and glucose was evident whether lipolysis was stimulated by ADA, dbcAMP, or ISO, we hypothesized that the expression of the rate-limiting enzyme for lipolysis, hormone-sensitive lipase (HSL), was increased. Our results show that insulin plus glucose-treated cells contain approximately 40% more HSL protein than control cells, in good agreement with the increase in maximally stimulated lipolysis. We conclude that hyperglycemic-hyperinsulinemic conditions increase basal and maximal adipocyte lipolysis by a mechanism that is not glutamine-dependent and involves maintenance of cellular concentrations of HSL. The results also provide evidence that factors other than translocation of HSL to the lipid droplet are necessary to activate the enzyme.