EFFECTS OF MILD HEAT-SHOCK ON GLYCOGENESIS AND ITS REGULATION BY INSULIN IN CULTURED FETAL HEPATOCYTES

The effects of a mild heat shock were investigated using cultured 15-d ay-old fetal rat hepatocytes in which an acute glucocorticoid-dependen t glycogenic response to insulin was present. After exposure from 15 m in to 2 h at 42.5 degrees C, cell surface [I-125]insulin binding progr essively decreased down to 60% of the value shown in cells kept at 37 degrees C, due to a decrease in the apparent number of insulin binding sites with little change in insulin receptor affinity. In parallel cu ltures, protein labeling with [S-35]methionine exhibited stimulated sy nthesis of specific proteins, in particular, 73-kDa Hse (heat shock co gnate) and 72-kDa Hsp (heat shock protein). When cells were returned t o 37 degrees C after 2 h at 42.5 degrees C, cell surface insulin bindi ng showed a two-third restoration within 3 h (insulin receptor half-li fe = 13 h), with similar concomitant return of Hsps72,73 synthesis to preinduction levels. The rate of [C-14]glucose incorporation into glyc ogen measured at 37 degrees C after 1- to 2-h heal treatment revealed a striking yet transient increase in basal glycogenesis (up to 5-fold) . At the same time, the glycogenesis stimulation by insulin was reduce d (from 3.2 to 1.4-fold), whereas that induced by a glucose load was m aintained. induction of thermotolerance after a first heating was obta ined for the heat shock-dependent events except for the enhanced basal glycogenesis. In insulin-unresponsive cells grown in the absence of g lucocorticoids, heat shock decreased the glycogenic capacity without m odifying the glucose load stimulation, supporting the hypothesis that insulin and thermal stimulation of glycogenesis share at least part of the same pathway. Inverse variations were observed between Hsps72,73 synthesis and both cell surface insulin receptor level and insulin gly cogenic response in fetal hepatocytes experiencing heat stress. (C) 19 95 Wiley-Liss, Inc.