Thyroid hormone and dehydroepiandrosterone permit gluconeogenic hormone responses in hepatocytes

The importance of the sn-glycerol-3-phosphate (G-3-P) electron transfer shu ttle in hormonal regulation of gluconeogenesis was examined in hepatocytes from rats with decreased mitochondrial G-3-P dehydrogenase activity (thyroi dectomized) or increased G-3-P dehydrogenase activity [triiodothyronine (T- 3) or dehydroepiandrosterone (DHEA) treated]. Rates of glucose formation fr om 10 mM lactate, 10 mM pyruvate, or 2.5 mM dihydroxyacetone were somewhat less in hypothyroid cells than in cells from normal rats but gluconeogenic responses to calcium addition and to norepinephrine (NE), glucagon (G), or vasopressin (VP) were similar to the responses observed in cells from norma l rats. However, with 2.5 mM glycerol or 2.5 mM sorbitol, substrates that m ust be oxidized in the cytosol before conversion to glucose, basal gluconeo genesis was not appreciably altered by hypothyroidism but responses to calc ium and to the calcium-mobilizing hormones were abolished. Injecting thyroi dectomized rats with T-3 2 days before preparing the hepatocytes greatly en hanced gluconeogenesis from glycerol and restored the response to Ca2+ and gluconeogenic hormones. Feeding dehydroepiandrosterone for 6 days depressed gluconeogenesis from lactate or pyruvate but substantially increased gluco se production from glycerol in euthyroid cells and restored responses to Ca 2+ in hypothyroid cells metabolizing glycerol, Euthyroid cells metabolizing glycerol or sorbitol use the G-3-P and malate/aspartate shuttles to oxidiz e excess NADH generated in the cytosol, The transaminase inhibitor aminooxy acetate (AOA) decreased gluconeogenesis from glycerol 40%, but had little e ffect on responses to Ca2+ and NE. However, in hypothyroid cells, with mini mal G-3-P dehydrogenase, AOA decreased gluconeogenesis from glycerol more t han 90%. Thus, the basal rate of gluconeogenesis from glycerol in the euthy roid cells is only partly dependent on electron transport from cytosol to m itochondria via the malate/aspartate shuttle and almost completely dependen t in the hypothyroid state, and the hormone enhancement of the rate in euth yroid cells involves primarily the G-3-P cycle. These data are consistent w ith Ca2+ being mobilized by gluconeogenic hormones and G-3-P dehydrogenase being activated by Ca2+ so as to permit it to transfer reducing equivalents from the cytosol to the mitochondria. (C) 2000 Academic Press.