Glucagon- and Dibutyryl Cyclic AMP-produced Inhibition of Cholesterol Ester Hydrolase in Isolated Rat Hepatocytes: Role of Calcium

The regulation of neutral cytosolic cholesterol ester hydrolase was studied in isolated rat liver cells. Addition of glucagon to cell suspensions caused a decrease in the enzyme activity which was significant at 1 nM concentration. The cyclic nucleotide analogue bibutyryl cyclic AMP (10 and 100 microM) also inhibited the esterase activity. In the absence of calcium, glucagon did not produce any effect on the enzyme. To see if calcium was involved in a regulatory mechanism, cholesterol ester hydrolase activity was measured in cytosol from cells preincubated in a medium without calcium and containing EGTA. This treatment produced a marked reduction in cytosolic Ca2+ concentration with a concomitant threefold stimulation of the esterase activity. Readdition of calcium to Ca2(+)-deprived cells diminished the activation due to calcium deficiency. The present results suggest that 1) cholesterol ester hydrolase could be modulated by a cAMP-mediated mechanism elicited by glucagon in which Ca2+ appears to be involved and 2) the enzyme activity may also be regulated by changes in the intracellular calcium concentration.