Characterization of a bile salt-dependent cholesteryl ester hydrolase activity secreted from HepG2 cells

HepG2 cells and medium were assayed for cholesteryl ester hydrolase (CEH) activity in the presence and absence of sodium cholate. Although bile salt-dependent CEH activity was measured in the medium at 6 to 96 h (up to 4500 pmol/h per mg cell protein), there was very little activity detected in the corresponding cell homogenates (less than 70 pmol/h per mg cell protein). Activity in the medium was expressed only in the presence of trihydroxy bile salts and was maximal at 40 mM cholate and pH 7.5. Incubation of HepG2 cells with brefeldin A resulted in an 80 to 90% inhibition of secretion of the bile salt-dependent CEH activity, while only inhibiting total protein secretion by 42%. Bile salt-dependent CEH activity could also be detected in rat liver perfusates. Although there was measurable activity in all of 14 livers analyzed (47 +/- 10 and 53 +/- 17 nmol/h per g liver per h perfusion during two 5-min collections after 15 and 30 min of perfusion, respectively), it did not correlate with the activity found in corresponding liver homogenates, as only four livers had detectable bile salt-dependent CEH activity. These results provide evidence for the secretion of a bile salt-dependent CEH activity, from both a hepatic cell line and the intact liver, that has similar properties to the enzyme previously isolated from rat liver homogenates and rat pancreas.