Effects on apoB-100 secretion and bile acid synthesis by redirecting cholesterol efflux from HepG2 cells

Abstract

This study determined the effects of apoA-I, HDL3, or hydroxy-␤ -cyclodextrin on apoB-100 secretion and bile acid synthesis by HepG2 cells. The principal observations were that: 1 ) ApoB-100 secretion into the medium was significantly less after the addition of any of the three agents. 2 ) Triglyceride mass was not significantly changed from control in the medium but was significantly, although modestly, reduced in the cells. 3 ) Neither free cholesterol (FC) nor cholesteryl ester (CE) mass in the medium was changed; by contrast, CE mass was reduced within the cells although FC was not. 4 ) Although the total mass of cholesterol in the medium was unaffected, the proportion associated with apoB-100 was reduced, whereas the proportion associated with the non-apoB-100 fraction was increased. 5 ) There was also an unanticipated, but substantial, increase in bile acid synthesis induced by apoA-I, HDL3, or hydroxy-␤ -cyclodextrin, which was time and concentration dependent, and which was associated with marked increases in cholesterol 7 ␣ -hydroxylase activity. There were no significant changes in ACAT activity and only modest increases in HMG-CoA reductase activity. These findings support previous clinical observations that an elevated apoB-100 can accompany a low HDL cholesterol in normotriglyceridemic subjects. They also point to physiologically important, although still only partially understood, metabolic relationships amongst hepatic apoB-100 secretion, cholesterol efflux, and bile acid synthesis. -Sniderman, A. D., Z. Zhang, J. Genest, and K. Cianflone. Effects on apoB-100 secretion and bile acid synthesis by redirecting cholesterol efflux from HepG2 cells. J. Lipid Res. 2003. 44: 527-532.

ApoA-I Lipidation in Primary Mouse Hepatocytes

“…The increase in apoA-I-associated [ 3 H]cholesterol in the VLDL/LDL fraction in ABCA1Ϫ/Ϫ, when compared with control hepatocytes, is in agreement with the recent reports of Sahoo et al (26) and Sniderman et al (44), which demonstrate a regulatory pool of cholesterol in hepatocytes that is shared for the formation of both VLDL and HDL. Immunoprecipitations of the total efflux media from [ 3 H]cholesterol-LDL-labeled hepatocytes with anti-human apoA-I, anti-murine apoB, or antimurine apoE showed as expected that apoB-containing lipoproteins were major exporters of cholesterol followed by apoA-I and apoE (Fig.…”

“…Previous studies have shown that inhibition of hydroxymethylglutaryl-CoA reductase also inhibited VLDL secretion in cultured hepatocytes (82,83). Cholesterol availability thus appears to be an important determinant of VLDL secretion, and depletion of the regulatory pool of cholesterol by apoA-I made less cholesterol available for VLDL synthesis and secretion (26,44).…”

ABCA1-dependent lipid efflux to apolipoprotein A-I mediates HDL particle formation and decreases VLDL secretion from murine hepatocytes

“…However, this possibility is suggested by the finding of elevated fasting plasma TG in patients with Tangier disease and in abca1 Ϫ/Ϫ mice (53, 54). Sniderman et al (57) have recently reported decreased apoB-100 secretion by HepG2 cells in response to incubation with apoA-I. These results provide further support for a regulatory pool of cholesterol in hepatocytes shared for formation of both VLDL and HDL.…”

Assessing the effects of LXR agonists on cellular cholesterol handling: a stable isotope tracer study

“…Cellular cholesterol metabolism and handling has been examined in terms of its synthesis by measuring the fraction of newly synthesized cholesterol (11,12), in terms of catabolism by measuring levels of bile acids through biochemical assays or HPLC (13,14), in terms of influx by measuring the radioactivity of absorbed [ for cholesterol metabolic studies and has the constitutive ability to synthesize and secrete lipoproteins (21,22). However, it is acknowledged that HepG2 cells differ from primary human hepatocytes in their production of bile acids, a catabolic product of cholesterol (23).…”

“…In this study, cellular cholesterol handling (e.g., synthesis, catabolism, influx, and efflux) was examined using a stable isotope labeling study and a two-compartment modeling scheme. In HepG2 cells, the incorporation of 13 C into cholesterol from [1][2][3][4][5][6][7][8][9][10][11][12][13] C]acetate was analyzed by mass isotopomer distribution analysis in conjunction with nonsteady state, multicompartment kinetic analysis to calculate the cholesterol fluxes. Incubation with synthetic, nonsteroidal LXR agonists (GW3965, T0901317, and SB742881) increased cholesterol synthesis (z10-fold), decreased cellular cholesterol influx (71-82%), and increased cellular cholesterol efflux (1.7-to 1.9-fold) by 96 h. As a consequence of these altered cholesterol fluxes, cellular cholesterol decreased (36-39%) by 96 h. The increased cellular cholesterol turnover was associated with increased expression of the LXR-activated genes ABCA1, ABCG1, FAS, and sterol-regulatory element binding protein 1c.…”