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Experimental Studies |
Medical Research Council Group in Molecular Endocrinology, CHUL Research Center and Laval University, Quebec, Canada G1V 4G2
Address all correspondence and requests for reprints to: Dr. Alain Bélanger, Medical Research Council Group in Molecular Endocrinology, CHUL Research Center, 2705 Laurier Boulevard, Quebec, Canada G1V 4G2.
Dehydroepiandrosterone (DHEA)- and pregnenolone (PREG)-fatty acid esters (FA) are formed in plasma high density lipoproteins (HDL), whereas they accumulate in very low density lipoproteins (VLDL), low density lipoproteins (LDL), and HDL. We have hypothesized that these lipoidal steroids could be transferred from HDL to VLDL and LDL by the cholesteryl ester (CE) transfer protein (CETP), which mediates CE transfer activity in human plasma. In this study, we further investigated this hypothesis. Lipoproteins and lipoprotein-deficient plasma (LPDP) were purified and analyzed by Western blots. LPDP was rich in CETP, in contrast to lipoprotein preparations, which contained very low amounts. Using these preparations in in vitro transfer assays, CE transfer from radiolabeled steroid ester-HDL to VLDL or LDL was only observed in the presence of LPDP. In contrast, time- and temperature-dependent transfer of DHEA-FA and PREG-FA were observed in the absence of LPDP. The addition of LPDP had no effect on the DHEA-FA transfer rate, whereas the PREG-FA transfer rate was increased. Moreover, in the absence of LPDP, no decrease in the transfer levels of DHEA-FA and PREG-FA was observed after the removal of CETP from lipoprotein preparations by immunoaffinity column chromatography. The PREG-FA transfer activity of LPDP was studied using the anti-CETP monoclonal antibody TP-2, which is known to block the CE transfer activity of CETP. In the presence of LPDP, this antibody led to a dose-dependent decrease in CE transfer activity, whereas PREG-FA transfer activity was unaffected.
In conclusion, we have shown that DHEA-FA and PREG-FA are transferred from HDL to VLDL and LDL by a CETP-independent mechanism. There is a major difference in the transport of lipoidal steroids by human lipoproteins compared to that of CE.
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