THE SELECTIVE UPTAKE OF THE CHOLESTERYL ESTERS OF LOW-DENSITY LIPOPROTEINS PARALLELS THE ACTIVITY OF PROTEIN-KINASE-C

The analysis of the association of I-125-LDL and [H-3]cholesteryl ethe rs (CEt)-LDL with HepG2 cells revealed a selective uptake of cholester yl esters (CE) of the LDL, as in the order of three-fold more CE were associated with the cells than LDL-proteins for an incubation of 4 h. To determine if a trans-signalling pathway is involved in this selecti ve uptake, HepG2 cells were pre-treated for 2 h with either a Protein Kinase A activator [8-(4-chlorophenylthioadenosine 3'-5' cyclic monoph osphate (CPT-cAMP)] or a Protein Kinase C activator [phorbol 12-myrist ate 13-acetate (PMA)]. We found that CPT-cAMP had a minimal effect, wh ile PMA was able to significantly increase the selective uptake of the CE of LDL. Indeed, upon a 2 h pre-incubation of HepG2 cells with PMA at a concentration of 160 mu M, an increase of more than 3-fold in CE selective uptake was registered and was shown to occur by the lipoprot ein binding sites (LBS) of HepG2 cells. Also, an incubation of the cel ls with 100 nM calphostin C, an inhibitor of protein kinase C, decreas ed the selective uptake by 41%. The effect of PMA is not abolished by either cycloheximide or actinomycin D. However, cycloheximide was show n to potentiate the effect of PMA on the LBS activity, suggesting that a protein which synthesis is affected by cycloheximide is involved in maintaining the LBS activity low. Our results show that the HepG2 cel l activity of CE selective uptake parallels the activity of Protein Ki nase C and suggest that the LBS could be a G-protein linked receptor.