L. Brissette et al., THE SELECTIVE UPTAKE OF THE CHOLESTERYL ESTERS OF LOW-DENSITY LIPOPROTEINS PARALLELS THE ACTIVITY OF PROTEIN-KINASE-C, Biochimica et biophysica acta, L. Lipids and lipid metabolism, 1301(1-2), 1996, pp. 133-140
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.