Phorbol esters inhibit low density lipoprotein processing by cultured human fibroblasts

Abstract

A 24 h pretreatment of MRC5 fibroblasts with the protein kinase C activator 12- O-tetradecanoylphorbol 13-acetate (TPA) induced a marked decrease in low density lipoprotein (LDL) internalization and degradation; the maximal effect (about 55% decrease) was observed for 10 −7 M TPA. LDL binding was reduced about 35–40%. A significant decrease (about 25%) in LDL internalization was observed after a 2 h incubation of cells with the drug, but longer incubation times (4–6 h) led to a greater effect. Another tumor promoter, phorbol 12,13-dibutyrate decreased LDL internalization by about 35%, whereas the non-tumor promoting 4α-phorbol 12,13-didecanoate had no effect. The protein kinase C inhibitor α-cobrotoxin partially antagonized the inhibitory effect of TPA on LDL internalization. The non-phorbol tumor promoter mezerein, another protein kinase C activator, decreased LDL uptake by about 50%. Finally, it was found that TPA had no significant effect on the affinity of the receptor for the LDL. These results suggest a role for protein kinase C in the LDL pathway in cultured human fibroblasts.