POSTTRANSCRIPTIONAL REGULATION OF APOLIPOPROTEIN-E EXPRESSION IN MOUSE MACROPHAGES BY PHORBOL ESTER

Phorbol ester-mediated differentiation of THP-1 cells (a human monocyt ic cell line) into mature macrophages is associated with a transcripti onal induction of apolipoprotein E (apoE) expression [Auwerx, Deeb, Br unzell, Peng and Chait (1988) Biochemistry 27, 2651-2655]. Endotoxin, on the other hand, which may also act through activation of protein ki nase C, is a potent inhibitor of apoE expression in mouse macrophages [Werb and Chin (1983)J. Biol. Chem. 258, 10642-10648]. The present exp eriments examine the effect of phorbol ester, an activator of protein kinase C, on the apoE expression in mouse thioglycollate-elicited peri toneal macrophages. Phorbol ester inhibits apoE expression in a specif ic, time- and dose-dependent manner. A 75 % inhibition in the rate of apoE secretion, but not that of total protein, was observed following a 4.5 h incubation with 160 nM phorbol ester, although nearly full inh ibition was obtained with 40 nM. The changes in apoE secretion were pa ralleled by similar changes in apoE synthesis, indicating synthesis as the primary site of action. The decreased rates of apoE synthesis are shown not to be due to increased apoE degradation. The profound inhib ition of apoE synthesis was not accompanied by significant changes in apoE mRNA levels at any concentration of phorbol ester (up to 16 muM), or length of treatment (up to 24 h), suggesting a post-transcriptiona l locus of regulation of apoE expression. Although the early changes i n apoE synthesis correlate with increased microsomal protein kinase C activity, the suppression of apoE expression persists even during cond itions of nearly complete (> 95 %) loss of protein kinase C activity, suggesting that the direct or indirect effect of protein kinase C on a poE expression is mediated by a stable phosphorylated protein, or that the observed effects are mediated through a protein kinase C species that is not readily downregulated by phorbol esters. The presented stu dies clearly demonstrate the potential importance of the translational regulation of apoE expression through the protein kinase C signal tra nsduction pathway.