Mitogenic stimulation alters the regulation of LDL receptor gene expression in human lymphocytes

Abstract

To address the possibility that influences other than ambient cholesterol concentrations regulate low density lipoprotein (LDL) receptor expression, the effect of mitogenic activation on the levels of LDL receptor mRNA in human lymphocytes was examined. Mitogenic stimulation of freshly isolated human peripheral blood mononuclear cells (PBMC) cultured in medium containing saturating concentrations of LDL resulted in cell cycle entry as evidenced by increased levels of mRNA for the interleukin-2 receptor, and also increased LDL receptor mRNA levels by 9-fold. Whereas LDL receptor gene expression was also induced when resting control PBMC were incubated in lipoprotein-deficient medium, mitogenic activation of PBMC cultured in the absence of LDL stimulated a further 3-fold increase in LDL receptor mRNA levels. The increase in LDL receptor mRNA levels in mitogen-stimulated PBMC was dependent on continued protein synthesis, was not the result of mRNA stabilization, and therefore most likely reflected enhanced gene transcription. It was unlikely that the increase in LDL receptor mRNA levels observed in mitogen-stimulated cells related merely to sterol deprivation since suppression of endogenous cholesterol synthesis with lovastatin increased LDL receptor mRNA only modestly. Moreover, mitogen-stimulated PBMC continued to synthesize cholesteryl esters, a storage form of cholesterol, confirming that they were not functionally deprived of sterols. Although mitogenic stimulation increased LDL receptor mRNA levels in PBMC, regulation by exogenous LDL was observed. Thus, LDL down-regulated LDL receptor gene expression in both control and mitogen-stimulated PBMC. Down-regulation was less effective in the latter; however, LDL down-regulated endogenous sterol synthesis to an equivalent extent in both control and mitogen-stimulated PBMC. By contrast, the oxygenated sterol, 25-hydroxycholesterol, and mevalonate, the precursor of endogenously synthesized sterols, down-regulated LDL receptor mRNA levels comparably in mitogen-stimulated and control PBMC. These data indicate that mitogenic stimulation provides an additional stimulus for LDL receptor gene expression over and above that of ambient sterols and, therefore, suggest that signals transduced during cellular activation play a role in regulation of LDL receptor mRNA levels.