A potential role for sterol 27-hydroxylase in atherogenesis

Objective: 27-hydroxycholesterol is the product of the mitochondrial cytoch rome P450 sterol 27-hydroxylase, a key enzyme in cholesterol metabolism pre sent in most tissues of the body. 27-hydroxycholesterol increases in abunda nce with progression of human atherosclerotic lesions, therefore the aim of this study was to determine the pattern of sterol 27-hydroxylase gene expr ession in normal and diseased arteries and to identify the cell types respo nsible for its expression, Methods: Reverse transcription-polymerase chain reaction (RT-PCR) analysis and in situ hybridisation, utilising a sterol 27 -hydroxylase cDNA probe, and immunohistochemistry, utilising an antibody to sterol 27-hydroxylase, together with an antibody to smooth muscle cell alp ha -actin and an antibody to CD68, a marker for macrophages, were used to s tudy expression of 27-hydroxylase in arterial specimens. In addition, RT-PC R was used to study expression of 27-hydroxylase in cultured macrophages an d smooth muscle cells. Results: Semi-quantitative RT-PCR analysis of normal and atherosclerotic human aortas showed that 27-hydroxylase is constitutiv ely expressed in the normal artery wall, and is substantially up-regulated in atherosclerosis. RT-PCR analysis of 27-hydroxylase expression in vitro d emonstrated that macrophages constitutively express high levels throughout their differentiation in culture whilst de-differentiated vascular smooth m uscle cells express very low levels. In situ hybridisation revealed that in normal artery and fatty streaks, expression of mRNA for 27-hydroxylase was low in the media, but higher in intimal smooth muscle cells. The macrophag es of fatty streaks expressed low or undetectable levels of 27-hydroxylase. However in advanced lesions the highest expression of 27-hydroxylase was d etectable in macrophages. Immunohistochemistry demonstrated that high level s of 27-hydroxylase protein occurred in macrophages near the shoulder regio n of plaques, at the edge of the lipid core. Conclusions: 27-hydroxylase ma y constitute a protective mechanism for removing cholesterol from macrophag es and smooth muscle cells. Genetic heterogeneity resulting in differences in sterol 27-hydroxylase activity between individuals may affect their abil ity to deal with accumulated cholesterol in the arterial intima, and hence their relative degree of predisposition to atherosclerosis. (C) 2001 Elsevi er Science Ireland Ltd. All rights reserved.