Cholesterol is a major component of the plasma membrane in mammalian cells, where it acts as a modulator of bulk physical state and integrity. In addition to its structural role,
cholesterol is essential for proliferation and other cell processes. The present study was undertaken to explore the stringency of the requirement for
cholesterol as a regulator of proliferation and cell cycle progression. Comparisons were made between
cholesterol and other
sterol analogs that differ from
cholesterol in three specific elements: the presence of a Delta5 double bond in ring B, the
hydroxyl group at C-3, and the presence of an aliphatic side chain. The human
leukemia cells HL-60 and MOLT-4 were cultured in
cholesterol-free medium and treated with different
sterols in the presence or absence of
SKF 104976, a competitive inhibitor of
lanosterol 14alpha-demethylase that allows the synthesis of
isoprenoid derivatives but not
cholesterol. Our results show that the beta-
hydroxyl group at C-3 and the unsaturated bond at Delta5 are necessary for cell proliferation and cell cycle progression. The
sterol analog 5alpha-cholestan-3beta-ol (
dihydrocholesterol), which is saturated at Delta5 and has an A/B ring junction in the trans configuration, was also able to support cell growth. However, 5beta-cholestan-3beta-ol and 5beta-cholestan-3alpha-ol, both of which have an A/B ring junction in the cis configuration, were totally ineffective in supporting cell growth. Indeed, they produced an inhibition of cell proliferation and arrested the cell cycle specifically in the G2/M phase. These effects of 5beta-cholestanols were abrogated by
cholesterol in a concentration-dependent manner. Moreover, 5beta-cholestanols potently inhibited
cholesterol biosynthesis and transcription driven by the
sterol response element. In addition to providing a description of the structural features of
sterols associated with their supporting action on cell proliferation in mammalian cells, the present results demonstrate that selected
cholesterol analogs may act as
cytostatic agents, interrupting cell cycle progression specifically in the G2/M phase.