CHOLESTEROL DEPLETION OF CAVEOLAE CAUSES HYPERACTIVATION OF EXTRACELLULAR SIGNAL-RELATED KINASE (ERK)

Previously we showed that activation of Erk in quiescent cells occurs in the caveolae fraction isolated from fibroblasts. Since the structur e and function of caveolae is sensitive to the amount of cholesterol i n the membrane, it might be that a direct link exists between the conc entration of membrane cholesterol and mitogen-activated protein (LAP) kinase activation. We acutely lowered the cholesterol level of the cav eolae fraction by incubating Rat-1 cells in the presence of either cyc lodextrin or progesterone, Cholesterol-depleted caveolae had a reduced amount of several key protein components of the MAP kinase complex, i ncluding Pas, GrbB, Erk2, and Src. Incubation of these cells in the pr esence of epidermal growth factor (EGF) caused a rapid loss of EGF rec eptor from the caveolae fraction, but the usual recruitment of c-Raf w as markedly inhibited. Despite the reduced amount of c-Raf and Erk2 in the cholesterol-depleted caveolae fraction, EGF caused a hyperactivat ion of the remaining caveolae Erk isoenzymes. This was followed by an increase in the amount of active Erk in the cytoplasm, The increased a mount of activated Erk produced under these conditions was linked to a 8-fold higher level of EGF-stimulated DNA synthesis. Even cholesterol depletion by itself stimulated Erk activation and DNA synthesis. Thes e results suggest that the MAP kinase pathway can connect the choleste rol level of caveolae membrane to the control of cell division.