Cholesterol Depletion of Caveolae Causes Hyperactivation of Extracellular Signal-related Kinase (ERK)

Abstract

Previously we showed that activation of Erk in quiescent cells occurs in the caveolae fraction isolated from fibroblasts. Since the structure and function of caveolae is sensitive to the amount of cholesterol in the membrane, it might be that a direct link exists between the concentration of membrane cholesterol and mitogen-activated protein (MAP) kinase activation. We acutely lowered the cholesterol level of the caveolae fraction by incubating Rat-1 cells in the presence of either cyclodextrin or progesterone. Cholesterol-depleted caveolae had a reduced amount of several key protein components of the MAP kinase complex, including Ras, Grb2, Erk2, and Src. Incubation of these cells in the presence of epidermal growth factor (EGF) caused a rapid loss of EGF receptor from the caveolae fraction, but the usual recruitment of c-Raf was markedly inhibited. Despite the reduced amount of c-Raf and Erk2 in the cholesterol-depleted caveolae fraction, EGF caused a hyperactivation of the remaining caveolae Erk isoenzymes. This was followed by an increase in the amount of active Erk in the cytoplasm. The increased amount of activated Erk produced under these conditions was linked to a 2-fold higher level of EGF-stimulated DNA synthesis. Even cholesterol depletion by itself stimulated Erk activation and DNA synthesis. These results suggest that the MAP kinase pathway can connect the cholesterol level of caveolae membrane to the control of cell division.