RECOMBINANT EXPRESSION OF CAVEOLIN-1 IN ONCOGENICALLY TRANSFORMED-CELLS ABROGATES ANCHORAGE-INDEPENDENT GROWTH

Caveolae are plasma membrane-attached vesicular organelles. Caveolin-1 , a 21-24-kDa integral membrane protein, is a principal component of c aveolae membranes in vivo. Both caveolae and caveolin are most abundan tly expressed in terminally differentiated cells: adipocytes, endothel ial cells, and muscle cells. Conversely, caveolin-1 mRNA and protein e xpression are lost or reduced during cell transformation by activated oncogenes such as v-abl and H-ras (G12V); caveolae are absent from the se cell lines. However, its remains unknown whether down-regulation of caveolin-1 protein and caveolae organelles contributes to their trans formed phenotype. Here, we have expressed caveolin-1 in oncogenically transformed cells under the control of an inducible-expression system. Regulated induction of caveolin-1 expression was monitored by Western blot analysis and immunofluorescence microscopy. Our results indicate that caveolin-1 protein is expressed well using this system and corre ctly localizes to the plasma membrane. Induction of caveolin-1 express ion in v-Abl-transformed and H-Ras (G12V)-transformed NIH 3T3 cells ab rogated the anchorage-independent growth of these cells in soft agar a nd resulted in the de novo formation of caveolae as seen by transmissi on electron microscopy. Consistent with its antagonism of Ras-mediated cell transformation, caveolin-1 expression dramatically inhibited bot h Ras/MAPK-mediated and basal transcriptional activation of a mitogen- sensitive promoter. Using an established system to detect apoptotic ce ll death, it appears that the effects of caveolin-1 may, in part, be a ttributed to its ability to initiate apoptosis in rapidly dividing cel ls. In addition, we find that caveolin-1 expression levels are reversi bly down-regulated by two distinct oncogenic stimuli. Taken together, our results indicate that down-regulation of caveolin-1 expression and caveolae organelles may be critical to maintaining the transformed ph enotype in certain cell populations.