Caveolin isoforms in resident and elicited rat peritoneal macrophages

Caveolin - an integral membrane protein - is the principal component of caveolae membranes in vivo, Multiple forms of caveolin have been identified: caveolin-1 alpha, caveolin-1 beta, caveolin-2 and caveolin-3. They differ in their specific properties and tissue distribution. When we studied the lysate of resident and elicited macrophages isolated from rat peritoneal cavity by Western blot analysis, we identified two different proteins (similar to 29 kDa and similar to 20 kDa) which were labelled with anti-caveolin antibodies, The similar to 20-kDa protein was labelled specifically only by antiVIP21/caveolin-1, while the similar to 29-kDa protein was labelled by anti-VLP21/caveolin-1 and anti-caveolin-2. The presence of the similar to 29-kDa protein was characteristic of resident macrophages, and only a smalt amount of the similar to 20-kDa protein was detected in these cells, Elicitation resulted in a significant increase in the amount of the similar to 20-kDa protein labelled by anti-VIP21/caveolin-1 only. According to its molecular mass and antibody-specificity, this protein might be identical with the caveolin-1 beta isoform. Our morphological (confocal and electron microscopical) studies have shown that in resident cells caveolin was present in the cytoplasm, in smaller vesicles and multivesicular bodies around the Golgi area. Only a very small amount of caveolae was found on the surface of these cells, In elicited macrophages, caveolae (labelled with the anti-VIP21/caveolin-1 antibody) appeared in large numbers on the cell surface, but caveolin detected by anti-caveolin-2 was also found in small vesicles and multivesicular bodies in the cytoplasm, According to these results, the absence of caveolae in resident cells can be explained by the absence of caveolin-1. The expression of the similar to 29-kDa (caveolin-related) protein in resident macrophages seems to be insufficient for caveolae formation. Elicitation significantly increased the expression of caveolin-1, and the increased amount of caveolin-1 resulted in caveolae formation on the cell surface.