Co-secretion of soluble caveolin-1 and pepsinogen in the oesophagus of the red-legged frog Rana aurora aurora.

Caveolin-1, an integral membrane protein, is the principal component of caveolae, which are specialised vesicular microdomains of the plasma membrane. Caveolae are found in most cell types, but they are most abundant in adipocytes, endothelial cells, fibroblasts, and muscle cells. Functionally, they have been implicated in endothelial transcytosis, potocytosis, and signal transduction. Recently, caveolin-1 has been found unexpectedly in the cytoplasm, mitochondria and elements of the secretory pathways of exocrine secretory cells. We have co-localised caveolin-1 and pepsinogen immunohistochemically in serous cells of oesophageal glands of the red-legged frog, Rana aurora aurora. Thus, according to its intracellular localisation pattern, caveolin-1 may be either a soluble protein, located in secretory droplets, or a protein that is inserted in caveolar membranes. Soluble caveolin-1, which is probably embedded in a lipid particle surrounded by a phospholipid shell, may be involved in intracellular and extracellular lipid transport. In the gut, caveolin-1-rich lipid particles can act as donor particles to facilitate (protein-mediated) intestinal uptake of cholesterol and phospholipids. Our findings strengthen the hypothesis that caveolin-1 has a physiological autocrine/paracrine function and demonstrate that secretion of this protein also occurs in vertebrates other than mammals, such as amphibians, which may be a useful alternative animal model to study caveolin-1.