SECRETION OF SOLUBLE INSULIN-LIKE GROWTH FACTOR-II MANNOSE 6-PHOSPHATE RECEPTOR BY RAT-TISSUES IN CULTURE

The insulin-like growth factor-II/mannose 6 phosphate receptor, which binds both IGF-II and mannose 6-phosphate containing proteins, such as lysosomal enzymes, has been detected in the serum of several species. Neither the source nor the role of this soluble, truncated form of the membrane receptor has been determined. This study has examined the ability of a variety of rat tissues in culture to synthesize and release soluble receptor. Explants (1mm3) of 21-day fetal, neonatal and adult rat tissues were cultured in serum-free medium and the conditioned medium analyzed for the presence of receptor. Using IGF-II binding and immunochemical techniques, receptor was detected in media from heart, kidney, liver, lung and muscle explants. [S-35]cysteine labeling indicated that de novo synthesis of the soluble receptor occurs in the cultured explants and can be inhibited by cycloheximide. This rat tissue explant culture system demonstrates that soluble receptor is released by a number of tissues, and should provide a useful model for further investigations into its function and regulation.