Genetics of receptors for bioactive polypeptides: A variant of swiss/3T3 fibroblasts resistant to a cytotoxic insulin accumulates lysosome‐like vesicles

Abstract

Recently we have isolated six variants of Swiss/3T3 mouse fibroblasts that are resistant to the cytotoxic insulin‐diphtheria toxin A fragment. All of the variants proved to have greatly reduced or no insulin binding capacity, and several variants showed altered morphologies and growth characteristics. We now report on the further characterization of one of these variants, CI‐3. which displays a massive accumulation of membranous vesicles in its cytoplasm. By electron microscopy these vesicles resemble lysosomes. They also appear to fluorcsce bright orange after treatment of viable cells with acridine orange. However, the specific activity of several lysosomal enzymes is depressed in CI‐3. Additionally, there is an apparent shift in the density of vesicles containing lysosomal enzymes in this variant. These alterations may be directly related to CI‐3′s resistance to the cytotoxic insulin and have some important bearings on the mechanism of insulin action.