The insulin receptors in two strains of Zajdela ascites hepatoma cells have been studied. Specific binding of insulin was investigated at 4, 15, and 22°, and the amount of insulin bound at steady state was proportional to the cell concentration. The optimum pH was 7.6.
Scatchard analysis of the insulin binding exhibited curvilinear plots. The number of insulin receptors was markedly reduced in Zajdela D and H cells as compared to normal hepatocytes. Furthermore, the estimated number of sites was much lower in Zajdela D hepatoma, the most transplanted tumor, compared to Zajdela H hepatoma. The dissociation constants were not modified by the transformation. Dissociation experiments have revealed enhancement of the dissociation rate of bound 125I-labeled insulin by native insulin.
Trypsin and papain digestion of Zajdela cells led to a marked inhibition of insulin binding. After digestion with glycosidases, sialidase, and/or galactosidase, the insulin binding was not decreased.
The specific binding of insulin to cells was diminished after pretreatment of Zajdela cells with concanavalin A. Scatchard analysis of insulin binding indicated that low concentrations of the lectin inhibited especially high-affinity sites. Preincubation of the cells with wheat germ agglutinin or the lectin Sophora japonica resulted in a biphasic effect on insulin binding.
Thus, the Zajdela D and H ascites hepatoma cells present a decreased number of specific insulin receptors compared to normal cells, this number being much lower in the most transplanted tumor.
This work has been supported by grants from Institut National de la Santé et de la Recherche Médicale (ATP 184.108.40.206) and from Centre National de la Recherche Scientifique (ERA 691).