Chinese hamster ovary cell variants resistant to monensin, an ionophoric antibiotic. II. Growth requirement for insulin and altered insulin‐receptor activity

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Abstract
From the Chinese hamster ovary (CHO) cell, genetic variants (MonR–31 and MonR–32) relatively resistant to monensin, an ionophoric antibiotic, have been isolated. Growth of both MonR‐31 and MonR‐32 clones required higher doses of serum than CHO. Addition of insulin to media containing a low dose of serum restored full colony formation, but growth of MonR‐31 or MonR‐32 cells required more insulin than CHO cells. Specific binding of [125l]insulin was observed in these cell lines. The two MonR clones bound about one‐half or less the [125l]insulin bound by CHO cells. Scatchard analysis for [125l]insulin binding at 4°C and 37°C showed altered number of binding sites, but not insulin affinity: The number of binding sites in the MonR cell was about a half or less that of the parental CHO cell. Down‐regulation of insulin receptor was assayed when both CHO and MonR cells were incubated with 1 μg/ml insulin. A 50–60% decrease in levels of insulin surface binding capacities was observed in CHC after exposure to insulin, whereas there was no decrease in MonR cell. The cellular uptake of 2‐[3H]deoxyglucose into CHO cells was significantly enhanced in the presence of insulin, but only slight, if any, increase was observed in MonR cells.