The effect of the hypoglycemic biguanide, phenformin, on the binding of insulin to MCF-7 cells, an in vitro line derived from a human breast cancer, has been investigated. Cells incubated for 24 h in the presence of 1.0 μg/ml of phenformin bound 62.2 ± 8.1% (mean ± SE) more 125I-insulin than did controls. The phenformin effect was dose-dependent over the concentration range of 0.1 μgsol;ml to 10.0 μg/ml. The increased binding was due to an increase in receptor number without a change in binding affinity. This demonstration of increased receptor number in response to phenformin exposure provides support for the hypothesis that one action of phenformin is to enhance tissue sensitivity to insulin.
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