Phenformin stimulation of insulin binding to human cultured lymphocytes

Anti-diabetic biguanides inhibit hormone-induced intracellular Ca2+ concentration oscillations in rat hepatocytes

Rat hepatocytes respond to glycogenolytic stimuli acting via phosphoinositide breakdown (e.g. alpha 1-adrenergic agonists, vasopressin) by oscillations of the free intracellular Ca2+ concentration ([Ca2+]i). We have investigated the action of metformin and phenformin, two anti-diabetic drugs of the biguanide type, on phenylephrine-induced [Ca2+]i oscillations. Metformin and phenformin lowered the frequency of the [Ca2+]i oscillations in a concentration-dependent manner with an IC50 of 0.1 mM and 1 microM, respectively. Simultaneous addition of the biguanides and insulin resulted in a further reduction of the frequency. By contrast, agents which increase the cellular cyclic AMP (cAMP) concentration (glucagon, forskolin, N,2'-O-dibutyryl-cAMP) reversed this inhibition. Furthermore, we investigated whether biguanides influenced the agonist-induced Ca2+ influx across the plasma membrane. When hepatocytes were loaded with the acetoxymethyl ester of fura-2 (fura-2/AM), addition of Mn2+ led to a quench of cellular fura-2, measured at the isosbestic excitation wavelength of 360 nm, until a new steady state was reached. Surprisingly, however, this addition of Mn2+ caused a marked increase of the fluorescence ratio simultaneously measured at 340 and 380 nm during the approach of the 360 nm signal to a new steady state. This observation can be understood on the basis of a compartmentalization of fura-2/AM into intracellular stores sensing the [Ca2+] therein. Subsequent application of phenylephrine resulted in a further decline of the fura-2 signal at 360 nm and a concomitant decrease of the fluorescence ratio. This second phase of the Mn2+ quench and the decrease of the fluorescence ratio could be diminished by addition of either 3 mM metformin or 30 microM phenformin. By contrast, when hepatocytes were loaded with fura-2/pentapotassium salt via a patch pipette, only the initial Mn(2+)-induced quench, measured at 360 nm, but no change of the fluorescence ratio, could be observed. The subsequent addition of phenylephrine and biguanides during the on-going quench caused no further changes, except for a fading oscillatory response. After loading hepatocytes with fluo-3 acetoxymethyl ester, the cells were permeabilized with 5 microM digitonin. Addition of inositol-1,4,5-trisphosphate (IP3) caused a rapid decrease of the remaining cellular fluorescence which could be effectively inhibited by 20 micrograms/ml heparin, indicating a release of Ca2+ from intracellular compartments mediated by IP3. This IP3-induced release of Ca2+ from intracellular stores could be diminished by prior addition of metformin and phenformin.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of metformin and glibenclamide on insulin receptors in fibroblasts and tumor cells in vitro

The effects of the oral hypoglycemic agents metformin and glibenclamide on receptor binding of insulin and insulin-induced receptor down-regulation were studied in cultured normal human fibroblasts, human breast tumors (cell lines MCF-7 and T-47D) and a human colon tumor (cell line HCT-8) in order to identify differences in receptor regulation between normal and transformed cells. Binding of 125I-insulin was not significantly altered in any cell type following 24-h exposure of cells to concentrations of these agents equivalent to the in vivo therapeutic range, either in the presence or absence of insulin. Glibenclamide (2 microM) and metformin (1-10 microM) induced a 13-28% reduction in insulin receptor down regulation in fibroblasts exposed to 1.7 x 10(-8)M-insulin, the loss of binding on exposure to insulin decreasing from 55% to 40-48%. Both agents induced a smaller percentage reduction in insulin-induced receptor down-regulation in HCT-8 cells exposed to 1.7 x 10(-8)M insulin for 4 h (26% binding loss reducing to 14-20% in the presence of drug) and in MCF-7 and T-47D cells exposed to 1.7 x 10(-7)M insulin for 4h (16% binding loss reducing to 10%). It remains to be shown whether these small differences in sensitivity of receptors in malignant and normal cells to regulation by insulin and oral hypoglycemic agents can be exploited clinically in cancer management programs.

The effect of phenformin and other adenosine triphosphate (ATP)-lowering agents on insulin binding to IM-9 human cultured lymphocytes

In the present study, we investigated the mechanism by which the antidiabetic drug phenformin increases insulin binding to its receptors in IM-9 human cultured lymphocytes. After a 24-hr preincubation, phenformin induced a twofold increase in specific 125I-insulin binding, and removal of phenformin was followed 6 hr later by a return in binding to control levels. This effect of phenformin on insulin binding was not a consequence of either inhibition of cell growth, changes in cellular cyclic adenosine monophosphate (AMP) levels, or changes in guanosine triphosphate (GTP) content. Since phenformin is known to inhibit various aspects of cellular energy metabolism, the relationship between 125I-insulin binding and energy metabolism in IM-9 cells was investigated. The phenformin-induced increase in insulin binding to IM-9 cells was related to a time- and dose-dependent decrease in ATP levels. Other agents that lowered ATP levels, including antimycin, dinitrophenol, and 2-deoxyglucose, also raised insulin binding. These studies indicated, therefore, that phenformin enhances insulin binding to receptors on IM-9 cells and that this effect on insulin receptors may be related to alterations in metabolic functions that are reflected by a lowering of ATP levels.