PERMOLYBDATE AND PERTUNGSTATE - POTENT STIMULATORS OF INSULIN EFFECTSIN RAT ADIPOCYTES - MECHANISM OF ACTION

In previous studies, tungstate and molybdate were found to mimic the b iological actions of insulin. It was suggested that these metallooxide s initially inhibit vanadate-sensitive protein phosphotyrosine phospha tase (PTPase). This, in turn, stimulates a staurosporine-sensitive cyt osolic protein tyrosine kinase (cytPTK), which activates several insul in bioeffects via insulin-independent pathways (Shisheva and Shechter, 1991, 1993; Elberg et al., 1994). Tungstate and molybdate, however, f acilitate bioeffects in rat adipocytes only at high (millimolar) conce ntrations (Goto et al., 1992). We report here that incubations of tung state or molybdate with hydrogen peroxide (H2O2) result in the formati on of pertungstate (pW, peroxide of tungstate) or permolybdate (pMo, p eroxide of molybdate). Pertungstate and permolybdate were found to sti mulate all or most of the insulin bioeffects in rat adipocytes. Moreov er, these permetallooxides are 80-180-fold more potent stimulators tha n the corresponding metallooxides. This shift in potency resembles tha t of pervanadate relative to vanadate in stimulating the same effect i n rat adipocytes (Fantus et al., 1989). pW and pMo are also active in normalizing blood glucose levels in streptozotocin-induced diabetic ra ts. Further studies aimed at understanding the higher efficacy of this permetallooxide revealed the following: (a)All three permetallooxides (pV, pW, pMo) are oxidizing agents relative to reduced glutathione (G SH). They oxidize stoichiometric amounts of GSH to GSSG. (b) All three metallooxides do not oxidize GSH to GSSG. (c) Both metallooxides and permetallooxides inhibit rat adipocytic PTPase at micromolar quantitie s (IC50 = 3-10 mu M). Permetallooxides, however, inhibited a larger PT Pase fraction (80-100%) compared to metallooxides (40-70% of the total ). (d) In cells, metallooxides solely activate cytPTK, whereas permeta llooxides activate both cytPTK and the insulin receptor tyrosine kinas e (insRTK). (e) Although both protein tyrosine kinases are stimulated, permetallooxides mediate their effects predominantly via the insulin receptor. (f) GSH partially protects PTPase from metallooxide inhibiti on in cell-free experiments. We propose that the higher efficacy of pe rmetallooxides in stimulating insulin responses originates from their oxidizing feature relative to GSH. Partial intracellular conversion of GSH to GSSG resulted in the inhibition of higher PTPase fraction, act ivation of the insRTK, and efficient triggering of insulin bioeffects via the activated receptor in a hormone-independent manner.