Activation of protein phosphorylation by oxidants in vascular endothelial cells: identification of tyrosine phosphorylation of caveolin

How does alendronate inhibit protein-tyrosine phosphatases?

Alendronate (4-amino-1-hydroxybutylidene 1,1-bisphosphonate) is a drug used in the treatment of osteoporosis and other bone diseases. The inhibition of protein-tyrosine phosphatases (PTPs) by alendronate suggests that PTPs may be molecular targets. As a clear understanding of the inhibition mechanism is lacking, our aim was to analyze the mechanism to provide further insight into its therapeutic effect. We show here that the inhibition of PTPs by alendronate in the presence of calcium followed first-order kinetic behavior, and kinetic parameters for the process were determined. Evidence is presented that the inhibition by alendronate/calcium is active site-directed. However, this process was very sensitive to assay constituents such as EDTA and dithiothreitol. Furthermore, the inhibition of PTPs by alendronate/calcium was eliminated by the addition of catalase. These observations suggest that a combination of alendronate, metal ions, and hydrogen peroxide is responsible for the inhibition of PTPs. The individual effects of alendronate, calcium, or hydrogen peroxide on the inactivation of CD45 were determined. Electrospray ionization mass spectrometry demonstrated that the mass of PTP1B increased by 34 +/- 2 units after the enzyme was inactivated with alendronate/calcium, due to the oxidization of the catalytic cysteine to sulfinic acid (Cys-SO2H). The inhibited PTP1B could be partially reactivated by treatment with reducing agents such as hydroxylamine (NH2OH) and N,N'-dimethyl-N, N'-bis(mercaptoacetyl)hydrazine, indicating the presence of other oxidized forms such as sulfenic acid (Cys-SOH). This further confirms that the inhibition is the result of oxidation of the catalytic cysteine. The relevance of this oxidative inhibition mechanism in a biological system is discussed.

The enhancement by wortmannin of protein kinase C-dependent activation of phospholipase D in vascular endothelial cells

Phosphatidic acid generation by phospholipase D (PLD) activation has been implicated in agonist- and oxidant-mediated endothelial cell signal transduction. We examined the effect of wortmannin on PLD activation in pulmonary artery endothelial and smooth muscle cells in culture. Pretreatment of bovine pulmonary artery endothelial cells (BPAECs) with wortmannin potentiated TPA- (100 nM), ATP- (100 microM), and bradykinin- (1 microM) induced [32P]PEt formation, an index of PLD activation. However, wortmannin by itself had no effect on PLD activity. The potentiating effect of wortmannin on TPA-induced PLD activation was dose- (1-10 microM) and time-dependent (5-30 min) and was inhibited by bisindoylmalemide, an inhibitor of protein kinase C (PKC). Furthermore, down-regulation of PKC by prolonged treatment with TPA (100 nM, 18 h) attenuated the wortmannin effect. This effect of wortmannin was specific for TPA- or agonist-induced PLD activation as no potentiation of [32P]PEt formation was observed with H2O2 (1 mM) or ionomycin (1 microM). The effect of wortmannin was not due to activation of PKC alpha as determined by western blot analysis of PKC alpha in the cytosol and membrane fractions. Also, genistein, an inhibitor of tyrosine kinases, did not attenuate the wortmannin-mediated potentiation of PLD thereby suggesting non-involvement of protein tyrosine phosphorylation. These results indicate that wortmannin potentiates PKC-dependent stimulation of PLD in vascular endothelial cells.