Endorepellin, the C-terminal domain of perlecan, is a powerful angiogenesis inhibitor. To dissect the mechanism of endorepellin-mediated endothelial silencing, we used an antibody array against multiple tyrosine kinase receptors. Endorepellin caused a widespread reduction in phosphorylation of key receptors involved in angiogenesis and a concurrent increase in phosphatase activity in endothelial cells and tumor xenografts. These effects were efficiently hampered by function-blocking antibodies against integrin ␣21, the functional endorepellin receptor. The Src homology-2 protein phosphatase-1 (SHP-1) coprecipitated with integrin ␣2 and was phosphorylated in a dynamic fashion after endorepellin stimulation. Genetic evidence was provided by lack of an endorepellin-evoked phosphatase response in microvascular endothelial cells derived from integrin ␣21 ؊/؊ mice and by response to endorepellin in cells genetically engineered to express the ␣21 integrin, but not in cells either lacking this receptor or expressing a chimera harboring the integrin ␣2 ectodomain fused to the ␣1 intracellular domain. siRNA-mediated knockdown of integrin ␣2 caused a dose-dependent reduction of SHP-1. Finally, the levels of SHP-1 and its enzymatic activity were substantially reduced in multiple organs from ␣21 ؊/؊ mice. Our results show that SHP-1 is an essential mediator of endorepellin activity and discover a novel functional interaction between the integrin ␣2 subunit and SHP-1. (Blood. 2009; 114:4897-4906) IntroductionThe development and maintenance of an efficient vascular system is vital for organ function and health in all higher organisms. Angiogenesis is abundant during fetal development but in the adult is a rare process mainly restricted to the female reproductive cycle and wound healing. However, deregulated angiogenesis is a prominent factor in many major diseases such as cancer, diabetes, and ischemia occurring after stroke. Controlled angiogenesis is achieved by interplay of proangiogenic and antiangiogenic factors. Proangiogenic growth factors, such as vascular endothelial growth factor (VEGF), signal by their respective receptor tyrosine kinase (RTK) to stimulate endothelial cell migration and proliferation, whereas angiogenic inhibitors counteract these actions. 1 The list of reported negative regulators of angiogenesis is long, ever growing, and truly versatile. Nevertheless, a common theme for many of them is that they are derived from limited proteolysis of extracellular matrix components and that they interact with integrin receptors. 2 The angiostatic protein endorepellin, located within the C-terminus of the heparan sulfate proteoglycan perlecan, consists of 3 laminin globular (LG1-3) domains separated by 4 epidermal growth factor-like repeats. 3 Endorepellin exerts its activity by a noncanonical cation-independent binding of the LG3 domain to the ␣2 I domain of the integrin ␣21, 4-7 a key receptor regulating angiogenesis. [8][9][10] This triggers a signaling cascade that leads to endothelial cell immobilizatio...
Previously, we showed that protein kinase B (Akt) activation increases intracellular ATP levels and decreases necrosis in renal proximal tubular cells (RPTC) injured by the nephrotoxicant S-(1, 2-dichlorovinyl)-l-cysteine (DCVC) (Shaik ZP, Fifer EK, Nowak G. Am J Physiol Renal Physiol 292: F292-F303, 2007). This study examined the role of Akt in improving mitochondrial function in DCVC-injured RPTC. Our data show a novel observation that phosphorylated (active) Akt is localized in mitochondria of noninjured RPTC, both in mitoplasts and the mitochondrial outer membrane. Mitochondrial levels of active Akt decreased in nephrotoxicant-injured RPTC, and this decrease was associated with mitochondrial dysfunction. DCVC decreased basal, uncoupled, and state 3 respirations; ATP production; activities of complexes I, II, and III; the mitochondrial membrane potential (DeltaPsi(m)); and F(0)F(1)-ATPase activity. Expressing constitutively active Akt in DCVC-injured RPTC increased the levels of phosphorylated Akt in mitochondria, reduced the decreases in basal and uncoupled respirations, increased complex I-coupled state 3 respiration and ATP production, enhanced activities of complex I, complex III, and F(0)F(1)-ATPase, and improved DeltaPsi(m). In contrast, inhibiting Akt activation by expressing dominant negative (inactive) Akt or using 20 microM LY294002 exacerbated decreases in electron transport rate, state 3 respiration, ATP production, DeltaPsi(m), and activities of complex I, complex III, and F(0)F(1)-ATPase. In conclusion, our data show that Akt activation promotes mitochondrial respiration and ATP production in toxicant-injured RPTC by 1) improving integrity of the respiratory chain and maintaining activities of complex I and complex III, 2) reducing decreases in DeltaPsi(m), and 3) restoring F(0)F(1)-ATPase activity.
Protein kinase B (Akt) activation is well known for its protective effects against apoptosis. However, the role of Akt in regulation of necrosis is unknown. This study was designed to test whether Akt activation protects against nephrotoxicant-induced injury and death in renal proximal tubular cells (RPTC). Exposure of primary cultures of RPTC to the nephrotoxic cysteine conjugate, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), resulted in 9% apoptosis and 30% necrosis at 24 h following the exposure. Akt was activated during 8 h but not at 24 h following toxicant exposure. No RPTC necrosis was observed during Akt activation. Blocking Akt activation using a phosphatidylinositol 3-kinase inhibitor, LY294002 (20 muM), or expressing dominant negative (inactive) Akt increased DCVC-induced RPTC necrosis to 42%. In contrast, Akt activation by expression of constitutively active Akt diminished necrosis to 15%. Modulation of Akt activity had no effect on DCVC-induced apoptosis. DCVC-induced RPTC injury was accompanied by decreases in respiration (51% of controls) and ATP levels (57% of controls). Akt inhibition exacerbated decreases in RPTC respiration and intracellular ATP content (both to 30% of controls). In contrast, Akt activation reduced DCVC-induced decreases in respiration (80% of controls) and prevented decline in ATP content. These data show that in RPTC, Akt activation reduces 1) toxicant-induced mitochondrial dysfunction, 2) decreases in ATP levels, and 3) necrosis. We conclude that Akt activation plays a protective role against necrosis caused by nephrotoxic insult in RPTC. Furthermore, we identified mitochondria as a subcellular target of protective actions of Akt against necrosis.
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