Dr. Robert Stanton
The critical reducing power in the cell is provided by NADPH. NADPH is the underpinning of the entire antioxidant system and also provides reducing power for other critical cellular processes such as nitric oxide production, NADPH oxidase, the cytochrome p450 system, and HMGCoA reductase in lipid synthesis, amongst many others. All of these systems ultimately require an adequate supply of NADPH as the reducing equivalent in order to operate. The pentose phosphate pathway (PPP) is the principal metabolic pathway that produces NADPH. Glucose 6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the PPP. Thus proper activity of G6PD is essential for the cell to defend against oxidative stress and to provide NADPH for other NADPH-requiring processes.
Research from this laboratory has shown: 1) that NADPH produced by G6PD plays a critical role in cell growth and in cell death 2) that G6PD,an enzyme traditionally thought to be under little control, is actually regulated both in its activity and intracellular location by a variety of intracellular signaling molecules, and 3) that alterations in G6PD activity will have significant effects on NADPH-requiring cellular events. Thus the PI has hypothesized that abnormal regulation of G6PD plays important roles in a variety of pathophysiologic processes. In particular, the laboratory has shown:
G6PD plays an important role in cell growth. Inhibitors of G6PD activity led to a decrease in 3H-thymidine incorporations. Cells overexpressing G6PD had increased cell growth as compared to cells transfected with vector alone. These changes in G6PD activity were paralleled by changes in NADPH levels. Thus it was shown that G6PD activity is important for cell growth by providing NADPH for redox regulation.
G6PD plays an important role in cell death. Inhibitors of G6PD enhanced oxidative stress- induced cell death. Overexpression of G6PD provided significant expression against oxidative stress-induced cell death. In addition the data showed that decreased G6PD activity led to an increase in apoptosis whereas overexpression of G6PD decreased apoptosis. Thus G6PD plays a critical role in defending cells against cell death processes.
Diabetes Mellitus is likely a condition of acquired G6PD impairment: Increased oxidative stress is of major importance in the pathogenesis of diabetic complications. It was determined whether cells exposed to high glucose have a properly responding PPP. Endothelial cells exposed to high glucose did not adequately activate the PPP when exposed to oxidant stress. This defective response of the PPP was also associated with a significant increase in cell death as compared to the low glucose group. Recent studies show that, at least in part, the high glucose-induced inhibition of G6PD is mediated by increased cAMP. Also recent studies suggest that one or more of the protein kinase C family members can affect activation of G6PD. More recently , animal studies show that there is impaired G6PD activity in diabetic animals as compared to control. Thus improving the enzyme activity may have a profound effect on cell function and reduce diabetic complications. Work is ongoing to further define the importance and mechanisms of these findings.
Impaired G6PD alters nitric oxide production: In collaboration with colleagues at Boston University (J. Loscalzo (Chairman of the Department of Medicine and world renown NO expert) and J. Leopold), we have shown that impairing G6PD activity reduces NADPH and thus reduces NO production by nitric oxide synthase which requires NADPH. Work in animals and humans suggest that decreases in G6PD activity underlie decreased NO production in vascular endothelial cells and subsequent cell damage and hypertension. Work is ongoing to further define the importance and mechanisms of these findings.