Glucose-6-phosphate dehydrogenase (G6PD) is an important site of metabolic control in the pentose phosphate pathway (PPP) which provides reducing power (NADPH) and pentose phosphates. The former is mainly involved in the detoxification of chemical reactive species; the latter in the regulation of cell proliferation. G6PD deficiency is the most common enzymopathy in the human population, characterized by decreased G6PD activity, mainly in red blood cells, but actually also in nucleated cells. This decreased activity is not due to enzyme synthesis impairment, but rather to reduced enzyme stability, which leads to a shortening of its half-life. Therefore, a major problem is to understand the underlying mechanisms linking G6PD deficiency to oxidative stress and cell proliferation. In order to address this issue, in the present study we utilized, as an experimental model, fibroblasts isolated from pterygium, an ocular proliferative lesion, from G6PD normal and deficient (PFs+ and PFs-, respectively) patients. Our choice was determined by the fact that pterygium is believed to be caused by chronic oxidative stress induced by UV exposure, and that pterygium fibroblasts resemble a tumorigenic phenotype. As controls we utilized fibroblasts isolated from conjunctiva from G6PD normal and deficient patients (NCFs+ and NCFs-, respectively) who had undergone cataract surgery.
Growth rate analysis revealed that PFs grow faster than NCFs, but while NCFs- grow more slowly than NCFs+, PFs- and PFs+ grow at the same rate. This was associated with significantly lower G6PD activity in NCFs+ compared to NCFs-, while no significant differences in the G6PD activity of PFs+ and PFs- were noted. This result was supported by the finding that in PFs-, G6PD mRNA levels were significantly higher than in PFs+. Another interesting finding of this study was increased green autofluorescence in both NCFs- and PFs- compared to corresponding positive cells, indicative of pronounced oxidative stress in deficient cells. Finally, abnormal accumulation of neutral lipids, mainly cholesterol esters was observed both in PFs- and PFs+ compared to NCFs- and NCFs+. Though further studies are necessary for better understanding the exact mechanism which links G6PD to oxidative stress and cell proliferation, our data allow to speculate on the role of G6PD on tumorigenesis, and to consider G6PD-deficient subjects at major risk to develop common and dreaded proliferative disorders, such as atherosclerosis and cancer.
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