| Introduction:Acute kidney injury (AKI) is a critical clinical problem with mortality at approximately50%. Renal ischemia-reperfusion (I-R) is a common cause of AKI in varied clinical setting including kidney transplantation, cardiopulmonary and aortic bypass surgery. Inflammation, oxidative stress, members of cell death path ways and certain enzymes of the cell cycle et al may contribute to AKI. Despite significant development in the understanding of the pathophysiology of I-R injury in kidney, there are still no specific therapeutic options for patients except for supportive care. The incident and prognosis of AKI varies in different age. This may coincide with a significantly increased cell susceptibility to injury and reduced functional recovery in the elderly. Stress can also induce rapidly cell senescence, independently of the number of cell divisions, so-called stress-induced premature senescence (SIPS). In mice, SIPS is stimulated through the p16/Rb pathway or ARF/p53pathway. SIRT1(silent information regulator) is a class III histone deacetylase (HDAC) initially identified from studies of ageing yeast. SIRT1can deacetylate p53, a key factor for senescence and apoptosis, in vitro. Our previous study showed that young mice are more resistant to acute kidney injury than adult mice. The aim of the present study was to investigate whether SIRT1mediate age-related different response to AKI and its relationship to SIPS.Methods:There are three groups of experiments:1) young mice with sham injury (6-8wk old); young mice with I-R injury; adult mice with sham injury (18-22wk old); adult mice with I-R injury.2) Additional27adult mice (18-22wk old) were randomized into2groups,13mice received SRT-1720(SIRT1activator) and the other14mice received vehicle as control.3)8wks to lOwks old SIRT1+/-(HT) and SIRT1+/+(WT) control mice were used (n=12for each group). Evaluation of SIRT1expression in kidney is detected by western blot, qRT-PCR and immunohistochemistry.Mice were subjected to a midline back incision and both renal pedicles were clamped for45min with microaneurysm clamps to make I-R model. Serum measurement (BUN and Creatinine), renal tissue evaluation (tubular injury score), apoptosis (TUNEL assay and cleaved caspase-3) and proliferation (PCNA staining and western blot) were done to evaluate renal injury. Members of cell death path ways and cell cycle (p53, p21, Bax and Bcl-2) were evaluated by Western blot.Results:24hours after renal ischemia-reperfusion (I-R) injury, adult mice (22wks) showed higher BUN than young mice (7wks)(190.9±20vs.38.6±6.8mg/dL, P<0.001). Additionally, in adult mice the BUN level remained very high after48h, which has significant difference with the young ones. Similar changes were found in the serum creatinine (adult I-R1.3±0.3vs. young I-R0.2±0.04mg/dL, P<.001). I-R induced tubular dilatation, necrosis, brush border loss and cast formation in I-R mice at48hours. Young mice had a markedly lower mean tubular injury score compared to the adult mice, no matter in cortex or in outer medulla. Adult mice with I-R injury displayed a greater number of TUNEL-positive cells, predominantly in the tubules of outer medulla, compared to young mice (adult I-R20.8±0.8vs. young I-R2.4±0.1cells/HPF, P<0.001). The kidneys from I-R treated adult mice displayed a smaller number of PCNA-positive tubular cells (adult I-R20.8±2.3vs. young I-R71.8±1.7cells/HPF, P<0.001) than that from young mice.SIRT1expression was very low in adult sham mice and had no significant change after I-R injury, while it was induced in young mice. It was2-fold increase in young mice after I-R injury than in adult mice (adult I-R1.0±0.2vs. young I-R2.2±0.3, P<0.01). In kidney, SIRT1mainly expressed on tubular epithelial cells, interstitial cells, arteriole smooth muscle cells and podocytes. I-R injury induced some regenerated tubular epithelial cells to express SIRT1in young kidneys. There were more p53and its downstream cell-cycle inhibitor p21expression in adult mice than young ones (p532.3±0.4vs.0.9±0.4, p212.4±0.4vs.0.3±0.04, P<0.05).To further detect the role of SIRT1in age-related responsibility to AKI, SRT-1720, a SIRT1activator, was given to adult mice. SRT-1720did improve renal function after I-R injury (BUN113.4±14.8vs.169.2±19.0mg/dL, P<0.05) by decreasing p53(52%) and p21(51%). By contrast, genetic ablation of one allele of SIRT1(SIRT1+/-) induced much worse renal dysfunction (BUN141.4±14.4vs.94.0±13.7mg/dL, P<0.05) than wild type (SIRT1+/+) mice, by increasing p53(3.9fold) and p21(10.1fold). Conclusion:These results suggest SIRT1is the key factor for age-related response to AKI. SIRT1activator (SRT1720) protects against I-R injury by attenuating apoptosis and promoting regeneration, while its deletion worsens tissue injury. SIRT1may improve cell senescence and suppress cell apoptosis by deacetylating p53. It also could stimulate cell proliferation and regeneration by decreasing p21. Given these broad actions of SIRT1, we propose that SIRT1is a novel survival factor and may have significant clinical implications as a therapeutic agent in ischemic episodes. |
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