OBJECTIVE
To investigate the contribution of p21 gene in renal tubular epithelial cells in p21 (+/+) and p21 (-/-) mice of young and old ages at different times after kidney ischemia/reperfusion injury (IRI).
METHODS
In p21 (+/+) and p21 (-/-) male mice at the ages of 2 and 12 months the kidneys were made ischemic by clamping the left renal artery for 45 minutes followed by declamping. On 0, 1, 3 and 7 days, 1, 3 and 6 months after reflow, renal tissue was processed for pathological study, determination of proliferating cell nuclear antigen (PCNA), apoptosis and senescence-associated beta-galactosidase (SA-beta-gal) analysis, using hematoxylin and eosin staining, immunohistochemistry, terminal deoxynucleotidyl transferase-mediated dUTP-biotin in situ nick-end labeling (TUNEL), and histochemical staining, respectively.
RESULTS
Renal tubule necrosis and cell apoptosis were more severe in p21 (-/-) mice and old mice as compared with p21 (+/+) mice and young mice (both P<0.05), respectively. In young p21 (+/+) mice, occasionally faint staining for SA-beta-gal activity began to appear after 1 month, and significantly increased 3 and 6 months after IRI (P<0.05), but there was no positive staining for SA-beta-gal in the contralateral kidney or both kidneys in p21 (-/-) mice at any time. Another manner of the expression of SA-beta-gal was detected in aged p21 (+/+) mice, as both kidneys showed intensely positive staining for SA-beta-gal at 0 day after IRI, it then subsided notably on 1 day in the IRI kidney (P<0.05), but increased again at 3 months, though still less intense than the contralateral kidney, albeit more intense than the young mice at the same time (P<0.05). Three months after IRI, in both the IRI kidney and the contralateral kidney, positive staining for SA-beta-gal almost reached the same level. On the contrary, only occasional faint staining for SA-beta-gal activity was observed in aged p21 (-/-) mice at any time. No significant difference in positive staining of nuclear PCNA was found between in young and aged p21 (+/+) mice (P>0.05), although the numbers of positively stained nuclear PCNA were more in number in young mice than in aged mice. But in p21 (-/-) mice, significantly more cells were positively stained for PCNA, especially in young mice and in IRI kidneys (P<0.05). Correlation analysis between senescent and apoptotic cells in aged mice made at 1 day after IRI showed striking negative correlation between both of them [p21 (+/+) mice: r=-0.82; P<0.001; p21 (-/-) mice: r=-0.76, P<0.001].
CONCLUSION
IRI can promote the senescence process of normal tubular cells, and can accelerate death (necrosis and apoptosis) process of senescent tubular cells. p21 gene may play an important role in the senescence changes in tubular epithelial cells after kidney ischemia/reperfusion injury.