The aim of this study was to determine the effects of kidney pathology on overall survival and longevity and the combined effects of chronic suppression of growth hormone (GH)/insulin-like growth factor-1 (IGF-1) activity and lifelong caloric restriction on age-associated nephropathy.
We analyzed the kidneys of rats with suppressed GH activity through genetic manipulation with an antisense GH transgene. Rats were fed normally or with a 30% calorie-restricted diet for 24-26 months. The kidneys of male wild-type young (6 months) and old (24-26 months) rats were compared with male hemizygote transgenic young (6 months) and old (24-26 months) rats fed with either regular diet or 30% calorie-restricted diet for their entire life span.
The transgenic rats had relatively less pituitary GH-secreting cells, and the plasma levels of IGF-1 were decreased by 53% in homozygote rats (tg/tg) and by 28% in hemizygote rats (tg/wt) compared to wild-type rats (wt/wt) of the same age (6 months). Wild-type rats fed the regular diet developed age-associated nephropathy as they aged, showing severe inflammatory cell infiltration, glomerulosclerosis, and tubulointerstitial fibrosis. In addition, about 83% of the wild-type rats allowed to survive naturally showed signs of nephropathy. In contrast, only 26% of the naturally surviving hemizygote rats showed features of nephropathy, despite the fact that these rats lived 8% longer (maximum survival 171 weeks) than the wild-type rats (maximum survival 158 weeks). When chronic suppression of GH/IGF-1 activity was combined with lifelong caloric restriction, however, age- associated nephropathy was nonexistent in hemizygote transgenic rats, and they showed about 30% increase in survival (maximum survival 204 weeks). There was no significant difference in the rate of neoplastic or nonneoplastic lesions (other than in the kidney) in the regularly fed wild-type rats or in the calorie-restricted hemizygote transgenic rats that survived longer.
We concluded that kidney pathology is an important determinant of overall survival, and that prevention of kidney pathology by dietary restriction, combined with chronic suppression of GH/IGF-1 activity, significantly extends overall survival and longevity.