Insulin-like growth factor I-enhanced renal expression of osteopontin after acute ischemic injury in rats

Pretreatment of rats with insulin-like growth factor I (IGF-I) ameliorates the course of acute ischemic renal injury. Differential display PCR was used to identify genes that are expressed in kidney after induction of acute ischemic renal injury in rats pretreated with vehicle or IGF-I. One amplification product that showed enhanced expression in kidneys of rats rendered ischemic compared to kidneys of sham-operated rats was identified as osteopontin. Sequence analysis of full-length complementary DNAs revealed a single species. Renal tissue was obtained for study 12 h and 1, 2, 3, 5, 7, 14, and 28 days postinjury. Levels of whole kidney osteopontin messenger RNA (mRNA) in rats rendered ischemic 1 day previously were elevated approximately 18-fold compared to levels measured in sham-operated controls, as determined by Northern analysis. No differences were noted 12 h postinjury. Levels of osteopontin mRNA remained elevated for 14 days after ischemia, but were no longer elevated at 28 days. IGF-I pretreatment resulted in enhanced levels of osteopontin mRNA 12 h, 1 day, and 5 days postinjury. In situ hybridization demonstrated that the elevated expression of osteopontin 1 day postinjury was localized predominantly to cells in the distal tubule and medullary thick ascending limb of Henle's loop. Immunostaining showed an identical localization for elevated protein expression. Five days postinjury, osteopontin peptide and mRNA were clearly detected in regenerating proximal tubules in addition to distal tubule and medullary thick ascending limb. We propose that endogenous osteopontin serves to promote tissue regeneration and tissue remodeling within 1 day after acute ischemic injury of kidney. IGF-I- enhanced expression of osteopontin at an earlier time postischemia may ameliorate the course of injury.