Thyroid hormone induction of ornithine decarboxylase in ischemic acute renal failure

Strikingly conserved gene expression changes of polyamine regulating enzymes among various forms of acute and chronic kidney injury

The polyamines spermidine and spermine and their common precursor molecule putrescine are involved in tissue injury and repair. Here, we test the hypothesis that impaired polyamine homeostasis contributes to various kidney pathologies in mice during experimental models of ischemia-reperfusion, transplantation, rhabdomyolysis, cyclosporine treatment, arterial hypertension, diabetes, unilateral ureteral obstruction, high oxalate feeding, and adenine-induced injuries. We found a remarkably similar pattern in most kidney pathologies with reduced expression of enzymes involved in polyamine synthesis together with increased expression of polyamine degrading enzymes. Transcript levels of amine oxidase copper-containing 1 (Aoc1), an enzyme which catalyzes the breakdown of putrescine, were barely detectable by in situ mRNA hybridization in healthy kidneys. Aoc1 was highly expressed upon various experimental kidney injuries resulting in a significant reduction of kidney putrescine content. Kidney levels of spermine were also significantly reduced, whereas spermidine was increased in response to ischemia-reperfusion injury. Increased Aoc1 expression in injured kidneys was mainly accounted for by an Aoc1 isoform that harbors 22 additional amino acids at its N-terminus and shows increased secretion. Mice with germline deletion of Aoc1 and injured kidneys showed no decrease of kidney putrescine content; although they displayed no overt phenotype, they had fewer tubular casts upon ischemia-reperfusion injury. Hyperosmotic stress stimulated AOC1 expression at the transcriptional and post-transcription levels in metanephric explants and kidney cell lines. AOC1 expression was also significantly enhanced after kidney transplantation in humans. These data demonstrate that the kidneys respond to various forms of injury with down-regulation of polyamine synthesis and activation of the polyamine breakdown pathway. Thus, an imbalance in kidney polyamines may contribute to various etiologies of kidney injury.

A comparison of thyroxine- and polyamine-mediated enhancement of rat facial nerve regeneration

Thyroid hormones and spermidine, a motor neuron trophic polyamine (PA), have been shown to enhance peripheral motor nerve regeneration; however, the mechanism by which these treatment modalities exert their effect is unknown. Similarities in treatment outcome suggest that these molecules may be working via a common mechanism. Such an explanation is plausible since thyroid hormone is a potent inducer of ornithine decarboxylase (ODC), which is the rate-limiting enzyme involved in polyamine synthesis. This study was designed to morphologically evaluate the effects of exogenous thyroxine and spermidine on the regeneration of the rat facial nerve. Myelinated fiber density, axonal size, and degree of myelination were assayed by light and electron microscopy 21 days following facial nerve crush. Strikingly, the two treatment modalities had identical effects on all parameters tested. Each significantly enhanced the density of myelinated axons in regenerating nerves relative to the vehicle control. In addition, relative to the control treatment, both thyroxine and spermidine significantly increased the cross-sectional area of regenerating axons (P < 0.05). Interestingly, neither of the drug treatments had any effect on remyelination at the position where this parameter was analyzed. The concurrent administration of both thyroxine and spermidine did not synergistically enhance motor neuron regeneration. These data support the hypothesis that thyroxine and spermidine enhance neural regeneration by a common mechanism.