Different Murine Models of Kidney Injury Reveal a Common Pattern of Dysregulation within the Polyamine System in Favour of its Catabolic Pathways.

The polyamines putrescine, spermidine and spermine are organic polycations that regulate many cell functions including proliferation and differentiation. It is known that certain genes of the polyamine system are dysregulated after kidney ischemia reperfusion injury. Here we examined the hypothesis that different forms of acute and chronic kidney injury lead to similar changes in the expression patterns of the polyamine system. In different models of acute and chronic kidney injury expression of genes involved in polyamine homeostasis were analyzed by RT-qPCR and RNAScope. In these models, expression of catabolic enzymes (Aoc1 and Sat1) was upregulated, and the anabolic enzymes (Odc1, Sms) were downregulated. The putrescine-degrading enzyme AOC1 exhibits the most striking changes. Interestingly, it can act together with ODC1 as gatekeepers of the polyamine system. The detected increase of Aoc1 takes place in the injured but regenerating proximal tubules. As a screening for stimuli of increased Aoc1 expression, we used mouse embryonic kidney explants. Here we observed changes of Aoc1expression under hypoxia and hyperosmotic conditions. These changes were further examined in mouse models of hypoxia. However, in vivo, hypoxia did not lead to changes of Aoc1 expression. Hyperosmolarity was confirmed as a stimulus by using the kidney cell lines M15 and 209/MDCT as well as cultured primary proximal tubules. Using reporter gene and RNA-stability assays, we could show that the increase in Aoc1 expression is based on mRNA-stabilization and transcriptional activation of one certain isoform. The activated isoform contains an additional set of 22 amino acids N-terminally that lead to an altered subcellular localization. In conclusion, different models of kidney injury exhibit a similar pattern of dysregulation of the polyamine system with the most striking change being the upregulation of Aoc1 in proximal tubules. Using hyperosmolarity as a stimulus, we provide first insights into the regulation of Aoc1 under harmful conditions.