Ornithine decarboxylase along the mouse and rat nephron

Hyperglycemia-induced oxidative stress in isolated proximal tubules of mouse: the in vitro effects of myricitrin and its solid lipid nanoparticle

CONTEXT

The hyperglycemia (Hyper) induces oxidative stress in kidney tubular cells. Myricitrin (Myr) has an antioxidant effect along with low bioavailability.

OBJECTIVE

The present research investigated the effects of Myr and its solid lipid nanoparticles (SLN) on isolated proximal tubules exposed to the hyperglycemic condition.

MATERIALS AND METHODS

In this experimental study, the proximal tubules of mice were dissected by the microdissection method and the tubules were prepared for experimental or Real Time-PCR measurement.

RESULTS

The malondialdehyde level, transforming growth factor-β, nuclear factor kappa B and Bax genes expression increased in Hyper and decreased in Hyper + Myr and its SLN-treated groups compared to Hyper. Superoxide dismutase, total antioxidant capacity, the viability of proximal tubules and Bcl-2 gene expression decreased in untreated Hyper and increased in all treatment groups compared to Hyper.

CONCLUSION

The hyperglycemia-induced oxidative disorder, inflammation and apoptosis in proximal tubules and that administrating Myr and its SLN improved them.

Linkage of organic anion transporter-1 to metabolic pathways through integrated "omics"-driven network and functional analysis

The main kidney transporter of many commonly prescribed drugs (e.g. penicillins, diuretics, antivirals, methotrexate, and non-steroidal anti-inflammatory drugs) is organic anion transporter-1 (OAT1), originally identified as NKT (Lopez-Nieto, C. E., You, G., Bush, K. T., Barros, E. J., Beier, D. R., and Nigam, S. K. (1997) J. Biol. Chem. 272, 6471-6478). Targeted metabolomics in knockouts have shown that OAT1 mediates the secretion or reabsorption of many important metabolites, including intermediates in carbohydrate, fatty acid, and amino acid metabolism. This observation raises the possibility that OAT1 helps regulate broader metabolic activities. We therefore examined the potential roles of OAT1 in metabolic pathways using Recon 1, a functionally tested genome-scale reconstruction of human metabolism. A computational approach was used to analyze in vivo metabolomic as well as transcriptomic data from wild-type and OAT1 knock-out animals, resulting in the implication of several metabolic pathways, including the citric acid cycle, polyamine, and fatty acid metabolism. Validation by in vitro and ex vivo analysis using Xenopus oocyte, cell culture, and kidney tissue assays demonstrated interactions between OAT1 and key intermediates in these metabolic pathways, including previously unknown substrates, such as polyamines (e.g. spermine and spermidine). A genome-scale metabolic network reconstruction generated some experimentally supported predictions for metabolic pathways linked to OAT1-related transport. The data support the possibility that the SLC22 and other families of transporters, known to be expressed in many tissues and primarily known for drug and toxin clearance, are integral to a number of endogenous pathways and may be involved in a larger remote sensing and signaling system (Ahn, S. Y., and Nigam, S. K. (2009) Mol. Pharmacol. 76, 481-490, and Wu, W., Dnyanmote, A. V., and Nigam, S. K. (2011) Mol. Pharmacol. 79, 795-805). Drugs may alter metabolism by competing for OAT1 binding of metabolites.

Sex-differential expression of ornithine aminotransferase in the mouse kidney

The mouse kidney expresses the gene of ornithine aminotransferase (Oat). Previous works suggest that Oat is differentially expressed in female and male mouse kidney (Alonso E, Rubio V. Biochem J 259: 131-138, 1989; Levillain O, Diaz JJ, Blanchard O, Dechaud H. Endocrinology 146: 950-959, 2005; Manteuffel-Cymborowska M, Chmurzynska W, Peska M, Grzelakowska-Sztabert B. Int J Biochem Cell Biol 27: 287-295, 1995; Natesan S, Reddy SR. Comp Biochem Physiol B Biochem Mol Biol 130: 585-595, 2001; Yu H, Yoo PK, Aguirre CC, Tsoa RW, Kern RM, Grody WW, Cederbaum SD, Iyer RK. J Histochem Cytochem 51: 1151-1160, 2003). This study was designed to provide a detailed description of the sexual dimorphism of Oat expression in the mouse kidney and to test the influence of sex hormones on its regulation. Experiments were performed on male and female Swiss OF1 mice during their postnatal development, at adulthood, and in orchidectomized and ovariectomized mice. Kidneys, dissected renal zones, and mitochondria were used to analyze OAT mRNA and protein levels and measure OAT activity. The results revealed that before puberty, Oat expression was similar between female and male kidneys whereas from puberty until adulthood Oat expression increased in the female kidney, becoming approximately 2.5-fold higher than in the male kidney. This sex-differential expression of Oat was associated with a sex-specific distribution of Oat along the corticopapillary axis and within the nephron. OAT was three- to fourfold more expressed in the female than the male cortex. In males, Oat was highly expressed in the medulla, mainly in the thick ascending limbs. Renal Oat distribution in orchidectomized mice resembled that in the females. Ovariectomy did not influence Oat expression. Sex differences are explained by the physiological increase in plasma testosterone in males. Expression of medium-chain acyl-CoA synthetase protein confirmed this finding. We report sexual dimorphism of Oat expression in the mouse kidney and show that Oat is naturally downregulated in the presence of testosterone.

Testosterone down-regulates ornithine aminotransferase gene and up-regulates arginase II and ornithine decarboxylase genes for polyamines synthesis in the murine kidney

The enzymes ornithine aminotransferase (OAT) and ornithine decarboxylase (ODC) share L-ornithine as a common substrate and arginase II produces this amino acid. In the murine kidney, testosterone induced ODC gene expression and polyamine production, but it is unknown how OAT gene is expressed under androgen treatment. These experiments were designed to study the influence of testosterone on the renal expression of OAT gene. Pharmacological and physiological doses of testosterone were injected into female and castrated male mice. Total RNA and soluble proteins extracted from whole kidneys were analyzed by Northern and Western blots, respectively. The results clearly indicate that pharmacological doses of testosterone simultaneously down-regulated the level of OAT protein and up-regulated the expression of arginase II and ODC genes. Variations of the levels of OAT protein and arginase II mRNA and protein were strongly correlated with testosteronemia. Orchidectomy increased the renal level of OAT protein and decreased that of ODC and arginase II. These effects were reversed by injecting a physiological dose of testosterone into castrated male mice. In conclusion, OAT and ODC genes are inversely regulated by testosterone in the mouse kidney. Consequently, in kidneys of testosterone-treated mice, L-arginine-derived ornithine produced by arginase II might be preferentially used by ODC for putrescine production rather than by OAT. This metabolic fate of L-ornithine was facilitated by decreasing OAT gene expression. In contrast, in female and castrated male mice devoided of testosterone, OAT gene is highly expressed and L-ornithine is converted into L-glutamate.

Localization and differential expression of arginase II in the kidney of male and female mice

Arginase II (AII) has been almost exclusively studied in male mammalian kidneys. Our investigations were conducted to localize AII gene expression in the female mouse kidney, and to analyze the differential expression of AII gene at the transcriptional and translational levels in the kidneys of female and male mice. Total RNAs and soluble proteins extracted from renal zones and whole kidneys were analyzed by Northern and Western blots, respectively. Mitochondrial and cytosolic proteins were analyzed by Western blot. L-[guanidino-14C]arginine hydrolysis by AII was detected in microdissected tubules and the 14CO2 released from [14C]urea hydrolysis was quantified. The results of these experiments showed that: (1) both AII mRNA and protein were highly expressed in the deep cortex and the outer stripe of the outer medulla, (2) urea was produced mainly in the proximal straight tubules (PST), (3) the 38-kDa AII protein was more abundant in the mitochondria than the cytosol, and (4) the renal content of AII mRNA and protein was about three-fold higher in female than in male mice. In conclusion, in both genders, AII gene expression is restricted to the PST and localized into mitochondria. AII gene is differentially expressed in the kidney of female and male mice since higher levels of AII mRNA, protein and activity were observed in the kidneys of the former than those of the latter. Renal AII gene expression was gender-dependent in mice but not in rats. Finally, in the PST of females, L-arginine-derived ornithine may be a precursor for the renal production of L -glutamate and L-glutamine because high levels of AII, ornithine aminotransferase and glutamine synthetase are expressed in this nephron segment.

Ornithine metabolism in male and female rat kidney: mitochondrial expression of ornithine aminotransferase and arginase II

In the kidney, l-ornithine is reabsorbed along the proximal convoluted tubule (PCT), transported by basolateral carriers, and produced by arginase II (AII). Here, the renal metabolic fate of l-ornithine was analyzed in male and female rats. Kidneys and renal zones were dissected and used for Western blot analysis, immunofluorescence, and electron microscopic studies. Ornithine aminotransferase (OAT) and AII were localized using specific antibodies. Ornithine oxidation was determined by incubating microdissected tubules with l-[1-14C] or l-[U-14C]ornithine in the presence or absence of energy-providing substrates. Ornithine decarboxylase (ODC) mRNAs were localized by in situ hybridization. The 48-kDa OAT protein was detected in male and female kidneys, but its level was fourfold higher in the latter. OAT relative distribution increased from the superficial cortex toward the outer medulla to reach its highest level. Almost all OAT protein was localized in cortical and medullary proximal straight tubules (CPST and OSPST, respectively). In proximal straight tubule (PST), AII protein distribution overlapped that of OAT. No gender difference in AII protein level was found. OAT and AII were colocalized within PST mitochondria. l-[1-14C]ornithine decarboxylation occurred in all tubules, but predominantly in proximal tubules. l-[1-14C]ornithine decarboxylation was enhanced when l-[1-14C]ornithine was given to tubules as the sole substrate. The use of l-[U-14C]ornithine demonstrated the complete oxidation of ornithine. In conclusion, the OAT gene was expressed more in female rat proximal tubules than in male. Because OAT and AII proteins overlapped in PST mitochondria, l-arginine-derived ornithine may be preferentially converted to l-glutamate, as proven by ornithine oxidation. However, the coexpression of ODC, glutamate decarboxylase, and glutamine synthetase in PST suggests that l-ornithine can also be metabolized to putrescine, GABA, and l-glutamine. The fate of l-ornithine may depend on the cellular context.

Influence of testosterone on regulation of ODC, antizyme, and N1-SSAT gene expression in mouse kidney

Polyamines are involved in the control of the cell cycle and cell growth. In murine kidney, testosterone enhances gene expression of ornithine decarboxylase (ODC), the first enzyme in polyamine biosynthesis. In this study, we document the time course effect of testosterone on 1) gene expression of ODC, antizyme 1 (AZ1), and spermidine/spermine-N1-acetyltransferase (N1-SSAT); 2) ODC activity in proximal convoluted tubules (PCT) and cortical proximal straight tubules (CPST); and 3) renal polyamine levels. Female mice were treated with testosterone for a period of 1, 2, 3, and 5 consecutive days. ODC gene expression was extremely low in kidneys of untreated female mice compared with that of males. Consequently, the renal putrescine level was sevenfold lower in females than in males, whereas spermidine and spermine levels did not differ between sexes. In female kidneys, testosterone treatment sharply increased ODC mRNA and protein levels as well as ODC activity. Testosterone increased the expression of ODC in PCT and CPST over different time courses, which suggests that ODC activity is differentially regulated in distinct tubules. The expression of AZ1 and N1-SSAT mRNA was similar in male and female mouse kidneys. Testosterone treatment enhanced AZ1 and N1-SSAT mRNA levels in a time-dependent manner by unknown molecular mechanisms. Putrescine and spermidine levels increased after testosterone treatment in female kidneys. Surprisingly, although ODC protein and activity were undetectable in female kidneys, the levels of AZ1 mRNA and protein were similar to those in males. Therefore, one may propose that ODC protein could be continuously degraded by AZ1 in female kidneys.