Isolation and culture of juxtaglomerular and renomedullary interstitial cells

Diuretic, Natriuretic, and Vasodepressor Activity of a Lipid Fraction Enhanced in Medium of Cultured Mouse Medullary Interstitial Cells by a Selective Fatty Acid Amide Hydrolase Inhibitor

The relationship between the endocannabinoid system in the renal medulla and the long-term regulation of blood pressure is not yet understood. To investigate the possible role of the endocannabinoid system in renomedullary interstitial cells, mouse medullary interstitial cells (MMICs) were obtained, cultured, and characterized for their responses to treatment with a selective inhibitor of fatty acid amide hydrolase, PF-3845 (N-3-pyridinyl-4-[[3-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenyl]methyl]-1-piperidinecarboxamide). Treatment of MMICs with PF-3845 increased cytoplasmic lipid granules detected by Sudan Black B staining and multilamellar bodies identified by transmission electron microscopy. High-performance liquid chromatography (HPLC) analyses of lipid extracts of MMIC culture medium revealed a 205-nm absorbing peak that showed responsiveness to PF-3845 treatment. The biologic activities of the PF-3845-induced product (PIP) isolated by HPLC were investigated in anesthetized, normotensive surgically instrumented mice. Intramedullary and intravenous infusion of PIP at low dose rates (0.5-1 area units under the peak/10 min) stimulated diuresis and natriuresis, whereas these parameters returned toward baseline at higher doses but mean arterial pressure (MAP) was lowered. Whereas intravenous bolus doses of PIP stimulated diuresis, the glomerular filtration rate, and medullary blood flow (MBF) and reduced or had no effect on MAP, an intraperitoneal bolus injection of PIP reduced MAP, increased MBF, and had no effect on urine parameters. These data support a model whereby PF-3845 treatment of MMICs results in increased secretion of a neutral lipid that acts directly to promote diuresis and natriuresis and indirectly through metabolites to produce vasodepression. Efforts to identify the structure of the PF-3845-induced lipid and its relationship to the previously proposed renomedullary antihypertensive lipids are ongoing.

Hypoxia-inducible factor-1α contributes to the profibrotic action of angiotensin II in renal medullary interstitial cells

To examine whether hypoxia-inducible factor (HIF)-1α mediates the profibrotic effects of angiotensin II, we treated cultured renal medullary interstitial cells with angiotensin II and found that it increased HIF-1α levels. This was accompanied by a significant upregulation of collagen I/III, the tissue inhibitor of metalloproteinase-1, elevation of the proliferation marker proliferating cell nuclear antigen, and a transdifferentiation marker vimentin. All these effects of angiotensin II were completely blocked by siRNA for HIF-1α but not HIF-2α. Overexpression of a prolyl-hydroxylase domain-containing protein 2 (PHD2) transgene, the predominant renal HIF prolyl-hydroxylase, attenuated the effects of angiotensin II and its gene silencing enhanced the effects of angiotensin II. Removal of hydrogen peroxide eliminated angiotensin II-induced profibrotic effects. A 2-week infusion of rats with angiotensin II increased the expression of HIF-1α and α-smooth muscle actin, another marker of transdifferentiation, in renal medullary interstitial cells in vivo. Thus, our study suggests that HIF-1α mediates angiotensin II-induced profibrotic effects through activation of cell transdifferentiation. We propose that redox regulation of prolyl-PHD2 plays a critical role in angiotensin II-induced activation of HIF-1α in renal cells.

Transcriptional regulation of heme oxygenases by HIF-1alpha in renal medullary interstitial cells

The present study was designed to test the hypothesis that hypoxia-inducible factor-1alpha (HIF-1alpha)-mediated transcriptional activation contributes to increased expression of heme oxygenase (HO) genes in renal medullary interstitial cells (RMICs). By Northern blot analysis, HO-1 mRNA expression was found to significantly increase in response to reduction of PO(2) in culture medium. However, HO-2 mRNA was not altered by hypoxia. This hypoxia-induced upregulation of HO-1 mRNA was significantly blocked by HIF-1alpha inhibition with ferrous ammonium sulfate. To further determine the role of HIF-1alpha in the activation of HO-1, the inducers of HIF-1alpha were used to address whether induction of HIF-1alpha stimulates HO-1 mRNA expression. Both desferrioxamine and CoCl(2) markedly increased HIF-1alpha mRNA and protein levels and resulted in the upregulation of HO-1 mRNA but not HO-2. Furthermore, inhibition of HIF-1alpha degradation by CBZ-LLL, an inhibitor of ubiquitin-proteasome, significantly increased HIF-1alpha protein and HO-1 mRNA but not HO-2 in these cells. Using cis-element oligodeoxynucleotide transfection to specifically decoy HIF-1alpha and block HIF-1alpha binding, increased mRNA expression of HO-1 in response to hypoxia and CoCl(2) was attenuated. In vitro nuclear run-on assays further confirmed that hypoxia and alterations of HIF-1alpha mRNA or protein levels significantly affected the formation of HO-1 mRNA. Taken together, our results indicate that HO-1, but not HO-2, is transcriptionally activated by hypoxia through HIF-1alpha-mediated mechanism in RMICs. This hypoxia-induced transcriptional activation may be one of the important mechanisms mediating increased expression of HO-1 in the renal medulla.

Oxygen-dependent expression of hypoxia-inducible factor-1alpha in renal medullary cells of rats

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a transcription factor that regulates the oxygen-dependent expression of a number of genes. This transcription factor may contribute to the abundant expression of many genes in renal medullary cells that function normally under hypoxic conditions. The present study was designed to determine the characteristics of HIF-1alpha cDNA cloned from the rat kidney and the expression profile of HIF-1alpha in different kidney regions and to explore the mechanism activating or regulating HIF-1alpha expression in renal medullary cells. A 3,718-bp HIF-1alpha cDNA from the rat kidney was first cloned and sequenced using RT-PCR and TA cloning technique. It was found that 823 amino acids deduced from this renal HIF-1alpha cDNA had 99%, 96%, and 90% identity with rat, mouse, or human HIF-1alpha deposited in GenBank, respectively. The 3'-untranslated region of HIF-1alpha mRNA from the rat kidney contained seven AUUUA instability elements, five of which were found to be conserved among rat, mouse, and human HIF-1alpha. Northern blot analyses demonstrated a corticomedullary gradient of HIF-1alpha mRNA expression in the kidney, with the greatest abundance in the renal inner medulla. Western blot analyses also detected a higher HIF-1alpha protein level in the nuclear extracts from the renal medulla than the renal cortex. A classic loop diuretic, furosemide (10 mg/kg ip), markedly increased renal medullary Po(2) levels from 22.5 to 52.2 mmHg, which was accompanied by a significant reduction of HIF-1alpha transcripts in renal medullary tissue. In in vitro experiments, low Po(2), but not elevated osmolarity, was found to significantly increase HIF-1alpha mRNA in renal medullary interstitial cells and inner medullary collecting duct cells. These results indicate that HIF-1alpha is more abundantly expressed in the renal medulla compared with the renal cortex. Increased abundance of HIF-1alpha mRNA in the renal medulla may represent an adaptive response of renal medullary cells to low Po(2).