Published online before print September 4, 2007, doi: 10.1161/HYPERTENSIONAHA.107.097519
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(Hypertension. 2007;50:844.)
© 2007 American Heart Association, Inc.
Editorial Commentaries |
Is Peroxisome Proliferator-Activated Receptor-
a New "Pal" of Renin?
From the Department of Molecular Physiology and Biophysics (E.T.W., C.D.S.), Molecular Biology Graduate Program (H.I.), and Department of Internal Medicine (H.L.K., C.D.S.), Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City.
Correspondence to Curt D. Sigmund, Departments of Internal Medicine and Physiology and Biophysics, 3181B Medical Education and Biomedical Research Facility, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242. E-mail curt-sigmund{at}uiowa.edu
Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that have a growing list of pleiotropic effects and have been implicated in inflammation, Alzheimer’s disease, cancer, diabetes, obesity, and cardiovascular diseases, including atherosclerosis and hypertension. The 
-subtype of the receptor (PPAR) has been shown to regulate transcription of target genes by binding as a heterodimer with retinoid X receptor (RXR) to a hexameric direct repeat called the PPAR response element (PPRE). The consensus sequence of the PPRE is AGGTCANAGGTCA, where N is a single nucleotide separating the 2 receptor-binding half sites (italicized). For classic PPAR response genes, PPAR/RXR
heterodimers sit on the PPRE and recruit corepressors in the absence of ligand. On ligand binding, a conformational change in PPAR is induced, resulting in a replacement of the corepressors with coactivators, thus achieving ligand-activated transcription of the target gene.
Thiazolidinediones (TZDs) are high-affinity synthetic ligands of PPAR used clinically to improve insulin sensitivity in type-2 diabetes. Reports that TZDs also have cardioprotective effects, such as reducing blood pressure, suggest a potential link between PPAR activity and the cardiovascular system. Compelling genetic evidence supporting this includes reports showing that patients with dominant-negative mutations in PPAR exhibit severe early onset hypertension.1 These data also indirectly suggest that the beneficial effects of TZDs on blood pressure may not be derived solely from improved glycemic controls but perhaps also from direct effects of PPAR in cardiovascular tissues, such as the blood vessel. Consequently, understanding the fundamental mechanisms controlling blood pressure by PPAR is essential.
One of the major pathways in blood pressure regulation is the renin-angiotensin system. TZD treatment prevents the increase in blood pressure caused by infusion of angiotensin-II (Ang II) in rats2 and decreases blood pressure and improves endothelial function in double-transgenic mice exhibiting lifelong hypertension.3 PPAR activation by TZDs causes a downregulation of Ang II type I receptor gene expression via a PPAR-dependent mechanism in vascular smooth muscle cells.4 Therefore, PPAR may play a role in the regulation of Ang II action.
Renin mediates the rate-limiting step in the synthesis of Ang II, and, as such, the transcriptional mechanisms controlling its synthesis have been the subject of extensive investigation. A transcriptional enhancer has been identified upstream of the promoter in the mouse, rat, and human renin genes,5 which has been reported to be required to control the baseline level of human renin expression in vivo.6 Among the transcription factor binding sites in the enhancer is the hormone response element (HRE). Several members of the nuclear hormone receptor superfamily, including retinoic acid receptor and RXR, have been shown to bind to the HRE and to regulate renin gene expression.7 In addition, vitamin D has been reported to negatively modulate renin gene expression through a vitamin D receptor–dependent mechanism, which may involve the HRE.7,8 Because the HRE is homologous to a PPRE, and PPAR, retinoic acid receptor-, RXR, and vitamin D receptor are all members of the same subfamily of ligand-activated transcription factors, it should not be surprising that PPAR may have to be included among those transcription factors thought to regulate renin expression.
In this issue of Hypertension, Todorov et al9 present data suggesting a role for PPAR in the stimulation of renin expression. They show that pharmacological activators of PPAR activate renin gene expression in a PPAR-dependent manner in human renin-expressing Calu-6 cells. Although the effect was largest with the synthetic PPAR ligand rosiglitazone, renin mRNA was also induced by the naturally occurring fatty acids linoleic acid and oleic acid. Calu-6 cells are renin-expressing cells derived from a pulmonary carcinoma that use a posttranscriptional pathway to regulate renin expression in response to cAMP.10 Importantly, the authors demonstrated that the PPAR-mediated induction in Calu-6 cells was transcriptional and that both rosiglitazone and oleic acid increased renin mRNA expression in cultured native JG cells.
Perhaps the most intriguing finding from this report was that the PPAR-mediated activation of the renin promoter did not map to the HRE in the renin enhancer but instead mapped to a sequence in the minimal promoter (–148 to –134) termed "Pal3" (Figure). Mutation of the Pal3 sequence in the context of a reporter construct containing only the minimal renin promoter fused to luciferase attenuated baseline transcription and abolished the induction by rosiglitazone. However, mutation of the Pal3 sequence when the renin enhancer was present in the reporter construct had only a minimal effect on baseline promoter activity, and, interestingly, partial responsiveness to rosiglitazone was retained. Thus, the full induction by PPAR may require sequences in both the enhancer and proximal promoter.
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This is not the first time that a Pal3 sequence has been identified to interact with PPAR. In a study by Okuno et al,11 sequences binding either PPAR/RXR heterodimers or PPAR/PPAR homodimers were selected using a PCR-mediated random site selection assay. Although the classical PPRE with a spacer of 1 nucleotide (termed "DR1") was identified when PPAR and RXR were added to the assay in equimolar amounts, the Pal3 sequence was identified when a 30-fold molar excess of PPAR over RXR was used (ie, favoring homodimer formation). The sequence of the Pal3 site as a position weight matrix is shown in the Figure, along with the closest homologies in the 5' flanking region of the human, rat, and mouse renin genes. By combining small interfering RNA and supershift analysis, Todorov et al9 were able to confirm both PPAR and RXR binding to the human renin Pal3 sequence. Notably, 2 complexes were supershifted when PPAR antisera was added to the gel shift reaction. One of these complexes was significantly reduced in the presence of PPAR and RXR small interfering RNA and consequently is likely to be a PPAR/RXR heterodimer. However, another supershift complex was retained and even increased in the presence of PPAR and RXR small interfering RNA. Because the PCR-mediated random site selection assay predicts the binding of PPAR homodimers to the Pal3 site, the authors argue that knockdown of PPAR could paradoxically increase the formation of PPAR homodimers at the same time as its capacity to compete for RXR is reduced. PPAR homodimers are most likely to form when the molar ratio of PPAR:RXR is high, a situation opposite to what should be expected in the presence of PPAR small interfering RNA. Therefore, their explanation appears counterintuitive. One aspect that must be considered in interpreting these results is the specificity of the PPAR antisera. Accordingly, the identity of the proteins binding in this complex, whether they contain PPAR homodimers, and whether other nuclear proteins bind to the Pal3 site require additional investigation and definitive proof.
Despite some differences in interpretation, the data in total support a role for PPAR in the regulation of renin gene expression, at least in Calu-6 cells in culture. Clearly, then, the most important outstanding question from this research is: to what extent does PPAR control renin expression in vivo? Unfortunately, the literature on the effect of TZDs on plasma renin activity is difficult to interpret, because some studies report an increase, whereas others report a decrease or no change. Tsai et al12 used gene targeting to knock in and replace 1 allele of mouse PPAR with a PPAR gene carrying a dominant-negative mutation. These mice exhibited a modest increase in blood pressure, with a concomitant increase in angiotensinogen expression in inguinal adipose tissue and Ang II type 1 receptor expression in gonadal adipose. However, there was no alteration of renin mRNA in the kidney or adipose tissue in these mice.12 Similarly, there was no change in renal renin mRNA in the kidney of knockout mice lacking PPAR that were rescued from embryonic lethality by expression of PPAR in trophoblast cells.13 Although one may conclude that PPAR does not regulate renin expression in vivo, other factors, such as feedback inhibition on renin expression, cannot be ruled out. Thus, the next obvious step is to generate a juxtaglomerular cell-specific knockout of PPAR, and, indeed, tools exist to accomplish this. A flox/flox allele of PPAR has been available for sometime,14 and a number of mouse models expressing cre-recombinase under the control of the renin promoter, including a knock-in allele of cre-recombinase to the renin locus, have been reported.15 Such a study will ultimately reveal the extent to which PPAR regulates renin gene expression and the physiological cues to which it responds.
PPAR is an attractive molecule given its role as a fatty acid sensor and the increased incidence of hypertension in obese patients. It remains possible that PPAR may represent a link among obesity, metabolic dysfunction, and activity of either the circulating or tissue renin-angiotensin system in hypertension. There is no doubt that the mechanisms regulating the renin-angiotensin system by PPAR will prove to be quite complex given the positive stimulation on renin transcription reported in this issue, along with the negative impact on the Ang II type 1 receptor reported previously. That TZDs are generally thought to modestly lower blood pressure suggests that a delicate balance must exist between the effects of PPAR on the renin-angiotensin system (renin versus Ang II type 1 receptor) and other vasoconstrictors, such as endothelin-1.
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Acknowledgments |
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Source of Funding
Funded by NIH ROI 48058.
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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 Top References |
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1. Barroso I, Gurnell M, Crowley VE, Agostini M, Schwabe JW, Soos MA, Maslen GL, Williams TD, Lewis H, Schafer AJ, Chatterjee VK, O’Rahilly S. Dominant negative mutations in human PPARgamma associated with severe insulin resistance, diabetes mellitus and hypertension. Nature. 1999; 402: 880–883.[Medline] [Order article via Infotrieve]
2. Diep QN, El Mabrouk M, Cohn JS, Endemann D, Amiri F, Virdis A, Neves MF, Schiffrin EL. Structure, endothelial function, cell growth, and inflammation in blood vessels of angiotensin II-infused rats: role of peroxisome proliferator-activated receptor-gamma. Circulation. 2002; 105: 2296–2302.
3. Ryan MJ, Didion SP, Mathur S, Faraci FM, Sigmund CD. PPAR(gamma) agonist rosiglitazone improves vascular function and lowers blood pressure in hypertensive transgenic mice. Hypertension. 2004; 43: 661–666.
4. Sugawara A, Takeuchi K, Uruno A, Ikeda Y, Arima S, Kudo M, Sato K, Taniyama Y, Ito S. Transcriptional suppression of type 1 angiotensin II receptor gene expression by peroxisome proliferator-activated receptor-gamma in vascular smooth muscle cells. Endocrinology. 2001; 142: 3125–3134.
5. Pan L, Gross KW. Transcriptional regulation of renin: an update. Hypertension. 2005; 45: 3–8.
6. Zhou X, Davis DR, Sigmund CD. The human renin kidney enhancer is required to maintain baseline renin expression but is dispensable for tissue-specific, cell-specific and regulated expression. J Biol Chem. 2006 281: 35296–35304.
7. Shi Q, Gross KW, Sigmund CD. Retinoic acid-mediated activation of the mouse renin enhancer. J Biol Chem. 2001; 276: 3597–3603.
8. Li YC, Kong J, Wei M, Chen Z-F, Liu SQ, Cao L-P. 1,25-Dihydroxyvitamin D3 is a negative endocrine regulator of the renin-angiotensin system. J Clin Invest. 2002; 110: 229–238.[CrossRef][Medline] [Order article via Infotrieve]
9. Todorov VT, Desch M, Schmitt-Nilson N, Todorova A, Kurtz A. Peroxisome proliferator-activated receptor-gamma is involved in the control of renin gene expression. Hypertension. 2007; 50: 939–944.
10. Sinn PL, Sigmund CD. Human renin mRNA stability is increased in response to cAMP in Calu-6 cells. Hypertension. 1999; 33: 900–905.
11. Okuno M, Arimoto E, Ikenobu Y, Nishihara T, Imagawa M. Dual DNA-binding specificity of peroxisome-proliferator-activated receptor gamma controlled by heterodimer formation with retinoid X receptor alpha. Biochem J. 2001; 353: 193–198.[CrossRef][Medline] [Order article via Infotrieve]
12. Tsai YS, Kim HJ, Takahashi N, Kim HS, Hagaman JR, Kim JK, Maeda N. Hypertension and abnormal fat distribution but not insulin resistance in mice with P465L PPARgamma. J Clin Invest. 2004; 114: 240–249.[CrossRef][Medline] [Order article via Infotrieve]
13. Duan SZ, Ivashchenko CY, Whitesall SE, D’Alecy LG, Duquaine DC, Brosius FC, Gonzalez FJ, Vinson C, Pierre MA, Milstone DS, Mortensen RM. Hypotension, lipodystrophy, and insulin resistance in generalized PPARgamma-deficient mice rescued from embryonic lethality. J Clin Invest. 2007; 117: 812–822.[CrossRef][Medline] [Order article via Infotrieve]
14. Matsusue K, Haluzik M, Lambert G, Yim SH, Gavrilova O, Ward JM, Brewer B Jr, Reitman ML, Gonzalez FJ. Liver-specific disruption of PPARgamma in leptin-deficient mice improves fatty liver but aggravates diabetic phenotypes. J Clin Invest. 2003; 111: 737–747.[CrossRef][Medline] [Order article via Infotrieve]
15. Sequeira Lopez ML, Pentz ES, Nomasa T, Smithies O, Gomez RA. Renin cells are precursors for multiple cell types that switch to the renin phenotype when homeostasis is threatened. Dev Cell. 2004; 6: 719–728.[CrossRef][Medline] [Order article via Infotrieve]
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Hypertension 2007 50: 939-944.
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