Blood Pressure AsSAuLTed by the Circadian Clock

Cell Metabolism Previews Blood Pressure AsSAuLTed by the Circadian Clock Kristin Eckel-Mahan1 and Paolo Sassone-Corsi1,* 1Department of Pharmacology, School of Medicine, University of California, Irvine, CA 92697-4625, USA *Correspondence: [email protected] DOI 10.1016/j.cmet.2010.01.004 While Cry proteins are necessary for circadian rhythmicity, they now appear to play a seminal role in blood pressure regulation. In a recent issue of Nature Medicine, Doi et al., 2009 show how the circadian clock may use Cry proteins to protect from salt-sensitive hypertension. Circadian rhythms (rhythms that take place in approximately 24 hr) are oscillatory programs present in most organisms. Biological circadian oscillations are upheld by circadian rhythmicity within CNS and peripheral tissues and include oscillatory activity in the sleep/wake cycle, feeding, and hormone secretion. New links between metabolism and the circadian clock have also been established (Eckel-Mahan and Sassone-Corsi, 2009). Supported at the cellular level by complex transcriptional and translational feedback loops, biological rhythms are necessary for normal physiology. Indeed, aberrant circadian oscillations at the cellular level have been shown to contribute to a number of pathologies, including the metabolic syndrome, obesity, and accelerated aging (Kondratov et al., 2006; Turek et al., 2005). The provocative new study presented in Nature Medicine (Doi et al., 2009) introduces salt-sensitive hypertension to the growing list of risks for organisms that suffer from circadian disturbance. Hypertension (high blood pressure) is an expanding health problem that increases the risk of heart attacks and strokes. Some hypertension remains undiagnosed; however, commonly acknowledged risk factors for the disease range from age and family history to stress and physical inactivity. While high-salt diets contribute to hypertension in some patients, it isn’t really clear at the molecular level why some people are ‘‘salt-sensitive’’ while others remain ‘‘saltinsensitive.’’ Salt-sensitive hypertension can occur when the kidneys retain too much salt. The subsequent rise in blood pressure is frequently followed by further elevations as the body attempts to force salt out of the kidneys. At least 50% of people with hypertension are ‘‘salt sensitive,’’ a percentage large enough for physicians to suggest lowering dietary salt for hypertensive patients (Weinberger, 2006). But who would have thought that one’s circadian rhythm had much to do with hypertension—and more specifically, salt sensitivity? Doi et al., 2009 reveals that an abnormal circadian rhythm may be a strong risk factor for salt-sensitive hypertension. In this study, the authors found that arrhythmic mice (Cry null mice, lacking the clock components CRY1 and CRY2) have a malfunctioning renin-angiotensin-aldosterone system (RAAS) resulting from the aberrant production of the mineralocorticoid aldosterone in the zona glomerulosa cells of the adrenal gland. When blood pressure is low, aldosterone is released from the zona glomerulosa in response to renininduced angiotensin II production. Aldosterone causes the retention of salt and water from the urine in the kidneys, over time increasing the body’s water content and leading to an overall increase in blood pressure. Normally, an organism responds to a high salt load by suppressing the RAAS system, and blood pressure remains remarkably constant (Chrysant, 2000). Blood pressure rises in Cry null mice, however, after introduction to a high-salt diet, but is counteracted by aldosterone inhibitors. In this study, Doi et al. navigate unchartered waters to discover the enzyme (a 3beta-hydroxysteroid dehydrogenase-isomerase isoform) responsible for the elevated aldosterone in Cry null mice, and to identify its human counterpart and potential clinical relevance. The CRY proteins are encoded by the Cryptochrome genes, which are transcriptionally regulated by the circadian activators CLOCK and BMAL1. CRY proteins provide negative feedback on the clock by inhibiting CLOCK:BMAL1- mediated transcription at promoters containing E box elements. Due to their arrhythmic phenotypes, Clock mutant and Bmal1 null animals have been studied extensively. Their arrhythmia is due to a decrease in E box-mediated transcription. Cry null mice, however, show elevated levels of E box-mediated transcription, not providing the necessary negative feedback for the CLOCK:BMAL1 complex (see Figure 1). In this context, it is interesting that the absence of CRY proteins specifically reveals an interaction of the circadian system with blood pressure. The novel aspects of this study extend beyond the observation that Cry null mice experience salt-sensitive hypertension. Using steroidogenic gene microarrays, the authors discovered a small group of genes in the 3beta-hydroxysteroid dehydrogenase-isomerase enzyme family that are dysregulated in the adrenal gland of Cry null mice. Interestingly, these oxidoreductases are required for the synthesis of aldosterone. Of the Hsd3b gene isoforms expressed in mouse adrenal glands, Hsd3b6 expression not only oscillates in WT animals but is elevated in the zona glomerulosa of Cry null mice, with an expression profile indicative of complete resistance to circadian control. The uncoupling of aldosterone from the RAAS system was revealed by the ability of trilostane, an inhibitor of Hsd3b to drop the plasma aldosterone concentration in Cry null mice. While WT animals showed no change in blood pressure when switched from a low-salt to a high-salt diet, Cry null mice experienced elevated blood pressure, which was reversed using the aldosterone antagonist epleronone. Similar circadian control is probably conserved in humans; Doi and colleagues successfully identify the human HSD3B1 isoform as a functional homolog of the mouse Hsd3b6. Cell Metabolism 11, February 3, 2010 ª2010 Elsevier Inc. 97 Cell Metabolism Previews Figure 1. Possible Modes of Hsd3b6 Regulation in the Zona Glomerulosa of WT and Cry Null Mice Hsd3b6 could be regulated by the circadian clock via the expression of the circadian output gene, DBP, or by other factors controlling its transcription, such as Nur77. The DBP gene, which contains circadian-responsive E box elements, is highly expressed in Cry null mice, and may be essential for D box activation at the Hsd3b6 promoter. Conversely, Hsd3b expression is highly responsive to the transcription factor Nur77 (Lavoie and King, 2009), a transcriptional activator thought to be regulated by the circadian clock in some tissues. Epigenetic studies will reveal whether the Hsd3b6 gene promoter is affected at the level of transcription factor accessibility. The role of Cry proteins in HAT and HDAC activity at target promoters suggests that the expression of Hsd3b6 may be affected by epigenetic alterations at the chromatin. While Clock mutant and Bmal1 null mice show metabolic disturbances or accelerated aging (among other pathologies), Cry null mice expose a novel role for the clock in physiology. But how exactly does the circadian clock contribute to salt-sensitive hypertension? Aldosterone levels do oscillate in mammals, but scant molecular data has linked salt-sensitive hypertension to the circadian clock. Doi et al. speculate that the Hsd3b6 promoter, containing two D box elements, might respond to elevated DBP expression, a circadian output gene that binds to D box elements and that remains elevated in Cry null mice (Figure 1). Alternative possibilities for Hsd3b6 upregulation remain, however. For example, CRY proteins may mediate their transcriptional repression via the recruitment of the histone deacetylase mSin3b to target gene promoters (Naruse et al., 2004). While there is no evidence that specific HDAC proteins act at the Hsd3b6 gene promoter, few studies have been performed to address how chromatin remodeling occurs at this region. Second, CRY proteins have been reported to inhibit the activity of some HATs, such as p300 (Etchegaray et al., 2003). In the absence of CRY proteins, the conformation of Hsd3b6-containing chromatin may enable constitutive transcription. Finally, the Hsd3b6 promoter appears to bind NR4A family transcription factors (Lavoie and King, 2009), among which Nurr77 is highly expressed in the adrenal gland. Nurr77 is circadian respon- 98 Cell Metabolism 11, February 3, 2010 ª2010 Elsevier Inc. sive in some tissues (Humphries et al., 2004) and remains a possible mechanism by which Cry proteins regulate Hsd3b6 expression (Figure 1). Whatever the mechanism, the interaction between the circadian clock and the RAAS is a physiologically prodigious discovery as the RAAS strictly controls blood pressure in vivo. Further studies will be necessary to determine the extent to which the circadian clock intersects with adrenal markers in humans. It is interesting to speculate whether ‘‘larks’’ or ‘‘owls,’’ for example, show a stronger propensity for salt-sensitive hypertension. Also, what about patients with severe sleep disorders? 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