Characterization and functional analyses of the human G protein-coupled receptor kinase 4 gene promoter

Primary Pediatric Hypertension: Current Understanding and Emerging Concepts

The rising prevalence of primary pediatric hypertension and its tracking into adult hypertension point to the importance of determining its pathogenesis to gain insights into its current and emerging management. Considering that the intricate control of BP is governed by a myriad of anatomical, molecular biological, biochemical, and physiological systems, multiple genes are likely to influence an individual's BP and susceptibility to develop hypertension. The long-term regulation of BP rests on renal and non-renal mechanisms. One renal mechanism relates to sodium transport. The impaired renal sodium handling in primary hypertension and salt sensitivity may be caused by aberrant counter-regulatory natriuretic and anti-natriuretic pathways. The sympathetic nervous and renin-angiotensin-aldosterone systems are examples of antinatriuretic pathways. An important counter-regulatory natriuretic pathway is afforded by the renal autocrine/paracrine dopamine system, aberrations of which are involved in the pathogenesis of hypertension, including that associated with obesity. We present updates on the complex interactions of these two systems with dietary salt intake in relation to obesity, insulin resistance, inflammation, and oxidative stress. We review how insults during pregnancy such as maternal and paternal malnutrition, glucocorticoid exposure, infection, placental insufficiency, and treatments during the neonatal period have long-lasting effects in the regulation of renal function and BP. Moreover, these effects have sex differences. There is a need for early diagnosis, frequent monitoring, and timely management due to increasing evidence of premature target organ damage. Large controlled studies are needed to evaluate the long-term consequences of the treatment of elevated BP during childhood, especially to establish the validity of the current definition and treatment of pediatric hypertension.

Role of GRK4 in the regulation of arterial AT1 receptor in hypertension

G-protein-coupled receptor kinase 4 (GRK4) gene variants, via impairment of renal dopamine receptor and enhancement of renin-angiotensin system functions, cause sodium retention and increase blood pressure. Whether GRK4 and the angiotensin type 1 receptor (AT(1)R) interact in the aorta is not known. We report that GRK4 is expressed in vascular smooth muscle cells of the aorta. Heterologous expression of the GRK4γ variant 142V in A10 cells increased AT(1)R protein expression and AT(1)R-mediated increase in intracellular calcium concentration. The increase in AT(1)R expression was related to an increase in AT(1)R mRNA expression via the NF-κB pathway. As compared with control, cells expressing GRK4γ 142V had greater NF-κB activity with more NF-κB bound to the AT(1)R promoter. The increased AT(1)R expression in cells expressing GRK4γ 142V was also associated with decreased AT(1)R degradation, which may be ascribed to lower AT(1)R phosphorylation. There was a direct interaction between GRK4γ and AT(1)R that was decreased by GRK4γ 142V. The regulation of AT(1)R expression by GRK4γ 142V in A10 cells was confirmed in GRK4γ 142V transgenic mice; AT(1)R expression was higher in the aorta of GRK4γ 142V transgenic mice than control GRK4γ wild-type mice. Angiotensin II-mediated vasoconstriction of the aorta was also higher in GRK4γ 142V than in wild-type transgenic mice. This study provides a mechanism by which GRK4, via regulation of arterial AT(1)R expression and function, participates in the pathogenesis of conduit vessel abnormalities in hypertension.

A novel role for c-Myc in G protein-coupled receptor kinase 4 (GRK4) transcriptional regulation in human kidney proximal tubule cells

The G protein-coupled receptor kinase 4 (GRK4) negatively regulates the dopaminergic system by desensitizing the dopamine-1-receptor. The expressional control of GRK4 has not been reported, but here we show that the transcription factor c-Myc binds to the promoter of GRK4 and positively regulates GRK4 protein expression in human renal proximal tubule cells (RPTCs). Addition of phorbol esters to RPTCs not only increased c-Myc binding to the GRK4 promoter but also increased both phospho-c-Myc and GRK4 expression. The phorbol ester-mediated increase in GRK4 expression was completely blocked by the c-Myc inhibitor, 10074-G5, indicating that GRK4 is downstream of phospho-c-Myc. The autocrine production of angiotensin II (Ang II) in RPTCs increased the phosphorylation and activation of c-Myc and subsequently GRK4 expression. 3-Amino-4-thio-butyl sulfonate, an inhibitor of aminopeptidase A, increased RPTC secretion of Ang II. 3-Amino-4-thio-butyl sulfonate or Ang II increased the expression of both phospho-c-Myc and GRK4, which was blocked by 10074-G5. Blockade of the Ang II type 1 receptor with losartan decreased phospho-c-Myc and GRK4 expression. Both inhibition of c-Myc activity and blockade of Ang II type 1 receptor restored the coupling of dopamine-1-receptor to adenylyl cyclase stimulation in uncoupled RPTCs, whereas phorbol esters or Ang II caused the uncoupling of normally coupled RPTCs. We suggest that the Ang II type 1 receptor impairs dopamine-1-receptor function via c-Myc activation of GRK4. This novel pathway may be involved in the increase in blood pressure in hypertension that is mediated by increased activity of the renin-angiotensin system and decreased activity of the renal dopaminergic system.

Evolutionary and functional analyses of cytochrome P4501A promoter polymorphisms in natural populations

The functional importance of variable, transcriptional regulatory sequences within and among natural populations is largely unexplored. We analysed the cytochrome P4501A (CYP1A) promoter in three populations of the minnow, Fundulus heteroclitus, because two SNPs in the promoter and first intron of CYP1A are under selection in populations adapted to pollutants. To define the importance of these SNPs, 1630 bp of the CYP1A promoter and first intron and exon were sequenced in eight individuals from three populations: a population from a polluted environment resistant to some aromatic pollutants and two flanking reference populations. CYP1A is induced by many aromatic pollutants, but in populations adapted to pollutants, CYP1A has been shown to be refractory to induction. We were interested in understanding whether variation in the CYP1A promoter explains mechanism(s) of adaptation to these aromatic pollutants. The CYP1A promoter was extremely variable (an average of 9.3% of the promoter nucleotides varied among all populations) and exhibited no fixed differences between populations. As CYP1A is poorly inducible in adapted fish, we hypothesized that CYP1A promoter regions might vary functionally between populations. Unexpectedly, in vitro analysis showed significantly greater transcription from CYP1A promoters found in the population from the polluted environment relative to promoters found in both reference populations. Thus, despite extensive variation among populations and lack of fixed differences between populations, individuals from a polluted environment have significantly enhanced promoter activity. These data demonstrate that intraspecific variation, which provides the raw material for natural selection to act on, can occur while maintaining promoter function.

HK-2 human renal proximal tubule cells as a model for G protein-coupled receptor kinase type 4-mediated dopamine 1 receptor uncoupling

HK-2 human renal proximal tubule cells (RPTC) are commonly used in the in vitro study of "normal" RPTCs. We discovered recently that HK-2 cells are uncoupled from dopamine 1 receptor (D(1)R) adenylyl cyclase (AC) stimulation. We hypothesized that G protein-coupled receptor kinase type 4 (GRK4) single nucleotide polymorphisms may be responsible for the D(1)R/AC uncoupling in HK-2. This hypothesis was tested by genotyping GRK4 single nucleotide polymorphisms, measuring D(1)-like receptor agonist (fenoldopam)-stimulated cAMP accumulation, quantifying D(1)R inhibition of sodium transport, and testing the ability of GRK4 small interfering RNA to reverse the D(1)R/AC uncoupling. We compared HK-2 with 2 normally coupled human RPTC cell lines and 2 uncoupled RPTC cell lines. The HK-2 cell line was found to have 4 of 6 potential GRK4 single nucleotide polymorphisms known to uncouple the D(1)R from AC (namely, R65L, A142V, and A486V). AC response to fenoldopam stimulation was increased in the 2 normally coupled human RPTC cell lines (FEN: 2.02+/-0.05-fold and 2.33+/-0.19-fold over control; P<0.001; n=4) but not in the 2 uncoupled or HK-2 cell lines. GRK4 small interfering RNA rescued the fenoldopam-mediated AC stimulation in the uncoupled cells, including HK-2. The expected fenoldopam-mediated inhibition of sodium hydrogen exchanger type 3 was absent in HK-2 (n=6) and uncoupled RPTC cell lines (n=6) but was observed in the 2 normally coupled human RPTC cell lines (-25.41+/-4.7% and -27.36+/-2.70%; P<0.001; n=6), which express wild-type GRK4. Despite the fact that HK-2 cells retain many functional characteristics of RPTCs, they are not normal from the perspective of dopaminergic function.