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

G protein-coupled receptor kinases in hypertension: physiology, pathogenesis, and therapeutic targets

G protein-coupled receptors (GPCRs) mediate cellular responses to a myriad of hormones and neurotransmitters that play vital roles in the regulation of physiological processes such as blood pressure. In organs such as the artery and kidney, hormones or neurotransmitters, such as angiotensin II (Ang II), dopamine, epinephrine, and norepinephrine exert their functions via their receptors, with the ultimate effect of keeping normal vascular reactivity, normal body sodium, and normal blood pressure. GPCR kinases (GRKs) exert their biological functions, by mediating the regulation of agonist-occupied GPCRs, non-GPCRs, or non-receptor substrates. In particular, increasing number of studies show that aberrant expression and activity of GRKs in the cardiovascular system and kidney inhibit or stimulate GPCRs (e.g., dopamine receptors, Ang II receptors, and α- and β-adrenergic receptors), resulting in hypertension. Current studies focus on the effect of selective GRK inhibitors in cardiovascular diseases, including hypertension. Moreover, genetic studies show that GRK gene variants are associated with essential hypertension, blood pressure response to antihypertensive medicines, and adverse cardiovascular outcomes of antihypertensive treatment. In this review, we present a comprehensive overview of GRK-mediated regulation of blood pressure, role of GRKs in the pathogenesis of hypertension, and highlight potential strategies for the treatment of hypertension. Schematic representation of GPCR desensitization process. Activation of GPCRs begins with the binding of an agonist to its corresponding receptor. Then G proteins activate downstream effectors that are mediated by various signaling pathways. GPCR signaling is halted by GRK-mediated receptor phosphorylation, which causes receptor internalization through β-arrestin.

Exercise ameliorates skeletal muscle insulin resistance by modulating GRK4-mediated D1R expression

Exercise has been recommended as a nonpharmaceutical therapy to treat insulin resistance (IR). Previous studies showed that dopamine D1-like receptor agonists, such as fenoldopam, could improve peripheral insulin sensitivity, while antipsychotics, which are dopamine receptor antagonists, increased susceptibility to Type 2 diabetes mellitus (T2DM). Meanwhile, exercise has been proved to stimulate dopamine receptors. However, whether the dopamine D1 receptor (D1R) is involved in exercise-mediated amelioration of IR remains unclear. We found that the D1-like receptor antagonist, SCH23390, reduced the effect of exercise on lowering blood glucose and insulin in insulin-resistant mice and inhibited the contraction-induced glucose uptake in C2C12 myotubes. Similarly, the opposite was true for the D1-like receptor agonist, fenoldopam. Furthermore, the expression of D1R was decreased in skeletal muscles from streptozotocin (STZ)- and high-fat intake-induced T2DM mice, accompanied by increased D1R phosphorylation, which was reversed by exercise. A screening study showed that G protein-coupled receptor kinase 4 (GRK4) may be the candidate kinase for the regulation of D1R function, because, in addition to the increased GRK4 expression in skeletal muscles of T2DM mice, GRK4 transgenic T2DM mice exhibited lower insulin sensitivity, accompanied by higher D1R phosphorylation than control mice, whereas the AAV9-shGRK4 mice were much more sensitive to insulin than AAV9-null mice. Mechanistically, the up-regulation of GRK4 expression caused by increased reactive oxygen species (ROS) in IR was ascribed to the enhanced expression of c-Myc, a transcriptional factor of GRK4. Taken together, the present study shows that exercise, via regulation of ROS/c-Myc/GRK4 pathway, ameliorates D1R dysfunction and improves insulin sensitivity.

Pathologic HDAC1/c-Myc signaling axis is responsible for angiotensinogen transcription and hypertension induced by high-fat diet

High-fat diet (HFD)-induced obesity is a cause of resistant hypertension. We have shown a possible link between histone deacetylases (HDACs) and renal angiotensinogen (Agt) upregulation in the HFD-induced hypertension, whereas the underlying mechanisms remain to be elucidated. Here, using a HDAC1/2 inhibitor romidepsin (FK228) and siRNAs, we determined roles of HDAC1 and HDAC2 in HFD-induced hypertension and found the pathologic signaling axis between HDAC1 and Agt transcription. Treatment with FK228 canceled the increased blood pressure of male C57BL/6 mice induced by HFD. FK228 also blocked upregulation of renal Agt mRNA, protein, angiotensin II (Ang II) or serum Ang II. Activation and nuclear accumulation of both HDAC1 and HDAC2 occurred in the HFD group. The HFD-induced HDAC activation was associated with an increase in deacetylated c-Myc transcription factor. Silencing of HDAC1, HDAC2 or c-Myc in HRPTEpi cells decreased Agt expression. However, only HDAC1 knockdown, but not HDAC2, increased c-Myc acetylation, suggesting selective roles in two enzymes. Chromatin immunoprecipitation assay revealed that HFD induced the binding of HDAC1 and deacetylated c-Myc at the Agt gene promoter. A putative c-Myc binding sequence in the promotor region was necessary for Agt transcription. Inhibition of c-Myc downregulated Agt and Ang II levels in kidney and serum, ameliorating HFD-induced hypertension. Thus, the abnormal HDAC1/2 in the kidney may be responsible for the upregulation of the Agt gene expression and hypertension. The results expose the pathologic HDAC1/c-myc signaling axis in kidney as a promising therapeutic target for obesity-associated resistant hypertension.

Inverse Salt Sensitivity of Blood Pressure Is Associated with an Increased Renin-Angiotensin System Activity

High and low sodium diets are associated with increased blood pressure and cardiovascular morbidity and mortality. The paradoxical response of elevated BP in low salt diets, aka inverse salt sensitivity (ISS), is an understudied vulnerable 11% of the adult population with yet undiscovered etiology. A linear relationship between the number of single nucleotide polymorphisms (SNPs) in the dopamine D₂ receptor (DRD2, rs6276 and 6277), and the sodium myo-inositol cotransporter 2 (SLC5A11, rs11074656), as well as decreased expression of these two genes in urine-derived renal proximal tubule cells (uRPTCs) isolated from clinical study participants suggest involvement of these cells in ISS. Insight into this newly discovered paradoxical response to sodium is found by incubating cells in low sodium (LS) conditions that unveil cell physiologic differences that are then reversed by mir-485-5p miRNA blocker transfection and bypassing the genetic defect by DRD2 re-expression. The renin-angiotensin system (RAS) is an important counter-regulatory mechanism to prevent hyponatremia under LS conditions. Oversensitive RAS under LS conditions could partially explain the increased mortality in ISS. Angiotensin-II (AngII, 10 nmol/L) increased sodium transport in uRPTCs to a greater extent in individuals with ISS than SR. Downstream signaling of AngII is verified by identifying lowered expression of nuclear factor erythroid 2-related factor 2 (NRF2), CCCTC-binding factor (CTCF), and manganese-dependent mitochondrial superoxide dismutase (SOD2) only in ISS-derived uRPTCs and not SR-derived uRPTCs when incubated in LS conditions. We conclude that DRD2 and SLC5A11 variants in ISS may cause an increased low sodium sensitivity to AngII and renal sodium reabsorption which can contribute to inverse salt-sensitive hypertension.

Interactions between the intrarenal dopaminergic and the renin-angiotensin systems in the control of systemic arterial pressure

Systemic arterial hypertension is one of the leading causes of morbidity and mortality in the general population, being a risk factor for many cardiovascular diseases. Although its pathogenesis is complex and still poorly understood, some systems appear to play major roles in its development. This review aims to update the current knowledge on the interaction of the intrarenal renin-angiotensin system (RAS) and dopaminergic system in the development of hypertension, focusing on recent scientific hallmarks in the field. The intrarenal RAS, composed of several peptides and receptors, has a critical role in the regulation of blood pressure (BP) and, consequently, the development of hypertension. The RAS is divided into two main intercommunicating axes: the classical axis, composed of angiotensin-converting enzyme, angiotensin II, and angiotensin type 1 receptor, and the ACE2/angiotensin-(1-7)/Mas axis, which appears to modulate the effects of the classical axis. Dopamine and its receptors are also increasingly showing an important role in the pathogenesis of hypertension, as abnormalities in the intrarenal dopaminergic system impair the regulation of renal sodium transport, regardless of the affected dopamine receptor subtype. There are five dopamine receptors, which are divided into two major subtypes: the D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) receptors. Mice deficient in any of the five dopamine receptor subtypes have increased BP. Intrarenal RAS and the dopaminergic system have complex interactions. The balance between both systems is essential to regulate the BP homeostasis, as alterations in the control of both can lead to hypertension.

GRK4, A Potential Link between Hypertension and Breast Cancer

Hypertension and breast cancer are two common diseases occurring in women. Clinical studies have shown increased breast cancer incidence in hypertensive women. Several lines of evidence demonstrate that G protein-coupled Receptor Kinase 4 (GRK4) could be a common risk factor for hypertension and breast cancer. This article reviews our current understanding of molecular mechanisms of GRK4 in hypertension and breast cancer.

The Hypertension Related Gene G-Protein Coupled Receptor Kinase 4 Contributes to Breast Cancer Proliferation

PURPOSE

Clinical studies have shown that breast cancer risk is increased in hypertensive women. The underlying molecular mechanism remains undetermined. The current study tests our hypothesis that G protein coupled receptor kinase 4 (GRK4) is a molecule that links hypertension and breast cancer. GRK4 plays an important role in regulation of renal sodium excretion. Sustained activation of GRK4 as in the circumstances of single nucleotide polymorphism (SNPs) causes hypertension. Expression of GRK4 in the kidney is regulated by cMyc, an oncogene that is amplified in breast cancer.

METHODS

Western analysis was used to evaluate GRK4 protein expression in seven breast cancer cell lines. GRK4 gene single nucleotide polymorphism in breast cancer cell lines and in breast cancer cDNA arrays were determined using TaqMan Genotyping qPRC. The function of GRK4 was evaluated in MCF-7 cells with cMyc knock-down and GRK4 re-expression and in MDA-MB-468 cells expressing inducible GRK4 shRNA lentivirus constructs. Nuclei counting and 5-Bromo-2'-deoxy-uridine (BrdU) labeling were used to evaluate cell growth and proliferation.

RESULTS

Genotyping of GRK4 SNPs in breast cancer cDNA arrays (n = 94) revealed that the frequency of carrying two hypertension related SNPs A142 V or R65 L is threefold higher in breast cancer patients than in healthy people (P = 7.53E-11). GRK4 protein is expressed in seven breast cancer cell lines but not the benign mammary epithelial cell line, MCF-10A. Three hypertension related SNPs in the GRK4 gene were identified in the breast cancer cell lines. Except for BT20, all other breast cancer lines have 1-3 GRK4 SNPs of which A142 V occurs in all 6 lines. MDA-MB-468 cells contain homozygous A142 V and R65 L SNPs. Knocking down cMyc in MCF-7 cells significantly reduced the growth rate, which was rescued by re-expression of GRK4. We then generated three stable GRK4 knock-down MDA-MB-468 lines using inducible lentiviral shRNA vectors. Doxycycline (Dox) induced GRK4 silencing significantly reduced GRK4 mRNA and protein levels, growth rates, and proliferation. As a marker of cell proliferation, the percentage of BrdU-labeled cells decreased from 45 ± 3% in the cells without Dox to 32 ± 5% in the cells treated with 0.1 µg/mL Dox.

CONCLUSIONS

GRK4 acts as an independent proliferation promotor in breast cancer. Our results suggest that targeted inhibition of GRK4 could be a new therapy for both hypertension and breast cancer.

In Utero Exposure to Fine Particulate Matter Causes Hypertension Due to Impaired Renal Dopamine D1 Receptor in Offspring

BACKGROUND/AIMS

Adverse environment in utero can modulate adult phenotypes including blood pressure. Fine particulate matter (PM2.5) exposure in utero causes hypertension in the offspring, but the exact mechanisms are not clear. Renal dopamine D1 receptor (D1R), regulated by G protein-coupled receptor kinase type 4 (GRK4), plays an important role in the regulation of renal sodium transport and blood pressure. In this present study, we determined if renal D1R dysfunction is involved in PM2.5-induced hypertension in the offspring.

METHODS

Pregnant Sprague-Dawley rats were given an oropharyngeal drip of PM2.5 (1.0 mg/kg) at gestation day 8, 10, and 12. The blood pressure, 24-hour sodium excretion, and urine volume were measured in the offspring. The expression levels of GRK4 and D1R were determined by immunoblotting. The phosphorylation of D1R was investigated using immunoprecipitation. Plasma malondialdehyde and superoxide dismutase levels were also measured in the offspring.

RESULTS

As compared with saline-treated dams, offspring of PM2.5-treated dams had increased blood pressure, impaired sodium excretion, and reduced D1R-mediated natriuresis and diuresis, accompanied by decreased renal D1R expression and GRK4 expression. The impaired renal D1R function and increased GRK4 expression could be caused by increased reactive oxidative stress (ROS) induced by PM2.5 exposure. Administration of tempol, a redox-cycling nitroxide, for 4 weeks in the offspring of PM2.5-treated dam normalized the decreased renal D1R expression and increased renal D1R phosphorylation and GRK4 expression. Furthermore, tempol normalized the increased renal expression of c-Myc, a transcription factor that regulates GRK4 expression.

CONCLUSIONS

In utero exposure to PM2.5 increases ROS and GRK4 expression, impairs D1R-mediated sodium excretion, and increases blood pressure in the offspring. These studies suggest that normalization of D1R function may be a target for the prevention and treatment of the hypertension in offspring of mothers exposed to PM2.5 during pregnancy.

Long-Term Exposure of Fine Particulate Matter Causes Hypertension by Impaired Renal D1 Receptor-Mediated Sodium Excretion via Upregulation of G-Protein-Coupled Receptor Kinase Type 4 Expression in Sprague-Dawley Rats

BACKGROUND

Epidemiological evidence supports an important association between air pollution exposure and hypertension. However, the mechanisms are not clear.

METHODS AND RESULTS

Our present study found that long-term exposure to fine particulate matter (PM2.5) causes hypertension and impairs renal sodium excretion, which might be ascribed to lower D1 receptor expression and higher D1 receptor phosphorylation, accompanied with a higher G-protein-coupled receptor kinase type 4 (GRK4) expression. The in vivo results were confirmed in in vitro studies (ie, PM2.5 increased basal and decreased D1 receptor mediated inhibitory effect on Na+-K+ ATPase activity, decreased D1 receptor expression, and increased D1 receptor phosphorylation in renal proximal tubule cells). The downregulation of D1 receptor expression and function might be attributable to a higher GRK4 expression after the exposure of renal proximal tubule cells to PM2.5, because downregulation of GRK4 by small-interfering RNA reversed the D1 receptor expression and function. Because of the role of reactive oxygen species on D1 receptor dysfunction and its relationship with air pollution exposure, we determined plasma reactive oxygen species and found the levels higher in PM2.5-treated Sprague-Dawley rats. Inhibition of reactive oxygen species by tempol (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) reduced blood pressure and increased sodium excretion in PM2.5-treated Sprague-Dawley rats, accompanied by an increase in the low D1 receptor expression, and decreased the hyperphosphorylated D1 receptor and GRK4 expression.

CONCLUSIONS

Our present study indicated that long-term exposure of PM2.5 increases blood pressure by decreasing D1 receptor expression and function; reactive oxygen species, via regulation of GRK4 expression, plays an important role in the pathogenesis of PM2.5-induced hypertension.

α1 -Adrenoceptor activation of PKC-ε causes heterologous desensitization of thromboxane receptors in the aorta of spontaneously hypertensive rats

BACKGROUND AND PURPOSE

In the aorta of adult spontaneously hypertensive (SHR), but not in that of normotensive Wistar-Kyoto (WKY), rats, previous exposure to phenylephrine inhibits subsequent contractions to PGE2 . The present experiments were designed to examine the mechanism(s) underlying this inhibition.

EXPERIMENTAL APPROACH

Isometric tension was measured in isolated rings of SHR and WKY aortae. Gene expression and protein presence were measured by quantitative real-time PCR and Western blotting respectively.

KEY RESULTS

In aorta of 18 weeks SHR, but not age-matched WKY, pre-exposure to phenylephrine inhibited subsequent contractions to PGE2 that were mediated by thromboxane prostanoid (TP) receptors. This inhibition was not observed in preparations of pre-hypertensive 5-week-old SHR, and was significantly larger in those of 36- than 18-week-old SHR. Pre-exposure to the PKC activator, phorbol 12,13-dibutyrate, also inhibited subsequent contractions to PGE2 in SHR aortae. The selective inhibitor of PKC-ε, ε-V1-2, abolished the desensitization caused by pre-exposure to phenylephrine. Two molecular PKC bands were detected and their relative intensities differed in 36-week-old WKY and SHR vascular smooth muscle. The mRNA expressions of PKC-α, PKC-ε, PK-N2 and PKC-ζ and of G protein-coupled kinase (GRK)-2, GRK4 and β-arrestin2 were higher in SHR than WKY aortae.

CONCLUSIONS AND IMPLICATIONS

These experiments suggest that in the SHR but not the WKY aorta, α1 -adrenoceptor activation desensitizes TP receptors through activation of PKC-ε. This heterologous desensitization is a consequence of the chronic exposure to high arterial pressure.