Regulator of G Protein Signaling 5 (RGS5)
Human RGS5 was first cloned in 1998 and reported to be highly expressed in the heart, lung, skeletal muscle, and small intestine and to a lesser extent in the brain, placenta, liver, and colon (Seki et al. 1998). Human, mouse, and rat RGS5 cDNAs are 90% homologous (Fig. 2b) and show similar tissue distribution. Schwartz and colleagues first established a prominent vascular role of RGS5 by demonstrating high expression in the human aorta (Adams et al. 2000). Subsequently, RGS5 was shown to be a marker for developing mural cells of the vasculature, namely, vascular smooth muscle cells (vSMC) and pericytes (Bondjers et al. 2003; Cho et al. 2003) as well as tumor pericytes (Berger et al. 2005). Although not exclusively expressed in the vasculature, RGS5 is emerging as a major player in vessel maturation, cardiovascular physiology, and pathology. Moreover, RGS5 controls contraction, migration, and fibrogenesis of hepatic stellate cells and as such may play a functional role in liver fibrosis (Bahrami et al. 2014). RGS5 is also associated with fat metabolism, obesity and hepatic steatosis, and inflammation in the adipose tissue, liver, and skeletal muscle (Deng et al. 2012). RGS5 has been identified as an inhibitor of hedgehog signaling (Hh) downstream of its GPCR Smoothened (Smo) (Mahoney et al. 2013). Since Hh signaling is involved in crucial cellular processes such as proliferation, vascular development and remodeling, tissue fibrosis, and cancer, RGS5 may be linked to Hh-related pathologies.
RGS5 Expression During Development
RGS5 expression is first detectable in E12.5 murine embryos in the aortic sac and blood vessels. By E14.5, RGS5 is highly expressed in most organs with the exception of the liver and lung. From E14.5 onward, strong in situ hybridization signals are detected in the aorta; central nervous system; kidney, including renal arteries and mesangial cells of glomeruli; blood vessels of the developing heart, and lung mesenchymal cells (Cho et al. 2003). Interestingly, RGS5 mRNA expression patterns in the mouse embryo resemble pericyte-specific platelet-derived growth factor receptor (PDGFR)β expression in developing arteries. PDGFRβ gene-deficient mice lack RGS5 expression in brain capillaries and small arteries, but show persistent expression of RGS5 in mural cells of the developing aorta and renal arteries (Bondjers et al. 2003; Cho et al. 2003). RGS5 expression steadily declines in adult mice when regional differences become evident. For instance, RGS5 expression in the descending aorta is 15-fold higher than in carotid arteries. Vascular bed-specific differences include increased methylation of CpG dinucleotides in the RGS5 promoter in carotid arteries consistent with epigenetic silencing in adult mice (Zhang et al. 2012). Even though RGS5 is prominently expressed during embryonic development, RGS5 gene-deficient mice develop normally to produce viable and fertile offspring (Ganss 2015).
Role of RGS5 in Cardiovascular Function and Pathology
In humans, RGS5 gene polymorphisms have been associated with hypertension. RGS5 has been shown to regulate angiotensin II signaling via the angiotensin receptor 1 (ATR1)/Gαq signaling axsis in vivo (Holobotovskyy et al. 2013) (Fig. 1). Indeed, RGS5 gene-deficient mice are hypertensive and display increased vascular sensitivity to angiotensin II signaling and fibrosis. This indicates a prominent role of RGS5 in modulating vascular reactivity to vasoconstrictors and vessel wall remodeling (Holobotovskyy et al. 2013). In pregnant mice, maternal RGS5 heterozygosity predisposes to gestational hypertension due to increased sensitivity to angiotensin II and insufficient vascular adaptation in response to a rise in plasma volume. In humans, reduced RGS5 expression in myometrial vessels of pregnant women correlates with preeclamptic pregnancies, thus implicating vascular RGS5 expression levels with hypertensive pregnancy complications (Holobotovskyy et al. 2015). Besides angiotensin II, RGS5 has also been shown to attenuate endothelin-1, sphingosine-1-phosphate, and platelet-derived growth factor (PDGF)-induced signaling in vascular cells (Cho et al. 2003).
Moreover, RGS5 expression is downregulated following mitogenic stimulation of vSMC in vitro and neointima formation in vivo. In contrast, overexpression of RGS5 reduces vascular proliferation and thus injury-induced neointima growth by blocking downstream mitogen-activated protein (MAP) kinase signaling (Daniel et al. 2016). Reduced RGS5 levels have also been implicated in atherosclerosis, the narrowing of arteries due to plaque buildup. These findings have been confirmed in human atherosclerotic lesions and apolipoprotein E (ApoE) knockout mice on a RGS5 gene-deficient background which show accelerated development of plaques due to increased MEK/ERK and NF-кB signaling activities (Cheng et al. 2015). RGS5 is also important for arteriogenesis, a vascular remodeling process where collateral arterial growth is stimulated to bypass obstruction of large conduit arteries (Arnold et al. 2014). In RGS5 knockout mice, quiescent vSMC which exhibit reduced Rho kinase activity fail to induce collateral arterial growth following injury.
Besides its vascular activity, RGS5 is also expressed in cell types of the adult heart where it controls diverse signaling pathways. Consistent with its role in vascular fibrosis, RGS5 protects the heart from hypertrophy and fibrosis during pressure overload in a process involving MEK/ERK and transforming growth factor (TGF)β signaling (Li et al. 2010). Similarly, in RGS5 overexpressing mouse hearts, cardiomyocytes are protected from apoptosis during ischemic injury through inhibition of JNK1/2 and p38 signaling (Wang et al. 2016). In addition, RGS5 regulates parasympathetic activation in the heart, heart rate, and rhythm; this is effected by negatively regulating atrial muscarinic receptor-activated potassium channels (IKAch) (Qin et al. 2016).
Role of RGS5 in Asthma
Beta-adrenergic signaling and β2 adrenergic receptor agonists are important for the treatment of asthma since they promote airway smooth muscle cell (aSMC) relaxation and air flow. In some patients, however, prolonged β2 adrenergic agonist application can lead to airway hypersensitivity, bronchoconstriction, and increased asthmatic symptoms. This is accompanied by downregulation of RGS5 in aSMCs, calcium (Ca++) mobilization, and enhanced aSMC contractility. Similar to vascular hypercontractility in response to angiotensin II, RGS5 gene-deficient mice exhibit increased bronchial sensitivity following muscarinic receptor stimulation (Yang et al. 2011). Thus, RGS5 genetic polymorphism emerges as potential marker predicting the response to β2 adrenergic bronchodilators in asthma patients (Labuda et al. 2013).
Role of RGS5 in Cancer
RGS5 is expressed in pericytes, the mural cells of the microvasculature, which play a crucial role in vessel maturation and stabilization. Neovessels which are induced in growing tumors in the process of angiogenesis are characterized by highly proliferative endothelial cells and a reduced number or loosely attached pericytes. Overall, tumor angiogenesis creates blood vessels which are structurally and functionally heterogeneous, chaotic, and leaky. RGS5 is abundantly expressed in these neovessels (Berger et al. 2005). Moreover, high expression of RGS5 is causally involved in vascular abnormalities since loss of RGS5 in RGS5-deficient tumor-bearing mice induces pericyte maturation and restores vessel integrity in a process described as vessel normalization; since vessel normalization increases efficacy of anticancer therapy, regulating RGS5 levels in cancer has important therapeutic implications (Hamzah et al. 2008). RGS5-controlled signaling networks which are involved in tumor vessel “abnormalization“ and “normalization“ processes are so far unkown. In human cancers, RGS5 has been identified as a potential driver of metastatic tumor spreading when overexpressed in cancer cells (Hu et al. 2013) and also as a marker of angiogenic tumor vasculature (endothelial cells and pericytes), for instance, in ovarian cancer, melanoma, and renal carcinoma (Silini et al. 2012). RGS5 as part of a pericyte maturation gene signature has predictive value for overall survial and responsiveness to anti-angiogenic therapy, for instance, in metastatic colorectal cancer (Volz et al. 2015).
Regulation of RGS5 Expression and Protein Turnover
RGS5 gene transcription and protein degardation are dynamically regulated consistent with its role in fine-tuning GPCR signaling in response to adaptive biological processes. For instance, the murine RGS5 gene harbors an active peroxisome proliferator-activated receptor (PPAR) binding site, and PPAR agonists enhance gene transcription. In contrast, PPAR antagonists suppress RGS5 levels which in turn increases vascular sensitivity to anigotensin II and vascular tone (Holobotovskyy et al. 2015). Moreover, RGS5 is one of few known targets of the ubiquitin-proteasome-dependent N-end rule degradation pathway which requires the presence of a cysteine residue at position 2 (C2) of the N-terminus (Fig. 2a) (Lee et al. 2005). Nitric oxide-mediated oxidation of C2 is required for its arginylation by arginyltransferase; arginylation then leads to degradation by E3 ubiquitin ligases. Due to the requirement of oxygen (O2) for RGS5 degradation, low O2 levels during ischemia or hypoxia could result in RGS5 protein accumulation. Indeed, RGS5 is increased under hypoxic conditions and during tumor angiogenesis and protects cells from apoptosis during ischemic injury (Berger et al. 2005; Wang et al. 2016).
Given the prevalence of GPCR signaling and their importance as pharmacological targets, interest in RGS molecules as modulators of GPCR signaling is steadily growing. RGS5 has been implicated in major signaling pathways which control cell proliferation, differentiation, inflammation, and fibrogenesis with a very strong prominence of regulating MAP kinase signaling across different cell types and tissues. RGS5 shares many features with other B/R4 family members; how receptor specificity is regulated under physiological conditions is still largely unknown, and functional analyses are hampered by the lack of specific pharmacological inhibitors for RGS5. Nevertheless, RGS5 displays unique features, for instance, its prominent role in cardiovascular physiology/pathology including angiogenesis and arteriogenesis; its ability to specifically regulate contractility and hypertrophy in vSMCs, aSMCs, and pericytes; its rapid proteasomal degradation in response to oxygen levels; and its role as a vascular target for PPAR family transcription regulators. These characteristics imply a crucial role for RGS5 in cardiovascular disease and make it a prime candidate for therapeutic intervention.
- Bondjers C, Kalen M, Hellstrom M, Scheidl SJ, Abramsson A, Renner O, et al. Transcription profiling of platelet-derived growth factor-B-deficient mouse embryos identifies RGS5 as a novel marker for pericytes and vascular smooth muscle cells. Am J Pathol. 2003;162:721–9.PubMedPubMedCentralCrossRefGoogle Scholar
- Volz NB, Stintzing S, Zhang W, Yang D, Ning Y, Wakatsuki T, et al. Genes involved in pericyte-driven tumor maturation predict treatment benefit of first-line FOLFIRI plus bevacizumab in patients with metastatic colorectal cancer. Pharm J. 2015;15:69–76.Google Scholar
- Wang Z, Huang H, He W, Kong B, Hu H, Fan Y, et al. Regulator of G-protein signaling 5 protects cardiomyocytes against apoptosis during in vitro cardiac ischemia-reperfusion in mice by inhibiting both JNK1/2 and P38 signaling pathways. Biochem Biophys Res Commun. 2016;473:551–7.PubMedCrossRefGoogle Scholar