Regulating the regulators: Epigenetic, transcriptional, and post-translational regulation of RGS proteins

Abstract

Regulators of G protein signaling (RGS) are a family of proteins classically known to accelerate the intrinsic GTPase activity of G proteins, which results in accelerated inactivation of heterotrimeric G proteins and inhibition of G protein coupled receptor signaling. RGS proteins play major roles in essential cellular processes, and dysregulation of RGS protein expression is implicated in multiple diseases, including cancer, cardiovascular and neurodegenerative diseases. The expression of RGS proteins is highly dynamic and is regulated by epigenetic, transcriptional and post-translational mechanisms. This review summarizes studies that report dysregulation of RGS protein expression in disease states, and presents examples of drugs that regulate RGS protein expression. Additionally, this review discusses, in detail, the transcriptional and post-transcriptional mechanisms regulating RGS protein expression, and further assesses the therapeutic potential of targeting these mechanisms. Understanding the molecular mechanisms controlling the expression of RGS proteins is essential for the development of therapeutics that indirectly modulate G protein signaling by regulating expression of RGS proteins.

Keywords: Epigenetics; MicroRNA; Post-translational modification; Proteasomal degradation; RGS; Regulator of G protein signaling; Therapeutics; Transcription factors.

Figure 1. RGS gene expression and protein stability are regulated by multiple mechanisms

Multiple regulatory mechanisms participate in determining the level of RGS proteins. Epigenetic modifications, mainly histone deacetylation and DNA methylation that are mediated by histone deacetylases (HDACs) or DNA methyltransferases (DNMTs), tighten the chromatin structure at RGS genes promoters, thereby obstructing the access of transcription factors and other proteins that are essential for transcription initiation, ultimately resulting in suppression of RGS gene expression. Multiple transcription factors directly bind RGS genes promoters to activate or repress transcription, adding another layer of regulation to the expression of RGS genes. In addition to transcription regulation, RGS mRNAs are targeted by both microRNAs and mRNA-binding proteins to either degrade or stabilize the respective mRNA, which critically determines the final levels of translated RGS proteins. Finally, active RGS proteins participate in various G protein dependant and independent signalling pathways in different cellular compartments. The activity and stability of RGS proteins is influenced by post-translational modifications such as phosphorylation, association with specific binding partners, and cellular localization of RGS proteins. Many RGS proteins undergo proteasomal degradation while some are degraded via lysosomal degradation. Regardless, this degradation is a critical step of regulation that ultimately governs the level of active cellular RGS proteins at a given time.

Figure 2. RGS2 protein levels are determined by multiple regulatory mechanisms, several of which can be manipulated by small molecules

RGS2 transcription is suppressed by the DNA methyltransferase 1 (DNMT1) enzyme and activated by the transcription factor CREB. Accordingly, the DNMT1 inhibitor 5-AZA enhances RGS2 transcription, whereas inhibiting CREB-mediated transcription using the small molecule KG-501 results in suppressed transcription. In addition to transcriptional regulation, RGS2 is also regulated at the mRNA levels, by miR-22 and has-miR-4717-5p, and at the protein level, by proteasome-mediated degradation with the assistance of other proteins such as FOXO44, CUL4B, and DDB1. Because many of these regulatory mechanisms are usually not selective and can influence other proteins, manipulating RGS2 expression by combining drugs that targets multiple mechanisms of regulation is potentially more advantageous.