Background: The G-protein coupled receptor kinase 5 (GRK5) plays a crucial role in regulating G protein-coupled receptors (GPCRs), which are a diverse group of cell surface receptors. GRK5 phosphorylates GPCRs, resulting in their desensitization and internalization. In addition, GRK5 also exerts effects that are independent of GPCRs; GRK5 itself can enter the nucleus either bind to DNA or to other transcriptional regulators to modulate the expression of target genes. GRK5 has previously been linked to the pathophysiology of inflammation, immunological responses, migration and invasion of cancer cell, and post-ischemic remodeling of the heart. The role of GRK5 in rheumatic conditions or in fibrotic tissue remodeling remains unknown to date.

Objectives: In this study, we aimed to examine the role of GRK5 in the pathophysiology of fibrotic tissue remodeling.

Methods: The expression of GRK5 was assessed in patients with systemic sclerosis (SSc) and in experimental models of SSc using real-time PCR, Western Blot and immunofluorescence staining. GRK5 expression was modulated in vitro and in vivo using knockdown and knockout methods, as well as adenoviral overexpression. The effect of GRK5 on experimental fibrosis was examined in three animal models of systemic sclerosis (SSc): dermal fibrosis induced by bleomycin, pulmonary fibrosis induced by bleomycin, and fibrosis caused by overexpression of a constitutively active TGF receptor type I (TBRI ^act ). The target genes of GRK5 in fibroblasts were identified by RNA-Seq analysis.

Results: Elevated mRNA, protein levels and enhanced nuclear accumulation of GRK5 protein were detected in fibroblasts within fibrotic skin and lung samples compared to non-fibrotic control samples. The GRK5 expression was also upregulated in murine models of SSc. TGFβ, but not other profibrotic mediators such as EGF, FGF9, or IL-1, upregulated the expression of GRK5. Knockdown of GRK5 in fibroblasts decreased their susceptibility to the pro-fibrotic effects of TGFβ, with impaired fibroblast-to-myofibroblast transition and collagen release. In addition, the targeted knockout of GRK5 in fibroblasts ameliorated fibrosis in experimental models of dermal and pulmonary fibrosis induced by bleomycin or by overexpression of TBRI ^act . However, forced overexpression of GRK5 promoted fibroblast-to-myofibroblast differentiation and collagen release. Adenoviral overexpression of GRK5 also aggravated bleomycin-induced dermal fibrosis. RNA-Seq analysis revealed that GRK5 regulates the expression of profibrotic genes such as COL1A1 , ACTA2 , CTGF and PAI-1 .

Conclusion: We demonstrate that GRK5 is sufficient and required for the activation of fibroblasts in response to TGFβ. Inactivation of GRK5 demonstrates profound antifibrotic effects in several preclinical models of systemic sclerosis (SSc). Inhibition of GRK5 might thus be a novel target for antifibrotic therapies.

REFERENCES: NIL.

Acknowledgements: NIL.

Disclosure of Interests: None declared.