Discovery of RGS2-FBXO44 interaction inhibitors using a cell-based NanoBit assay

Regulators of G protein signaling (RGS) proteins negatively regulate signaling through G protein-coupled receptors, and reduced RGS protein function is involved in numerous pathologies. However, therapeutic intervention is challenging, as RGS proteins lack druggable binding pockets and enzymatic activity. Instead, targeting mechanisms that control RGS protein expression show promise as an alternative. Pharmacological stabilization of RGS2 would be a feasible therapeutic strategy in pathologies associated with reduced RGS2 protein levels, such as hypertension, heart failure, and asthma. RGS2 is rapidly degraded through the ubiquitin-proteasomal system, and we recently identified the E3 ligase that recognizes RGS2. F-box Only Protein 44 (FBXO44) acts as the substrate recognition site for RGS2 in this E3 ligase complex, and we hypothesize that inhibiting the RGS2-FBXO44 interaction will lead to enhanced RGS2 levels. Here, we developed a NanoLuc Binary Technology (NanoBiT) assay that detects the interaction between RGS2 and FBXO44. This assay was used to screen 1600 compounds from the Life Chemicals protein-protein interaction fragment library. We identified a promising hit, denoted compound 10, that inhibits the RGS2-FBXO44 interaction with a potency of 19.6 μM, through direct binding to RGS2. The resulting increase in RGS2 protein levels is dependent on FBXO44, as siRNA-mediated FBXO44 knockdown attenuates the effect of compound 10. Altogether, compound 10 represents the first example of a small-molecule inhibitor of the RGS2-FBXO44 interaction and a first step toward the development of molecular probes with a defined mechanism to stabilize RGS2 protein levels. SIGNIFICANCE STATEMENT: This study provides a strategy to identify molecules that selectively inhibit RGS2 protein degradation as well as the first example of a compound with the ability to inhibit RGS2 interaction with the E3 ligase component FBXO44. This study provides proof of concept that a small-molecule RGS2-FBXO44 interaction inhibitor will increase RGS2 protein levels. Future development of compounds with this mechanism of action would be clinically useful in pathologies associated with low RGS2 protein levels, including hypertension, heart failure, and asthma.