No abstract

The application of the aryloxy triester phosphoramidate prodrug technology to ISS-610-Met improves its ability to inhibit STAT3 in cells.

SPAK and OSR1 are two protein kinases that play critical roles in regulating ion homeostasis. They are activated under osmotic stress through phosphorylation by their upstream WNK kinases at a conserved threonine site on their T‐loops. Additionally, WNK kinases phosphorylate SPAK and OSR1 at a highly conserved serine residue on their S‐motif, the function of which remains elusive. Using affinity pull down and mass spectrometry, we identified the E3 ubiquitin ligase complex Cullin 4‐DDB1‐WDR3/WDR6 as a binder to OSR1 kinase in a SPAK/OSR1 S‐motif phosphorylation‐dependent manner. This binding was found to be compromised by S‐motif phosphorylation following osmotic stress. Using proteasomal and neddylation inhibitors, we subsequently showed that OSR1 ubiquitylation was abolished under osmotic stress when its S‐motif is phosphorylated. These results provide the first example of an E3 ubiquitin ligase system that binds the OSR1 kinase and, thus, links the CRL4 complex to ion homeostasis.

Since the discovery of WNK mutations that cause an inherited form of hypertension in humans, there has been increasing interest in targeting WNK signaling as a novel strategy for modulating blood pressure. This notion is now supported by numerous mouse models with impaired WNK signaling that exhibit reduced blood pressure. Biochemical analyses of the various protein components that make up this signaling pathway have identified a number of plausible molecular targets that are amenable to targeting by small molecules. To date, a selection of small-molecule WNK signaling inhibitors have been identified and have shown promise in suppressing the activity of WNK signaling in cells and in animals. In this Minireview, we briefly discuss the WNK signaling pathway and provide an overview of the various druggable targets within this cascade, as well as the different WNK signaling inhibitors discovered to date.

SPAK and OSR1 are two protein kinases that play important roles in regulating the function of numerous ion co-transporters. They are activated by two distinct mechanisms that involve initial phosphorylation at their T-loops by WNK kinases and subsequent binding to a scaffolding protein termed MO25. To understand this latter SPAK and OSR1 regulation mechanism, we herein show that MO25 binding to these two kinases is enhanced by serine phosphorylation in their highly conserved WEWS motif, which is located in their C-terminal domains. Furthermore, we show that this C-terminal phosphorylation is carried out by WNK kinases in vitro and involves WNK kinases in cells. Mutagenesis studies revealed key MO25 residues that are important for MO25 binding and activation of SPAK and OSR1 kinases. Collectively, this study provides new insights into the MO25-mediated activation of SPAK and OSR1 kinases, which are emerging as important players in regulating ion homeostasis.