Pervanadate induces Mammalian Ste20 Kinase 3 (MST3) tyrosine phosphorylation but not activation

Molecular mechanisms involved in regulating protein activity and biological function of MST3

Mammalian sterile 20-like (Ste20-like) protein kinase 3 (MST3) or serine/threonine-protein kinase 24 (STK24) is a serine/threonine protein kinase that belongs to the mammalian STE20-like protein kinase family. MST3 is a pleiotropic protein that plays a critical role in regulating a variety of events, including apoptosis, immune response, metabolism, hypertension, tumor progression, and development of the central nervous system. The MST3-mediated regulation is intricately related to protein activity, post-translational modification, and subcellular location. Here, we review the recent progress on the regulatory mechanisms against MST3 and its-mediated control of disease progression.

Epithelial polarization in the 3D matrix requires MST3 signaling to regulate ZO-1 position

Apical-basal cell polarity must be tightly controlled for epithelial cyst and tubule formation, and these are important functional units in various epithelial organs. Polarization is achieved through the coordination of several molecules that divide cells into an apical domain and a basolateral domain, which are separated from tight and adherens junctions. Cdc42 regulates cytoskeletal organization and the tight junction protein ZO-1 at the apical margin of epithelial cell junctions. MST kinases control organ size through the regulation of cell proliferation and cell polarity. For example, MST1 relays the Rap1 signal to induce cell polarity and adhesion of lymphocytes. Our previous study showed that MST3 was involved in E-cadherin regulation and migration in MCF7 cells. In vivo, MST3 knockout mice exhibited higher ENaC expression at the apical site of renal tubules, resulting in hypertension. However, it was not clear whether MST3 was involved in cell polarity. Here, control MDCK cells, HA-MST3 and HA-MST3 kinase-dead (HA-MST3-KD) overexpressing MDCK cells were cultured in collagen or Matrigel. We found that the cysts of HA-MST3 cells were fewer and smaller than those of control MDCK cells; ZO-1 was delayed to the apical site of cysts and in cell-cell contact in the Ca2+ switch assay. However, HA-MST3-KD cells exhibited multilumen cysts. Intensive F-actin stress fibers were observed in HA-MST3 cells with higher Cdc42 activity; in contrast, HA-MST3-KD cells had lower Cdc42 activity and weaker F-actin staining. In this study, we identified a new MST3 function in the establishment of cell polarity through Cdc42 regulation.

MST3 Involvement in Na+ and K+ Homeostasis with Increasing Dietary Potassium Intake

K⁺ loading inhibits NKCC2 (Na-K-Cl cotransporter) and NCC (Na-Cl cotransporter) in the early distal tubules, resulting in Na⁺ delivery to the late distal convoluted tubules (DCTs). In the DCTs, Na⁺ entry through ENaC (epithelial Na channel) drives K⁺ secretion through ROMK (renal outer medullary potassium channel). WNK4 (with-no-lysine 4) regulates the NCC/NKCC2 through SAPK (Ste20-related proline-alanine-rich kinase)/OSR1 (oxidative stress responsive). K⁺ loading increases intracellular Cl⁻, which binds to the WNK4, thereby inhibiting autophosphorylation and downstream signals. Acute K⁺ loading-deactivated NCC was not observed in Cl⁻-insensitive WNK4 mice, indicating that WNK4 was involved in K⁺ loading-inhibited NCC activity. However, chronic K⁺ loading deactivated NCC in Cl⁻-insensitive WNK4 mice, indicating that other mechanisms may be involved. We previously reported that mammalian Ste20-like protein kinase 3 (MST3/STK24) was expressed mainly in the medullary TAL (thick ascending tubule) and at lower levels in the DCTs. MST3^−/− mice exhibited higher ENaC activity, causing hypernatremia and hypertension. To investigate MST3 function in maintaining Na⁺/K⁺ homeostasis in kidneys, mice were fed diets containing various concentrations of Na⁺ and K⁺. The 2% KCl diets induced less MST3 expression in MST3^−/− mice than that in wild-type (WT) mice. The MST3^−/− mice had higher WNK4, NKCC2-S130 phosphorylation, and ENaC expression, resulting in lower urinary Na⁺ and K⁺ excretion than those of WT mice. Lower urinary Na⁺ excretion was associated with elevated plasma [Na⁺] and hypertension. These results suggest that MST3 maintains Na⁺/K⁺ homeostasis in response to K⁺ loading by regulation of WNK4 expression and NKCC2 and ENaC activity.

Glycoprotein G-protein Coupled Receptors in Disease: Luteinizing Hormone Receptors and Follicle Stimulating Hormone Receptors

Signal transduction by luteinizing hormone receptors (LHRs) and follicle-stimulating hormone receptors (FSHRs) is essential for the successful reproduction of human beings. Both receptors and the thyroid-stimulating hormone receptor are members of a subset of G-protein coupled receptors (GPCRs) described as the glycoprotein hormone receptors. Their ligands, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and a structurally related hormone produced in pregnancy, human chorionic gonadotropin (hCG), are large protein hormones that are extensively glycosylated. Although the primary physiologic functions of these receptors are in ovarian function and maintenance of pregnancy in human females and spermatogenesis in males, there are reports of LHRs or FSHRs involvement in disease processes both in the reproductive system and elsewhere. In this review, we evaluate the aggregation state of the structure of actively signaling LHRs or FSHRs, their functions in reproduction as well as summarizing disease processes related to receptor mutations affecting receptor function or expression in reproductive and non-reproductive tissues. We will also present novel strategies for either increasing or reducing the activity of LHRs signaling. Such approaches to modify signaling by glycoprotein receptors may prove advantageous in treating diseases relating to LHRs or FSHRs function in addition to furthering the identification of new strategies for modulating GPCR signaling.

Measuring the Kinase Activity of GCKIII Proteins In Vitro

One of the CCM genes, CCM3/PDCD10, binds to the protein kinase family GCKIII, which comprises MST3/STK24, SOK1/STK25, and MST4/STK26. These proteins have been shown to have the same effect as CCM3, both in endothelial cells and in animal models such as zebrafish and are most likely involved in CCM pathogenesis. We describe here an in vitro kinase assay of GCKIII proteins which can be used to study their regulation in endothelial and other cells under different circumstances.