Search

Summary: Cell volume regulation is a vital system for cellular activities. When perturbed by hypoosmotic or hyperosmotic stress, cells immediately induce the cell volume recovery system, regulatory volume decrease (RVD) or regulatory volume increase (RVI), respectively. In contrast to the knowledge about effector molecules, the molecular mechanisms linking osmosensing to RVD/RVI induction remain unknown. Additionally, few reciprocal responders in the bidirectional osmotic stress response have been identified. We previously reported that ASK3 bidirectionally switches its kinase activity under osmotic stress. Herein we demonstrate that ASK3 controls both RVD and RVI under osmotic stress. Using a high-content genome-wide small interfering RNA (siRNA) screen, we identify PP6 as a direct ASK3 inactivator. Furthermore, PP6 rapidly interacts with ASK3 in an osmolality-dependent manner, and it inactivates ASK3 to induce RVI and, thereby, cell survival under hyperosmotic stress. These findings suggest that the PP6-ASK3 interaction is a core module in the bidirectional osmotic stress response. : Osmotic stress comprises two types of information: intensity and directionality. Nevertheless, few bidirectional cellular systems have been identified. Using a high-content genome-wide siRNA screen, Watanabe et al. unveil that a PP6-ASK3 module bidirectionally interprets and converts osmotic stress signals to control cell volume homeostasis. Keywords: apoptosis signal-regulating kinase 3, ASK3, protein phosphatase 6, PP6, high-content genome-wide siRNA screen, osmotic stress, cell volume regulation

The With No lysine [K] (WNK)-Ste20-related proline/alanine-rich kinase (SPAK)/oxidative stressresponsive kinase 1 (OSR1) pathway has been reported to be a crucial signaling pathway for triggering pseudohypoaldosteronism type II (PHAII), an autosomal dominant hereditary disease that is characterized by hypertension. However, the molecular mechanism(s) by which the WNK-SPAK/ OSR1 pathway is regulated remain unclear. In this report, we identified WNK4 as an interacting partner of a recently identified MAP3K, apoptosis signal-regulating kinase 3 (ASK3). We found that WNK4 is phosphorylated in an ASK3 kinase activity-dependent manner. By exploring the ASK3-dependent phosphorylation sites, we identified Ser575 as a novel phosphorylation site in WNK4 by LC-MS/MS analysis. ASK3-dependent WNK4 Ser575 phosphorylation was mediated by the p38MAPK-MAPK-activated protein kinase (MK) pathway. Osmotic stress, as well as hypotonic low-chloride stimulation, increased WNK4 Ser575 phosphorylation via the p38MAPK-MK pathway. ASK3 was required for the p38MAPK activation induced by hypotonic stimulation but was not required for that induced by hypertonic stimulation or hypotonic low-chloride stimulation. Our results suggest that the p38MAPK-MK pathway might regulate WNK4 in an osmotic stress-dependent manner but its upstream regulators might be divergent depending on the types of osmotic stimuli., Scientific Reports, 6, 18710; 2016

Apoptosis and inflammation generally exert opposite effects on tumorigenesis: apoptosis serves as a barrier to tumour initiation, whereas inflammation promotes tumorigenesis. Although both events are induced by various common stressors, relatively little is known about the stress-induced signalling pathways regulating these events in tumorigenesis. Here, we show that stress-activated MAP3Ks, ASK1 and ASK2, which are involved in cellular responses to various stressors such as reactive oxygen species, differentially regulate the initiation and promotion of tumorigenesis. ASK2 in cooperation with ASK1 functioned as a tumour suppressor by exerting proapoptotic activity in epithelial cells, which was consistent with the reduction in ASK2 expression in human cancer cells and tissues. In contrast, ASK1-dependent cytokine production in inflammatory cells promoted tumorigenesis. Our findings suggest that ASK1 and ASK2 are critically involved in tumorigenesis by differentially regulating apoptosis and inflammation.

Methylglyoxal (MG) is a reactive endogenous metabolite that is produced from the process of degradation of triose-phosphates. Under hyperglycemic conditions the rate of MG formation increases as a result of elevated concentrations of precursors. It has been established that MG elicits oxidative stress signaling, leading to the activation of MAP kinases, p38 MAPK and JNK, yet it remains largely unknown about a role of cell-cycle checkpoint regulation in MG-induced signaling. Here, we show that checkpoint kinases, Chk1 and Chk2, as well as their upstream ATM kinase are phosphorylated and activated following MG treatment of cultured cells. This MG-induced activation of Chk1 and Chk2 were inhibited by either aminoguanidine (AG), an inhibitor of production of advanced glycation end products (AGEs) or N-acetyl-l-cysteine (NAC), an anti-oxidant in dose dependent manners, indicating that oxidative stress via AGEs is involved critically in the activation of Chk1 and Chk2 by MG. Furthermore, it was found that cell-cycle synchronized cells exhibited G(2)/M checkpoint arrest following MG treatment, and that siRNA-mediated knock-down of Chk2, but not Chk1, results in a failure of MG-induced G(2)/M arrest. Thus, the results indicate a critical role for Chk2 in MG-induced G(2)/M cell-cycle checkpoint arrest.

Changes in the osmolality of body fluids pose a serious danger to cells and living organisms, which have developed cellular systems to sense and respond to osmotic stress and to maintain homoeostasis of body fluid. However, these processes are incompletely understood in mammals. Here we show that apoptosis signal-regulating kinase 3 (ASK3) is predominantly expressed in the kidney and alters its kinase activity bidirectionally in response to osmotic stress. We further demonstrate that ASK3 interacts with WNK1, mutation in which causes an inherited form of hypertension in humans. Knockdown of Ask3 by short interfering RNA enhances the activation of the WNK1-SPAK/OSR1 signalling pathway. Moreover, Ask3 knockout mice exhibit a hypertensive phenotype, in addition to hyperactivation of SPAK/OSR1 in renal tubules. Our results suggest that ASK3 is a unique bidirectional responder to osmotic stress and that it has a role in the control of blood pressure as an upstream suppressor of the WNK1-SPAK/OSR1 signalling pathway.

Apoptosis signal-regulating kinase (ASK) 1 is a mitogen-activated protein kinase kinase kinase (MAP3K) in the c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase pathways that play multiple important roles in cytokine and stress responses. Here we show that ASK2, a highly related serine/threonine kinase to ASK1, also functions as a MAP3K only in a heteromeric complex with ASK1. We found that endogenous ASK2 was constitutively degraded in ASK1-deficient cells, suggesting that ASK1 is required for the stability of ASK2. ASK2 in a heteromeric complex with a kinase-negative mutant of ASK1 (ASK1-KN) effectively activated MAP2K and was more competent to respond to oxidative stress than ASK2 alone. Knockdown of ASK2 revealed that ASK2 was required for oxidative stress-induced JNK activation. These results suggest that ASK2 forms a functional MAP3K complex with ASK1, in which ASK1 supports the stability and the active configuration of ASK2. Moreover, ASK2 was found to activate ASK1 by direct phosphorylation, suggesting that ASK1 and ASK2 in a heteromeric complex facilitate their activities to each other by distinct mechanisms. Such a formation of functional heteromeric complex between different MAP3Ks may be advantageous for cells to cope with a wide variety of stimuli by fine regulation of cellular responses.