DNA damage-induced phosphatase Wip1 in regulation of hematopoiesis, immune system and inflammation.

PP2C serine-threonine phosphatase, Wip1, is an important regulator of stress response. Wip1 controls a number of critical cellular functions: proliferation, cell cycle arrest, senescence and programmed cell death, apoptosis or autophagy. Ppm1d, the gene encoding Wip1 phosphatase, is expressed in hematopoietic progenitors, stem cells, neutrophils, macrophages B and T lymphocytes in bone marrow and peripheral blood. The Wip1-/- mice display immunodeficiency, abnormal lymphoid histopathology in thymus and spleen, defects in B- and T-cell differentiation, as well as susceptibility to viral infection. At the same time, Wip1 knockout mice exhibit pro-inflammatory phenotype in skin and intestine in the model of inflammatory bowel disease (IBD) with elevated levels of inflammation-promoting cytokines TNF-α, IL-6, IL-12, IL-17. Several Wip1 downstream targets can mediate Wip1 effects on hematopoietic system including, p53, ATM, p38MAPK kinase, NFkB, mTOR. Here, we summarized the current knowledge on the role of Wip1 in the differentiation of various hematopoietic lineages and how Wip1 deficiency affects the functions of immune cells.

HSP110 promotes colorectal cancer growth through STAT3 activation.

Heat shock protein 110 (HSP110) is induced by different stresses and, through its anti-apoptotic and chaperoning properties, helps cells survive these adverse situations. In colon cancers, HSP110 is abnormally abundant. We have recently shown that colorectal cancer patients with microsatellite instability (MSI) had an improved response to chemotherapy because they harbor an HSP110-inactivating mutation (HSP110DE9). In this work, we used patient biopsies, human colorectal cancer cells grown in vitro and in vivo (xenografts), and intestinal crypts to demonstrate that HSP110 is also involved in colon cancer growth. We showed that HSP110 induces colon cancer cell proliferation and that this effect is associated with STAT3 activation, specifically an increase in STAT3 phosphorylation, nuclear translocation and transcription factor activity. STAT3 inhibition blocks the proliferative effect of HSP110. From a molecular standpoint, we demonstrated that HSP110 directly binds to STAT3, thereby facilitating its phosphorylation by JAK2. Finally, we showed a correlation between HSP110 expression and STAT3 phosphorylation in colon cancer patient samples. Thus, the expression of HSP110 in colon cancer contributes to STAT3-dependent tumor growth and the frequent inactivating mutation of this chaperone is probably an important event underlying the improved prognosis in colon cancer displaying MSI.

Wee1 inhibition potentiates Wip1-dependent p53-negative tumor cell death during chemotherapy.

Inactivation of p53 found in more than half of human cancers is often associated with increased tumor resistance to anti-cancer therapy. We have previously shown that overexpression of the phosphatase Wip1 in p53-negative tumors sensitizes them to chemotherapeutic agents, while protecting normal tissues from the side effects of anti-cancer treatment. In this study, we decided to search for kinases that prevent Wip1-mediated sensitization of cancer cells, thereby interfering with efficacy of genotoxic anti-cancer drugs. To this end, we performed a flow cytometry-based screening in order to identify kinases that regulated the levels of γH2AX, which were used as readout. Another criterion of the screen was increased sensitivity of p53-negative tumor cells to cisplatin (CDDP) in a Wip1-dependent manner. We have found that a treatment with a low dose (75 nM) of MK-1775, a recently described specific chemical inhibitor of Wee1, decreases CDDP-induced H2AX phosphorylation in p53-negative cells and enhances the Wip1-sensitization of p53-negative tumors. We were able to reduce CDDP effective concentration by 40% with a combination of Wip1 overexpression and Wee1 kinase inhibition. We have observed that Wee1 inhibition potentiates Wip1-dependent tumor sensitization effect by reducing levels of Hipk2 kinase, a negative regulator of Wip1 pathway. In addition, during CDDP treatment, the combination of Wee1 inhibition and Wip1 overexpression has a mild but significant protective effect in normal cells and tissues. Our results indicate that inhibition of the negative regulators of Wip1 pathway, Wee1 and Hipk2, in p53-negative tumors could potentiate efficiency of chemotherapeutic agents without concomitant increase of cytotoxicity in normal tissues. The development and clinical use of Wee1 and Hipk1 kinase chemical inhibitors might be a promising strategy to improve anti-cancer therapy.

Role of Gadd45a in Wip1-dependent regulation of intestinal tumorigenesis.

Conversion of intestinal stem cells into tumor-initiating cells is an early step in Apc(Min)-induced polyposis. Wild-type p53-induced phosphatase 1 (Wip1)-dependent activation of a DNA damage response and p53 has a permanent role in suppression of stem cell conversion, and deletion of Wip1 lowers the tumor burden in Apc(Min) mice. Here we show that cyclin-dependent kinase inhibitor 2a, checkpoint kinase 2, and growth arrest and DNA damage gene 45a (Gadd45a) exert critical functions in the tumor-resistant phenotype of Wip1-deficient mice. We further identified Gadd45a as a haploinsufficient gene in the regulation of Wip1-dependent tumor resistance in mice. Gadd45a appears to function through its ability to activate the Jnk-dependent signaling pathway that in turn is a necessary mediator of the proapoptotic functions of p53 that respond to activation of the ß-catenin signaling pathway. We propose that silencing of Gadd45a is sufficient to override p53 activation in the presence of active ß-catenin under conditions of an enhanced DNA damage response.

[Degradation of Cdc25A phosphatase in HeLa cells under normal and stress conditions]. / Degradatsiia fosfatazy Cdc25A v kletkakh HeLa v normal'nykh i stressovykh usloviiakh.

Cdc25A phosphatase regulates cell cycle progression by removing the inhibitory phosphates from cyclin-dependent kinases. Activity of Cdc25A depends on its phosphorylation status. During normal cell cycle progression and after DNA damage phosphorylation by Chk1 (or Chk2) triggers Cdc25A degradation via ubiquitin-proteasome pathway. In this study we investigate the role of various phosphorylation sites (Ser123, Ser75, Ser17 and Ser115) in the regulation of Cdc25A stability. We have shown that only S75A mutation abrogates Cdc25A degradation both in normal and stress conditions. We also studied the influence of stable form of Cdc25A on checkpoint progression after DNA damage. We have found out that delay in DNA synthesis after UV and IR does not depend on Cdc25A activity. However, the presence of stable Cdc25A increases the number of mitotic cells after these stresses.

[The role of stress-kinase p38 in cell cycle blocking delay under osmotic stress]. / Rol' kinazy P38 v ostanovke kletochnogo tsikla v usloviiakh stressa.

Alteration in osmotic conditions leads to activation of some defensive mechanisms resulting in blocking the cell cycle progression. One of stress kinases, activated after osmotic stress, is p38 MAPK. In the present study the role of p38 both in intra-S-phase and G2/M checkpoint is shown. Besides, p38-dependent degradation of Cdc25A phosphatase after osmotic stress is demonstrated. Expression of stable form of Cdc25A results in a partial abrogation of G2/M block of the cell cycle, but has no effect on DNA synthesis arrest.