The pathways to tumor suppression via route p38

Cumulating researches on the relationship between P38 MAPK signaling pathway and gastric carcinoma

The cascade reaction of mitogen-activated protein kinases (MAPKs) is one of the vital intracellular signal transduction systems, participating in many physiological progressions, such as cell growth, proliferation, differentiation and apoptosis. P38 is a member of MAPKs, mediating many cell reactions induced by stress, inflammatory cytokines or bacterial products and playing a key role in the regulation of cell cycle. For different cell lines of gastric carcinoma, P38 has different functions. The same phenomenon can be seen when the cells are presented under different stimulus. P38 pathway may be one candidate target of cancer therapy.

HPV16 E7-dependent transformation activates NHE1 through a PKA-RhoA-induced inhibition of p38alpha

Background

Neoplastic transformation originates from a large number of different genetic alterations. Despite this genetic variability, a common phenotype to transformed cells is cellular alkalinization. We have previously shown in human keratinocytes and a cell line in which transformation can be turned on and followed by the inducible expression of the E7 oncogene of human papillomavirus type 16 (HPV16), that intracellular alkalinization is an early and essential physiological event driven by the up-regulation of the Na/⁺H⁺ exchanger isoform 1 (NHE1) and is necessary for the development of other transformed phenotypes and the in vivo tumor formation in nude mice.

Methodology

Here, we utilize these model systems to elucidate the dynamic sequence of alterations of the upstream signal transduction systems leading to the transformation-dependent activation of NHE1.

Principal Findings

We observe that a down-regulation of p38 MAPK activity is a fundamental step in the ability of the oncogene to transform the cell. Further, using pharmacological agents and transient transfections with dominant interfering, constitutively active, phosphorylation negative mutants and siRNA strategy to modify specific upstream signal transduction components that link HPV16 E7 oncogenic signals to up-regulation of the NHE1, we demonstrate that the stimulation of NHE1 activity is driven by an early rise in cellular cAMP resulting in the down-stream inhibition of p38 MAPK via the PKA-dependent phosphorylation of the small G-protein, RhoA, and its subsequent inhibition.

Conclusions

All together these data significantly improve our knowledge concerning the basic cellular alterations involved in oncogene-driven neoplastic transformation.

A connection between MAPK pathways and circadian clocks

Circadian clocks and mitogen-activated protein kinase (MAPK) signaling pathways are fundamental features of eukaryotic cells. Both pathways provide mechanisms for cells to respond to environmental stimuli, and links between them are known. We recently reported that the circadian clock in Neurospora crassa regulates daily rhythms in accumulation of phosphorylated, and thus active, OS-2 MAPK, a relative of mammalian p38 MAPK, when cells are grown in constant conditions. In the absence of acute stress, rhythmically activated MAPK then signals to downstream effector molecules to regulate rhythmic expression of target genes of the pathway. Clock regulation of MAPK signaling pathways provides a mechanism to coordinately control major groups of genes such that they peak at the appropriate times of day to provide a growth and survival advantage to the organism by anticipating stresses. MAPK pathways are well known for their role in cell proliferation and tumor suppression. New evidence reveals that some mammalian clock components also function as tumor suppressors and rhythms in phospho-MAPK have been observed in higher eukaryotes. Thus, the role of the clock in regulation of the activity of MAPK pathways provides important clues into the function of the circadian clock as a tumor suppressor.

Mitogen-activated protein (MAP) kinase/MAP kinase phosphatase regulation: roles in cell growth, death, and cancer

Mitogen-activated protein kinase dual-specificity phosphatase-1 (also called MKP-1, DUSP1, ERP, CL100, HVH1, PTPN10, and 3CH134) is a member of the threonine-tyrosine dual-specificity phosphatases, one of more than 100 protein tyrosine phosphatases. It was first identified approximately 20 years ago, and since that time extensive investigations into both mkp-1 mRNA and protein regulation and function in different cells, tissues, and organs have been conducted. However, no general review on the topic of MKP-1 exists. As the subject matter pertaining to MKP-1 encompasses many branches of the biomedical field, we focus on the role of this protein in cancer development and progression, highlighting the potential role of the mitogen-activated protein kinase (MAPK) family. Section II of this article elucidates the MAPK family cross-talk. Section III reviews the structure of the mkp-1 encoding gene, and the known mechanisms regulating the expression and activity of the protein. Section IV is an overview of the MAPK-specific dual-specificity phosphatases and their role in cancer. In sections V and VI, mkp-1 mRNA and protein are examined in relation to cancer biology, therapeutics, and clinical studies, including a discussion of the potential role of the MAPK family. We conclude by proposing an integrated scheme for MKP-1 and MAPK in cancer.

Activating PTPN11 mutants promote hematopoietic progenitor cell-cycle progression and survival

OBJECTIVE

Mutations in PTPN11, which encodes the protein tyrosine phosphatase Shp2, are commonly found in juvenile myelomonocytic leukemia (JMML). We hypothesized that PTPN11 mutations promote cell-cycle progression and confer enhanced survival to hematopoietic progenitors.

MATERIALS AND METHODS

Murine bone marrow low-density mononuclear cells were transduced with pMIEG3, pMIEG3-WT Shp2, pMIEG3-Shp2D61Y, or pMIEG3-Shp2E76K followed by cell-cycle and survival functional analysis as well as biochemical analysis for key cell-cycle and programmed cell-death regulatory proteins.

RESULTS

A higher proportion of hematopoietic progenitors bearing the gain-of-function Shp2 mutants were residing in the S or G2 phase of the cell cycle in response to low doses of granulocyte-macrophage colony-stimulating factor compared to cells transduced with empty vector (MIEG3) or with WT Shp2. Likewise, Shp2D61Y- or Shp2E76K-expressing hematopoietic cells demonstrated reduced apoptosis based on Annexin-V staining and produced increased progenitor colonies after 48 hours in minimal media compared to cells transduced with empty vector or WT Shp2. To differentiate enhanced survival vs hyperproliferation, cells were stained with PKH26 to distinguish undivided cells from divided progeny. Shp2D61Y- or Shp2E76K-expressing PKH26+ cells similarly demonstrated reduced apoptosis. Upon biochemical analysis, expression of Akt- and Erk-responsive cell-cycle and programmed cell-death regulatory proteins were altered, including increased levels of cyclin D1, Bcl2, and BclXL and reduced levels of p27, p21, and Bim.

CONCLUSION

Collectively, these data demonstrate that gain-of-function Shp2 mutants promote hematopoietic progenitor cell-cycle progression and survival and imply that agents targeting the cell cycle or promoting apoptosis may have therapeutic potential in JMML.

Dual-specificity MAP kinase phosphatases (MKPs) and cancer

There are ten mitogen-activated protein kinase (MAPK) phosphatases (MKPs) that act as negative regulators of MAPK activity in mammalian cells and these can be subdivided into three groups. The first comprises DUSP1/MKP-1, DUSP2/PAC1, DUSP4/MKP-2 and DUSP5/hVH-3, which are inducible nuclear phosphatases. With the exception of DUSP5, these MKPs display a rather broad specificity for inactivation of the ERK, p38 and JNK MAP kinases. The second group contains three closely related ERK-specific and cytoplasmic MKPs encoded by DUSP6/MKP-3, DUSP7/MKP-X and DUSP9/MKP-4. The final group consists of three MKPs DUSP8/hVH-5, DUSP10/MKP-5 and DUSP16/MKP-7 all of which preferentially inactivate the stress-activated p38 and JNK MAP kinases. Abnormal MAPK signalling will have important consequences for processes critical to the development and progression of human cancer. In addition, MAPK signalling also plays a key role in determining the response of tumour cells to conventional cancer therapies. The emerging roles of the dual-specificity MKPs in the regulation of MAPK activities in normal tissues has highlighted the possible pathophysiological consequences of either loss (or gain) of function of these enzymes as part of the oncogenic process. This review summarises the current evidence implicating the dual-specificity MKPs in the initiation and development of cancer and also on the outcome of treatment.

[Nuclear FoxO1 as a bridge between metabolism and mitogenesis]

The transcription factor FoxO1 plays a central role in the metabolic adaptations to fasting. Furthermore, FoxO1 and its homologs encode enzymes buffering ROS (reactive oxygen species) or repairing damaged DNA, and are involved in stalling the ageing process across evolution. Our recent work conducted in primary cultures of rat hepatocytes suggests new mechanisms by which FoxO1 exerts some of these functions. They revealed an unexpected role for FoxO1 in amplifying metabolic, survival, mitogenic and stress signals, and the existence of multiple feed-back loops by which FoxO1 integrates and controls these pathways. Furthermore, these effects were found to be independent of FoxO1 direct binding to DNA.

Cancer stem cells and the ontogeny of lung cancer

Lung cancer is the leading cause of cancer death in the world today and is poised to claim approximately 1 billion lives during the 21st century. A major challenge in treating this and other cancers is the intrinsic resistance to conventional therapies demonstrated by the stem/progenitor cell that is responsible for the sustained growth, survival, and invasion of the tumor. Identifying these stem cells in lung cancer and defining the biologic processes necessary for their existence is paramount in developing new clinical approaches with the goal of preventing disease recurrence. This review summarizes our understanding of the cellular and molecular mechanisms operating within the putative cancer-initiating cell at the core of lung neoplasia.

Phosphorylation of fibroblast growth factor (FGF) receptor 1 at Ser777 by p38 mitogen-activated protein kinase regulates translocation of exogenous FGF1 to the cytosol and nucleus

Exogenous fibroblast growth factor 1 (FGF1) signals through activation of transmembrane FGF receptors (FGFRs) but may also regulate cellular processes after translocation to the cytosol and nucleus of target cells. Translocation of FGF1 occurs across the limiting membrane of intracellular vesicles and is a regulated process that depends on the C-terminal tail of the FGFR. Here, we report that translocation of FGF1 requires activity of the alpha isoform of p38 mitogen-activated protein kinase (MAPK). FGF1 translocation was inhibited after chemical inhibition of p38 MAPK or after small interfering RNA knockdown of p38alpha. Translocation was increased after stimulation of p38 MAPK with anisomycin, mannitol, or H2O2. The activity level of p38 MAPK was not found to affect endocytosis or intracellular sorting of FGF1/FGFR1. Instead, we found that p38 MAPK regulates FGF1 translocation by phosphorylation of FGFR1 at Ser777. The FGFR1 mutation S777A abolished FGF1 translocation, while phospho-mimetic mutations of Ser777 to Asp or Glu allowed translocation to take place and bypassed the requirement for active p38 MAPK. Ser777 in FGFR1 was directly phosphorylated by p38alpha in a cell-free system. These data demonstrate a crucial role for p38alpha MAPK in the regulated translocation of exogenous FGF1 into the cytosol/nucleus, and they reveal a specific role for p38alpha MAPK-mediated serine phosphorylation of FGFR1.

Determinants that control the distinct subcellular localization of p38alpha-PRAK and p38beta-PRAK complexes

p38alpha and p38beta MAPKs (mitogen-activated protein kinases) share about 80% of their protein sequence identity, but have quite different biological functions. One such difference is in regulating the subcellular localization of their downstream kinases, such as PRAK (p38-regulated/activated protein kinase or MK5). The p38alpha-PRAK complex is found in the nucleus, whereas the p38beta-PRAK complex is exclusively localized to the cytosol. By generating a series of chimeric and point mutants of p38alpha and p38beta, we found two amino acid residues (Asp(145) and Leu(156) in p38alpha, Gly(145) and Val(156) in p38beta) that determine the distinct subcellular locations of p38alpha-PRAK and p38beta-PRAK. The subcellular localization of MK2 (MAPK-activated protein kinase 2), another downstream kinase of p38, was regulated in the same manner as that of PRAK. We found that nuclear import, but not export, determines the subcellular localization of p38alpha-PRAK and p38beta-PRAK. The published structure of the p38alpha-MK2 complex suggests Leu(156) of p38alpha is involved in the interaction with the nuclear localization signal in PRAK. The difference at this residue between p38alpha and p38beta may affect the nuclear localization signal in PRAK differently, and thereby influence the import of the complexes. Asp(145) in p38alpha (or Gly(145) in p38beta) is located on a different surface patch, and further random mutagenesis revealed that mutation of Asp(145), Thr(123), and Gln(325), the residues that can directly interact with importin alpha as predicted by modeling, but not mutation of the other 7 amino acid residues that cannot reach importin alpha, re-locate p38alpha-PRAK to the cytosol, suggesting that interaction with import machinery is involved in determining the subcellular localization of the p38alpha-PRAK and p38beta-PRAK complexes. Last, we show that nuclear localization of PRAK is required for its role in inhibiting the proliferation of NIH3T3 cells. In conclusion, multiple determinants control the distinct subcellular localization of p38alpha-PRAK and p38beta-PRAK complexes, and the location of PRAK plays a role in its function.