Abstractp38 MAPK signaling has been implicated in the regulation of processes leading to cancer development and progression. Chronic inflammation is a known risk factor for tumorigenesis, yet the precise mechanism of this association remains largely unknown. The related p38aMAPK (MAPK14) proteins p38g (MAPK12) and p38d (MAPK13) were recently shown to modulate the immune response, although their role in tumorigenesis remains controversial and their function in inflammation-associated cancer has not been studied. We analyzed the role of p38g and p38d in colon cancer associated to colitis using the azoxymethane/dextran sodium sulphate (AOM/DSS) colitis-associated colon cancer model in wild-type (WT), p38g-, p38d-, and p38g/ d-deficient (p38g/d À/À ) mice. We found that p38g/d deficiency significantly decreased tumor formation, in parallel with a decrease in proinflammatory cytokine and chemokine production. Analysis of leukocyte populations in p38g/d À/À mouse colon showed less macrophage and neutrophil recruitment than in WT mice. Together, our results establish that p38g and p38d are central to colitis-associated colon cancer formation through regulation of hematopoietic cell response to injury, and validate p38g and p38d as potential targets for cancer therapy. Cancer Res; 74(21); 6150-60. Ó2014 AACR.

The p38 mitogen-activated protein kinase (p38MAPK) signal transduction pathway is an important regulator of cell processes, whose deregulation leads to the development and progression of cancer. Defining the role of each p38MAPK family member in these processes has been difficult. To date, most studies of the p38MAPK pathways focused on function of the p38α isoform, which is widely considered to negatively regulate malignant transformation; nonetheless, few reports address the p38γ and p38δ isoforms. Here, we used embryonic fibroblasts derived from mice lacking p38γ or p38δ and show evidence that these isoforms participate in several processes involved in malignant transformation. We observed that lack of either p38γ or p38δ increased cell migration and metalloproteinase-2 secretion, whereas only p38δ deficiency impaired cell contact inhibition. In addition, lack of p38γ in K-Ras-transformed fibroblasts led to increased cell proliferation as well as tumorigenesis both in vitro and in vivo. Our results indicate that p38γ and p38δ have a role in the suppression of tumor development.

Activation of p38γ modulates the integrity of the complex formed by the human discs large protein (hDlg) with cytoskeletal proteins, which is important for cell adaptation to changes in environmental osmolarity. Here we report that, in response to hyperosmotic stress, p38γ also regulates formation of complexes between hDlg and the nuclear protein polypyrimidine tract-binding protein-associated-splicing factor (PSF). Following osmotic shock, p38γ in the cell nucleus increases its association with nuclear hDlg, thereby causing dissociation of hDlg-PSF complexes. Moreover, hDlg and PSF bind different RNAs; in response to osmotic shock, p38γ causes hDlg-PSF and hDlg-RNA dissociation independently of its kinase activity. These findings identify a novel nuclear complex and suggest a previously unreported function of p38γ, which is independent of its catalytic activity and could affect mRNA processing and/or gene transcription to aid cell adaptation to osmolarity changes in the environment.

Human disc‐large (hDlg) is a scaffold protein critical for the maintenance of cell polarity and adhesion. hDlg is a component of the p38γ MAP kinase pathway, which is important for the adaptation of mammalian cells to changes in environmental osmolarity. Here we report a strong decrease in the levels of hDlg protein in the human epithelial cell line HeLa when exposed to osmotic shock. This is independent of the phosphorylation state of hDlg, is prevented by preincubating the cell with the caspase inhibitor z‐VAD and is part of the apoptotic process triggered by cellular stress. Although, both caspase 3 and caspase 6 are strongly activated by osmotic shock, the time course of caspase 6 activation parallels hDlg degradation, suggesting that this caspase may be responsible for the proteolysis. Mutating hDlg Asp747 to Ala abolishes caspase‐induced cleavage, but does not affect the early stage of apoptosis or cell attachment. Our findings show that osmotic stress triggers hDlg degradation through a mechanism different from the one mediated by proteasomes, and we identify hDlg as a caspase substrate during the apoptotic process, although its proteolysis may not be implicated in the progression of early apoptosis.

<p>Figure S1. Effect of p38gamma and p38delta deletion in early stages of colitis-associated colon cancer induced by AOM/DSS treatment. Figure S2. p38gamma and p38deltaexpression in AOM/DSS-treated and untreated mouse colon. WT, p38gamma /-, p38delta/- or p38gamma/delta/- mice were AOM/DSS-treated and untreated. At 9 weeks post-AOM injection, colon extracts were prepared for mRNA and protein analysis. Figure S3. p38gamma or p38deltadeletion has no significant effect on body weight and colon length in the course of DSS-induced colitis. WT and p38gamma/delta/- mice were treated with 3% DSS in drinking water for 5 days. Figure S4. p38gamma/delta-/- mice show increased IEC apoptosis after DSS treatment. Figure S5. ERK1/2 phosphorylation, but not IkappaBalpha proteolysis, was reduced in p38gamma/delta-/- colon cells compared to WT. Figure S6. Effect of p38gamma/deltadeletion in the expression of chemokine receptors and in the recruitment of neutrophils, macrophages and Treg cells. Table SI. Primer sequences used for gene expression.</p>