Extracellular acidic environments induce phosphorylation of ZAP-70 in Jurkat T cells

Atypical p38 Signaling, Activation, and Implications for Disease

The mitogen-activated protein kinase (MAPK) p38 is an essential family of kinases, regulating responses to environmental stress and inflammation. There is an ever-increasing plethora of physiological and pathophysiological conditions attributed to p38 activity, ranging from cell division and embryonic development to the control of a multitude of diseases including retinal, cardiovascular, and neurodegenerative diseases, diabetes, and cancer. Despite the decades of intense investigation, a viable therapeutic approach to disrupt p38 signaling remains elusive. A growing body of evidence supports the pathological significance of an understudied atypical p38 signaling pathway. Atypical p38 signaling is driven by a direct interaction between the adaptor protein TAB1 and p38α, driving p38 autophosphorylation independent from the classical MKK3 and MKK6 pathways. Unlike the classical MKK3/6 signaling pathway, atypical signaling is selective for just p38α, and at present has only been characterized during pathophysiological stimulation. Recent studies have linked atypical signaling to dermal and vascular inflammation, myocardial ischemia, cancer metastasis, diabetes, complications during pregnancy, and bacterial and viral infections. Additional studies are required to fully understand how, when, where, and why atypical p38 signaling is induced. Furthermore, the development of selective TAB1-p38 inhibitors represents an exciting new opportunity to selectively inhibit pathological p38 signaling in a wide array of diseases.

IL4-induced gene 1 is secreted at the immune synapse and modulates TCR activation independently of its enzymatic activity

Amino-acid catabolizing enzymes produced by mononuclear phagocytes play a central role in regulating the immune response. The mammalian phenylalanine-catabolizing enzyme IL4-induced gene 1 (IL4I1) inhibits effector T lymphocyte proliferation and facilitates regulatory T-cell development. IL4I1 expression by macrophages of various human tumors may affect patient prognosis as it facilitates tumor escape from the T-cell response in murine models. Its enzymatic activity appears to participate in its effects, but some actions of IL4I1 remain unclear. Here, we show that the presence of IL4I1 during T-cell activation decreases early signaling events downstream of TCR stimulation, resulting in global T-cell inhibition which is more pronounced when there is CD28 costimulation. Surprisingly, the enzymatic activity of IL4I1 is not involved. Focal secretion of IL4I1 into the immune synaptic cleft and its binding to CD3⁺ lymphocytes could be important in IL4I1 immunosuppressive mechanism of action.

Expression of acidosis-dependent genes in human cancer nests

Previous studies investigating cancer cells cultured at acidic pH have shown that the expression level of ~700 genes were more than two-fold higher than those of the cells cultured in alkaline medium at pH 7.5. The aim of the present study was to confirm whether these acidosis-induced genes are expressed in human cancer tissues. Therefore, 7 genes were selected from our previous study, which encoded interleukin 32 (IL-32), lysosomal H⁺ transporting ATPase, V0 subunit d2 (ATP6V0D2), tumor necrosis factor receptor superfamily, member 9 (TNFRSF9), amphiregulin, schwannoma-derived growth factor (AREG), v-erb-b2 erythroblastic leukemia viral oncogene homolog 3 (ErbB3), PRR5-ARHGAP8 (LOC553158) and dimethylglycine dehydrogenase (DMGDH), and their expression was examined in human clinical specimens from patients with cancer. In addition, the expression of the gene encoding manganese superoxide dismutase (MnSOD) was examined. The specimens from patients with colon, stomach and renal cancer showed increased MnSOD, IL-32, and TNFRSF9 transcripts compared to those from non-tumorous regions of the same patients. Notably, an elevated expression of ATP6V0D2 was found in the specimens from patients with stomach cancer, whereas the expression was decreased in those from patients with colon and renal cancer. The expression of LOC553158 was upregulated in colon and stomach cancer specimens. These results indicate that the investigation of gene expression under acidic conditions is useful for the development of novel cancer markers and/or chemotherapeutic targets.

Acidic extracellular microenvironment and cancer

Acidic extracellular pH is a major feature of tumor tissue, extracellular acidification being primarily considered to be due to lactate secretion from anaerobic glycolysis. Clinicopathological evidence shows that transporters and pumps contribute to H⁺ secretion, such as the Na⁺/H⁺ exchanger, the H⁺-lactate co-transporter, monocarboxylate transporters, and the proton pump (H⁺-ATPase); these may also be associated with tumor metastasis. An acidic extracellular pH not only activates secreted lysosomal enzymes that have an optimal pH in the acidic range, but induces the expression of certain genes of pro-metastatic factors through an intracellular signaling cascade that is different from hypoxia. In addition to lactate, CO₂ from the pentose phosphate pathway is an alternative source of acidity, showing that hypoxia and extracellular acidity are, while being independent from each other, deeply associated with the cellular microenvironment. In this article, the importance of an acidic extracellular pH as a microenvironmental factor participating in tumor progression is reviewed.

Acidic environments enhance the inhibitory effect of statins on proliferation of synovial cells

Many previous studies in animal models and clinical investigations have suggested that statins are useful chemotherapeutics against rheumatoid arthritis, whereas in vitro experiments using synovial cell lines showed no significant effect of statins on cell proliferation until now. Since synovial fluid in rheumatoid joint knee was found to be acidic, we examined the effect of statins on human synovial sarcoma cell line SW982 cells in acidic medium. Statins suppressed the proliferation of SW982 cells at pH6.7, while the suppression was very weak in pH7.5 medium. It was shown that the suppression was caused by the decrease in geranylgeranyl diphosphate, suggesting that a geranylgeranylated protein(s) has an essential role in cell proliferation of SW982 cells under acidic conditions. Our present data clearly implied that statins had high efficacy against SW982 cells in acidic medium whose pH is close to that of rheumatoid arthritis loci in patients. These results lead us to anticipate that screening of chemicals having high therapeutic efficacy in acidic medium promotes the development of new microenvironment-dependent medicines for chemotherapies against rheumatoid arthritis.

Gene Expressions for Signal Transduction under Acidic Conditions

Although it is now well known that some diseased areas, such as cancer nests, inflammation loci, and infarction areas, are acidified, little is known about cellular signal transduction, gene expression, and cellular functions under acidic conditions. Our group showed that different signal proteins were activated under acidic conditions compared with those observed in a typical medium of around pH 7.4 that has been used until now. Investigations of gene expression under acidic conditions may be crucial to our understanding of signal transduction in acidic diseased areas. In this study, we investigated gene expression in mesothelioma cells cultured at an acidic pH using a DNA microarray technique. After 24 h culture at pH 6.7, expressions of 379 genes were increased more than twofold compared with those in cells cultured at pH 7.5. Genes encoding receptors, signal proteins including transcription factors, and cytokines including growth factors numbered 35, 32, and 17 among the 379 genes, respectively. Since the functions of 78 genes are unknown, it can be argued that cells may have other genes for signaling under acidic conditions. The expressions of 37 of the 379 genes were observed to increase after as little as 2 h. After 24 h culture at pH 6.7, expressions of 412 genes were repressed more than twofold compared with those in cells cultured at pH 7.5, and the 412 genes contained 35, 76, and 7 genes encoding receptors, signal proteins including transcription factors, and cytokines including growth factors, respectively. These results suggest that the signal pathways in acidic diseased areas are different, at least in part, from those examined with cells cultured at a pH of around 7.4.

Respiration and the F₁Fo-ATPase enhance survival under acidic conditions in Escherichia coli

Besides amino acid decarboxylation, the ADP biosynthetic pathway was reported to enhance survival under extremely acidic conditions in Escherichia coli (Sun et al., J. Bacteriol. 193∶ 3072–3077, 2011). E. coli has two pathways for ATP synthesis from ADP: glycolysis and oxidative phosphorylation. We found in this study that the deletion of the F₁Fo-ATPase, which catalyzes the synthesis of ATP from ADP and inorganic phosphate using the electro-chemical gradient of protons generated by respiration in E. coli, decreased the survival at pH 2.5. A mutant deficient in hemA encoding the glutamyl tRNA reductase, which synthesizes glutamate 1-semialdehyde also showed the decreased survival of E. coli at pH 2.5. Glutamate 1-semialdehyde is a precursor of heme synthesis that is an essential component of the respiratory chain. The ATP content decreased rapidly at pH 2.5 in these mutants as compared with that of their parent strain. The internal pH was lowered by the deletion of these genes at pH 2.5. These results suggest that respiration and the F₁Fo-ATPase are still working at pH 2.5 to enhance the survival under such extremely acidic conditions.