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In many tumors pronounced extracellular acidosis resulting from glycolytic metabolism is found. Since several environmental stress factors affect the mitochondrial activity the aim of the study was to analyze the impact of acidosis on cellular oxygen consumption and which signaling pathways may be involved in the regulation. In two tumor cell lines and normal fibroblasts cellular oxygen consumption rate (OCR) and mitochondrial function were measured after 3 h at pH 6.6. Besides the activation of ERK1/2, p38 and PI3K signaling in the cytosolic and mitochondrial compartment, the mitochondrial structure and proteins related to mitochondria fission were analyzed. The acidic extracellular environment increased OCR in tumor cells but not in fibroblasts. In parallel, the mitochondrial membrane potential increased at low pH. In both tumor lines (but not in fibroblasts), the phosphorylation of ERK1/2 and PI3K/Akt was significantly increased, and both cascades were involved in OCR modulation. The activation of signaling pathways was located predominantly in the mitochondrial compartment of the cells. At low pH, the mitochondrial structure in tumor cells showed structural changes related to elongation whereas mitochondria fragmentation was reduced indicating mitochondria fusion. However, these morphological changes were not related to ERK1/2 or PI3K signaling. Acidic stress seems to induce an increased oxygen consumption, which might further aggravate tumor hypoxia. Low pH also induces mitochondria fusion that is not mediated by ERK1/2 or PI3K signaling. The mechanism by which these signaling cascades modulate the respiratory activity of tumor cells needs further investigation., Competing Interests: Declaration of competing interest The authors declare no competing conflicts of interests., (Copyright © 2024. Published by Elsevier Inc.)

Background: The acidic extracellular environment of tumors has been shown to affect the malignant progression of tumor cells by modulating proliferation, cell death or metastatic potential. The aim of the study was to analyze whether acidosis-dependent miRNAs play a role in the signaling cascade from low pH through changes in gene expression to functional properties of tumors in vitro and in vivo., Methods: In two experimental tumor lines the expression of 13 genes was tested under acidic conditions in combination with overexpression or downregulation of 4 pH-sensitive miRNAs (miR-7, 183, 203, 215). Additionally, the impact on proliferation, cell cycle distribution, apoptosis, necrosis, migration and cell adhesion were measured., Results: Most of the genes showed a pH-dependent expression, but only a few of them were additionally regulated by miRNAs in vitro (Brip1, Clspn, Rif1) or in vivo (Fstl, Tlr5, Txnip). Especially miR-215 overexpression was able to counteract the acidosis effect in some genes. The impact on proliferation was cell line-dependent and most pronounced with overexpression of miR-183 and miR-203, whereas apoptosis and necrosis were pH-dependent but not influenced by miRNAs. The tumor growth was markedly regulated by miR-183 and miR-7. In addition, acidosis had a strong effect on cell adhesion, which could be modulated by miR-7, miR-203 and miR-215., Conclusions: The results indicate that the acidosis effect on gene expression and functional properties of tumor cells could be mediated by pH-dependent miRNAs. Many effects were cell line dependent and therefore do not reflect universal intracellular signaling cascades. However, the role of miRNAs in the adaptation to an acidic environment may open new therapeutic strategies., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021. Published by Elsevier Inc.)

Epithelial-to-mesenchymal transition (EMT) is an important process of tumor progression associated with increased metastatic potential. EMT can be activated by external triggers such as cytokines or metabolic parameters (e.g. hypoxia). Since extracellular acidosis is a common finding in tumors, the aim of the study is to analyze its impact on the expression of EMT markers in vitro and in vivo as well as the functional impact on cell adhesion. Therefore, three tumor and two normal epithelial cell lines were incubated for 24 h at pH 6.6 and the expression of EMT markers was studied. In addition, mRNA expression of transcription and metabolic factors related to EMT was measured as well as the functional impact on cell adhesion, either during acidic incubation or after priming cells in an acidic milieu. E-cadherin and N-cadherin were down-regulated in all tumor and normal cell lines studied, whereas vimentin expression increased in only two tumor and one normal cell line. Down-regulation of the cadherins was seen in total protein and to a lesser extent in surface protein. In vivo an increase in N-cadherin and vimentin expression was found. Acidosis up-regulated Twist1 and Acsl1 but down-regulated fumarate hydratase (Fh). Cell adhesion during acidic incubation decreased in AT1 prostate carcinoma cells whereas preceding acidic priming increased their subsequent adhesion. Low tumor pH is able to modulate the expression EMT-related proteins and by this may affect the stability of the tissue structure., (Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.)

Inflammatory mediators produced by the tumor cells are of importance for immune response but also for malignant progression. The aim of the study was to analyze the expression of monocyte chemoattractant protein-1, interleukin-6 (IL-6), tumor necrosis factor-α, inducible isoform of nitric oxide synthase (iNOS), cyclooxygenase-2, and osteopontin in vitro in two different tumor cell lines under hypoxia (pO 2 ≈1.5 mmHg) and/or acidosis (pH=6.6) for up to 24 hours since hypoxia and acidosis are common characteristics of solid tumors. Additionally, the same tumor cell lines implanted in vivo were made hypoxic and acidotic artificially for 24 hours, after which the cytokine expression was measured. Finally, the activation of ERK1/2 and p38 by acidosis/hypoxia and their impact on cytokine expression were studied. The results indicate that acidosis and hypoxia have fundamentally different (often opposing) effects on cytokine expression. In addition, these effects were tumor cell line specific. When combining hypoxia and acidosis, the overall changes reflect an additive effect of both conditions alone, indicating that hypoxia and acidosis act by independent mechanisms. The in vivo changes corresponded well with the results obtained in the isolated tumor cells. Only iNOS expression was downregulated in vivo but increased in cell culture. For IL-6 expression, the acidosis-induced changes were dependent on ERK1/2 activation. In conclusion, it was demonstrated that the environmental pO 2 and pH strongly affect the expression of inflammatory mediators in tumor cells. In vivo, most of the inflammatory mediators were downregulated, which could limit the activation of immune cells and by this foster the immune escape of tumors., (Copyright © 2017 The Authors. Published by Elsevier Inc. All rights reserved.)

The expression and activity of P-glycoprotein (pGP) play a role in the multidrug resistance of tumors. Because solid-growing tumors often show pronounced hypoxia or extracellular acidosis, this study attempted to analyze the impact of an acidic environment on the expression and activity of pGP and on the cytotoxicity of chemotherapeutic agents. For this, prostate carcinoma cells were exposed to an acidic extracellular environment (pH 6.6) for up to 24 hours. pGP activity was more than doubled after 3 to 6 hours of incubation in acidic medium, whereas cellular pGP expression remained constant, indicating that increased transport rate is the result of functional modulation. In parallel, the cytotoxic efficacy of daunorubicin showed pronounced reduction at low pH, an effect that was reversible on coincubation with a pGP inhibitor. A reduction of intracellular Ca2+ concentration by 35% under acidic conditions induced a higher transport rate of pGP, an effect comparable to that found on inhibition of protein kinase C (PKC). These data indicate that pGP activity is increased by low extracellular pH presumably as a result of lowered intracellular calcium levels and inhibition of PKC. These findings may explain the reduced cytotoxicity of chemotherapeutic agents in hypoxic/acidic tumors.