The Acidic Tumor Microenvironment Affects Epithelial-Mesenchymal Transition Markers as Well as Adhesion of NCI-H358 Lung Cancer Cells

Epithelial-mesenchymal transition (EMT), which is involved in metastasis formation, requires reprogramming of gene expression mediated by key EMT transcription factors. However, signals from the cellular microenvironment, including hypoxia, can also modulate the process of EMT. Hypoxia is often associated with a reduction in the extracellular pH of the tumor microenvironment (acidosis). Whether acidosis alone has an impact on the expression of the EMT markers E-cadherin, N-cadherin, and vimentin was studied in NCI-H358 lung cancer cells. Reducing extracellular pH decreased E-cadherin mRNA, while vimentin and N-cadherin mRNA were doubled. However, at the protein level, E-cadherin and N-cadherin were both reduced, and only vimentin was upregulated. E-cadherin and N-cadherin expression at the cell surface, which is the relevant parameter for cell-cell and cell-matrix interaction, decreased too. The reduction of cell surface proteins was due to diminished protein expression and not changes in cellular localization, since localization of EMT markers in general was not affected by acidosis. Acidosis also affected NCI-H358 cells functionally. Adhesion was decreased when the cells were primed in an acidic medium before measuring cell adherence, which is in line with the reduced expression of cadherins at the cell surface. Additionally, migration was decreased after acidic priming. A possible mechanism for the regulation of EMT markers involves the action of microRNA-203a (miR-203a). In NCI-H358 lung cancer cells, miR-203a expression was repressed by acidosis. Since a decrease in the level of miR-203a has been shown to induce EMT, it might be involved in the modulation of EMT marker expression, adhesion, and migration by the acidic tumor microenvironment in NCI-H358 lung cancer cells.

Functional Impact of Acidosis-Regulated MicroRNAs on the Migration and Adhesion of Tumor Cells

Tumor tissue shows special features in metabolism in contrast to healthy tissue. Besides a distinctive oxygen deficiency, tumors often show a reduced extracellular pH (acidosis) resulting from an intensified glycolysis not only under hypoxic but also under normoxic conditions (Warburg effect). As shown in previous studies, cell migration is increased in AT1 prostate carcinoma cells after incubation at pH 6.6, and this leads to an increased number of lung metastases in vivo. However, the signaling pathway causing these functional changes is still unknown. Possible mediators could be acidosis-regulated microRNAs (miR-7, miR-183, miR-203, miR-215). The aim of the study was therefore to analyze whether a change in the expression of these microRNAs has an impact on the tumor cell migration and adhesion. Studies were performed with AT1 rat prostate cancer cells which were incubated for 24 h at pH 7.4 or 6.6. Keeping AT1 tumor cells at low pH increased the migratory capacity by about 100%. But also the decrease of miR-203 and miR-215 expression (at normal pH) led to an increase in migration velocity by 50%. In contrast, cell adhesion was increased by about 75% at low pH. However, an increase in miR-215 expression at pH 6.6 reduced the adhesion by trend. These results clearly indicated that the extracellular pH has an impact on migration and adhesion of tumor cells. In this mechanism, pH-regulated microRNAs could play a role since changes in the expression of these microRNAs (especially miR-203) are also able to modulate the migratory behavior.

The Role of MicroRNA Expression for Proliferation and Apoptosis of Tumor Cells: Impact of Hypoxia-Related Acidosis

The metabolic microenvironment in tumors is characterized by hypoxia and acidosis. Extracellular pH sometimes decreases to even below 6.0. Previous experiments showed that tissue pH has an impact on tumor cell proliferation and apoptosis. However, the mechanism of how cell cycle progression is affected by decreased pH is not fully understood yet. One possible mechanism includes changes in the expression of miRNAs. The aim of this study was to analyze the impact of pH-regulated miRNAs (miR-183 and miR-215) on proliferation, apoptosis, and necrosis of tumor cells. Therefore, AT1 prostate and Walker-256 mammary carcinoma cells were transfected with the miRNAs or with the respective antagomirs and incubated at pH 7.4 and 6.6 for 24 h. AT1 cells underwent a G0/G1 cell cycle arrest under acidic conditions and showed a marked reduction of the number of actively DNA-synthesizing cells. In Walker-256 cells, acidosis induced a reduction of apoptosis and additionally a significant increase in necrotic cell death. Transfection of tumor cells with miR-183 or miR-215, which were significantly downregulated under acidic conditions, had no impact on cell death of AT1 or Walker-256 cells. Overexpression of miR-183, which is also downregulated by acidosis, intensified G0/G1 cell cycle arrest in AT1 cells. Previous studies revealed that hypoxia-related tumor acidosis affects the expression of different small noncoding RNAs. However, not all of these acidosis-regulated miRNAs seem to have an impact on proliferation, apoptosis, and necrosis of tumor cells. While miR-215 had no influence, miR-183 seems to be an interesting candidate that could amplify the impact of extracellular acidosis on malignant behavior of tumor cells.

Acidosis-Induced Regulation of Egr1 and Ccn1 In Vitro and in Experimental Tumours In Vivo

Extracellular acidosis is a characteristic of solid tumours, resulting from hypoxia-induced glycolytic metabolism as well as from the "Warburg effect" (aerobic glycolysis). The acidic environment has shown to affect functional tumour properties (proliferation, migration, invasion) and thus the aim of the study was to identify signalling mechanisms, mediating these pH-dependent effects. Therefore, the serum response factor (Srf) and the activation of the serum response element (SRE) by acidosis were analysed in AT-1 prostate carcinoma cells. Furthermore, the expression of downstream targets of this cascade, namely the early growth response 1 (Egr1), which seems to be involved in tumour proliferation, and the cellular communication network factor 1 (Ccn1), which both contain SRE in their promotor region were examined in two tumour cell lines. Extracellular acidification led to an upregulation of Srf and a functional activation of the SRE. Egr1 expression was increased by acidosis in AT-1 cells whereas hypoxia had a suppressive effect. In experimental tumours, in vivo Egr1 and Ccn1 were also found to be acidosis-dependent. Also, it turned out that pH regulated expression of Egr1 was followed by comparable changes of p21, which is an important regulator of the cell cycle.This study identifies the Srf-SRE signalling cascade and downstream Egr1 and Ccn1 to be acidosis-regulated in vitro and in vivo, potentially affecting tumour progression. Especially linked expression changes of Egr1 and p21 may mediate acidosis-induced effects on cell proliferation.

Role of the mTOR Signalling Pathway During Extracellular Acidosis in Tumour Cells

The metabolic microenvironment of solid tumours is often dominated by extracellular acidosis which results from glycolytic metabolism. Acidosis can modulate gene expression and foster the malignant progression. The aim of the study was to analyse the effects of extracellular acidosis on the mTOR signalling pathway, an important regulator of anabolic and catabolic processes like cell proliferation and autophagy. The study was performed in two tumour cell lines, AT-1 prostate and Walker-256 mammary carcinoma cells. Cells were incubated at pH 7.4 or 6.6 for 3 h and 24 h. Then RNA and protein were extracted and analysed by qPCR and western blot. mTOR and P70-S6 kinase (P70-S6K), an important downstream target of mTOR, as well as the autophagic flux were studied. The effect of acidosis on P70S6K phosphorylation was compared to pharmacological mTOR inhibition with LY294002 and rapamycin. In both cell lines the total mTOR expression was not altered by acidosis, however, the mTOR phosphorylation was reduced after 3 h but not after 24 h. The P70S6K phosphorylation was reduced at both time points comparable to changes by pharmacological mTOR inhibitors. The autophagic flux, also a target of mTOR and measured by LC3-II expression, was increased in both cell lines after 24 h of acidosis. The results of this study indicate that mTOR signalling is inhibited by extracellular acidosis which then lead to a reduced activity of the P70-S6 kinase (modulating gene expression) and increased autophagy possibly mediated by ULK1/2 activity. These finding may offer new perspectives for therapeutic interventions in acidic tumours.