## 1 Introduction  
Colon cancer is one of the most frequent human malignancies in the world [1; 2; 3]. Despite the progress in surgical techniques and systemic treatments, the majority of diagnosed patients die from the metastatic spread of tumor cells, generally into the liver and the peritoneum [4; 5; 6; 2]. The biology of metastasis formation is described as an invasion-metastas cascade in which tumor cells leave their primary site and form new colonies in distant tissues [7; 8]. This process consists of five steps: local invasion of tumor cells into the surrounding matrix (Step 1), intravasation of tumor cells into the circulatory system (Step 2), systemic transportation of tumor cells (Step 3), extravasation of tumor cells into parenchyma of distant tissue sites (Step 4), and colonization of distant organs and the establishment of macroscopic tumors (Step 5). The pivotal roles in the promotion of the invasion-metastasis cascade are the mutual and interdependent interactions between cancer cells and their microenvironment [9]. Although it is known that the tumor environment is a complex network of inflammatory and immune cells, carcinoma-associated fibroblasts, hypoxia, signal molecules, and extracellular matrix components [9], the initial mechanisms responsible for the initiation of tumor cell invasion are still poorly understood. Evidence suggests that environmental acidification in cancers occurs during the initial avascular phase of a carcinoma [10; 11; 12]. During the phase of rapid growth, in which tumors become vascularized, the supply of oxygen is limited because of the blood's sluggish perfusion through the heterogeneously distributed vascular network [13]. These cells encounter an oxygen-poor environment and, through the Pasteur effect, switch from oxidative phosphorylated metabolism toward anaerobic glycolysis [14; 15]. As a result, the proton (H\({}^{+}\)) concentration increases within the lumen due to the diffusion limitation and increased production of lactate from hypoxic-glycolytic cells [16]. Consequently, a highly acidic lumen medium is achieved. An appropriate intracellular pH has many diverse effects on cells, including an effect on proliferation rate [16; 17], differentiation [18; 19; 20], metabolism [21; 22], protein synthesis, and glycosylation [23]. For example, studies have demonstrated a relation between changes in pH and cell proliferation activity using human peripheral blood mononuclear cells (T cells) and murine lymphocytes. The results indicate that the increased proliferation rate induced a reduction in the pH values [18; 24]. At the same time, intracellular acidification has been shown to be cytotoxic because of the induction of apoptosis [25; 26]. To overcome the low intracellular pH, as a result of accelerated metabolism, adequate for uncontrolled cell proliferation activity as well as cancer progression, cancer cells use a broad range of mechanisms for removing acids in order to maintain an appropriate intracellular pH and avoid cancer cell death. In this respect, lactic acid and H\({}^{+}\) efflux through monocarboxylate transporters and Na-driven proton extrusion have been described, respectively [27; 28]. As a consequence, a reverse pH gradient is formed and the pH of the extracellular space of cancer cells becomes more acidic compared with the lumen [13; 28; 29; 30]. An earlier study by [PERSON] at al. included a comparative analysis of the extracellular pH values of 77 different human tumors and normal subcutaneous tissue. According to the study, the average pH values, corresponding to the tumor environment, were within the pH range of 7.25 \(\pm\) 0.99, whereas the pH value in normal tissue was pH = 7.54 \(\pm\) 0.09. They also reported that extracellular pH values are not related to the tumor histology, degree of differentiation, tumor size, patient age, or treatment history [31]. Because extracellular pH can affect cultured cancer cells in so many ways, it is important to clarify whether it plays a role in metastasis formation.