Urinary bladder cancer is one of the most common malignancies of the urinary tract, and the ninth most common cancer worldwide. A high frequency of bladder tumors (77%) occur in men; it is 7th most common cancer type in men and 17th in women (1,2). With a steady increase in its incidence with age, bladder cancer is most frequently diagnosed in the 5 to 7th decades (3). Almost 75% of all bladder cancers are confined to mucosa (Ta) and submucosa (T1) at the time of initial diagnosis, whereas the remainder have progressed to muscle-invasive disease. Until recently, Ta and T1 bladder tumors were identified as ‘superficial bladder cancers’; however, with the revelation that certain subgroups can progress to invasive disease, they are now categorized as ‘non-muscle-invasive diseases’. To date, numerous etiological factors, including chemical agents, smoking, diet and genetic factors, have been identified as potential causes of bladder cancer (4,5). In addition, the effects of various biochemical markers have been considered (6).

Ion channels and calcium homeostasis play a role in all basic cellular mechanism, including apoptosis, proliferation and differentiation, to maintain homeostasis (7,8). Transient receptor potential (TRP) channels were first described in Drosophila (9). TRP channels are cation-selective channels, which are predominantly localized to the plasma membrane (10,11) and are associated with various diseases, such as cancer (7,8). Variable expression levels of one or more TRP proteins are important for the initiation and progression of certain cancer types (8).

At the cellular level, TRP channels act as multifunctional sensors; they may be activated by physical or chemical stimuli, or by the binding of specific ligands (8). More than 50 different TRP channels have been identified among different species, and >20 TRP channel types may be found in mammals (8). Depending on sequence homology, the 28 mammalian TRP channels are categorized into 6 subfamilies: Vanilloid receptor (TRP cation channel subfamily V); canonical (TRP cation channel subfamily C); melastatin [or long; TRP cation channel subfamily M (TRPM)]; polycystin (TRP cation channel subfamily P); mucolipin (TRP cation channel subfamily ML); and ankyrin (TRP cation channel subfamily A) (12,13).

The TRPM subfamily includes 8 subgroups (TRPM1, TRPM2, TRPM3, TRPM4, TRPM5, TRPM6, TRPM7 and TRPM8), which are considered to have roles in tumorigenesis, cell proliferation and differentiation (11,14). The TRPM subfamily takes its name from the melastatin protein, which is defined as potential tumor suppressor. Most members of TRPM subfamily are permeable to calcium (4,11). Changes in the expression levels of TRPM ion channels can affect various physiological functions and lead to the initiation of pathological processes (4).

Previous studies have reported on the associations between certain cancer types and TRPM ion channels (7,15–21); however, to the best of our knowledge, there have been no studies in the literature that have investigated the role of these ion channels in bladder cancer. In this original study, we aimed to determine whether bladder cancer is associated with the mRNA expression levels of TRPM ion channel genes, and whether this could merit further study with the aim of establishing novel treatment modalities.

TRP channels are involved in cell differentiation, cell growth and apoptosis, and increases in Ca²⁺ induce apoptosis in cells (10). Therefore, TRP channels localized on the tumor cell membrane are predicted to be novel targets for drug therapy, immunotherapy and gene therapy. There is only a small amount information available regarding changes in the expression of these channels during cell differentiation and carcinogenesis (10). Further research studies on the expression pathways of these channels are required to contribute to the understanding of carcinogenesis and the implementation of TRP agonists in cancer treatment (10).

Intracellular calcium regulation is one of the basic mechanisms by which cell replication and apoptosis are controlled. It has an essential role in tumor formation (24). Although the selective suppression of TRPM protein expression or transmigration to the nucleus does not exert any effects on normal cells, this is an important therapeutic target for inducing apoptosis of cancer cells (24).

Certain Ca²⁺ and Na²⁺ permeable members of TRP channel family are expressed at various levels in cancer cells; however, to date, there have been few studies reporting this expression of the TRP channel family in cancer cells (25). Such alterations in the expression of TRP have not yet been fully elucidated, and it is presently unclear whether this is one of the cornerstones of cancer progression or whether it occurs secondary to other changes (25). As an answer to this question, some other markers in bladder and prostate cancer, such as BLCA-4 (a nuclear matrix protein that has homology with the ELK-3 gene, a member of the ETS transcription factor family) could predict the course of the cancer. This suggests that some TRP proteins may also be of value in the course of bladder cancer (26). The existence of ion channels in bladder cancer and their potential effects on malignant phenotypes have not been studied previously (15).

TRPM7 channels are Ca²⁺-permeable, non-selective cation channels. The activation of TRPM7 channels has been shown to be involved in cellular Mg²⁺ homeostasis, Ca²⁺-associated neuronal damage under ischemic conditions, and diseases caused by abnormal magnesium absorption (27). TRPM2 channels are non-voltage-gated cation channels and have a significant permeability to an important second messenger: Ca²⁺ (27).

In the present study, a significant increase was observed in TRPM7 expression in bladder cancer tissues compared to healthy bladder tissue, whereas no differences were observed with regard to TRPM2 and TRPM4. Furthermore, significant decreases in the expression levels of TRPM5 and TRPM8 were found in bladder cancer tissues. This finding was in accordance with a study conducted by Mizuno et al (16), who reported increased expression of TRPM7 mRNA in bladder cancer cells compared with normal urothelial cells. TRPM7 was also suggested to be associated with breast and gastric cancers (17,18). Additionally, TRPM7 has been used as a therapeutic target in the treatment of cancers such as breast and gastric cancer (17,18). Overexpression of these channels could be a novel marker for carcinogenic and tumor cells (24). The increased expression of TRPM7 in bladder cancer in the current study may confirm this concept.

TRPM8 channels are Ca²⁺-permeable channels that are widely expressed and may be used as prognostic and diagnostic markers, particularly in prostate cancer (19). In a recent study by Xiao et al (20), the increased expression of TRPM8 in metastatic bladder cancer was reported. By contrast, a significant decrease in the expression of TRPM8 mRNA in bladder cancer cells was observed in the present study. This difference may be due to the low patient number or the difference between the two patient cohorts regarding to tumor stage. For example, all cancer patients in the present study were diagnosed with superficial bladder cancer, whereas the study by Xiao et al comprised patients with metastatic cancer. These findings demonstrate the importance of the current study.

TRPM4 and TRPM5 channels are impermeable to Ca²⁺. However, Ca²⁺ has an important role in their activities, as these two channels are activated by the increase of intracellular calcium. Thus, TRPM4 and TRPM5 are calcium-activated, non-selective cationic channels that are found in numerous tissue types (28). The expression levels of TRPM4 and TRPM5 are known to increase in certain types of cancer (21). It has been hypothesized that various expression levels (high and low) of TRPM5 may be associated with tumorigenesis: For example, an increase in the expression of TRPM5 mRNA was previously detected in cases of Wilms tumor and rhabdomyosarcoma (29); however, in the present study, TRPM5 immunoreactivity was revealed to be extremely low in bladder cancer tissues and normal bladder tissues. Recent studies have suggested that TRPM4 is one of the predominant regulators of the excitation-contraction mechanisms of bladder smooth muscle (30,31) and, thus, it may serve as an interesting therapeutic target in the treatment of overactive bladder. However, an association between TRPM4 and bladder cancer has not yet been determined. The present results are consistent with the literature (15), as no difference in TRPM4 expression between bladder cancer tissue and normal tissue was detected.

In the literature, limited data is available regarding the immunostaining and expression of TRPM channels in normal and cancerous bladder tissues. In the present study, intense expression of TRPM2 was observed in bladder epithelium, mucosal and smooth muscle cells in controls and cancer patients, whereas TRPM7 expression was found to be restricted to epithelial cells in the two study groups and was not observed in other cell types. Despite the absence of TRPM5 immunoreactivity in any of the tissues, a limited level of mRNA was detected by RT-qPCR in each group. TRPM4 and TRPM8 immunoreactivity was also found to be extremely limited and confined to epithelial cells.

In conclusion, to the best of our knowledge, the present study is the first in the literature to focus on TRP channels in bladder cancer. As shown in the aforementioned studies, TRPM7 may have a role in tumor pathogenesis, progression, migration and invasion. With the aid of TRPM7 analysis, it may be possible to predict the progression of low-grade tumors to high-grade tumors. In this study, due to the small number of patients, it was not possible to make significant comparisons between the cases categorized as non-muscle invasive, muscle invasive, low-grade and high grade. In future studies with larger patient volumes, the correlation of TRPM7 expression with bladder cancer progression may be established.