First findings of gap junction proteins in human urothelial carcinoma

Excerpt

Connexin 43 is a gap junction protein, and a recent rat study by Lee reported it to be involved in the ovariectomy-induced overactive bladder [1]. Gap junctions mediate intercellular communication and are also involved in the regulation of proliferation and differentiation [2]. They are formed by connexins, of which 21 different molecules are known in humans [3]. In the human bladder, gap junctions have been found in the smooth muscle cells of the detrusor muscle [4]. The gap junction protein connexin 43 has additionally been detected in so-called interstitial cells beneath the urothelium [5]. In that study, human urothelium was found to be negative for connexins 40, 43 and 45. In contrast hereto, connexin 43 has been detected in the bovine urinary bladder and its tumors [6]. Interestingly, connexin 26 has been detected in human urothelium [7]. That report also revealed an altered, weakened expression of connexin 26 in bladder tumors. A possible role of gap junctions and connexins in carcinogenesis was discussed [8, 9]: A former concept was that a loss of connexins and reduced gap junctional intercellular communication correlated with malignancy and that thus the induction of connexins and gap junctions should be beneficial for patients suffering from urothelial carcinoma. In contradiction thereto, the current view is that enhanced gap junctional intercellular communication capacity or increased expression of connexins supports bladder carcinogenesis [10]. To the authors’ knowledge, the expression of connexins in human bladder cancer has not been investigated previously. Therefore, the aim was to investigate the synthesis of a range of connexins (26, 29, 32, 36 and 43) in tissue samples of human urothelial carcinoma. With approval from the ethical committee, tissue was obtained from nine patients with urothelial carcinoma during curative tumor surgery (five males, 64–91 years of age; four females, 46–84 years old). Slides were immune-stained with polyclonal antibodies from Thermo Fischer Scientific, Rockford, IL, USA, using a dilution of 1:50 (Cx43) or 1:100 (all others): anti-connexin 26, anti-connexin 29, anti-connexin 32, anti-connexin 36, anti-connexin 43, secondary antibody anti-rabbit-horseradish peroxidase developed with diamino-benzidine. Connexin 29, used as negative control, yielded no signal (Fig. 1a). Connexin 43 was clearly stained on murine heart tissue as positive control (Fig. 1b). The diagnosis of urothelial carcinoma was confirmed in all of the nine cases. Connexin 26 was found in the tumor cells of two of the nine patients. The connexins 29, 32 and 36 were not found in the tumor cells at all. However, in contrast thereto, connexin 43 was detected in the tumor cells of eight of the nine patients (Fig. 1c, d). It can therefore be concluded that they are unlikely to play a role in human urothelial carcinoma. The finding of connexin 26 expression in two of the nine carcinomas indicates a minor role for this connexin. The main result of this preliminary assessment was that connexin 43 can frequently be found in human urothelial carcinomas. This is a new finding which has not been published previously. Since normal human urothelium does not produce connexin 43, the findings presented are most likely to be tumor-specific or tumor-associated. Therefore, urothelial carcinomas producing connexin 43 can be included in the discussion on the various roles of connexin 43. One decade ago it was assumed that a loss of connexin 43 led to malignancy and that it might, therefore, be a target for genetic therapeutical approaches [8, 9]. The more recent interpretation of the role of connexin 43 is that it promotes carcinogenesis [10]. It enhanced the adherence of tumor cells to the stroma as well as the migration and—probably—dissemination of cancer cells. It is interesting to note that one early evaluation did, however, postulate that “increased gap junctional intercellular communication capacity or increased connexin(s) expression” increased rat bladder carcinogenesis as tested in cell lines and tumors [11]. The high prevalence of the expression of connexin 43 in this investigation supports the latter view, as far as it is possible to draw such a conclusion from preliminary observations. In conclusion, the synthesis of connexin 43 in urothelial cancer cells is a cofactor in the genesis and growth of urothelial carcinomas.

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