The Gap Junction-independent Tumor-suppressing Effect of Connexin 43

Abstract

The gap junction gene connexin 43 (Cx43) showed tumor-suppressing effects on various tumor cell lines. We have previously demonstrated that Cx43 inhibited expression of S phase kinase-associated protein 2 (Skp2), the human F-box protein that regulates the ubiquitination of p27. Cx43 did not alter the mRNA level of SKP2, but it promoted the degradation of the Skp2 proteins (Zhang, Y. W., Nakayama, K., Nakayama K. I., and Morita, I. (2003) Cancer Res. 63, 1623-1630). In this study, we showed that the specific gap junction inhibitor 18 ␤-glycyrrhetinic acid did not influence the inhibitory effect of Cx43 on Skp2 expression. Further, the deletion mutation analyses demonstrated that the C-terminal domain of Cx43 that did not form gap junctions was sufficient to inhibit expression of Skp2, whereas the N-terminal domain that formed the gap junctions did not show such an effect. Like the full-length Cx43, the Cterminal domain also increased the protein instability of Skp2, whereas the N terminus did not. Moreover, the C-terminal domain was as effective as the full-length Cx43 in inhibiting cell proliferation; however, the Nterminal domain did not show any inhibitory effect on cell proliferation. Therefore, these data revealed a gap junction-independent pathway for Cx43 to inhibit tumor growth by suppressing the Skp2 expression.

The stage-specific function of gap junctions during tumourigenesis

“…A down-regulation of cyclin D1 upon Cx43 transfection was revealed in human breast tumour cells [28] and in normal cell lines [29]. A similar correlation was observed for E9 mouse lung carcinoma and osteosarcoma cells [30].…”

“…A down-regulation of cyclin D1 upon Cx43 transfection was revealed in human breast tumour cells [28] and in normal cell lines [29]. A similar correlation was observed for E9 mouse lung carcinoma and osteosarcoma cells [30]. Mechanistic studies demonstrated that the exogenous expression of Cx43 might exert its inhibitory effect on cell proliferation via the inhibition of the expression of S phase kinase-associated protein 2 (skp2), the protein that promotes the ubiquitination of cyclin-dependent kinase inhibitor p27kip.…”

“…A down-regulation of cyclin D1 in osteosarcoma cells was shown upon Cx43 transfection, which correlated with up-regulation of p27kip [31]. The deletion mutation analyses revealed that the C-terminal domain of Cx43 that did not form gap junctions was still sufficient to inhibit skp2 expression, while coupling inhibitors did not affect the inhibitory effect of Cx43 on skp2 expression, indicating that couplingindependent mechanisms of Cx43 interfere with the cell cycle [30,31]. It was also recently shown that the phosphorylation of Cx43 on S262 by protein kinase C cancels HEK-293 cell growth inhibition by Cx43 independently of the channel-forming ability [32].…”

CCN3 (NOV) Interacts with Connexin43 in C6 Glioma Cells

“…without enhancing GJIC (20). One is therefore driven to search for other mechanisms by which connexins may alter growth.…”

“…Recently, several reports have described that Cx43 plays a role in suppressing tumor growth independent of gap junction formation (19 -22). The C-terminal domain of Cx43 seems to be important in this tumor-inhibiting effect (20). Cx43 may also regulate the cell cycle by acting on the S-phase kinaseassociated protein (21).…”