High incidence of spontaneous and chemically induced liver tumors in mice deficient for connexin32

Abstract

Connexins are subunits of gap junction channels, which mediate the direct transfer of ions, second messenger molecules and other metabolites between contacting cells. Gap junctions are thought to be involved in tissue homeostasis, embryonic development and the control of cell proliferation [1,2]. It has also been suggested that the loss of intercellular communication via gap junctions may contribute to multistage carcinogenesis [3-5]. We have previously shown that transgenic mice that lack connexin32 (Cx32), the major gap junction protein expressed in hepatocytes, express lower levels of a second hepatic gap junction protein, Cx26, suggesting that Cx32 has a stabilizing effect on Cx26 [6]. Here, we report that male and female one-year-old mice deficient for Cx32 had 25-fold more and 8-fold more spontaneous liver tumors than wild-type mice, respectively. Incorporation of bromodeoxyuridine (BrdU) into the liver was higher for Cx32-deficient mice than for wild-type mice, suggesting that their hepatocyte proliferation rate was higher. Furthermore, intraperitoneal injection, two weeks after birth, of the carcinogen diethylnitrosamine (DEN) led, after one year, both to more liver tumors in Cx32-deficient mice than in controls, and to accelerated tumor growth. Loss of Cx32 protein from hepatic gap junctions is therefore likely to cause enhanced clonal survival and expansion of mutated ('initiated') cells, which results in a higher susceptibility to hepatic tumors. Our results demonstrate that functional gap junctions inhibit the development of spontaneous and chemically induced tumors in mouse liver.

Effect of the tumor promoter phenobarbital on the pattern of global gene expression in liver of connexin32‐wild‐type and connexin32‐deficient mice

“…It is noteworthy, however, that the incidence of liver tumors is much higher in male as compared to female mice demonstrating the involvement of sex hormones in liver tumor development. 39 Even though the long-term effects of Cx32-deficiency on chemically induced hepatocarcinogenesis in mouse liver are rather dramatic, [5][6][7] we did not observe a concomitant strong change in the expression of a particular gene that could explain the strainspecific differential behavior. A comparable microarray analysis carried out with a conditional Cx26-deficient mouse also showed only very few genes that were moderately altered in expression in liver of the gene knockout mice (T.O., unpublished observation).…”

“…5,25 Because there is no concomitant increase in liver weight in Cx32ko mice, hepatocyte death rates must be also upregulated, resulting in tissue homeostasis at an elevated level of cell turnover. In our present study, only 5 genes, including Cx32, were significantly altered in Cx32ko as compared to Cx32wt mice under the criteria used.…”

Putting tumours in context

“…Much remains to be learned about how the specific combination of connexins facilitates tissue interactions, but it is clear, again, that generalizations should be avoided, as the expression patterns (and probably the function) of connexins are tissue dependent and change during tumour progression 17,18 . For example, some breast cancer cells upregulate connexin 32 (Cx32) 19 , but loss of Cx32 contributes to hepatocellular carcinoma 20,21 ; Cx43 inhibits tumorigenicity of lung, cervical and bladder carcinoma cells [22][23][24] , but has no effect on squamous cell carcinomas 25 ; and other connexins can facilitate cell adhesion during metastasis 26 .…”

The relationship between aging and carcinogenesis: a critical appraisal

“…No significant differences were found in the mutation spectra and the mutation incidence between p53 (/( and p53 '/' mice [239,240], whereas the incidence of spontaneous tumors in p53 (/( mice were increased as compared with wild-type control [241,242]. Gap junction-deficient mice (Cx32 (/( ) have an extremely increased susceptibility to spontaneous and chemically induced carcinogencsis [226]. Mice with a defect in the xeroderma pigmentosum group A (XPA ) gene have a complete deficiency in nucleotide excision repair and have a more than 1000-fold higher risk to developing UV-induced skin cancer as well as increased susceptibility of internal organs to mutagenesis and development of cancer after exposure to chemical carcinogens [217,243].…”