Transgenic Disruption of Gap Junctional Intercellular Communication Enhances Early but Not Late Stage Hepatocarcinogenesis in the Rat

Abstract

Much experimental evidence supports the conclusion that loss of gap junctional intercellular communication (GJIC) contributes to carcinogenesis. Transgenic rats featuring a dominant negative mutant of the connexin 32 gene under albumin promoter control (Cx32 Tg-High and Cx32 Tg-Low lines, respectively with high and low copy numbers of the transgene) have disrupted GJIC, as demonstrated by scrape dye-transfer assay in vivo as previous report by Asamoto et al. (2004). In the present study, we investigated the susceptibility of these transgenic rats to a single intraperitoneal administration of diethylnitrosamine (DEN), and found a significant increase in preneoplastic glutathione S-transferase placental form (GST-P) positive lesions in the livers of Cx32 Tg-High but not Cx32 Tg-Low rats. However, incidences of adenomas and hepatocellular carcinomas were not elevated at the end of the experiment (52 weeks). In addition, we investigated the promotional effect of phenobarbital (PB) on Cx32 Tg-High rats pretreated with DEN and found enhanced formation of GST-P positive lesions, in contrast to the lack of promoting effects reported for Cx32 deficient mice. The results indicate that although both high and low expression of the dominant negative connexin 32 mutant gene in our rats is able to inhibit gap junctional capacity, only high expression is effective at enhancing susceptibility to early stage DEN-induced liver carcinogenesis.

Hepatic gap junctions in the hepatocarcinogen-resistant DRH rat

“…Genetic ablation of Cx32 and overexpression of wildtype or abnormal Cx32 proteins have been used to study hepatocarcinogenesis in rodents or metastasis in the human carcinoma after treatment with the following the tumor initiating carcinogen, N-nitrosodiethylamine, with or without the tumor promoters, phenobarbital or Wy-14,643 (Temme et al 1997;Moennikes et al 1999Moennikes et al , 2000Moennikes et al , 2003Dagli et al 2004;Hokaiwado et al 2005); X-ray irradiation (King and Lampe 2004b); or without any carcinogen (Temme et al 1997;Li et al 2007). Although it is generally accepted that Cx proteins are involved in carcinogenesis, it remains controversial as to exactly how they relate to cancer development and even whether they inhibit or promote cancer development (Moennikes et al 2000(Moennikes et al , 2003Hokaiwado et al 2005;Ott et al 2006;Conklin et al 2007;Li et al 2007).…”

“…Although it is generally accepted that Cx proteins are involved in carcinogenesis, it remains controversial as to exactly how they relate to cancer development and even whether they inhibit or promote cancer development (Moennikes et al 2000(Moennikes et al , 2003Hokaiwado et al 2005;Ott et al 2006;Conklin et al 2007;Li et al 2007). These apparently conXicting hypotheses may be attributed to the fact that the function of Cx proteins during carcinogenesis depend on the activity of functional gap junctions or on the localization of Cxs relative to the membrane or cytoplasm.…”

“…Indeed, expression of some Cx proteins was shown to either increase or remain unaltered (Sakamoto et al 1992;Neveu et al 1994) in pre-neoplastic or transformed tumor cells (Krutovskikh et al 1994;Yamasaki et al 1995). In addition, transgenic disruption of the gap junction function enhanced the early, but not the late, stages of hepatocarcinogenesis (Hokaiwado et al 2005). …”

Changes in cell morphology affect the quantification of intercellular communication

“…Among the systems mediating cell-cell interaction, GJIC is unique in the sense that cells directly transfer small molecules (<1000 Da) from one cell to its neighbor. Several studies demonstrated that GJIC disruption contribute to carcinogenic process (Hokaiwado et al, 2005;Yi et al, 2006). The first data of down-regulated GJIC in cancer were reported by Loewenstein and Kano (1966) and further studies in several types of cancer confirmed that malignant cells show aberrant GJIC (Yang et al, 2003;Pu et al, 2004;Frank et al, 2006).…”

Limited Effects of Disrupted Gap Junctions in the Liver on Multi-organ Carcinogenesis Induced by Diisopropanolnitrosamine in the Mutant Connexin 32 Transgenic Rat

“…In the liver of this transgenic rat, membrane localization of normal endogenous Cx32 protein is disturbed and gap junction capacity measured by the scrape dye-transfer assay in vivo was markedly decreased compared to the wild-type case. We have found that these GJIC-decreased rats are highly susceptible to induction of preneoplastic foci in the liver by a single injection of DEN 21 .…”