Gap junctions as targets for cancer chemoprevention and chemotherapy.

Abstract

The development of the most efficacious strategy for the prevention and treatment of cancers is based on understanding the underlying mechanisms of carcinogenesis. This includes the knowledge that the carcinogenic process is a multi-step, multi-mechanism process and that no two cancers are alike, in spite of some apparent universal characteristics, such as their inability to have growth control, to terminally differentiate, to apoptose abnormally and to have an apparent extended or immortalized life span. The multi-step process, involving the "initiation" of a single cell via some irreversible process, with the clonal expansion of this initiated cell by suppressing growth control and inhibiting apoptosis (promotion step), leads to a situation whereby additional genetic and epigenetic events can take place (progression step) to confer the necessary phenotypes of invasiveness, and metasis (neoplastic stage). While it is clear that, in principle, prevention of each of these three steps is possible, in practical terms, while it would make sense to minimize the initiation step, one can never reduce this step to zero. On the other hand, since the promotion step is the rate-limiting step of carcinogenesis, intervening to block this step makes the most sense. Also, by understanding the ultimate biological function that confers growth control, terminal differentiation or apoptosis for cells, there is even some hope of treating some, but not all, malignant cells such that they can regain some ability to perform these vital cellular functions. Gap junctional intercellular communication (GJIC) has been speculated to be a necessary, if not sufficient, biological function of metazoan cells for the regulation of growth control, differentiation and apoptosis of normal progenitor cells. Normal, contact-inhibited fibroblast and epithelial cells have functional GJIC, while most, if not all, tumor cells have dysfunctional homologous or heterologous GJIC. Cancer cells are characterized by the lack of growth control, inability to terminally differentiate or apoptose under normal conditions and have extended or immortalized life spans. Chemical tumor promoters, growth factors and hormones have been shown to inhibit GJIC. Several oncogenes and anti-sense connexin genes have been shown to down-regulate GJIC function. Anti-oncogene drugs, anti-tumor promoting natural and synthetic chemicals, as well as GJIC-deficient neoplastic cells, transfected with various connexin genes, have been shown to re-gain GJIC and growth control with the loss of tumorigenicity. Therefore, the hypothesis for a rational approach to identify anti-tumor promoting chemopreventive drugs and anti-carcinogenic treatments is to use the prevention of the down regulation of GJIC by the tumor promoters and the restoration of GJIC in neoplastic cells. While previous and many current strategies for chemoprevention and therapy have been based on treating specific oncogene products or cell signalling mechanisms, as well as advance molecular modifications of older strategies, none have specifically used the prevention of GJIC by agents during the rate limiting step of carcinogenesis or the restoration of GJIC in neoplastic cells which are deficient in GJIC. Since there are multiple mechanisms by which GJIC is down regulated during the tumor promotion phase and since stable GJIC deficiencies in neoplastic cells can be the result of transcriptional, translational or posttranslational mechanisms, it should be clear there would not be one "golden bullet" approach to resolve either the chemoprevention or therapeutic approach. Even with the hypothesis that GJIC, which depends on the transcription of normal connexin genes, their normal translation, trafficking, assembly and function, it should be clear that cells with normal connexin genes and potentially normal GJIC might not have functional GJIC because of dysfunction of other defects in cancer cells, namely cell-adhesion or cell-matrix problems (both of which are necessary for GJIC to occur). In essence, if dietary or chemopreventive/therapy is to be effective, the strategy must either ameliorate the growth stimulatory effects of exogenous chemicals, growth factors or hormones, that trigger various mitogenic/anti-apoptotic signal transducing systems that inhibit GJIC.