Treatment of human cell lines with 5-azacytidine may result in profound alterations in clonogenicity and growth rate

The cellular basis of tumor progression

Variability in disease presentation and course is a hallmark of cancer. Variability is seen among similarly diagnosed cancers in different patients or animal hosts and in the same cancer at different periods of time. This latter type of variability, termed "tumor progression," was defined by Foulds in a series of six rules that describe the independent behavior of individual cancers and the independent evolution of different cancer characteristics. Tumor progression is believed to result from variability among subpopulations of tumor cells within individual cancers and from selection of these subpopulations by conditions within the cancer environment, such that different subpopulations come to prominence over the course of cancer development and growth. Interactions among subpopulations, however, modulate tumor behavior as well as tumor evolution. The leading hypothesis for the origin of tumor subpopulations is the genetic instability of cancer cells. There are a number of possible mechanisms of genetic instability, some internal to cancer cells (mutation, amplification, mutator phenotypes, DNA repair deficiencies) and some present in the tumor microenvironment (endogenous mutagens). There are also potential epigenetic mechanisms of variability, including alterations in gene regulation, differentiation, adaptation, and cell fusion. Regardless of mechanism, the heterogeneity of tumor subpopulations poses a number of challenges to the practice of cancer research, including the design of reproducible and meaningful experiments. Tumor heterogeneity also has significant consequences for the clinical assessment of tumor prognosis and the development of effective treatment regimens.

Influence of 5-azacytidine on differentiation and growth in rat nervous system tumor cell lines

In vitro studies with permanent cell lines derived from ENU-induced rat CNS and PNS tumors have shown that substances acting on second messenger systems as well as glucocorticoids have a cytostatic effect at high cell densities, but enhance proliferation at low densities. The nontoxic cytostatic effects are partially accompanied by the induction of differentiation monitored by immunocytochemistry with antibodies against GFAP, S-100 and PKC (alpha,beta). Cell differentiation with or without the influence of the drugs applied could be highly facilitated in some (CNS), but not all (PNS) types of tumor cells from neuroectodermal origin by pretreatment of cultures with 5-azacytidine under demethylating noncytostatic conditions. The implications of our results are relevant for molecular neurobiology and might offer new therapeutical approaches and may provide models for neurotoxicology.

Can cancer chemotherapy enhance the malignant behaviour of tumours?

Cancer chemotherapy is currently undergoing an intensive reappraisal because of its unimpressive performance against the major common cancers. There are a number of possible reasons for this lack of success; one considered here is that under some circumstances anti-neoplastic drug treatment actually increases the malignant behaviour of tumours. Support for this idea comes mainly from experimental studies in which drug treatments increased metastatic spread. Investigation of this phenomenon shows that drug induced modifications of the host, including immunosuppression and vascular damage, can indeed facilitate metastasis. In addition, new data are presented demonstrating that the direct action of drugs on the tumour cells themselves can have similar enhancing effects. The possible mechanisms underlying such direct effects are discussed and the ability of anti-cancer drugs to cause genetic mutations, amplify genes, and alter gene expression are considered. While the nature and extent of this facilitation of tumour malignancy is not fully understood, it is suggested that this possibility should be considered in the design of treatment protocols.

Is your initiator really necessary?

There is little hard evidence for the involvement of specific genotoxic initiators in the pathogenesis of the common carcinoma. Recent findings suggest that sporadic carcinogenesis is a dynamic and probabilistic process which requires a critical mass of abnormal cells for its expression, and that this requirement may distinguish the evolution of carcinomas from that of paediatric or haematologic malignancies. The proposal that specific carcinogens are neither necessary nor sufficient for tumourigenesis is consistent with the growing realization that aberrant expression of specific oncogenes is neither necessary nor sufficient for cellular transformation. These new perspectives have major implications for basic research strategy and public health policy.