Epidemiological studies suggest that both genetic and environmental factors influence the incidence of sporadic breast cancer in women in different geographic areas and of different ethnicities. However, positive identification of these factors and determination of their functional contributions to cancer etiology have proven difficult. Carcinogenesis involves the sequential acquisition of a complex set of phenotypes over extended time periods. The ability to assay the effects of suspect environmental and genetic factors on presumptive target cells using surrogate endpoints corresponding to the acquired phenotypes aids in the positive identification of such factors. Determining causality is best performed in cultured cells, where complicating variables can be monitored and controlled, and cells can be monitored in real time. Although such culture systems are primitive in comparison to intact tissue, they can be used to model certain aspects of tumor progression. To define molecular events and cellular characteristics that allow tumorigenic progression to proceed in the human breast, others and we have used human mammary epithelial cell (HMEC) cultures derived from surgically discarded phenotypically normal tissue. A notable difference between normal and tumor breast tissue is the ability of the latter to give rise to cells that can proliferate indefinitely in culture. Such “immortality” is thought to be a key predisposing factor for carcinogenesis because it allows the progeny of a single cell to sequentially accumulate the multiple errors needed to gain invasive and metastatic properties. Studies of HMEC and other cell types have led to the hypothesis that human cells have developed extremely stringent mechanisms to prevent immortality, and that malignant precursors must evade this protective gauntlet to give rise to frank malignancies. The p16 tumor suppressor protein is one “sentinel” that responds to potentially oncogenic microenvironmental stresses by causing growth arrest. In rare cases, however, the gene that encodes p16 can be shut down permanently or mutated, allowing HMEC to grow for extended periods and acquire potentially malignant changes. We have assessed the ability of radiation – a known breast carcinogen, to affect evasion of p16-mediated growth arrest by cultured HMEC, and have found that moderate doses of radiation can increase this pre-malignant aberration. This is one example of the use of a simple experimental system employing cultured HMEC to test a potentially carcinogenic function of genetic and environmental factors.
Supported by the UCSF Breast Cancer and the Environment Research Center and a NASA Specialized Center of Research (NSCOR).