bcl-2: Role in epithelial differentiation and oncogenesis

The precise regulation and maintenance of balance between cell proliferation and cell death in multicellular organisms is critical for tissue homeostasis. bcl-2 initiates a new gene family involved in the regulation of cell death and survival without affecting cell proliferation. Expression of Bcl-2 has been reported in a wide range of hematopoietic cells, nonneoplastic epithelia (both hormone-responsive and nonresponsive), and epithelial malignancies. Although the major group of epithelial cells expressing Bcl-2 protein are in the proliferating zones, expression is not directly related to cell proliferation. Bcl-2 is also associated with stem cells committed to differentiation and morphogenesis. The survival advantage provided by Bcl-2 prolongs the life span of epithelial cells with differentiation potential and allows proliferation, differentiation, and morphogenesis to proceed. The gene expression in hormone-responsive organs may contribute to the sustained life of those terminally differentiated epithelial cells and a decrease in Bcl-2 levels leads to cell death by apoptosis. Overexpression of bcl-2 protects epithelial cells from death, but it is neither able to immortalize normal cells, nor to cause tumorigenic transformation of immortalized epithelial cells. Heterogeneous expression of Bcl-2 in epithelial malignancies suggests that the gene is differentially regulated. Furthermore, its expression in association with precancerous lesions suggests a role in the early stage of tumorigenesis. The effects of Bcl-2 expression on sensitivity of epithelial cells to drug, radiation, and hormone therapies vary depending on the type of tumor. Expression of Bcl-2 is associated with resistance to hormone therapy and recurrence in prostate carcinomas, whereas in lung and breast carcinomas it is associated with a better prognosis. Studies now being performed should clarify the underlying mechanisms of differential gene regulation in different tissues and show the clinical significance of the expression of bcl-2 and other members of the bcl-2 gene family.