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Shyam Patel

Breast Cancer is the most common female malignancy. In women, it is the second most common cause of cancer related death, and the first cause of mortality in women 40-59 years old. It is estimated that 211,240 American women will be diagnosed with breast cancer in 2005, and 40,410 women will die as a result of this disease. Although a number of effective treatments have been developed, treatment of metastatic disease is only palliative. Development of more efficacious treatments against breast cancer is greatly needed, and depends on unraveling the intricacies of the biological pathways involved in cancer progression and in chemotherapy resistance.

Breast cancer, like other types of cancer, arise from the accumulation of genetic changes that allow cells to bypass the homeostatic mechanisms that govern normal cell growth and death regulation. Deviation from the norm includes the induction of proliferative stimuli and the simultaneous suppression of compensatory cell death. Programmed Cell Death is a “suicide” mechanism, which under normal conditions is essential for an organism’s biological development and homeostasis, through removal of cells. For example, the formation of fingers and toes in humans is through the apoptotic removal of tissue between them during embryonic development. It also functions to remove cells that may pose a threat to the organism’s survival, for example, cells that have DNA damage, have been infected, or have a proliferation problem.  It is called “programmed” cell death because it is highly regulated and choreographed by the cell's own machinery in its objective to dismantle a cell.

Two types of programmed cell death known are apoptosis and autophagy. Apoptosis (type I programmed cell death) is an ordered cell death process that involves dismantling cells into membrane-enclosed vesicles, which are engulfed by phagocytes, thereby preventing the release of intracellular components from dying cells. Proper control of this process is required for normal development and disease prevention. Apoptotic cell death is an essential and preferred method for the removal of cells during development. 

            Autophagy (type II programmed cell death) is an evolutionary conserved catabolic lysosomal pathway that results in degradation of long-lived proteins and organelles. Cells undergo this process when faced with insufficient supply of nutrients in the environment and begin to breakdown their internal organelles for the nutrition. Autophagy may help promote cell survival, either by degradation of damaged organelles and toxic metabolites, or by the degradation of “self” proteins, resulting in production of amino acids, to provide an alternative energy source during starvation, and following growth factor withdrawal. However, constitutive activation of autophagy in response to stress,  can lead to excessive self-digestion and degradation of essential cellular components, leading to cell death. Thus, the recognition of autophagy as a mechanism of cell survival and death. Defects in autophagy promote breast cancer: mutations in the essential autophagy gene beclin1 are found in 40-75% of breast cancers, however, how this contributes to cancer development, progression, and treatment response is not known. Furthermore, the roles of disabled apoptosis and defective autophagy in breast cancer and chemotherapy responsiveness are not well established.

            The role of beclin1-haploinsufficiency in mammary tumorigenesis is being assessed through the immortalization of mouse mammary epithelial cells (MMECs) from beclin1+/- mice. Autophagic and apoptotic levels were determined by subjecting bim-/-, bim-/-beclin1+/-, beclin1+/- Bcl-2 overexpressing immortalized mouse mammary epithelial cells (iMMECs) to ischemic stress, nutrient deprivation, for a period of time. From our viability studies, where we exposed beclin wild type, beclin +/-, beclin wild type overexpressing Bcl-2, and beclin+/- overexpressing Bcl-2 cells to 5 days of ischemia, we ascertained that those cells with an overexpression of the anti-apoptotic protein Bcl-2, saw an increase in relative survivability against ischemic conditions. Those cell lines with an overexpression of Bcl-2, which hinders apoptotic cell death, allowed the mechanism of autophagic cell death to take over, which allowed those cells to live longer than cells that did not overexpress Bcl-2. Thus, showing that autophagy can serve the role of a survival mechanism. In the future, to monitor the level of autophagy under ischemic conditions, we will need to expose cells for immunofluorescence for eGFP-LC3 translocation.  Also to determine the in vivo tumorigenic potential of apoptosis and/or autophagy defective iMMECs, we need to assay for chemotherapy responsiveness with targeted therapeutics, which can manipulate apoptosis and/or autophagy, either to restore apoptosis or through an siRNA knockdown of Bcl-2 , to improve chemotherapeutic responsiveness to develop more efficacious breast cancer treatments.   

References:

Degenhardt, K., Chen, G., Lindsten, T., and White, E. (2002a). Bax and Bak mediate p53-independent suppression of tumorigenesis. Cancer Cell 2,193-203.

Tan, T. T., Degenhardt, K., Nelson, D. A., Beaudoin, B., Nieves-Neira, W., Bouillet, P.,Villunger, A., Adams, J. M., and White, E., (2005). Key roles of BIM-driven apoptosis in epithelial tumors and rational chemotherapy. Cancer Cell 7, 227-238.

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