Lack of functional pRb results in attenuated recovery of mRNA synthesis and increased apoptosis following UV radiation in human breast cancer cells

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Abstract
Lack of functional pRb results in attenuated recovery of mRNA synthesis and increased apoptosis following UV radiation in human breast cancer cells. We have previously demonstrated that a human breast cancer cell line, MDA-MB-468, which lacks the retinoblastoma protein (pRb), is particularly sensitive to low doses of ultraviolet (UV) radiation. These cells are 15–20-fold more sensitive to UV radiation than cells with wild-type pRb. In order to understand the mechanisms of the high apoptotic response of MDA-MB-468 cells to UV radiation, we examined the effects of UV on these cells with regards to both membrane-mediated events and DNA damage. We found that MDA-MB-468 cells were resistant to all ligand-induced death receptor signaling. In addition, although UV activated caspase 8 in MDA-MB-468 cells, a peptide inhibitor of caspase 8 failed to inhibit UV-induced apoptosis. We then tested the possibility that nuclear events mediated the enhanced sensitivity to UV-induced apoptosis in these cells. Unlike UV-resistant cells, MDA-MB-468 cells were unable to recover mRNA synthesis after 5 J/m2 UVC. We also found that the pRb-null DU-145 cells similarly had attenuated recovery of mRNA synthesis after UV radiation. In UV-resistant cells with wild-type pRb, the inactivation of pRb with HPV-16 E7 resulted in significant inhibition in their ability to recover mRNA synthesis and increased levels of apoptosis following UV radiation. Furthermore, pRb-null cells were deficient in repair of UV radiation-induced DNA damage. These data suggest that the sensitivity of MDA-MB-468 cells to UV radiation is due to defects in repair of DNA damage and recovery of mRNA synthesis rather than to membrane death receptor pathways. Inactivation of pRb may contribute to an increased sensitivity to UV radiation by attenuating repair of DNA lesions and recovery of mRNA synthesis following UV radiation.