| The underlying causes for different apoptotic responses in neoplastic cells are still not fully understood. The retinoblastoma protein (pRb) is known to be a critical regulator of cell cycle progression and has been implicated in the regulation of apoptosis. The work presented here demonstrates that in human breast cancer (HBC) cells, pRb regulates UV radiation-induced apoptosis, potentially through DNA repair. Initial experiments determined that MDA-MB-468 HBC cells, which lack the RB gene, were highly sensitive to UV radiation as measured by both apoptotic fraction and clonogenicity. Similarly, DU-145 cells, which possess a mutant, nonfunctional form of pRb, were also extremely sensitive to UV radiation. In addition, use of HPV-16 E7 to inactivate pRb in normal human mammary epithelial cells and in HBC cells with wild-type pRb (SUM-102PT cells) increased the levels of UV radiation-induced apoptosis. These data demonstrate a relationship between sensitivity to UV radiation and lack of pRb.; This work also evaluated the mechanisms behind the UV radiation-induced apoptosis in the MDA-MB-468 cells. The sensitivity of MDA-MB-468 cells to UV radiation was not due to alterations in membrane signaling, but rather to the impaired ability of these pRb-null cells to both recover mRNA synthesis and repair the UV radiation-induced lesions. This information suggests that pRb may play a role in the repair of UV radiation-induced DNA lesions, thus resulting in increased sensitivity to UV radiation in a pRb-null setting.; Since UV radiation- and cisplatin-induced lesions are both repaired by the nucleotide excision repair pathway, the responses of MDA-MB-468 and SUM-102PT cells to cisplatin and various other chemotoxic agents were evaluated. The pRb-null MDA-MB-468 cells were extremely sensitive to cisplatin, correlating with their extreme sensitivity to UV radiation. This further supports the hypothesis that pRb has a role in DNA repair. Taken together, this work increases understanding of pRb as a regulator of apoptosis, and suggests that pRb may have a previously unknown role in DNA repair after UV radiation. |
