Oncogene
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Drug resistance that occurs during cancer chemotherapy has been a major problem in controlling neoplastic progression. To study the cellular mechanisms of acquired drug resistance we developed 1,25-dihydroxyvitamin D3 (1,25D3)-resistant sublines of promyelocytic leukemia HL60 cells which have increased proliferation rates (Exp. Cell Res., 224, 312, 1996; Cancer Res., 50, 5513, 1996). ⋯ Interestingly, the resistant cell lines constitutively express high levels of retinoblastoma protein (pRb), and pRb is highly phosphorylated, indicating that the G1 cyclin/Cdk complexes in the resistant cells are physiologically active. The results suggest that the increased activity of cyclin D/Cdk6, and perhaps cyclin E/Cdk2, lead to rapid hyperphosphorylation of pRb and consequently a shorter early G1 phase, and that in the resistant cells the increased ratio of cyclin E to p27Kip1 results in activation of Cdk2 and contributes to the abrogation of the 1,25D3-induced block to the S phase entry. Additionally, it is apparent that constitutively increased levels of pRb are compatible with increased rates of cell proliferation.
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Loss of attachment to an extracellular matrix substrate arrests the growth of untransformed cells in the G1 phase. This anchorage-dependent cell cycle arrest is linked to increased expression of the p21Cip1 (p21) and p27Kip1 (p27) cyclin-dependent kinase inhibitors. The result is a loss of cdk2-associated kinase activity, especially that of cyclin E-cdk2. ⋯ The cytoplasmic p21 and p27 were bound to cyclin E-cdk2, as well as to complexes containing cyclin A and cyclin D. The nuclear cyclin E-cdk2 complexes from the transformed cells grown in suspension contained only low levels of p21 and p27 and had histone H1 kinase activity. Thus, at least three mechanisms contribute to keeping cyclin E-cdk2 complexes active in suspended anchorage-independent cells: cyclin E and cdk2 are upregulated, as reported previously, cdk inhibitors are sequestered away from the nucleus by cytoplasmic cyclin-cdk complexes, and the binding of the inhibitors to nuclear cyclin E-cdk2 complexes is impaired.
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To investigate the effects of the expression of Bcl-2 protein in bladder cancer on the apoptosis induced by cisplatin or adenoviral-mediated p53 gene (Ad5CMV-p53) transfer, we transfected the bcl-2 gene into KoTCC-1, a human bladder cancer cell line that does not express the Bcl-2 protein. The Bcl-2-transfected KoTCC-1 (KoTCC-1/B) exhibited significantly higher resistance to both cisplatin and Ad5CMV-p53 transfer than did either the parental KoTCC-1 (KoTCC-1/P) or the vector-only transfected cell line (KoTCC-1/C). The flow cytometric analysis of the propidium iodide-stained nuclei and DNA fragmentation analysis after cisplatin or Ad5CMV-p53 treatment revealed DNA degradation in both KoTCC-1/P and KoTCC-1/C, whereas KoTCC1/B showed a marked inhibition of DNA degradation. ⋯ Furthermore, the cisplatin and Ad5CMV-p53 treatments each reduced the volume of the subcutaneous tumors established in nude mice formed by KoTCC-1/P or KoTCC-1/C; in contrast, their reductive effects on the tumors formed by KoTCC-1/B were significantly suppressed. The intraperitoneal tumor cell implantation model revealed that the prognoses of mice injected with KoTCC-1/B were significantly inferior to those of the mice injected with either KoTCC-1/P or KoTCC-1/C after treatment with cisplatin or Ad5CMV-p53. These findings suggest that the expression of Bcl-2 in bladder cancer cells interferes with the therapeutic effects of cisplatin and Ad5CMV-p53 through the inhibition of the apoptotic pathway.
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A high prevalence of RET rearrangements is found in papillary thyroid carcinomas (PTC) of children from Belarus after the Chernobyl reactor accident. The ELE/RET rearrangement (PTC3) is prevailing. ⋯ Sequencing of the breakpoint regions at the genomic DNA level revealed DNA modifications that might be relevant for illegitimate recombination after DNA doublestrand breaks. The high prevalence of ELE/RET rearrangements and various subtypes appears to be typical for radiation-induced thyroid carcinomas of children after the Chernobyl reactor accident.
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Comparative Study
The effect of HER-2/neu overexpression on chemotherapeutic drug sensitivity in human breast and ovarian cancer cells.
Recent studies indicate that oncogenes may be involved in determining the sensitivity of human cancers to chemotherapeutic agents. To define the effect of HER-2/neu oncogene overexpression on sensitivity to chemotherapeutic drugs, a full-length, human HER-2/neu cDNA was introduced into human breast and ovarian cancer cells. In vitro dose-response curves following exposure to 7 different classes of chemotherapeutic agents were compared for HER-2- and control-transfected cells. ⋯ In vivo, HER-2/neu-overexpressing breast and ovarian cancer xenografts were responsive to different classes of chemotherapeutic drugs compared to control-treated xenografts with no statistically significant differences between HER-2/neu-overexpressing and nonoverexpressing xenografts. We found no instance in which HER-2/neu-overexpressing xenografts were rendered more sensitive to chemotherapeutic drugs in vivo. HER-2/neu-overexpressing xenografts consistently exhibited more rapid regrowth than control xenografts following initial response to chemotherapy suggesting that a high rate of tumor cell proliferation rather than intrinsic drug resistance may be responsible for the adverse prognosis associated with HER-2/neu overexpression in human cancers.