Cancer research
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The major mechanism of action of STI571 is a competitive interference with the ATP-binding site of the Bcr/Abl tyrosine kinase. In the BCR/ABL-positive cell line KBM5, we studied cellular events associated with the in vitro acquisition of resistance to STI571. The emergence of the STI571-resistant phenotype was accompanied by only a marginal increase in the number of copies of the BCR/ABL gene and its level of expression. ⋯ The contribution of this mutation to STI571-resistant phenotype remains unknown. Preliminary data showing partial reversibility of resistance in these cells suggest that resistance may be multifactorial. No other mutations were identified in the kinase domain of the BCR/ABL gene.
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Using human acute leukemia HL-60/Bcr-Abl (with ectopic expression of p185 Bcr-Abl) and K562 cells (with endogenous expression of p210 Bcr-Abl) subjected to a continuous selection pressure of up to 1.0 micro M Gleevec (imatinib mesylate, STI-571), we have isolated Gleevec-resistant K562 R (+Bcr-Abl), K562 R (-Bcr-Abl), and HL-60/Bcr-Abl R cells, which display disparate level and activity of Bcr-Abl tyrosine kinase (TK). As compared with their sensitive counterparts, Gleevec-resistant cell types were >/=5-fold resistant to Gleevec-induced apoptosis. Bcr-Abl protein levels were significantly increased in HL-60/Bcr-Abl R and K562 R (+Bcr-Abl) cells, but K562 R (-Bcr-Abl) cells showed a marked decline in the mRNA and protein levels and activity of Bcr-Abl. ⋯ Treatment with 17-AAG or PD180970 also induced apoptosis of CD34+ leukemic cells from three patients with chronic myeloid leukemia in blast crisis who had progressive leukemia while receiving Gleevec therapy. Taken together, these findings indicate that in addition to overexpression or mutations in Bcr-Abl, resistance to Gleevec may also develop due to a loss of Bcr-Abl expression. These findings also support the rationale to test the in vivo efficacy of 17-AAG and PD180970 against STI-571-resistant Bcr-Abl-positive acute leukemias.
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Many antigens associated with human tumors are overexpressed in tumor cells as compared with normal tissues; these "self" tumor-associated antigens are also expressed during fetal development, and it is, thus, not surprising that they are either weakly immunogenic or functionally nonimmunogenic in the tumor-bearing host. In the studies reported here, we have used different vaccines and vaccine strategies in an attempt to develop antitumor immunity in a stringent animal model. The tumor antigen used was human carcinoembryonic antigen (CEA). ⋯ The results demonstrate that (a) continued boosting with vaccine is required to maintain CEA-specific T-cell responses, and boosting with rF-CEA/TRICOM is superior to boosting with rF-CEA; (b) a diversified vaccination protocol consisting of primary vaccination with rV-CEA/TRICOM followed by boosting with rF-CEA/TRICOM is more efficacious than homogeneous vaccination with rF-CEA/TRICOM in the induction of both CEA-specific T-cell responses and antitumor activity; and (c) the use of cytokines, local granulocyte macrophage colony-stimulating factor (GM-CSF) and low-dose systemic interleukin 2, in combination with vaccine is essential in inducing antitumor activity, as compared with the use of cytokines alone, or the use of vaccines without cytokine. Both GM-CSF and interleukin 2 were shown to contribute to the induction of CEA-specific T-cell responses. These studies thus provide a "proof of concept" that potent vaccines and vaccine strategies, in combination with cytokines, may be essential to obtain the level of T-cell responses directed against a self-antigen that is necessary to achieve antitumor responses.
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Hypoxia-inducible factor 1 (HIF-1), a heterodimer of HIF-1alpha and HIF-1beta subunits, is a transcriptional activator central to the cellular response to low oxygen that includes metabolic adaptation, angiogenesis, metastasis, and inhibited apoptosis. Thioredoxin-1 (Trx-1) is a small redox protein overexpressed in a number of human primary tumors. We have examined the effects of Trx-1 on HIF activity and the activation of downstream genes. ⋯ Transfection of a redox-inactive Trx-1 (C32S/C35S) markedly decreased levels of HIF-1alpha protein, HIF-1 transactivating activity, and VEGF protein in MCF-7 cells compared with empty vector controls. In vivo studies using WEHI7.2 cells transfected with Trx-1 showed significantly increased tumor VEGF and angiogenesis. The results suggest that Trx-1 increases HIF-1alpha protein levels in cancer cells and increases VEGF production and tumor angiogenesis.
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The c-kit tyrosine kinase inhibitor STI571 exhibits a substantial therapeutic activity in patients with chronic myeloid leukemia and gastrointestinal stromal tumors respectively associated with constitutive activation of the BCR-ABL and c-kit tyrosine kinases. Human colorectal tumors also express the c-kit proto-oncogene. The present study focuses on the anticancer activity of STI571 in human colorectal tumor cells in vitro and in vivo. ⋯ These cellular effects were associated with a decrease in tumor growth. We also demonstrated that stem cell factor is a proangiogenic factor in vivo and in vitro. These encouraging results warrant further preclinical investigations and clinical trials on the use of the c-kit inhibitor STI571 as a chemotherapeutic agent in colon cancer prevention and in treatment of advanced colorectal cancers associated with liver metastases.