The oncogenic Ras proteins play critical roles in the development and maintenance of cancer phenotypes and serve as important targets for anticancer treatment. However, oncogenic Ras-targeted therapeutic agents are not yet available. Therefore, it is urgent to develop anticancer agents that can effectively eliminate Ras-mutant cancer cells. We hypothesize that agents that induce synthetic lethality in cancer cells expressing oncogenic Ras genes but not in normal isogenic cells will be valuable prototypes for developing Ras-targeted anticancer therapeutics. In searching for such agents, we screened a chemical library and identified a compound (designated oncrasin-1) that kills immortalized and tumorigenic human ovarian epithelial cells expressing oncogenic K-Ras but not their isogenic normal counterparts. Oncrasin-1 can effectively kill various lung cancer cells with K-Ras mutations. The cytotoxic effects correlated with apoptosis induction by the compounds and could be blocked by K-Ras siRNA or protein kinase C iota (PKCiota) siRNA, suggesting that Ras and/or PKCiota activities are required for oncrasin-induced apoptosis. Treatment of sensitive cancer cells with oncrasin-1, -60, or -231 led to suppression of the phosphorylation of the C-terminal domain (CTD) of the largest subunit of RNA polymerase II, whish is consistent with previous reports that the continuous activity of RNA polymerase II is required to prevent oncogene-induced apoptosis in transformed cells and that mutations compromising CTD function is synthetically lethal with elevated levels of Ras activity in yeast. Treatment with oncrasin-1 also led to co-aggregation of PKCiota and splicing factors in megaspliceosomes and to disruption of the interaction between PKCiota and CDK9/cyclin T1 complex, which phosphorylates the CTD. Thus, we hypothesized that oncrasin compounds are a novel class of CTD inhibitors with selective anticancer activity. The in vivo administration of oncrasin-1 suppressed the growth of human lung tumor xenografts by >70% and prolonged the survival of tumor-bearing nude mice without causing detectable toxicity. Testing some of those analogues on NCI-60 cell lines showed that oncrasins are active against several cell lines derived from lung, colon, breast, ovary, and kidney cancers and oncrasin-60 lies outside the category of adequately studied classes of antitumor agents, indicating its novel anticancer mechanisms. Thus, oncrasin compounds are potentially a novel class of CTD inhibitors that are synthetically lethal to cancers with increased Ras/PKCiota activity. However, in vivo evaluation of the pharmacokinetics, antitumor efficacy, and safety of oncrasin compounds is necessary before they can be evaluated clinically. The goal of this proposal is to determine anti- lung cancer activity of the most active analogues by evaluating their in vitro activities in several lung cancer cell lines with or without Ras gene mutations, their pharmacokinetics, and their in vivo activity and toxicity. The proposed studies will provide solid preclinical data for possible clinical evaluation of oncrasin compounds and may lead to the development of new therapeutic agents that are useful for the treatment of lung cancers.
We have identified a group of new agents that are selectively toxic to cancer cells with increased Ras activity but not to normal cells. The molecular characterization revealed those compounds inhibit the phosphorylation and function of C-terminal domain of the largest subunit of RNA polymerase II. The goal of this proposal is to perform pre-clinical studies on their antitumor activities in lung cancer cells in vitro and in vivo that are required for future clinical trials.
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