Attempts were made to measure at least 3,000 cells per sample; however in the samples treated with Onc the cells undergoing apoptosis were detached and fewer cells remained per slide for the analysis. While transfection of cells with GAPDH siRNA reduced expression of this protein by nearly 70%, the expression was restored in the cells exposed to 0.8 M Onc for 48 or 72 h. The data thus Rifapentine (Priftin) provide evidence that one of the targets of Onc is usually siRNA, likely within the RNA-induced silencing complex (RISC). In light of the findings that microRNAs are involved in tumor pathogenesis as well as in enhancing cell resistance to anticancer therapy the present data may provide explanation for both, the antitumor Onc activity and its propensity to enhance effectiveness of cytotoxic drugs. Keywords:microRNAs, ranpirnase, gene regulation, RISC, glyceraldehyde 3-phosphate dehydrogenase, laser scanning cytometry, mesothelioma Onconase (Onc), generic name ranpirnase is usually a basic protein of 12,000 MW isolated from oocytes or early embryos of Northern Leopard frog (Rana pipienss) that shows poor ribonucleolytic activity and belongs to pancreatic RNase Rifapentine (Priftin) A superfamily (reviewed in refs.1-5). Onc is usually cytostatic and cytotoxic to a variety of tumor cell lines and enhances sensitivity of cells to antitumor modalities with a diverse mechanism of action.6-12Onc also inhibits growth of tumors in the in vivo animal models.13,14In clinical trials for unresectable malignant mesothelioma Onc was shown to display antitumor activity5,15,16particularly in the cases that failed a prior chemotherapy regimen.17Such patients when treated with Onc plus doxorubicin experienced a significant (p = 0.016) prolongation of the median survival time compared to the patients treated with doxorubicin alone.17 The cytostatic effects of Onc were shown to be mediated by downregulation of cyclin D3, induction of p16INK4A, p21WAF1/CIP1and p27KIPand decreased pRb phosphorylation resulting in cell arrest in G1.18The cytotoxicity manifests after a prolonged (2448 h) cells exposure to Onc in form of classical apoptosis with characteristic changes in cell morphology, fragmentation of DNA,6activation of caspases, serine proteases and transglutaminase.19-21 The target of Onc is intracellular RNA; Onc chemically altered to extinguish its enzymatic ribonuclease activity is usually inactive.10,21,22The ribonucleolytic activity of Onc within the cell is sustained by its lack of sensitivity to ribonuclease inhibitor protein and exceptional conformational stability.23It is unclear however, which RNA species within the cell are Onc targets whose destruction may explain why Onc exerts its cytostatic and cytotoxic effects that are preferential to tumor cells and sensitizes cells to diverse antitumor modalities.7-12It has been observed that degradation of rRNA in Onc-treated cells coincided with inhibition of protein synthesis which Rifapentine (Priftin) Rifapentine (Priftin) led the authors to propose that rRNA is the preferential target and that suppression of translation is the primary cause of Onc cytostatic and cytotoxic activity.10Degradation of tRNA was subsequently postulated as the mechanism explaining cytotoxic activity of Onc.24Several findings, however, are incompatible with this mechanism. Specifically: (i) Onc induces an increase rather than decrease in expression of some proteins such as inhibitors of CDKs.18(ii) Recent analysis of cDNA array data revealed that Onc induces transcription of over 50 genes some related to cell cycle progression and apoptosis.3(iii) The nonspecific inhibition of translation such as caused by cycloheximide triggers entirely different pattern of cell response in terms of cell cycle arrest25and induction of apoptosis.26(iv) Normal cells having uncorrupted G1restriction point25are more sensitive to inhibition of translation rather than tumor cells that often have the restriction point compromised.27Yet Onc is more effective to tumor than to normal cells. The discrepancy between the observed effects of Onc and the proposed mechanism of its action based on inhibition of translation10,24prompted us to Rabbit Polyclonal to Transglutaminase 2 postulate that Rifapentine (Priftin) one of the Onc targets is the non-coding RNA (microRNA) involved in regulation of gene expression through RNA interference (RNAi).28Targeting RNAi, as will be discussed further, may explain both the preferential effectiveness of Onc towards tumor cells as well as its ability to sensitize cells to other antitumor agents. In the present study, therefore, we explored whether the gene-silencing properties of small interfering RNA (siRNA) can be affected by Onc. The data show that silencing of the abundant, ubiquitously expressed housekeeping gene glyceraldehyde 3-phosphate dehydrogenase (GAPDH) by siRNA.
