Abstract
SummaryThe mechanistic target of rapamycin (mTOR) controls cell growth and enlargement and has been found to be aberrant in a wide variety of malignancies. Although mTOR is already an attractive antineoplastic target, overexpression or aberrant expression of mTOR may also provide an opportunity to further increase the size differential between malignant and normal cells, providing an opportunity to amplify and exploit cell size differences between neoplastic cells and their normal counterparts using physiochemical treatment modalities. Therefore, this study sought to quantify the concentration response and time course effects of rapamycin on cell cycle entry, cell enlargement, and cell proliferation in U937 human monocytic leukemia and human hematopoietic stem cells (hHSCs). In addition, the effects of combination treatment with mTOR inhibitors (rapamycin, everolimus, and temsirolimus) and cytoskeletal-directed agents (cytochalasin B and vincristine) in leukemic cells (U937, THP1, K562, Molt-4, and L1210) were assessed for potential drug synergy. While both U937 cells and hHSCs exhibited a marked reduction in cell volume, U937 cells were able to proliferate in the presence of rapamycin ranging from 0.5 nM to 10 μM (10,000 nM), whereas hHSCs were able to proliferate only at lower concentrations, and were completely inhibited from proliferation by 8 nM rapamycin. These effects were observed with as little as 0.5 nM rapamycin, demonstrating the profound affinity the compound has for FK-binding protein 12 (FKBP12), which subsequently forms the FKBP12/rapamycin complex to inhibit mTOR. Rapamycin continued to exert effects on cell size and proliferation even at 10 μM, without producing marked cytotoxicity. Although cytochalasin B and vincristine were unable to substantially enlarge rapamycin-treated leukemia cells, it appears that rapamycin and its associated analogs everolimus and temsirolimus have notable synergistic potential with microfilament-disrupting cytochalasin B and microtubule-disrupting vincristine as assessed by comparative effects on cell growth, annexin V staining, IC30 isobolograms, and Chou-Talalay statistics. These observations indicate a potentially novel therapeutic rationale for hematological malignancies and for other cancers to elicit the preferential destruction of neoplastic cells that aberrantly express mTOR.
Highlights
The mechanistic target of rapamycin has received considerable attention as a potential target for cancer chemotherapy
The 10 μM rapamycin was removed prior to vincristine administration, but due to the high concentration of rapamycin prior to removal, a residual concentration of 40 nM rapamycin was left in solution, along with the 8 nM vincristine
Vincristine-treated U937 cells were compared to controls that received only rapamycin to determine whether cell size effects of rapamycin could be affected by the microtubule-altering agent in neoplastic or normal cells. 4 nM vincristine and 2 μM cytochalsin B were tested with a 12 h and 48 h exposure, respectively after treatment with 50 nM rapamycin for 24 h
Summary
The mechanistic target of rapamycin (mTOR) has received considerable attention as a potential target for cancer chemotherapy. Aberrant mTOR regulation has been found in a considerable diversity of cancer types, including breast, lung, and pancreatic carcinomas, as well as hematological malignancies [2,3,4,5]. Virtually all research involving the mTOR pathway has been directed toward inhibiting mTOR as a key oncoprotein This is to be expected, as oncogenes and their associated oncoproteins facilitate cancer pathology through increasing growth and proliferation, as well as establishing an immortalized cellular phenotype [6]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
