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Phosphorous Metabolite and Cell Cycle Kinetic Response of Two Human Squamous Cell Carcinomas to Radiation¹

MR Research Center, Division of Radiology [S. V., L. C., A. M., T. N.] and Department of Immunopathology [R. V.], The Cleveland Clinic Foundation, Cleveland, Ohio 44195-5129, and University of Chicago Center for Radiation Therapy, Department of Radiation and Cellular Oncology, Chicago, Illinois 60616 [S. V., M. B., R. W.]

Phosphorous metabolism and cell cycle phase kinetics in response to radiation of two perfused human squamous cell carcinoma cell lines, SQ20B (radioresistant) and SQ38 (relatively radiosensitive), embedded in both basement membrane (Matrigel) and agarose gel threads were studied. The findings for these human cancer cells in response to 2- and 50-Gy irradiation are as follows. (a) Well perfused pure cancer cells (both SQ20B and SQ38) in both proliferative (cells embedded in Matrigel) and static (cells embedded in agarose threads) states did not show significant alteration in either phosphorous bioenergetics or membrane metabolites at 24 and 48 h after irradiation, although a large fraction of the population was clonogenically impaired. Previously reported, sensitively detected, metabolite alterations in response to radiation in rodent and human tumors in situ were not seen in these homogeneous cancer cell populations. (b) The radiosensitive squamous cell carcinoma cell line SQ38 exhibited G₁ block (from 54.38 ± 1.40% in control to 73.93 ± 1.01% after irradiation; mean ± SD) in response to low-dose 2-Gy irradiation and G₂ block (from 12.98 ± 2.15% in control to 25.6 ± 3.15% after irradiation) in response to high-dose 50-Gy irradiation, while the radioresistant cell line SQ20B showed only conventional G₂ block in response to both doses. The differential cell cycle phase response may indicate the difference in radioresistance. (c) The membrane metabolites (including phosphomonoesters and phosphodiesters) and phosphocreatine gradually increased from the early passages to late passages, suggesting that cell proliferation rates were increasing as the cells adapted to tissue culture. The results suggest that the radiation-induced metabolite changes observed in solid tumors in situ may not be a direct response to interim changes within the cancer cells but, rather, a consequence of radiation damage either to the vasculature or to other host-mediated factors.

¹ This work was supported by NIH Grants CA47227 (T. N.) and CA42596 (R. W.), the Sherman Research Fund of the Cleveland Clinic Foundation (T. N.), and a RSNA Seed Grant (S. V.).

² To whom requests for reprints should be addressed, at The Cleveland Clinic Foundation, Division of Radiology, L10, 9500 Euclid Ave., Cleveland, OH 44195-5129.

Received 4/ 3/92. Accepted 7/17/92.