Radiation survival of two human cervical carcinoma cell lines after multifraction irradiation

PURPOSE

Multifraction irradiation may contribute to radiation therapy treatment failure if selection of radiation resistant subpopulations occurs. We sought to determine whether surviving cells following daily fraction irradiation of two human cervical squamous cell carcinoma lines would express different radiation survival characteristics compared to the unirradiated parent.

METHODS AND MATERIALS

A late-passage line (HTB35) and an early-passage line (RECA) received daily 2 Gy x-irradiation. Two new stable HTB35 cell lines were established after 40 and 60 Gy (HTB35-40 and HTB35-60). A single line was established from RECA after 30 Gy (RECA-30). High dose rate (74 cGy/min) acute radiation survival curves were prepared from the three new lines and the unirradiated parents. Potentially lethal damage repair (PLDR) and sublethal damage repair (SLDR) responses were detailed for HTB35, HTB35-40 and HTB35-60. Low dose rate (1.27 cGy/min) survival was measured for HTB35 and HTB35-60. Clones were derived from HTB35 and from HTB35-60 and the surviving fraction at 2 Gy (SF2) values were determined.

RESULTS

The two parent lines (HTB35 and RECA) differed in acute radiation survival. The surviving lines following multifraction irradiation (HTB35-40, HTB35-60, and RECA-30) showed no change in acute radiation response compared to the appropriate parent. HTB35-40 and HTB35-60 were repair proficient, demonstrating similar PLDR and SLDR recovery ratios as the parent. Likewise, acute, low dose rate survival of HTB35 and HTB35-60 was similar. Nine clones derived from HTB35 lacked a consistent difference in SF2 compared to the original culture. A single clone of seven derived from HTB35-60 was consistently radiation resistant (SF2 = 0.81 +/- 0.06) compared to the original culture (SF2 = 0.50 +/- 0.09).

CONCLUSION

No evidence was obtained that cell lines generated following multiple daily fractions of x-irradiation in vitro possessed acute radiation survival or repair characteristics that were different from the unirradiated parent.

Heterogeneity of radiosensitivity in a human glioma cell line

Sixteen clones were isolated from an early-passage human glioma cell line (IN859) and have been found to show variation in several biological characteristics including DNA content, modal chromosome number, and morphology. In addition, heterogeneity of radiosensitivity was detected: the doses that gave a surviving fraction of 0.01 varied by a factor of approximately 1.5. The most sensitive (clone 6) and the most resistant (clone 9) clones were selected for further study; their surviving fractions at 2Gy (SF2) were 0.37 and 0.64, respectively. When compared at a fixed radiation dose the sensitive clone surprisingly demonstrated greater split-dose recovery than the resistant clone; it also showed greater low dose-rate sparing.

Endogenous thiol levels in heterogeneous murine tumor cells as a function of the physiological state and the response to X-irradiation

The endogenous thiols (PSH, protein sulfhydryls; NPSH, nonprotein sulfhydryls; and GSH, glutathione) were measured in the 66 and 67 murine carcinoma cells growing under different physiological conditions in vitro (e.g., proliferation, P; nutrient-deprived quiescence QI; and QI cells stimulated by refeeding the monolayer in situ and assayed 4 (St4) and 14 (St14) h later). The aerobic radiation response was also studied as a function of the physiological state and thiol concentration. The changes in PSH levels suggest that the proportion of thiol-containing proteins changed whenever the cells were in transition between different physiological states (e.g., when QI cells were stimulated by refeeding, the proportion of PSH was elevated dramatically over either QI or P cells). The NPSH and GSH levels were both down significantly in the QI vs. P cells as was the total thiol level (PSH plus NPSH). Fourteen h but not 4 h after stimulation, the NPSH and GSH levels had returned to or exceeded the P-cell levels. Also, the proportion of GSH in the NPSH fraction varied as a function of the physiological state. The 66 and 67 QI cells were both more radiosensitive than the respective P cells. Also, the 66 cell radiation-induced cytotoxicity had returned to the P response by about 4 h after refeeding but the stimulated 67 cells had not. However, no overall correlation was apparent between the various aerobic radiation responses and the pool sizes of either the total thiols or of the various subsets of thiols. The depressed total thiol level and the increased radiosensitivity of the QI cells could represent a cause-and-effect relationship or these parameters could be independent phenomena only related indirectly through the reduced metabolic activity of the quiescent cells.

Differential response to gamma radiation of human stomach cancer cells in vitro

In vitro effects of radiation were studied in two permanent cell lines (AGS and SII) from two patients with adenocarcinoma of the stomach and three permanent sublines from each cell line. Radiation survival parameters for AGS and SII parent cell lines and sublines were determined after in vitro irradiation of their cells with 0.5 to 10 Gy of 60Co gamma rays. The AGS and SII cell lines had different growth properties, DNA contents and radiation survival curves. Surviving fractions of SII parent cells (76 chromosomes) after 2.0 and 10 Gy were 1.22 and 17.8 times greater, respectively, than values for AGS parent cells (47 chromosomes). Sensitivities (D0) were 1.08 and 1.45 Gy for AGS and SII parent lines, respectively. The D0 values for AGS parent cells and sublines were similar (1.01 to 1.08 Gy), but SII parent cells and sublines had D0 values of 1.45, 1.36, 1.37 and 1.12 Gy (for SII-A). Also, the SII parent cells had survival fractions after 2.0 and 10 Gy that were 1.3 and 11.3 times greater, respectively, than values for the SII-A cells. These data show differences in radiation responses among stomach cancer cell lines and sublines that may relate to DNA content, but there was no consistent correlation between radiation response and a particular cell characteristic.

A radiobiological comparison of human tumor soft-agar clonogenic assays

Radiation survival curves have been generated for 3 human tumor cell lines as a means of comparing and evaluating the validity of human tumor soft-agar clonogenic assays. The assays investigated were the Hamburger-Salmon, Courtenay-Mills, Courtenay-Mills plus additions, soft agar (no additions), and soft agar plus additions. The additions were formulated to supplement the media used in soft agar assays of primary ovarian and cervical carcinoma specimens. Supplementing the media with additions led to a 2- to 3-fold increase in PE of CaSki cells but had no effect on the PEs of ME180 and OWI cells. Radiation survival curves were similar in all assays for CaSki and OWI but differed for ME180 cells. For ME180 cells, the Courtenay-Mills and soft agar assays plus additions produced the most radioresistant curves (Do = 2.2 Gy); the cells were more responsive when assayed by the Hamburger-Salmon method (Do = 1.5 Gy), and the soft agar and Courtenay-Mills assays gave the most radiosensitive curves (Do = 1.2 Gy). These results demonstrate that the PE of human tumor cell lines may be increased with no effect on radiation survival; radiation survival may be altered without changes in PE and neither may be altered by applying modifications and supplements to existing clonogenic assays.

Unequal modification of the thermal survival responses of clonal human colon carcinoma cell lines by decreased pH

We have investigated the responses of two closely related human colon tumour lines (clone A and clone D) to hyperthermic cell killing in vitro under two conditions of environmental acidity (pH values 7.40 and 6.75). At pH 7.40, under prolonged exposure to 42.5 degrees C, both lines exhibited biphasic response curves with thermotolerance appearing at about 4 h of continuous heating. It was found that the clone D human tumour line was more resistant to hyperthermic inactivation than the clone A line. At pH 6.75, both lines exhibited increased sensitivity to heating killing at 42.5 degrees C, but it was found that the effect was unequally demonstrated by the two human tumour lines (i.e. decreased pH was not acting simply as a dose modifying agent). Specifically, clone A exhibited a much greater hyperthermic sensitivity in the thermotolerant region of survival (i.e. a heating time of 4 h or more) than did clone D. Clinically, this result would suggest that although potential pH sensitization of heterogeneous tumours to hyperthermic cell killing may vary in a random manner among tumour cell subpopulations within a given tumour, the increased heat sensitivity found may produce isosurvival results for equal heating times for the different subpopulations.

Polar solvent modification of X ray induced potentially lethal damage in heterogeneous human colon tumor cells in vitro

Two subpopulations of tumor cells (clones A and D) obtained from a human colon adenocarcinoma were examined for their sensitivities to x-irradiation as unfed, early plateau phase cultures. Both the single dose survival curves and the kinetics of potentially lethal damage recovery (PLDR) were determined for the two tumor lines. Also, possible modification of PLDR by N,N-dimethylformamide (DMF), which has previously been shown to enhance the radiosensitivity of exponentially growing tumor cells, was investigated by adding DMF (0.8% v/v) to plateau phase cultures immediately after irradiation, and determining effects on the extent of PLDR. For non-DMF treated cells, the survival curve parameters of the diploid (clone D) and aneuploid (clone A) lines were very similar. The single-hit, multitarget values for n, Do (Gy), and Dq (Gy) were: 7.9, 0.82, and 1.70 for clone D; and 10.6, 0.83, and 1.96 for clone A. Using initial survival levels of 3.5% (clone D) or 5.5% (clone A) to investigate PLDR, it was found that the increase in survival (surviving fraction ratio or SFR) for clone D was 2.2, while the SFR for clone A was 1.6. DMF did not change either the kinetics or extent of PLDR in these two tumor lines when added to cultures immediately after irradiation. Our results indicate that significant heterogeneity in PLDR exists between these closely related tumor subpopulations.

Murine mammary tumour cells in vitro. I. The development of a quiescent state

Three mouse mammary tumour lines (66, 67, and 68H) derived from a single mouse mammary tumour were investigated for their growth kinetics and development of quiescent cells in unfed monolayer cultures. All three lines develop pure quiescent populations when grown in unfed plateau cultures. A dramatic cell-cycle redistribution accompanied the proliferating (P) to quiescent (Q) transition, with the percentage of cells having a G1 DNA content increasing from 50% in the P state to greater than 97% in the Q state. As the cultures progressed from exponential to plateau growth, a decrease of greater than or equal to 50% in cellular RNA was observed in all three lines. This property enables the clear identification of P v. Q cells by flow cytometry using the two-step acridine orange assay. Autoradiographic data verified that these plateau cells were quiescent since less than 2.5% of the cells incorporated [3H]TdR when labelled for approximately two doubling times. Further comparison of the P and Q cells showed that: (a) the Coulter volume of Q cells was approximately half that of P cells in all three lines; (b) viability, as measured by dye exclusion was greater than 95% in all cultures regardless of their proliferative state; and (c) colony-forming ability decreased as the cells entered the quiescent state. In each of these cell lines the development of Q-cell populations was marked by similar changes in all measured parameters. These quiescent tumour cells provide a relatively simple model to evaluate what, if any, important differences exist between the response of P v. Q cells to various therapeutic agents.

Heterogeneity in induced thermal resistance of rat tumor cell clones

Four 13762NF rat mammary adenocarcinoma clones were examined for their survival response to heating under conditions that induced transient thermal resistance (thermotolerance). Clones MTC and MTF7 were isolated from the subcutaneous locally growing tumor, whereas clones MTLn2 and MTLn3 were derived from spontaneous lung metastases. There was heterogeneity among these clones in thermotolerance induced by either fractionated 45 degrees C or continuous 42 degrees C heating, but the order of sensitivity was not necessarily the same. The clones developed thermal resistance at different rates and to different degrees within the same time intervals. There was heterogeneity between clones isolated from within either the primary site or metastatic lesions. However, clones derived from metastatic foci did not intrinsically acquire more or less thermotolerance to fractionated 45 degrees C or continuous 42 degrees C heating than did clones from the primary tumor. Further, there was no apparent relationship between any phenotypic properties that conferred more or less thermotolerance in vitro and any phenotypic properties that conferred enhanced metastatic success of these same clones by spontaneous (subcutaneous) or experimental (intravenous) routes in vivo. These tumor clones also differ in their karyotype, metastatic potential, cell surface features, sensitivity to x-irradiation and drugs, and ability to repair sublethal radiation damage. These results provide further credence to the concept that inherent heterogeneity within tumors may be as important in therapeutic success as other known modifiers of outcome such as site and treatment heterogeneity.

Phenotypic drift and heterogeneity in response of metastatic mammary adenocarcinoma cell clones to adriamycin, 5-fluoro-2'-deoxyuridine and methotrexate treatment in vitro

Rat 13762 mammary adenocarcinoma cell clones of differing spontaneous metastatic potentials were tested for their sensitivities to Adriamycin, 5-fluoro-2'-deoxyuridine (FUdR) and methotrexate in vitro. Cells were treated 4 hours with a single dose of drug, and colony formation was used to assay cell survival. Dose-response curves and survival parameters were calculated for local tumor-derived clones MTC and MTF7 and lung metastasis-derived clone MTLn3. The logarithmic curves were analyzed (slope and y-intercepts) for statistical comparisons. Heterogeneous sensitivities to Adriamycin and FUdR were observed, but there was no difference in the sensitivities of the clones to methotrexate cell killing. We could not find correlations between drug response and clonal origin, passage number or metastatic properties. The sensitivities of the clones to Adriamycin and FUdR changed upon in vitro passage, although drift in Adriamycin sensitivity during growth in culture was not statistically significant. The results demonstrate that clonal heterogeneity exists within the 13762 tumor and its metastases in their inherent responses to chemotherapy agents, and in the absence of host selective pressures tumor cells can spontaneously and reproducibly drift in their sensitivities to certain chemotherapeutic drugs.