Differential post-irradiation caffeine response in normal diploid versus SV40-transformed human fibroblasts: potential role of nuclear organization and protein-composition

Caffeine enhances radiosensitization to orthotopic transplant LM3 hepatocellular carcinoma in vivo

The aim of this study was to determine whether caffeine enhanced radiosensitization in an orthotopic transplant of LM3 human hepatocellular cancer in nude mice. LM3 hepatocellular carcinoma cells were infected with red fluorescent protein and irradiated, and cell cycle distribution and survival fraction were detected. A nude mouse model of orthotopic transplant of red fluorescent protein-expressing LM3 hepatocellular cancer was established. Nude mice were divided into four groups: control (NS); caffeine (Caff) alone; irradiation (IR) alone; and caffeine + IR (Caff + IR). Tumor growth curves were described. Expression of cyclin and apoptosis were evaluated by analysis of phosphorylated cyclin dependent kinase 1 (CDC2) Tyr15 (CDC2-Tyr15-P), cyclinB1, TUNEL staining, and caspase-3. Caffeine abrogated IR-induced G(2) phase arrest and decreased survival of irradiated LM3 cells. Caffeine enhanced radiosensitivity of LM3 hepatocellular cancer in vivo. Tumor growth delay time in the Caff + IR group was 14.3 days compared with the NS group, 14.1 days compared with the Caff alone group, and 7.2 days compared with the IR alone group. At 15 Gy, expression of CDC2-Tyr15-P in the Caff + IR group (26.0 +/- 8.9%) was significantly lower than in the IR alone group (68.4 +/- 10.6%), expression of cyclinB1 and proportion of TUNEL-positive cells in the Caff + IR group (30.4 +/- 8.7% and 59.2 +/- 9.5%, respectively) was significantly higher than in the IR alone group (7.0 +/- 3.7% and 24.2 +/- 7.2%, respectively), expression of caspase-3 was consistent with the TUNEL staining results. This study suggested that caffeine might enhance the radiosensitivity of LM3 hepatocellular cancer in vivo, and may be feasible for further clinical applications.

Radiosensitization of tumor cells by modulation of ATM kinase

PURPOSE

To elucidate the relationship between the radiation-induced activation of ataxia telangiectasia mutated (ATM) kinase, G2 arrest and the caffeine-induced radiosensitization.

METHOD

RKO cells (human colorectal cancer cells) and ATM kinase over-expressing RKO/ATM cells were used. The cellular radiosensitivity was determined with clonogenic survival assay and the cell cycle progression, including G2 arrest, was studied with flow cytometry. The activity of ATM kinase, check point 2 (Chk2) kinase and cycline B1/cell division cycle 2 (Cdc2) kinase was investigated. The radiosensitivity of RKO xenografts grown in nude mice was studied.

RESULTS

RKO/ATM cells were radioresistant as compared with RKO cells. There was a greater increase in ATM kinase activity and G2 arrest in RKO/ATM cells than in RKO cells. Caffeine also sensitized both RKO cells and RKO/ATM cells to radiation. The caffeine treatment suppressed the radiation-induced activation of ATM kinase, suppressed the activation of Chk2 kinase and inhibited the accumulation of cells in G2 phase. The activity of cycline B1/Cdc2 kinase increased earlier but decayed rapidly in the presence of caffeine. Caffeine enhanced radiation-induced growth delay of RKO xenografts.

CONCLUSIONS

Caffeine inhibited the radiation-induced activation of ATM kinase, thereby preventing the accumulation of cells in G2 phase. Consequently, radiosensitivity of cells increased in the presence of caffeine both in vitro and in vivo.

The vitamin-like dietary supplement para-aminobenzoic acid enhances the antitumor activity of ionizing radiation

PURPOSE

To determine whether para-aminobenzoic acid (PABA) alters the sensitivity of tumor cells to ionizing radiation in vitro and in vivo.

METHODS AND MATERIALS

Cellular proliferation was assessed by WST-1 assays. The effects of PABA and radiation on tumor growth were examined with chick embryo and murine models. Real-time reverse transcriptase-polymerase chain reaction and Western blotting were used to quantify p21CIP1 and CDC25A levels.

RESULTS

Para-aminobenzoic acid enhanced (by 50%) the growth inhibitory activity of radiation on B16F10 cells, whereas it had no effect on melanocytes. Para-aminobenzoic acid enhanced (50-80%) the antitumor activity of radiation on B16F10 and 4T1 tumors in vivo. The combination of PABA and radiation therapy increased tumor apoptosis. Treatment of tumor cells with PABA increased expression of CDC25A and decreased levels of p21CIP1.

CONCLUSIONS

Our findings suggest that PABA might represent a compound capable of enhancing the antitumor activity of ionizing radiation by a mechanism involving altered expression of proteins known to regulate cell cycle arrest.

Caffeine inhibits cell proliferation by G0/G1 phase arrest in JB6 cells

Caffeine is a major biologically active constituent in coffee and tea. Because caffeine has been reported to inhibit carcinogenesis in UVB-exposed mice, the cancer-preventing effect of caffeine has attracted considerable attention. In the present study, the effect of caffeine in quiescent (G0 phase) cells was investigated. Pretreatment with caffeine suppressed cell proliferation in a dose-dependent manner 36 h after addition of fetal bovine serum as a cell growth stimulator. Analysis by flow cytometry showed that caffeine suppressed cell cycle progression at the G0/G1 phase, i.e., 18 h after addition of fetal bovine serum, the percentages of cells in G0/G1 phase in 1 mM caffeine-treated cells and in caffeine-untreated cells were 61.7 and 29.0, respectively. The percentage of cells in G0/G1 phase at 0 h was 75.5. Caffeine inhibited phosphorylation of retinoblastoma protein at Ser780 and Ser807/Ser811, the sites where retinoblastoma protein has been reported to be phosphorylated by cyclin-dependent kinase 4 (cdk4). Furthermore, caffeine inhibited the activation of the cyclin D1-cdk4 complex in a dose-dependent manner. However this compound did not directly inhibit the activity of this complex. In addition, caffeine did not affect p16INK4 or p27Kip1 protein levels, but inhibited the phosphorylation of protein kinase B (Akt) and glycogen synthase kinase 3beta. Our results showed that caffeine suppressed the progression of quiescent cells into the cell cycle. The inhibitory mechanism may be due to the inhibition of cell growth signal-induced activation of cdk4, which may be involved in the inhibition of carcinogenesis in vivo.

Caffeine inhibits homology-directed repair of I-SceI-induced DNA double-strand breaks

We recently reported that two Chinese hamster mutants deficient in the RAD51 paralogs XRCC2 and XRCC3 show reduced radiosensitization after treatment with caffeine, thus implicating homology-directed repair (HDR) of DNA double-strand breaks (DSBs) in the mechanism of caffeine radiosensitization. Here, we investigate directly the effect of caffeine on HDR initiated by DSBs induced by a rare cutting endonuclease (I-SceI) into one of two direct DNA repeats. The results demonstrate a strong inhibition by caffeine of HDR in wild-type cells, and a substantial reduction of this effect in HDR-deficient XRCC3 mutant cells. Inhibition of HDR and cell radiosensitization to killing shows similar dependence on caffeine concentration suggesting a cause-effect relationship between these effects. UCN-01, a kinase inhibitor that effectively abrogates checkpoint activation in irradiated cells, has only a small effect on HDR, indicating that similar to radiosensitization, inhibition of checkpoint signaling is not sufficient for HDR inhibition. Recombination events occurring during treatment with caffeine are characterized by rearrangements reminiscent to those previously reported for the XRCC3 mutant, and immunofluorescence microscopy demonstrates significantly reduced formation of IR-specific RAD51 foci after caffeine treatment. In summary, our results identify inhibition of HDR as a significant contributor to caffeine radiosensitization.

Caffeine induces TP53-independent G(1)-phase arrest and apoptosis in human lung tumor cells in a dose-dependent manner

Caffeine is a model radiosensitizing agent that is thought to work by abrogating the radiation-induced G(2)-phase checkpoint. In this study, we examined the effect that various concentrations of caffeine had on cell cycle checkpoints and apoptosis in cells of a human lung carcinoma cell line and found that a concentration of 0.5 mM caffeine could abrogate the G(2)-phase arrest normally seen after exposure to ionizing radiation. Surprisingly, at a concentration of 5 mM, caffeine not only induced apoptosis by itself and acted synergistically to enhance radiation-induced apoptosis, but also induced a TP53-independent G(1)-phase arrest. Examination of the molecular mechanisms by which caffeine produced these effects revealed that caffeine had opposing effects on different cyclin-dependent kinases. CDK2 activity was suppressed by caffeine, whereas activity of CDC2 was enhanced by suppressing phosphorylation on Tyr15 and by interfering with 14-3-3 binding to CDC25C. These data indicate that the effect of caffeine on cell cycle checkpoints and apoptosis is dependent on dose and that caffeine acts through differential regulation of cyclin-dependent kinase activity.

Homologous recombination as a potential target for caffeine radiosensitization in mammalian cells: reduced caffeine radiosensitization in XRCC2 and XRCC3 mutants

The radiosensitizing effect of caffeine has been associated with the disruption of multiple DNA damage-responsive cell cycle checkpoints, but several lines of evidence also implicate inhibition of DNA repair. The role of DNA repair inhibition in caffeine radiosensitization remains uncharacterized, and it is unknown which repair process, or lesion, is affected. We show that a radiosensitive cell line, mutant for the RAD51 homolog XRCC2 and defective in homologous recombination repair (HRR), displays significantly diminished caffeine radiosensitization that can be restored by expression of XRCC2. Despite the reduced radiosensitization, caffeine effectively abrogates checkpoints in S and G2 phases in XRCC2 mutant cells indicating that checkpoint abrogation is not sufficient for radiosensitization. Another radiosensitive line, mutant for XRCC3 and defective in HRR, similarly shows reduced caffeine radiosensitization. On the other hand, a radiosensitive mutant (irs-20) of DNA-PKcs with a defect in non-homologous end-joining (NHEJ) is radiosensitized by caffeine to an extent comparable to wild-type cells. In addition, rejoining of radiation-induced DNA DSBs, that mainly reflects NHEJ, remains unaffected by caffeine in XRCC2 and XRCC3 mutants, or their wild-type counterparts. These observations suggest that caffeine targets steps in HRR but not in NHEJ and that abrogation of checkpoint response is not sufficient to explain radiosensitization. Indeed, immortalized fibroblasts from AT patients show caffeine radiosensitization despite the checkpoint defects associated with ATM mutation. We propose that caffeine radiosensitization is mediated by inhibition of stages in DNA DSB repair requiring HRR and that checkpoint disruption contributes by allowing these DSBs to transit into irreparable states. Thus, checkpoints may contribute to genomic stability by promoting error-free HRR.

Transforming growth factor-beta3 protection of epithelial cells from cycle-selective chemotherapy in vitro

The transforming growth factor-beta (TGF-beta) family of regulatory growth factors can reversibly arrest cell division in the G1 phase of the cell cycle. Previously, TGF-beta3 was shown to protect epithelial cells and hematopoietic cells from cytotoxic damage in vitro and in vivo, and to reduce the severity and duration of oral mucositis induced by 5-fluorouracil (5-FU) in vivo. In the present study, we tested whether TGF-beta3 can protect epithelial cells from a range of chemotherapy drugs with differing mechanisms of action, using the CCL64 cell line as a model system. We report that preincubation of cells with TGF-beta3 for 24 hr resulted in enhanced clonogenicity following exposure to vinblastine, vincristine, etoposide, taxol, ara-C, methotrexate, or 5-FU. Protection was measured in colony-forming assays, which demonstrated that the protected cells could re-enter the cell cycle and undergo multiple rounds of cell division. At high cytotoxic drug concentrations, absolute colony counts were increased for the cultures prearrested by TGF-beta3, as compared with the proliferating control cultures. The effects of TGF-beta3 were reduced for cisplatin and doxorubicin, drugs that are toxic to cells throughout the cell cycle. Thus, TGF-beta3 can effectively reduce the cytotoxicity of anticancer drugs that act predominantly in S or M phase of the cell cycle.

Reducing the radiation-induced G2 delay causes HeLa cells to undergo apoptosis instead of mitotic death

Cells exposed to radiation may undergo death through apoptosis or mitotic death. HeLa cells predominantly undergo mitotic death after irradiation. Treatment of these cells with caffeine has been shown to shorten the G2 delay after irradiation, and to decrease their survival. The kinase inhibitor staurosporine also decreases the radiation-induced G2 delay in HeLa cells. Here we extend these findings to show that the decrease in radiation-induced G2 delay mediated by caffeine or staurosporine is accompanied by a shift in the pathway of cell death from mitotic death to apoptotic death. The increase in apoptosis is further accompanied by decreased clonogenic survival after irradiation. Based on these findings we propose the hypothesis that one mechanism of enhancing cell killing by radiation is to trigger apoptosis by decreasing the G2 delay induced by irradiation.