Dynamic changes in ABA content in water-stressed Populus nigra: effects on carbon fixation and soluble carbohydrates.

BACKGROUND AND AIMS: Hydraulic and chemical signals operate in tandem to regulate systemic plant responses to drought. Transport of abscisic acid (ABA) through the xylem and phloem from the root to shoot has been suggested to serve as the main signal of water deficit. There is evidence that ABA and its ABA-glycosyl-ester (ABA-GE) are also formed in leaves and stems through the chloroplastic 2-C-methylerythritol-5-phosphate (MEP) pathway. This study aimed to evaluate how hormonal and hydraulic signals contribute to optimize stomatal (gs), mesophyll (gm) and leaf hydraulic (Kleaf) conductance under well-watered and water-stressed conditions in Populus nigra (black poplar) plants. In addition, we assessed possible relationships between ABA and soluble carbohydrates within the leaf and stem. METHODS: Plants were subjected to three water treatments: well-watered (WW), moderate stress (WS1) and severe stress (WS2). This experimental set-up enabled a time-course analysis of the response to water deficit at the physiological [leaf gas exchange, plant water relations, (Kleaf)], biochemical (ABA and its metabolite/catabolite quantification in xylem sap, leaves, wood, bark and roots) and molecular (gene expression of ABA biosynthesis) levels. KEY RESULTS: Our results showed strong coordination between gs, gm and Kleaf under water stress, which reduced transpiration and increased intrinsic water use efficiency (WUEint). Analysis of gene expression of 9-cis-epoxycarotenoid dioxygenase (NCED) and ABA content in different tissues showed a general up-regulation of the biosynthesis of this hormone and its finely-tuned catabolism in response to water stress. Significant linear relationships were found between soluble carbohydrates and ABA contents in both leaves and stems, suggesting a putative function for this hormone in carbohydrate mobilization under severe water stress. CONCLUSIONS: This study demonstrates the tight regulation of the photosynthetic machinery by levels of ABA in different plants organs on a daily basis in both well-watered and water stress conditions to optimize WUEint and coordinate whole plant acclimation responses to drought.

CSA and CSB proteins interact with p53 and regulate its Mdm2-dependent ubiquitination.

Mutations in Cockayne syndrome (CS) A and B genes (CSA and CSB) result in a rare genetic disease that affects the development and homeostasis of a wide range of tissues and organs. We previously correlated the degenerative phenotype of patients to the enhanced apoptotic response, exhibited by CS cells, which is associated with the exceptional induction of p53 protein, upon a variety of stress stimuli. Here we showed that the elevated and persistent levels of p53 displayed by CS cells are due to the insufficient ubiquitination of the p53 protein. We further demonstrated that CSA and CSB proteins associate in a unique complex with p53 and Mdm2; this interaction greatly stimulates the ubiquitination of p53 in an Mdm2-dependent manner. Tandem affinity purification and immunoprecipitations combined with mass spectrometry studies indicate that CSA and CSB associate within a Cullin Ring Ubiquitin Ligase complex responsible, under certain circumstances, for p53 ubiquitination. This study identifies CSA and CSB as the key elements of a regulatory mechanism that equilibrate beneficial and detrimental effects of p53 activity upon cellular stress. The deregulation of p53, in absence of either of the CS proteins, can potentially explain the early onset degeneration of tissues and organs observed in CS patients.

Radiation-induced bystander effect in healthy G(o) human lymphocytes: biological and clinical significance.

To study the bystander effects, G(0) human peripheral blood lymphocytes were X-irradiated with 0.1, 0.5 and 3 Gy. After 24h, cell-free conditioned media from irradiated cultures were transferred to unexposed lymphocytes. Following 48 h of medium transfer, viability, induction of apoptosis, telomere shortening, reactive oxygen species (ROS) levels and micronuclei (after stimulation) were analyzed. A statistically significant decrement in cell viability, concomitant with the loss of mitochondrial membrane potential, telomere shortening, increases in hydrogen peroxide (H(2)O(2)) and superoxide anion (O(2)(-)) with depletion of intracellular glutathione (GSH) level, and higher frequencies of micronuclei, were observed in bystander lymphocytes incubated with medium from 0.5 and 3 Gy irradiated samples, compared to lymphocytes unexposed. Furthermore, no statistically significant difference between the response to 0.5 and 3 Gy of irradiation in bystander lymphocytes, was found. However, when lymphocytes were irradiated with 0.1 Gy, no bystander effect with regard to viability, apoptosis, telomere length, and micronuclei was observed, although a high production of ROS level persisted. Radiation in the presence of the radical scavenger dimethyl sulfoxide (DMSO) suppressed oxidative stress induced by 3 Gy of X-rays with the effective elimination of bystander effects, suggesting a correlation between ROS and bystander signal formation in irradiated cells. The data propose that bystander effect might be mostly due to the reactions of radiation induced free radicals on DNA, with the existence of a threshold at which the bystander signal is not operative (0.1 Gy dose of X-rays). Our results may have clinical implications for health risk associated with radiation exposure.

Relationship between spontaneous or radiation-induced apoptosis and telomere shortening in G(0) human lymphocytes.

To examine the correlation between spontaneous or radiation-induced apoptosis and telomere shortening, G(0) human peripheral blood lymphocytes were irradiated with X-rays and analyzed for viability, apoptosis, and telomere length. Part of the lymphocytes was kept under liquid-holding conditions for 48 h, and then loaded onto Ficoll-Paque medium to separate apoptotic (high-density) from normal (normal-density) cells. Then all samples were examined for the same three end-points. To determine whether expression of p53 influences the telomere shortening associated with a spontaneous or radiation-induced apoptotic process, the lymphocytes were also analyzed for expression of p53 at 0 and 48 h recovery times (non-irradiated and irradiated samples) and after 2 weeks in liquid-holding conditions (non-irradiated sample). After 48 h in liquid-holding, the p53-dependent apoptotic lymphocytes in the irradiated cultures presented shortened telomeres. After a 2-week recovery time, non-irradiated cells showed a p53-dependent spontaneous apoptosis, but no telomere shortening. These results demonstrate that radiation-induced apoptosis correlates with shortened telomeres in G(0) human lymphocytes. Spontaneous and radiation-induced apoptosis are dependent on expression of p53. In contrast, p53 may not play an effective role in telomere shortening, because spontaneous apoptosis did not correlate with telomere shortening. As most tumours are compromised with respect to p53 function, our findings on the role of p53 in telomere shortening may prove critical for applying therapeutic modalities in the clinic, and may facilitate the design of agents that selectively disrupt telomere integrity in tumour cells.

Multidisciplinary Rectal Cancer Management: 2nd European Rectal Cancer Consensus Conference (EURECA-CC2).

BACKGROUND AND PURPOSE: During the first decade of the 21st century a number of important European randomized studies were published. In order to help shape clinical practice based on best scientific evidence from the literature, the International Conference on 'Multidisciplinary Rectal Cancer Treatment: Looking for an European Consensus' (EURECA-CC2) was organized in Italy under the endorsement of European Society of Medical Oncology (ESMO), European Society of Surgical Oncology (ESSO), and European Society of Therapeutic Radiation Oncology (ESTRO). METHODS: Consensus was achieved using the Delphi method. The document was available to all Committee members as a web-based document customized for the consensus process. Eight chapters were identified: epidemiology, diagnostics, pathology, surgery, radiotherapy and chemotherapy, treatment toxicity and quality of life, follow-up, and research questions. Each chapter was subdivided by a topic, and a series of statements were developed. Each member commented and voted, sentence by sentence thrice. Sentences upon which an agreement was not reached after voting round # 2 were openly debated during a Consensus Conference in Perugia (Italy) from 11 December to 13 December 2008. A hand-held televoting system collected the opinions of both the Committee members and the audience after each debate. The Executive Committee scored percentage consensus based on three categories: "large consensus", "moderate consensus", and "minimum consensus". RESULTS: The total number of the voted sentences was 207. Of the 207, 86% achieved large consensus, 13% achieved moderate consensus, and only 3 (1%) resulted in minimum consensus. No statement was disagreed by more than 50% of the members. All chapters were voted on by at least 75% of the members, and the majority was voted on by >85%. CONCLUSIONS: This Consensus Conference represents an expertise opinion process that may help shape future programs, investigational protocols, and guidelines for staging and treatment of rectal cancer throughout Europe.

Frequency of sister chromatid exchanges and micronuclei monitored over time in patients with early-stage breast cancer: results of an observational study.

Spontaneous chromosomal instability correlates with a high risk of cancer. The frequency of spontaneous sister chromatid exchanges (SCE) and micronuclei (MN) in peripheral blood lymphocytes was used for evaluation of spontaneous chromosomal instability in early-stage breast cancer patients to determine whether SCE and MN frequencies are biomarkers of damage from chemotherapy and radiotherapy. In 20 stage I-II breast cancer patients, SCE and MN were measured before surgery and at 4 weeks after. In patients who received adjuvant chemotherapy (CTx), they were also determined before starting radiotherapy (RTx). Other assessments were done 2, 6, and 12 months after RTx in almost all patients and at 18 months in 4 patients. Generalized estimating equations (GEE) were used to estimate population averaged effects at the different treatment and follow-up time points. Moreover, SCE and MN baseline values in patients were compared with those of a control group of 12 healthy women. A significant difference emerged between patients and healthy controls (P<0.0001 for SCE and P<0.0003 for MN; Mann-Whitney test); SCE increased significantly after CTx and MN increased significantly after RTx. In the GEE model, the smoking habit was associated with increased SCE in patients treated with CTx; age significantly affected MN frequencies. Both MN and SCE frequencies are increased in breast cancer patients and are indicators of CTx and RTx damage, respectively. The increased SCE levels in patients treated with CTx may be due to a synergic effect of smoking and chemotherapy.

Computed tomography for excision cavity localization and 3D-treatment planning in partial breast irradiation with high-dose-rate interstitial brachytherapy.

BACKGROUND AND PURPOSE: When high-dose-rate brachytherapy is used for partial breast irradiation (PBI) precise pre-implant definition of planning target volume (PTV) and implant geometry is required. After implantation, accurate PTV localization, catheter reconstruction and optimization of dose distribution are needed for good PTV coverage and dose conformity. We applied image-guidance using computed tomography (CT) for pre-implant PTV definition and post-implant dosimetry. MATERIALS AND METHODS: In 54 patients implant geometry was designed by external beam virtual simulation. A template was placed over dummy beam digitally reconstructed radiographs displaying PTV. Needle entrance and exit points were defined and marked on the patient's skin to serve as landmarks during implantation. After implantation, in 46/54 patients PTV was defined, catheters were reconstructed and active lengths in the catheters were specified using CT-based-3D planning system. Dosimetry was performed with a Plato-Nucletron treatment planning system. RESULTS: Post-implantation CT visualized precise catheter placement with respect to the PTV in all patients. CT-based treatment planning provided good coverage of PTV and homogeneous dose distribution. CONCLUSIONS: In post-operative PBI with high-dose-rate brachytherapy CT-based pre-implant definition of implant geometry ensures adequate PTV coverage. After implantation, CT-based 3D-treatment planning software ensures exact PTV localization and catheter reconstruction, and dose distribution optimization.

CSB protein is (a direct target of HIF-1 and) a critical mediator of the hypoxic response.

Cockayne syndrome (CS) is a rare genetic disease characterized by neurological problems, growth failure and premature ageing. Many of these features cannot simply be ascribed to the defect that CS cells display during transcription-coupled repair. Here, we show that CSB mutant cells are unable to react to hypoxic stimuli by properly activating the hypoxia-inducible factor-1 (HIF-1) pathway, a defect that is further enhanced in the event of a concomitant genotoxic stress. Furthermore, we show that CSB expression is under the control of HIF-1 and has a critical function during hypoxic response by redistributing p300 between HIF-1 and p53. Altogether, our data demonstrate that CSB is part of a feedback loop mechanism that modulates the biological functions of p53. The outcome of this study provides new insights into the understanding of the molecular basis of the CS phenotype and the involvement of the CSB protein in the hypoxic response pathway.

The multidisciplinary rectal cancer treatment: main convergences, controversial aspects and investigational areas which support the need for an European Consensus.

BACKGROUND AND PURPOSE: During the past decades staging and treatment of rectal cancer are used different in Europe and in North America. To promote a process to integrate the daily practice with the best evidence of the literature an International Conference was organized in Italy. Agreement between Experts, Centres, and specialists who participated in the Conference are reported. METHODS: Five aspects were analyzed and a questionnaire was tailored for this purpose. The questionnaire had 159 questions. During the Conference, at the beginning of each Session, the moderators showed the answers from the Experts and the Centres, and, at the end of the session, the audience voted in all controversial issues. Agreements were scored at three levels: minimum, if it was between 51 and 74% of votes for each group; moderate, between 75 and 94%; large, more than 94%. RESULTS: The main results are: staging: endoanal ultrasound was considered as mandatory in T staging, in the evaluation of sphincter infiltration, and in the restaging of T after chemoradiotherapy (chRT). Magnetic Resonance Imaging is mandatory in the evaluation of mesorectal fascia infiltration. Endoscopy had a moderate agreement for the definition of tumour location, and the barium enema as optional. Digital rectal examination is complementary for staging and PET-CT investigational for T, N and yT staging. Preoperative radiotherapy: for T4 stage chRT was always the preferred treatment, often with moderate agreement, for any tumour location and N status. For T3, chRT received the same agreement except for high location and N0-N1. For T2 stage, N2 and positive nodes outside the mesorectum, chRT received minimum agreement for low and middle tumours; for high tumours only positive nodes outside the mesorectum was agreed upon. Preoperative radiotherapy, negative specimen and sphincter preservation: chRT was agreed by many for all T stages and N presentations of lower third tumours, except for T1-2 N0-N1. Postoperative treatments: the selection for these treatments often received moderate agreement according to the infiltration of surrounding organs, positive nodal status and circumferential radial margins. Therapy of metastatic disease: an agreement was found for FOLFOX as first-line therapy and for FOLFIRI as second-line, although comparative studies show similar activity of FOLFOX and FOLFIRI regimens. CONCLUSIONS: This process represents an expertise opinion process that may contribute to increased scientific debate and to promote the development of 'guidelines', 'clinical recommendations' and ultimately a Consensus on the evolving approach to rectal cancer treatment.

Short-course versus split-course radiotherapy in metastatic spinal cord compression: results of a phase III, randomized, multicenter trial.

PURPOSE: Hypofractionated radiotherapy (RT) is often used in the treatment of metastatic spinal cord compression (MSCC). This randomized trial was planned to assess the clinical outcome and toxicity of two different hypofractionated RT regimens in MSCC. PATIENTS AND METHODS: Three hundred patients with MSCC were randomly assigned to a short-course RT (8 Gy x 2 days) or to a split-course RT (5 Gy x 3; 3 Gy x 5). Only patients with a short life expectancy entered the protocol. Median follow-up was 33 months (range, 4 to 61 months). RESULTS: A total of 276 (92%) patients were assessable; 142 (51%) treated with the short-course and 134 (49%) treated with the split-course RT regimen. There was no significant difference in response, duration of response, survival, or toxicity found between the two arms. When short- versus split-course regimens were compared, after RT 56% and 59% patients had back pain relief, 68% and 71% were able to walk, and 90% and 89% had good bladder function, respectively. Median survival was 4 months and median duration of improvement was 3.5 months for both arms. Toxicity was equally distributed between the two arms: grade 3 esophagitis or pharyngitis was registered in four patients (1.5%), grade 3 diarrhea occurred in four patients (1.5%), and grade 3 vomiting or nausea occurred in 10 patients (6%). Late toxicity was never recorded. CONCLUSION: Both hypofractionated RT schedules adopted were effective and had acceptable toxicity. However, considering the advantages of the short-course regimen in terms of patient convenience and machine time, it could become the RT regimen of choice in the clinical practice for MSCC patients.

Raltitrexed and radiotherapy as adjuvant treatment for stage II-III rectal cancer: a feasibility study.

AIMS AND BACKGROUND: Adjuvant 5-FU chemotherapy plus radiotherapy represents the standard treatment for radically resected rectal cancer at high risk of relapse according to the NIH Consensus Conference. The therapeutic gain was obtained with a high rate of severe treatment-related toxicity and a suboptimal patient compliance with this regimen. Raltitrexed is a specific thymidylate synthase inhibitor with a convenient administration schedule, acceptable toxicity and radiosensitizing properties, as the published phase I trials in combination with radiotherapy have shown. The aim of this prospective multicenter phase II study was to evaluate the feasibility, gastrointestinal and hematological acute toxicity of raltitrexed in combination with radiotherapy in rectal cancer patients. METHODS: From September 2000 to June 2004, 50 patients with radically resected stage II-III rectal adenocarcinoma were treated. All patients were evaluable for compliance and acute toxicity. Within 45-60 days of surgery, each patient underwent concomitant adjuvant radiochemotherapy. Radiotherapy was administered to the pelvis (plus perineum after abdominoperineal resection) with photon beam energy exceeding 5 MV, 3-4 fields, 45 Gy/25 fractions/5 weeks plus a boost delivered to the site of resected disease with 3-4 fields, 9 Gy/5 fractions/1 week to a total dose of 54 Gy. The boost dose was administered after complete exclusion of the small bowel from the treatment volumes; if this was not possible a total dose of 50.4 Gy was given. Raltitrexed was administered intravenously at a dose of 3 mg/m2 as a bolus injection on days 1 and 22 of radiotherapy one hour before treatment, for a total of two cycles. Each patient underwent weekly clinical evaluation and laboratory tests. Toxicity was assessed by the WHO scale. RESULTS: Forty-five (90%) patients completed the established treatment. Acute severe toxicity included grade III proctitis in 4/50 (8%), grade III-IV diarrhea in 4/50 (8%), grade III perineal dermatitis in 4/50 (8%) and grade III leukopenia in 2/50 (4%) patients; five patients (10%) experienced a transient grade Ill increase in their liver biochemistry values. CONCLUSIONS: Our data related to acute toxicity and patient compliance proved the feasibility of this adjuvant radiochemotherapy treatment. A longer follow-up is necessary to evaluate the effectiveness of this new regimen in terms of disease-free and overall survival.

T1-T2 breast cancer with four or more positive axillary lymph nodes: adjuvant locoregional radiotherapy with high-dose or standard-dose chemotherapy. Results of an observational study.

AIM AND BACKGROUND: The aim of this study was to investigate the efficacy of postoperative locoregional radiotherapy in patients with T1-T2 breast cancer and four or more positive axillary lymph nodes submitted to mastectomy or breast-conserving surgery followed by standard-dose or high-dose adjuvant chemotherapy. The incidence of locoregional relapses and the survival correlated with the number of positive nodes were recorded for each treatment arm. PATIENTS AND METHODS: From August 1992 to August 1999 86 breast cancer patients (median age, 54 years, T1-T2, N+ > or = 4) submitted to surgery were treated. Sixty-three patients received standard-dose chemotherapy while 23 patients with 10 or more positive nodes received high-dose chemotherapy. After four courses of standard-dose anthracycline-based chemotherapy peripheral blood stem cells were mobilized with cyclophosphamide (7 g/m2) and G-CSF (10-16 microg/kg/day/sc). High-dose chemotherapy consisted of etoposide 1000 mg/m2, thiotepa 500 mg/m2 and carboplatin 800 mg/m2. Hormone receptor-positive patients underwent hormone therapy. Following chemotherapy all 86 patients were given conventional radiotherapy to the breast or the chest wall and the supraclavicular fossa. The high-dose subgroup received radiotherapy to the internal mammary nodes +/- axilla. RESULTS: The median follow-up from the start of radiotherapy was 36.5 months. Locoregional relapses occurred in nine patients (10.4%); in four of them they were isolated (4.6%). Local relapses were four (4.6%) and regional relapses six (6.9%). Twenty-five patients (29%) had distant metastases. The five-year and eight-year overall actuarial survival rates were 82.6% +/- 4.8 and 60.1% +/- 8.8, respectively. No statistical differences were found when the number of positive nodes or the type of treatment of N+ 10 patients was included in the analysis. CONCLUSIONS: Breast cancer patients with four or more positive axillary lymph nodes are at high risk of developing locoregional and distant relapses. The results reported here demonstrate the efficacy of radiotherapy in the reduction of locoregional failure; no differences in survival and locoregional control in relation to treatment arm and number of positive nodes were found.

Breast-conserving therapy for stage I-II breast cancer in elderly women.

PURPOSE: To assess breast-conserving therapy results in elderly patients with early-stage breast cancer (clinical Stage I-II). METHODS AND MATERIALS: Between 1979 and 1998, 196 women (200 treated breasts) aged > or =70 years (median age, 72.5 years) were treated with breast-conserving therapy (lumpectomy or quadrantectomy with axillary lymph node dissection and radiotherapy). Pathologic axillary node involvement was found in 51 patients (28%). Two-thirds of patients received tamoxifen, and 16% received chemotherapy. RESULTS: At a median follow-up of 59 months, 3 patients (1.5%) had developed local recurrence and 20 (10.2%) distant metastases. The overall survival rate was 81% and 62% at 5 and 10 years, respectively. The corresponding disease-specific survival rates were 92% and 88%. Axillary nodal involvement was the only statistically significant risk factor for the development of metastases (p = 0.0035). Arm mobility impairment and arm lymphedema each occurred in 5 patients. In another 5 patients, a thromboembolic event occurred during tamoxifen treatment. CONCLUSION: Elderly women tolerate breast-conserving therapy, including radiotherapy, well and have excellent rates of locoregional control and disease-specific survival.

Long-term results of induction chemotherapy followed by concurrent chemotherapy and thoracic irradiation in limited small cell lung cancer.

BACKGROUND: Small cell lung cancer (SCLC) is a chemoresponsive tumor but overall survival remains poor even in limited disease (LD). With the aim of eradicating chemoresistant tumor cells and reducing toxicity, we investigated in this phase II trial the feasibility and outcome of a sequential approach of induction chemotherapy (CT) followed, in responding patients with LD-SCLC, by intensified platinum-based CT and concurrent thoracic irradiation (TI). MATERIALS AND METHODS: We treated 55 consecutive LD-SCLC patients with three 21-day cycles of cyclophosphamide, epiadriamycin and vincristine (CEV) as induction CT. In 44 (80%) patients there was an objective response and they received treatment intensification consisting of TI and concomitant CT with carboplatin and etoposide plus recombinant granulocite colony stimulating factor. Twenty-five (57%) patients were submitted to twice-daily thoracic irradiation (TDTI; 1.5 Gy per fraction, to a total dose of 45 Gy) and 19 (43%) to once-daily thoracic irradiation (ODTI; 2 Gy per fraction, to a total dose of 50 Gy). RESULTS: Median follow up was 75 months (range, 42-102). Of 44 patients submitted to intensification with TI plus CT, 32 (73%) had a complete and 12 (27%) a partial response. Median overall survival of all 55 patients was 17 months with actuarial survival probabilities of 2 and 5 years, 32 and 25%, respectively. Analysis of patient sub-groups showed a 5-month median survival in non-responders, 19 in TDTI and 17 in ODTI patients, respectively. Two and 5 year survival probabilities were 0% in non-responders, 40 and 35% in TDTI and 39 and 21% in ODTI patients, respectively. At present, 13 of 44 responders are still alive, of which nine (20%) have been progression-free from 45 to 93 months (median 60). Treatment failure was registered in 31 (70%) of 44 patients who received both induction and intensification treatment. One-half of patients had intrathoracic recurrence, eight of which only local and the remaining seven local and distant. Fourteen (32%) patients had brain metastases. Grade 3-4 neutropenia occurred in 24 (55%) patients with no differences between treatment groups. Grade 3 esophagitis was registered in four (9%) patients: in 3/25 (12%) and 1/19 (5%) of those who received TDTI and ODTI, respectively (P=not significant). Acute radiation pneumonitis occurred in three (12%) patients submitted to TDTI. No clinically debilitating pulmonary fibrosis, permanent esophageal stricture or toxic death was observed. CONCLUSIONS: In LD-SCLC patients late concurrent CT plus TI is feasible and effective. Our long-term results are similar to the best reported in the literature. Despite the high incidence of complete response obtained, however, one-half of the patients had intrathoracic relapse and one-third brain metastases.

Transcription coupled repair efficiency determines the cell cycle progression and apoptosis after UV exposure in hamster cells.

Nucleotide excision repair (NER) is a major pathway for the removal of bulky adducts and helix distorting lesions from the genomic DNA. NER is highly heterogeneous across the genome and operates principally at different levels of hierarchy. Transcription coupled repair (TCR), a special sub-pathway of NER and base excision repair (BER), is critical for cellular resistance after UV irradiation in mammalian cells. In this study, we have investigated the effects of UV-C irradiation on cell cycle progression and apoptosis in G1 synchronised isogenic hamster cell lines that are deficient in TCR and NER pathways. Our results revealed the existence of two apoptotic modes at low UV (2-4J/m2) doses in TCR deficient (UV61) and NER deficient (UV5) cells: one occurring in the first G1 and the other in the second G1-phase following the first division. At high UV doses (8-32J/m2), UV61 and UV5 cells underwent apoptosis without entry into S-phase after a permanent arrest in the initial G1. In contrast to repair deficient cells, parental TCR proficient AA8 cells did not show a significant G1 arrest and apoptosis at doses below 8J/m2. UV61 (proficient in repair of 6-4 photoproducts (PPs)) and UV5 (deficient in 6-4 PP repair) cells showed similar patterns of cell cycle progression and apoptosis. Taken together, these results suggest that the persistence of 6-4 PP and the replication inhibition may not be critical for apoptotic response in hamster cells. Instead, the extent of transcription blockage resulting from the TCR deficiency constitutes the major determining factor for G1 arrest and apoptosis.