DNA injuries, their repair, and carcinogenesis

Quantum yield for preferential photodimerization in long pyrimidine tracts

The quantum yield for cyclobutyl-pyrimidine dimerization in DNA has been observed to increase approximately linearly with increasing pyrimidine tract length. A model without adjustable parameters for this yield is proposed based on energy delocalization, vibronic symmetry switching, and saturation statistics that describe the average number of (base pairwise) breathing modes in a given tract of pyrimidines. This average number of modes is an approximately linear function of the tract length. Monte Carlo techniques are used to simulate base sequences and photochemical events, and indicate that this model is consistent with experiment for Tetrahymena pyriformis DNA.

Modification of N-methyl-N-nitrosourea-induced urinary bladder carcinogenesis in rats following stimulation of urothelial proliferation by a partial cystectomy

The present experiments are concerned with the question whether stimulation of urothelial proliferation modifies tumor development in the urinary bladder. To induce proliferative activity of the urothelium a partial cystectomy (one-third resection of the bladder) was performed in female Wistar rats. N-Methyl-N-nitrosourea (MNU) was used as a carcinogen which acts directly on the urothelium without requiring metabolic activation. MNU was given as a single intravesicular dose of 5 mg/kg body weight via a urethral catheter. After an experimental period of 15 months rats with an intact quiescent bladder showed a tumor incidence of 32.6%. Rats having received MNU 45 h following partial cystectomy - when proliferative activity reached its peak - had developed bladder tumors with a frequency of 17.9%. Initial administration of MNU followed 24 h later by a one-third resection of the bladder resulted in a tumor incidence of only 8.8%. The histologic types of tumors induced proved to be similar to those found with other carcinogens. However, by contrast the majority of urothelial tumors were characterized by a squamous metaplasia. There was no substantial difference between the various histologic tumor types found in the resting and regenerating bladder. The mechanisms responsible for the observed inhibition of tumor development in the regenerating bladder are unknown. It is assumed that an increased capacity of the proliferating urothelial cells to repair carcinogen-induced DNA damage may play an important role.

Inhibition of tumor development in the regenerating rat urinary bladder stimulated to proliferate by cyclophosphamide

The present study deals with the effect of stimulation of urothelial proliferation on experimental bladder carcinogenesis. To induce proliferative activity of the bladder mucosa cyclophosphamide (cp) was intraperitoneally administered to rats in a single dose (100 mg/kg). N-Butyl-N-(4-hydroxybutyl)-nitrosamine (BBN) was used as carcinogen and administered by gavage in 3 fractionated doses when proliferation of the urothelium was highest at 28 and 40 hr as well as 7 days following the injection of cp. Contrary to our original working hypothesis, tumor development proved to be inhibited in the bladder following initial stimulation of urothelial proliferation by cp. After administration of a low total dose of BBN (300 mg/kg) and an experimental period of 6 and 12 months none of the rats pretreated with cp developed a tumor in the regenerating bladder, whereas solitary transitional cell papillomas were observed in 6.7% of the control animals with a quiescent bladder. Following administration of BBN at a high total dose (1.300 mg/kg) and an induction time of 4, 6 and 12 months papillomas and non-invasive papillary transitional cell carcinomas occurred in only 21.6% of the rats initially receiving cp but in 48.1% of the control animals without stimulation of urothelial proliferation by cp. After treatment with BBN alone there was a far larger number of rats with multiple tumors in the quiescent bladder. The reduction in the incidence of tumors following administration of cp is not attributable to a prolongation of the latency period or induction time. It is an open question which mechanisms are responsible for the observed inhibition of experimental bladder carcinogenesis. An increased DNA repair induced synchronously with the stimulated replicative de-novo DNA synthesis or a decreased activity of urothelial enzymes metabolizing BBN to its ultimate carcinogen are proposed as the most likely explanations.

Inhibitory effect of partial cystectomy on experimental carcinogenesis in the urinary bladder

It was our aim in the present animal experiments to study the influence of stimulation of proliferative activity on carcinogenesis in the urinary bladder. Stimulation of urothelial proliferation was achieved by a one-third resection of the bladder. N-butyl-N-(4-hydroxybutyl)- nitrosamine (BBN), which was used as a carcinogen, was administered by gavage in three fractionated doses when proliferative activity was highest at 30, 45, and 70 h postoperatively. Contrary to our working hypothesis, the incidence of urinary bladder tumors proved to be significantly reduced by partial cystectomy. After administration of a low total dose of BBN (300 mg/kg bodyweight) and an experimental period of 6, 12, and 18 months, only 2.6% of the rats with a partial cystectomy, but 12.6% of the control animals with an intact bladder had developed papillomas and noninvasive papillary transitional cell carcinomas. Following administration of BBN at a higher total dose (1,300 mg/kg bodyweight), bladder tumors occurred after an induction period of 4, 6, and 12 months in 27.4% of the partially cystectomized and 48.1% of the nonoperated rats. Multiple tumors were found more frequently in the controls than in the operated animals. The reduction in the tumor incidence following one-third resection of the bladder evidently does not depend on a prolongation of the latency period or induction time. From findings in analogous experimental models it is conceivable that the observed inhibition of experimental bladder carcinogenesis is brought about by an increased capacity of the proliferating urothelial cells to repair carcinogen-induced DNA damage. Further studies are required to elucidate the significance of a stimulated proliferation for the repair system and neoplastic transformation of the urothelium.

The action of a mammalian endonuclease on psoralen-bound DNA

The sequential actions of two enzymes believed to be involved in DNA repair, namely a mammalian endonuclease and the bacterial DNA polymerase I on psoralen bound 32P-labeled DNA, was studied. When ultraviolet-irradiated DNA is exposed to the sequential action of the endonuclease, the formation of single-strand breaks prepares the DNA for the exonucleolytic excision of thymine dimers. The mammalian endonuclease purified from rat liver to electrophoretic homogeneity is inactive on normal DNA, DNA irradiated at 360 nm or DNA mixed with psoralen without irradiation. Incubation of psoralen-bound DNA labeled with 32P with the endonuclease releases the isotope in the acid soluble indicating that psoralen-bound DNA is susceptible to the endonucleolytic attack. Sedimentation of DNA on sucrose gradients indicates that there is no collapse of the DNA molecule after treatment with the endonuclease. Moreover, there is no release of the adduct in the acid soluble after treatment with DNA polymerase, indicating that the 5--3 min exonucleolytic activity of that enzyme is impaired by the remaining crosslinks. The crosslinks also inhibit the incorporation of [3H] dATP in presence of DNA polymerase I.

[Stimulating effect of partial cystectomy on regeneration of rat bladder urothelium and its significance for carcinogenesis (author's transl)]

Cell proliferation kinetics of the urinary bladder urothelium have been analyzed in a total of 218 female Wistar rats after partial cystectomy. After one-third resection of the bladder DNA synthesis started 15 h postoperatively in the bladder stump and reached its maximum at 25 h with a mean 3H-TdR index of 10.1%. In the area of resection the proliferative activity increased after 20 h and the highest 3H-TdR index was found to be 24.7% after 45 h. In the case of hemicystectomy the labeling index in the stump increased 15 h postoperatively and the highest 3H-T-dR index was determined at 20 h with 16.6%. The urothelial cells in the area of resection began to proliferate synchronously with those in the stump and the maximum of DNA synthesis was measured 35 h postoperatively with an 3H-TdR index of 24.2%. After 2 weeks the proliferative activity within the stump and operative region corresponded to that of the control urothelium. The basal cells showed absolutely the highest proliferative activity, the suprabasal cells exhibited on the other side the highest regenerative potential compared with the control urothelium. Partial cystectomy might possibly serve as an experimental model for testing low potential carcinogens, in the case where carcinogenic effects would be initiated, potentiated and accelerated via stimulation of the DNA synthesis.