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

Epithelial tumours of the lower urinary tract in humans and rodents

Bladder neoplasms that are morphologically similar to those occurring in the human bladder can be induced in rats and mice. Thus, these animal models can provide useful information in elucidating the histogenesis of the human bladder cancer. It is unknown whether human and rodent bladder cancers share similar molecular mechanism(s). There are species-specific differences in the type of neoplastic lesions. In rats, irrespective of the carcinogen used, the lesions tend to be polypoid exophytic masses and invasion is a late event, occurring only after a large dose of carcinogen and after an extended period of observation. On the other hand, mice tend to develop nodular invasive carcinomas readily preceded by the development of CIS. In both species, the neoplasms tend to show squamous differentiation, and metastatic spread is rare. Nevertheless, both species provide a useful model for the assessment of human risk of test chemicals because the principal neoplastic lesions that develop in response to test carcinogens are transitional cell neoplasms. Further study of cases of chemically, induced bladder cancer in rodents is essential to determine the applicability of the mode of action to humans.

Development of N-butyl-N-(hydroxybutyl)-nitrosamine-induced tumors in the partially resected, proliferating rat urinary bladder in dependence upon the time of onset of stimulated DNA synthesis

Tumor development was investigated in the partially resected, proliferating urinary bladder of rats in dependence upon the onset of stimulated de novo DNA synthesis related to carcinogen dosing. N-butyl-N-(4-hydroxybutyl)-nitrosamine (BBN) was used as carcinogen and administered by gavage in three fractionated doses (100 mg/kg body weight each) either during the phase of the most pronounced proliferation of the urothelium 30, 45 and 70 h after one-third resection of the bladder or 24 h and 1 week prior to partial cystectomy. When BBN was given during most increased DNA synthesis subsequent to one-third resection, the incidence of bladder tumors was reduced to 8.7% compared with 19.6% found in control animals with a non-resected, quiescent bladder. Tumor formation was neither inhibited nor enhanced when BBN was initially administered, followed by partial cystectomy 24 h or 1 week after the last carcinogen dose, yielding tumor incidences of 18.2% and 22.5%, respectively. Thus, the feeding of BBN during the period of maximum DNA synthesis inhibited tumor development in the partially resected bladder, while stimulation of cell replication subsequent to carcinogen administration did not influence the carcinogenic process initiated. The results obtained indicate that time of onset of stimulated DNA synthesis related to carcinogen dosing is the decisive factor in modifying urothelial carcinogenesis in the proliferating urinary bladder.

Cell cycle of normal bladder urothelium in developing and adult mice

The present research has employed a novel, nonradioactive technique to quantitatively study normal urothelial proliferation in foetal, neonatal, juvenile and adult mouse bladder. Using whole mount histological preparations, the total number of urothelial nuclei per mouse bladder, and per given urothelial cell layer, have been assessed to provide data of the (unstimulated) kinetic behaviour of basal urothelial cells (the proliferative population), to analyse characteristics of the normal urothelial cell cycle. The urothelial cell cycle time increases from 30.6 h (foetal) to 40 weeks (adult), the duration of mitosis from 0.23 h (foetal) to 2.71 h (adult) and the duration of DNA synthesis from 2.52 h (neonatal) to 10.83 h (adult). These are average values for the urothelial cell cycle, which do not preclude the possible existence of proliferative units. The ratio of superficial nuclei to basal and intermediate nuclei, possibly indicative of a urothelial proliferative unit, declines to reach a plateau (1:40) in adult mice. These findings indicate that rapid urothelial proliferation during early murine development was likely to be a) biologically useful, since intrauterine foetal metabolic activity may require a functional bladder urothelium at an early stage, b) kinetically similar to acutely regenerating adult urothelial cells after cytotoxic insult. During murine life, the range of durations of mitosis and DNA synthesis is much less than the range of cell cycle times. Normal unstimulated urothelium of adult mice was confirmed to proliferate slowly.

Synchronization of stimulated urothelial proliferation. Experimental models for cell cycle specific testing of bladder carcinogens

In the present experiments an attempt was made to synchronize urothelial proliferation in the urinary bladder of rats stimulated by either a partial cystectomy (one-third resection) or a single i.p. administration (100 mg/kg) of CP. To temporarily inhibit DNA synthesis HU was given intraperitoneally in multiple fractionated doses (0.1 mg/g each) at hourly intervals during the period of most pronounced proliferative activity between 33 and 55 h after partial cystectomy and between 26 and 44 h after injection of CP. Following partial cystectomy the 3H-TdR index rapidly increased after termination of the HU administration reaching peak values of 54% and 56% at 6 and 8 h, respectively. Thereafter, there was a sharp decline of the percentage of DNA synthesizing cells within 2 h to 24% at 10 h. Then 16 h after removal of the HU block the 3H-TdR index amounted to 15%. At 20 h the labeling increased again to 22%, indicating that the initially blocked cells were capable of going through another cell cycle. After 1 week the 3H-TdR index was 2.5% and after 15 days 0.2%. Synchronously with the decrease of DNA synthesis the mitotic index rapidly increased reaching a maximum value of 4.3% at 10 h. The total fraction of 3H-TdR-labeled cells (growth fraction) was 57%. Following administration of CP 3H-TdR incorporation increased steeply after the last injection of HU and at 6 h a maximum value of 50% was obtained. Subsequently, the 3H-TdR index gradually decreased to 11% after 12 h. At 8, 15, and 30 days labeling indices of 1.9%, 0.5%, and 0.3% were determined. The mitotic index was highest with 0.21-0.22% between 12 and 16 h after removal of the HU block. The growth fraction amounted to 53%. The results reported here show a satisfactory degree of synchrony of stimulated urothelial proliferation obtained by multiple fractionated doses of HU. In particular the cystectomy model will be useful for testing possible cell cycle specificity of urothelial carcinogenesis.

Renewal of normal urothelium in adult mice

Normal unstimulated urothelium of adult mice appears to be essentially quiescent, as demonstrated by an 3HTdR-pulse labelling index of 0.11%. There is no evidence that this labelling index may be artificially low. Results from continuous labelling experiments, where 3HTdR was supplied in the drinking water of mice for varying periods, render it possible that the urothelium satisfies its small renewal requirements via a low growth fraction of cycling cells. The remainder may consist of quiescent cells, which may be capable of resuming cell cycle progression when required, and of post-mitotic cells, which are incapable of further proliferation. There are three possible levels of adult urothelial proliferation with cell cycle times of about a) one year (if the urothelium is homogeneous and the growth fraction unity), b) three weeks (if the growth fraction is about 5%), and c) 15 h (when the urothelial cells are rapidly regenerating).

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.

Urothelial proliferation in growing mice

Developing murine urothelium undergoes pronounced proliferation until at least 10 days after birth. Thereafter, both mitotic and [3H]TdR-labelling indices fall sharply with age. The ratio of labelling to mitotic indices also alters dramatically during development, which is probably due to both endoreduplication and changes in the relative durations of the DNA synthesis and mitotic phases. This ratio reaches stability at 5 weeks of age. The adult labelling and mitotic indices were 0.11 and 0.019% respectively, indicating a very slow turnover.

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.

Cell cycle kinetics of regenerating urothelial cells of the rat urinary bladder after partial cystectomy

The cell cycle kinetics of bladder urothelial cells regenerating after partial cystectomy were investigated in 96 female Wistar rats using the percentage labelled mitoses method. In the area of resection a mean cell cycle time (TC) of 15 h was determined. The DNA synthesis phase (TS) lasted 6 h and the premitotic-postsynthetic phase together with the mitosis phase (TG2 + M) 1.5 h, thus giving a presynthetic-postmiotic phase (TG1) of 7.5 h. Similar values were found for the urothelial cells in the stump: the mean cycle time measured 14 h, the TS-phase 6 h, the TG6 + M-phase 2 h and the TG1-phase 6 h. These data are discussed with respect to known cell cycle parameters of bladder urothelium regenerating in response to cytotoxic agents and of neoplastic urothelial cells. The reported findings provide a basis for further investigations using weak carcinogens and threshold doses of potent carcinogens to test the working hypothesis that stimulation of proliferation following partial cystectomy is capable of initiating, accelerating and/or potentiating carcinogenic cell transformation in the urinary bladder.

Cytopathological effect of partial cystectomy of rats

The effect of partial cystectomy on induction of DNA synthesis in the urinary bladder epithelium was studied in a total of 147 male F344 rats. DNA synthesis started 12 hours after 75% or 50% resection of the bladder, and reached a maximum at 48 hours with a mean labelling index of 15.8% after 75% partial cystectomy and 13.8% after 50% partial cystectomy. In these two groups the proliferative activity of bladder epithelial cells returned to normal after 2 weeks and 1 week, respectively. Cytodifferentiation of epithelial cells, observed by transmission and scanning electron microscopy, remained normal during the regeneration process.