Persisting long-term effects of a single carcinogenic dose of methylnitrosourea on epidermal growth in mice

The cell proliferation in hairless mouse epidermis was studied before tumor development following a single application of a carcinogenic dose (2 mg) of N-methyl-N-nitrosourea (MNU). The number of basal and suprabasal cells, the mitotic index (MI) and the mitotic rate (MR) were scored in histological sections. The [3H]thymidine labeling index (LI) and the mean grain count (MGC) were scored in histological sections or in smears of basal cells. Flow cytometric two-parameter analyses of cellular DNA and protein content were performed on isolated epidermal basal cells. An increased MR and a slight but consistent epidermal hyperplasia were found. A 24-h study performed 25 weeks after MNU application showed that the MR and MI were altered in a circadian stage-dependent manner with considerably increased values around noon when the circadian rhythms had their peaks, followed by normal values around midnight. The LI was generally increased, but showed a normal circadian rhythm with high values at night and low values during day. The MGC was reduced at night and in the morning when the LI values were high. The results show that a single carcinogenic dose of MNU caused alterations in the epidermal growth kinetics that persisted until tumor development. The altered growth parameters, however, had circadian rhythms that were in phase with control rhythms. Assuming a constant size of the proliferative compartment, the increased mitotic activity indicated a considerable shortening of the mean cell cycle time.