Promotion mechanism of phenobarbital and partial hepatectomy in DENA hepatocarcinogenesis cell kinetics effect.

Diethylnitrosamine (DENA, 10 mg kg-1 per day) was fed to rats for 2, 4 and 6 weeks. One week after the cessation of DENA, animals were submitted either to partial hepatectomy or to phenobarbital administration. Partial hepatectomy did not promote neoplastic transformation, except after a 6-week DENA treatment. A minimum of phenobarbital was required to reach a significant promoting effect in DENA carcinogenesis. A too-limited treatment was ineffectual but could be compensated for by prolonged DENA administration. The phenobarbital treatment became unnecessary when neoplastic nodules were present. Phenobarbital continuously given after the carcinogen administration promoted neoplastic transformation even after a subcarcinogenic DENA treatment (2 weeks). It accelerated the pathological evolution and increased the tumour incidence. In these conditions, phenobarbital increased the proliferation advantage of preneoplastic cells over normal cells. In the different experimental modalities, the promoting effect was associated with the induction of chronic cell proliferation, the inhibition of the rapid response to the 2/3 partial hepatectomy and the mitotic circadian rhythm normally present during liver regeneration. It is concluded that the promotion mechanism could consist in disturbing the mitotic control in order to maintain, for a long time, a chronic low level of cell proliferation permitting the selective growth of preneoplastic cells and their subsequent transformation.