We have previously demonstrated that hepatocyte proliferation induced by the
mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)]
benzene (
TCPOBOP) is independent of changes in
cytokines, immediate early genes, and
transcription factors that are considered to be necessary for regeneration of the liver after partial
hepatectomy (PH) or
necrosis. To further investigate the differences between
mitogen-induced mouse hepatocyte proliferation and liver regeneration after PH, we have measured the expression of
cyclin D1,
cyclin D3,
cyclin E, and
cyclin A and of the
cyclin-dependent kinases CDK2, CDK4, and CDK6. The involvement of the
cyclin-dependent kinase inhibitors p21 and p27 and of the oncosuppressor gene p53 was also examined at different times after stimulation of hepatocyte proliferation. Results showed that a single administration of
TCPOBOP caused a very rapid increase in the levels of
cyclin D1, a G1
protein, when compared with two thirds PH (8 hours versus 30 hours). The early increase in
cyclin D1 protein levels was associated with a faster onset of increased expression of S-phase-associated
cyclin A (24 hours versus 36 hours with PH mice). Accordingly, measurement of
bromodeoxyuridine (
BrdU) incorporation revealed that, although approximately 8% of hepatocytes were
BrdU-positive as early as 24 hours after
TCPOBOP, no significant changes in
BrdU incorporation were observed at the same time point after two thirds PH. The expression of other
proteins involved in cell cycle control, such as
cyclin-dependent kinases (CDK4, CDK2, CDK6), was also analyzed. Results showed that expression of CDK2 was induced much more rapidly in
TCPOBOP-treated mice (2 hours) than in mice subjected to PH (36 hours). A different pattern of expression in the two models of hepatocyte proliferation, although less dramatic, was also observed for CDK4 and CDK6. Expression of the CDK inhibitors p21 and p27 and the oncosuppressor gene p53 variably increased after two thirds PH, whereas basically no change in
protein levels was found in
TCPOBOP-treated mice. The results demonstrate that profound differences in many
cell cycle-regulatory proteins exist between direct
hyperplasia and compensatory regeneration.
Cyclin D1 induction is one of the earlier events in hepatocyte proliferation induced by the primary
mitogen TCPOBOP and suggests that a direct effect of the
mitogen on this
cyclin may be responsible for the rapid onset of
DNA synthesis observed in
TCPOBOP-induced
hyperplasia.