THE RESPONSE OF HEPATOCYTES ISOLATED FROM PHENOBARBITONE TREATED MICETO MITOGENIC GROWTH-FACTORS

The ability was investigated of epidermal growth factor (EGF) and hepa tocyte growth factor (HGF) to stimulate DNA synthesis in hepatocytes i solated from C57B1/6J mice following 1, 3, 7, 30 and 90 days pre-treat ment with the hepatomegalic drug, phenobarbitone (PB). A 3-fold increa se in S-phase labelled hepatocytes was observed in the absence of grow th factors after 3 days treatment with PB, which was not seen at other investigated time points. This suggests that the proliferative influe nce present in vivo at this time interval is maintained in the ex vivo model. Maximum labelling indices of >5-fold the unstimulated control value were observed in hepatocytes isolated from control and 1 day PB pre-treated mice when cultured in the presence of 5 or 10 ng/ml EGF or HGF. Hepatocytes isolated from 3, 7, 30 or 90 day treated mice showed a considerably reduced responsiveness to growth factors; maximum labe lling indices did not exceed by a factor of 2 the value obtained in th e absence of growth factors. However, the apparent decrease in respons iveness to growth factors in hepatocytes isolated from 3 day pre-treat ed mice was due to an increased background level of proliferation and the attainment of a 'ceiling level' of DNA synthesis at approx. 35%. D NA synthesis was not further enhanced by addition of both EGF and HGF. This maximal level of stimulation may indicate that only a specific h epatocyte sub-population is capable of responding to growth factors un der the conditions employed. The loss in sensitivity to mitogenic stim uli after 7 days PB pre-treatment correlates with a reported decrease in receptor protein and mRNA levels in rats and coincides with the in vivo shift from hyperplasia to hypertrophy.