Previous work in our laboratory has characterized hepatocyte cell cycle regulation during the perinatal and postnatal periods in the rat. Our studies showed that the cell cycle regulator, cyclin D1, plays a central role in controlling hepatocyte proliferation. We found that the abundance of cyclin D1 is regulated posttranscriptionally, and that the p38 mitogen-activated protein kinase pathway acts as a negative regulator of both hepatocyte proliferation and cyclin Dl abundance. Finally, we have demonstrated a functional relationship between hepatocyte growth arrest in the term fetus, the perinatal activation of the p38 pathway and loss of cyclin D1. These results form the basis for the current proposal. We will test the following hypotheses: 1) Hepatic p38 activity is regulated by metabolic changes that occur during the perinatal period and during the newborn-to-adult transition. 2) p38 regulates cyclin D1 abundance through one or both of two mechanisms, control of cyclin D1 translation and/or control of cyclin D1 degradation via theubiquitin/proteasome pathway. 3) This physiological regulation of p38 accounts, at least in part, for regulation of hepatocyte proliferation in the intact animal. Based on these hypotheses, we have developed the following specific aims: 1) Investigate the physiological stimuli that account for the ontogeny of p38 regulation during normal perinatal and postnatal liver development. 2) Determine the mechanism(s) by which p38 controls cyclin D1 abundance. 3) Apply our findings to animal models in which hepatocyte proliferation is modulated, including perinatal development, liver regeneration after partial hepatectomy, and the in vivo effect of exogenous growth factors on hepatocyte cell cycle activation.
Aim 1 will employ manipulation of the metabolic milieu in primary fetal hepatocyte cultures and correlation with the in vivo hepatic metabolic environment.
Aim 2 will be carried out in primary cultures of fetal hepatocytes using transient transfection or adenovirus-mediated introduction of the p38-activating kinase, MKK6.
Aim 3 will extend our findings to well-established rodent models of hepatocyte growth regulation. We anticipate that these studies will provide novel insights into the regulation of hepatocyte proliferation during normal development and in the mature rat.
