Hepatic compensatory growth (HCG), or liver regeneration, is a critical physiological response to liver injury, disease or liver resection. The capacity for HCG diminishes with age, causing complications in individuals with liver disease or undergoing hepatic resection for treatment of hepatocellular carcinoma. Plasminogen activator inhibitor type 1 (PAI-1) is an immediate early gene expressed during HCG in the two month old rat. There are age-dependent changes in plasma PAI-1 and in both plasminogen activators, but little is known about hepatic PAI-1 gene expression or age-related alternations of its expression during HCG. The two primary mediators of HCG, hepatocyte growth factor (HCG) and transforming growth factor type beta (TGF-beta), can regulate PAI-1 gene expression in certain cell lines, but nothing is known about their capacity to regulate PAI-1 in primary hepatocytes. HGF and TGF-beta must be proteolytically cleaved for activation; components of plasminogen activation can accomplish this activation. Since PAI-1 is a primary modulator of plasminogen activation, it may serve to regulate the balance of mature HGF and TGF-beta by regulating their maturation. Such a feedback loop would connect a triad involving HGF, TGF-beta and PAI-1 which could function in the control of the initiation and progression of HCG. In this model, alterations in the regulation of any of these molecules would result in aberrations in HCG, as observed in the aged. The objective of this proposal is to test the hypotheses first, that the expression of PAI-1 during liver regeneration is altered with increasing age; and second, that the alterations in the expression of hepatic PAI-1 are due to age-dependent changes in the mechanisms of regulation of PAI-1 by HGF and TGF-beta. The regulation of PAI-1 gene expression during liver regeneration in the aged rat will be analyzed by studies of hepatic growth following 70% partial hepatectomy. The kinetics and site of mRNA induction will be determined by Northern blot analysis and in situ hybridization. The regulation of PAI-1 gene expression by HGF and TGF-beta will be studied in primary cultures of young and aged rat hepatocytes. Regulatory mechanisms will be studied by analysis of gene transcription and mRNA stability, as well as identification of the cis-acting DNA sequences and trans-acting proteins involved. Specific DNA protein interactions identified will be compared to those observed in vivo during liver regeneration. Age-dependent alterations of hepatic PAI-1 synthesis potentially may effect the balance of the major mediators of HCG. Understanding the mechanisms of regulation of PAI-1 during liver regeneration may allow the development of new strategies for management of liver disease in the aged.
| White, L A; Bruzdzinski, C; Kutz, S M et al. (2000) Growth state-dependent binding of USF-1 to a proximal promoter E box element in the rat plasminogen activator inhibitor type 1 gene. Exp Cell Res 260:127-35 |
