Broadly stated the goal is to find how a particular cell type is selectively induced to grow in response to a body need. Rat liver, a well studied model whose cells are normally in a state of growth arrest (G/o phase), undergoes a remarkable outburst of proliferation activity in response to a sudden excessive work load, imposed experimentally by partial hepatectomy.
The aim i s to explore molecular mechanisms through which a physiological work load may be translated into signals that initiate liver growth. The question is relevant to restoration or repair of lost or damaged liver and other tissues, as well as to future development of an artificial liver, and also to our understanding of neoplasia. Moreover, the hepatocyte culture studies that we propose, if successful, could lead to reduced animal usage. We plan to address the problem directly and in a novel way. The working hypothesis is that a large metabolic imbalance elicits hormonal responses which comprise one of the signaling mechanisms employed by the body to impose its demands on target tissues. The individual needs of the tissues themselves are mediated by peptide growth factors which act at short range, primarily within the tissue. The combined interaction of the hormone(s) and growth factors initiate growth. We will focus on whether, when and in what way combinations of these effectors alter the expression of four genes: two with direct roles in glucose metabolism (gt-1 and gt- 2), one that controls a battery of metabolic and related genes and seems also to depress growth (c/ebp), and one a recognized growth suppressor (rb), whose involvement in liver growth has not been studied. Pilot studies point to growth associated changes in expression of these genes (mRNA abundance). To pinpoint stages of the cell cycle at which these changes occur (most are early), we will monitor expression of known cell cycle marker genes (oncogenes and others related to growth). Conduct of this research involves extensive use of primary hepatocyte cultures and parallel studies in several whole animal models. A major determinant of hepatocyte performance in culture is the extracellular matrix to which the cells are attached. In almost universally used conventional culture systems, hepatocytes rapidly lose certain transcriptional capabilities and normal programs of gene expression are drastically altered. Other matrices offer partial solutions, and the gene studies can progress while we explore additional matrices that maintain a closer semblance of the hepatocyte phenotype. Nearly all of these experiments are in new, unexplored territory; we emphasize correlation with in vivo studies for assessment of physiological relevance. New approaches centered on molecular mechanisms offer a new look at hepatocyte growth control.
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