CELLULAR MEMBRANE RECEPTOR SIGNALS AND THE HEPATIC GROWTH RESPONSE 
Rebecca A. Taub, M.D., Associate Investigator 
The major interests of Dr. Taub's laboratory are 
1) characterization of the growth-response genes of 
hepatic cells, 2) analysis of insulin receptor gene 
expression, and 3) studies of receptor-ligand inter- 
actions through analyses of antireceptor monoclo- 
nal antibodies. 
I. Characterization of the Growth-Response Genes 
of Hepatic Cells. 
The liver is composed primarily of hepatocytes, 
cells of epithelial cell origin, that perform vital met- 
abolic and synthetic functions. Although the adult 
liver does not normally proliferate, it retains the ca- 
pacity to regenerate after partial hepatectomy or 
chemical injury. This regeneration appears to be ac- 
complished by the induction of DNA synthesis in 
virtually all of the remaining hepatic cells. Once the 
liver has regained its initial size, it reenters a quies- 
cent state. The growth factors, both circulating and 
local, involved in this tightly regulated process may 
include insulin, glucagon, epidermal growth factor 
(EGF) [or transforming grov^^h factor a (TGF-a)], 
TGF-P, and other putative hepatic growth factors. 
Dr. Taub's laboratory is approaching the complex 
series of early events leading to liver regeneration 
after partial hepatectomy in several ways. After 
identifying the immediate-early hepatic growth- 
response genes, the laboratory will be able to com- 
pare them with those genes previously shown to be 
involved in growth responses in other tissues, such 
as fibroblasts and lymphocytes. In fibroblasts and 
lymphocytes, the immediate growth-response gene 
products appear to fall into two large classes: 1) se- 
creted proteins that may function in autocrine 
growth regulation and 2) nuclear proteins that may 
regulate subsequent nuclear events. Although it is 
presumed that growth and autocrine factors vary 
significantly in different tissues, it is not yet known 
if the same pattern of expression of nuclear regula- 
tory proteins is common to most growing tissues. 
The long-term goal of this research is to understand 
the genetic regulation of growth in hepatic and 
other epithelial cells. 
The laboratory uses two liver cell systems in 
which to study hepatic growth. The first is an H35 
rat hepatoma cell line that becomes quiescent 
under serum-deprived conditions and initiates DNA 
synthesis in response to insulin. In this cell line, 
physiologic concentrations of insulin stimulate 
many of the immediate-early growth-response 
genes such as myc, fas, egr-1 and egr-2, and jun 
that are stimulated in mitogen-treated fibroblasts 
and regenerating liver. Because it is a cell line, 
growth conditions can be easily manipulated. How- 
ever, this H35 cell line does not possess all of the 
normal hepatic properties, and therefore regenerat- 
ing rat liver itself is a second liver cell system used 
to identify hepatic grov^h-response genes. 
The techniques of differential screening and 
cDNA subtraction analysis have yielded exciting 
results in identifying immediate-early growth- 
response genes in fibroblasts and lymphocytes. 
These techniques were applied to regenerating 
liver cDNA libraries prepared 3 h post-hepatectomy 
in the presence of cycloheximide, and —20 unique 
induced genes were isolated. A similar number of 
genes has been isolated from H35 cDNA libraries 
prepared from cells treated for 3 h with insulin in 
the presence of cycloheximide. Several criteria are 
used to identify an induced gene of particular inter- 
est: 1) The gene must be different from previously 
identified genes (such as myc, fas, egr,jun), as de- 
termined by hybridization studies and DNA se- 
quence analyses. 2) On genomic Southern analysis, 
the gene should be unique or a member of a gene 
family, and conserved across species. 3) Preferably 
the gene is specifically or predominantly expressed 
in regenerating liver (and H35 cells) and not in mi- 
togen-treated fibroblasts. However, some genes ap- 
pear to be expressed specifically in regenerating 
liver and not H35 cells. 
Studies are continuing to characterize the com- 
plete immediate-early genomic response in liver re- 
generation and to perform a more detailed analysis 
of specific induced hepatic genes. 
II. Regulation of Expression of the Human Insulin 
Receptor Gene. 
The insulin receptor (IR) is an essential protein 
present on the surface of virtually all cells. Little is 
known about the control of the level of this protein 
on cellular surfaces, but it has been found that the 
IR level correlates roughly with the level of IR gene 
transcripts within cells. The IR gene promoter is 
like other housekeeping promoters: it lacks a TATA 
box and has multiple transcriptional initiation sites, 
primarily within a 300 bp GC-rich region. The long- 
term goal of this project is to understand the rela- 
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