Transcription Factors in Cell Growth and Kidney Differentiation 
marker, studies like these are helping to define 
the circuitry among second messenger cascades. 
Additional studies currently under way are 
aimed at defining structure-function relation- 
ships of the Egr-1 protein. The protein contains 
several activation domains and a small modular 
repressor region whose sequence has been highly 
conserved in evolution. Nuclear localization is 
dependent upon a bipartite sequence, and DNA 
binding is determined by the three zinc fingers. 
A major challenge ahead is to define a func- 
tional role for the Egr l protein. Attempts at 
abrogating Egr- 1 activity may prove useful in de- 
fining a phenotype. In this regard, the use of anti- 
sense methodology — either as oligonucleotides 
or in the form of stable inducible vectors — is 
under investigation. The latter technique, when 
applied to c-Fos, has resulted in a remarkable and 
reversible inhibition of cell growth. Another ap- 
proach is also available to inhibit Egr- 1 activity: 
nature has designed its own inhibitor of Egr-1 
function in the form of a tumor-suppressor gene, 
one whose absence is likely to lead to Wilms' tu- 
mor (see below). 
Transcription Factors in Kidney 
Development 
A second recent focus of our laboratory is on 
trying to define molecular events that character- 
ize the development of the kidney. It is well 
known that metanephric mesenchymal (blaste- 
mal) cells convert into epithelial cells over a 4- to 
5 -day period (in the rat or mouse) in response to 
invasion of the ureteric bud. Little is known at the 
molecular and cellular level of the events that 
transpire during this conversion process. Our aim 
is to define a hierarchy of transcriptional regula- 
tors whose expression is modulated during 
nephrogenesis. Toward this end, several studies 
are under way. 
One strategy is to identify zinc finger transcrip- 
tional regulators expressed in the kidney during 
differentiation. Using a so-called H/C-link probe 
directed against a region often conserved in zinc 
finger proteins, we have isolated from a kidney 
library a dozen cDNA clones encoding zinc finger 
proteins. These are now being characterized by 
their developmental profile (Northern blots and 
in situ hybridization) and by limited sequence 
analysis. One of these clones hybridizes at high 
stringency to the candidate Wilms' tumor anti- 
oncogene (WTl) identified recently by reverse 
genetic means in the laboratories of David Hous- 
man (Massachusetts Institute of Technology) 
and Gail Bruns (Harvard University). Another 
clone has identified a zinc finger protein that 
is expressed very early in human kidney 
development. 
In a second approach, work is in progress to set 
up stem cell cultures from embryonic kidney for 
the development of cell lines arrested at different 
stages of differentiation. If these cells can be 
maintained and then made to differentiate in cul- 
ture under appropriate conditions, they will be 
invaluable for studying the early molecular and 
cellular events in nephrogenesis. 
Third, we are pursuing an interesting connec- 
tion between Egr-1 and WTl . WTl is a zinc finger 
protein; three of its four zinc fingers show a 60- 
70 percent similarity to the three zinc fingers of 
Egr l. Indeed, WTl and Egr-l bind to a common 
sequence, as shown by the work of Frank 
Rauscher (Wistar Institute). We have recently 
shown (in collaboration with Dr. Rauscher) that 
WTl acts as a transcriptional repressor, whereas 
Egr- 1 is a transcriptional activator. These findings 
may have exciting biological implications, be- 
cause several binding sites exist for the Egr-1/ 
Wilms' tumor proteins in the promoter sequence 
of the insulin-like growth factor II (IGF-II). 
IGF-II levels are known to be high in Wilms' 
tumor, and these levels fall during development. 
Thus the possibility exists that IGF-II is a target 
for the repressive action of the Wilms' tumor pro- 
tein and for positive regulation by Egr- 1 . If this 
model holds, it will be a way to explain the find- 
ings of aberrant regulation of IGF-II in Wilms' 
tumor and will also provide major insight into the 
mesenchymal cell-to-epithelial cell conversion 
in the kidney. 
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