and the elaboration of a specialized adipogenic 
phenotype. 
It is well established that the process of terminal 
adipocyte differentiation entails ordered changes in 
gene expression. Dr. Daniel Lane and his colleagues 
(Johns Hopkins University School of Medicine) have 
identified genes induced during fat cell differentia- 
tion that encode a variety of enzymes and fatty acid- 
binding proteins required for the synthesis and stor- 
age of triglycerides, lipids, and fats. Surprisingly, 
these fat-specific genes are not the direct targets of 
the aforementioned hormonal inducers. The opti- 
mal differentiation program, for example, entails 
the initial exposure of 3T3-L1 cells to insulin, dexa- 
methasone, and methylisobutylxanthine for a two- 
day period. Thereafter the last two hormones are 
removed from the culture medium, and only after an 
additional four days are the fat-specific genes in- 
duced. Apparently the inducing effects of dexameth- 
asone and methylisobutylxanthine are somehow re- 
layed during the process of terminal differentiation. 
Several years ago Dr. McKnight and Dr. Ed Birken- 
meier (a collaborative associate from the Jackson 
Laboratory) noted a correlation between adipocyte 
differentiation and the synthesis of a transcription 
factor called CCAAT/enhancer-binding protein 
(C/EBP). Neither the factor nor its encoding mRNA 
is present in proliferating 3T3-L1 preadipocytes. 
C/EBP is synthesized and accumulated to a substan- 
tial level, however, as the cells acquire their differ- 
entiated phenotype. Many of the fat-specific genes 
identified by Dr. Lane appear to be regulated by 
C/EBP. Binding sites for C/EBP occur in the pro- 
moters of many such genes, and evidence demon- 
strating a direct, activating role for C/EBP has been 
established in both test-tube and living cell assays. 
Two experiments, one conducted by Dr. Lane's 
laboratory and the other by Dr. McKnight's labora- 
tory, provided evidence confirming the role of 
C/EBP in fat cell differentiation. In the former case, 
antisense inhibition experiments were used to 
block the expression of C/EBP, and, hence, terminal 
adipocyte differentiation. Complementary experi- 
ments designed to express C/EBP prematurely dur- 
ing adipogenesis resulted in an accelerated differen- 
tiation program. 
Although such studies firmly establish the essen- 
tial role played by C/EBP in adipose conversion, 
they did not resolve the linkage between the hor- 
monal inducers of differentiation and the activation 
of the adipogenic genetic program. Neither the syn- 
thesis nor activity of C/EBP is influenced by any of 
the three adipogenic hormones. An apparent break- 
through came, however, when Dr. Zhaodan Cao 
(HHMI Associate) discovered genes encoding two 
C/EBP-related proteins, C/EBP|8 and C/EBP5. Ex- 
pression profiles of these genes correlate with the 
presentation and withdrawal of dexamethasone and 
methylisobutylxanthine. Moreover, it was shown 
that dexamethasone is a direct inducer of transcrip- 
tion of the gene encoding C/EBP5 and that methyl- 
isobutylxanthine is a direct inducer of the gene en- 
coding C/EBP/3. Dr. McKnight and his colleagues 
have tentatively speculated that the and d isoforms 
of C/EBP represent relay switches that pass on the 
effects of their inducing, adipogenic hormones. 
Tests of this hypothesis are now under way. Should 
the notion turn out to be correct, the work will have 
provided a satisfying extension to the pioneering 
studies of Dr. Green, as well as the groundwork for 
future molecular biological studies on the problem 
of cell differentiation and specialization. 
Dr. McKnight is also a staff member in the De- 
partment of Embryology at the Carnegie Institu- 
tion of Washington, Baltimore, and Adjunct Pro- 
fessor in the Departments of Biology and of 
Molecular Biology and Genetics at the Johns Hop- 
kins University School of Medicine. Effective in the 
next year, he will leave to assume the position of 
Research Director of Tularik, Inc. 
Articles 
Cao, Z., Umek, R.M., and McKnight, S.L. 1991. 
Regulated expression of three C/EBP isoforms 
during adipose conversion of 3T3-L1 cells. Genes 
Dev 5:1538-1552. 
Lamb, P., and McKnight, S.L. 1991. Diversity and 
specificity in transcriptional regulation: the bene- 
fits of heterotypic dimerization. Trends Biochem 
Sci 16:417-422. 
Thompson, C.C., and McKnight, S.L. 1991. Anat- 
omy of an enhancer. Trends Genet 8:232-236. 
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