Regulation of Keratin Expression During 
Differentiation and Development in Human Skin 
Elaine V. Fuchs, Ph.D. — Investigator 
Dr. Fuchs is also Professor in the Departments of Molecular Genetics and Cell Biology and of Biochemistry 
and Molecular Biology at the University of Chicago. She received her B.S. degree in chemistry from the 
University of Illinois and her Ph.D. degree in biochemistry from Princeton University, where she studied 
with Charles Gilvarg. Her postdoctoral research was done with Howard Green at the Massachusetts Insti- 
tute of Technology. Dr. Fuchs counts among her honors the R.R. Bensely Award from the American Asso- 
ciation of Anatomists. 
THE long-range objective of my research is to 
understand the biochemical mechanisms that 
operate and regulate the expression of human 
genes during development and differentiation in 
skin. Our present knowledge of the biochemistry 
of human skin and its diseases is limited. Al- 
though dermatologists have always directed their 
interest toward human skin biology and skin dis- 
eases, the field of molecular biology has only re- 
cently approached a level of understanding that 
permits probing the complex biochemistry of 
human skin. A major factor facilitating such stud- 
ies is the ability to grow human skin cells — in- 
cluding epidermal cells, dermal fibroblasts, mela- 
nocytes, and dermal papillae cells — in tissue 
culture. These culture systems provide essential 
experimental models for studying many genetic 
skin diseases and skin cancers. 
Much of our research on human skin has fo- 
cused on the epidermis. The epidermis com- 
prises about 20 cell layers, of which the outer- 
most is the skin surface. Only the inner or basal 
layer of the epidermis is truly living and under- 
goes DNA synthesis and cell division. Under an as 
yet unidentified trigger, a basal cell ceases to di- 
vide and makes a commitment to differentiate ter- 
minally. As the cell moves outward to the skin 
surface, it undergoes a variety of morphological 
and biochemical changes. The most pronounced 
of these changes is the production of a dense net- 
work of keratin filaments, which are tough, resil- 
ient protein fibers. Many skin diseases of the epi- 
dermis, including psoriasis and basal and 
squamous cell carcinomas, involve a malfunc- 
tioning of the diflferentiative process that is fre- 
quently associated with some abnormality in the 
production or organization of these keratin fila- 
ments. Our investigation is focused on the regula- 
tion of the expression of keratin proteins and 
their genes in human epidermis and in epidermal 
cells differentiating in tissue culture. 
The keratins are a group of 10-20 closely re- 
lated proteins (40-70 kDa) that form the 10-nm 
keratin filaments in the cytoplasm of epidermal 
cells. Only a subset (typically 2-6) of the kera- 
tins are ever expressed at any one time. As a nor- 
mal epidermal cell differentiates, it changes the 
subset of keratins that it makes. In addition, the 
cell increases its keratin synthesis, leaving the 
fully differentiated epidermal cell with 85 per- 
cent of its total protein as keratins. In diseases of 
the skin involving epidermal hyperproliferation, 
including psoriasis and squamous cell carci- 
nomas, a new subset of keratins not normally 
made in the epidermis is produced. This new set 
of keratins is diagnostic for hyperproliferative ab- 
normalities of the skin. 
A coordinated genetic and biochemical ap- 
proach is necessary to determine the regulation 
of the multiple keratins and to decipher their 
structural and functional roles in the differentiat- 
ing epidermal cell. A number of years ago, we 
showed that expression of different subsets of 
epidermal keratins is due to changes in the syn- 
thesis of different keratin mRNAs. We used DNA 
recombinant technology to show that the epider- 
mal keratin mRNAs are encoded by about 20 dif- 
ferent genes and that these genes are of two dis- 
tinct types. Type I encodes small keratins (40-53 
kDa); type II encodes larger keratins (53- 
67 kDa). 
Keratins are expressed as specific pairs of type 
I and II proteins. In the past year, we showed that 
the basic subunit of keratin filaments is a hetero- 
dimer, composed of one molecule of each of the 
two keratin types. Approximately 20,000 hetero- 
dimers are necessary to form a single 1 0-nm fila- 
ment; the assembly process is energy-indepen- 
dent and does not appear to require any auxiliary 
proteins or factors. 
Using DNA sequencing, we have determined 
the amino acid sequences for several different 
keratin pairs. The sequences that are likely to be 
involved in the assembly process are highly con- 
served among keratin pairs, whereas the se- 
quences that protrude along the surface of the 
keratin filament are different for different pairs of 
keratins. Hence the structures of keratin fila- 
ments assembled from different protein pairs are 
similar, but they are coated with different se- 
quences and are likely to interact with different 
proteins inside the cell. In this way the cytoskele- 
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