ELUCIDATING THE MOLECULAR MECHANISMS UNDERLYING EPIDERMAL GROWTH, 
DIFFERENTLVTION, AND DEVELOPMENT IN HUMAN SKIN 
Elaine Fuchs, Ph.D., Associate Investigator 
The long-range objective of this laboratory is to 
understand the regulation of the expression of 
human genes during differentiation and develop- 
ment in epithelial tissues, particularly those of the 
skin. An understanding of the biochemical mecha- 
nisms underlying differentiation is a prerequisite to 
elucidating how these processes go awry in epithe- 
lial diseases. Epithelial tissues have a common pro- 
tective function, which is manifested by the produc- 
tion of an extensive cytoskeletal architecture. The 
unique 8 nm keratin filaments of this framework ac- 
count for 1- 5% of the protein in simple epithelial 
tissues, 30% of the protein in mitotically active epi- 
dermal cells, and up to 85% of the protein in fully 
differentiated epidermal squames and hair cells. 
There are more than 20 different keratins. These 
keratins can be subdivided into two types (I and II), 
both of which are essential for filament assembly. 
Keratins are frequently expressed as specific type I 
and type II pairs, and the pattern of pairwise ex- 
pression varies with epithelial cell type and with 
relative stages of differentiation and development. 
Expression of keratins is also sensitive to the cellu- 
lar environment, suggesting that keratins are tai- 
lored to suit the varied structural and functional 
needs of each epithelial cell. 
I. Regulation of Epidermal Differentiation by Vitamin A 
Previously this laboratory has optimized condi- 
tions for cultivation of human epidermal cells; most 
of their differentiative functions, including stratifi- 
cation and expression of the differentiation-specific 
keratins Kl and KIO, are maintained. In the past 
year these methods were extended to produce two 
additional model systems: one for studying trans- 
formation of keratinocytes by human papillo- 
maviruses and one for studying squamous cell car- 
cinomas (SCCs) in vitro. For both systems the 
morphological and biochemical features of abnor- 
mal differentiation are very similar to those that 
occur in the disease state in vivo. 
By culturing normal epidermal and SCC cells on 
floating coUagen-fibroblast lattices, the laboratory 
has investigated the effects of vitamin A and its ana- 
logues on normal and abnormal differentiation and 
on growth. Normal epidermal cultures produce a 
single layer of mitotically active basal cells that ex- 
press the type I keratin K14 and the type II keratin 
K5. As these cells undergo a commitment toward 
terminal differentiation, they downregulate the ex- 
pression of these basal keratins and induce the 
expression of keratins KIO and Kl. SCC-13, a cell 
line from a human squamous cell carcinoma of the 
skin, produces 2-3 layers of basal cells, with a 
much greater proportion of cells in S phase (DNA 
synthesis) than normal. As these cells undergo a 
commitment toward terminal differentiation, they 
downregulate the expression of K5 and K14 and in- 
duce the expression of K6 and Kl6, a pair of kera- 
tins not normally expressed in the epidermis in 
vivo but induced in a variety of epidermal diseases 
associated with hyperproliferation, e.g., psoriasis 
and squamous cell carcinomas. Surprisingly, the ex- 
pression of these hyperproliferation-associated ker- 
atins occurs in the nondividing population of SCC- 
13 cultures. Moreover, when retinoids are added to 
the culture medium at a concentration 10-fold 
higher than physiological, they increase the prolif- 
eration of keratinocyte cultures, and yet they in- 
hibit differentiation of both normal and SCC-13 
cells. This inhibition includes the suppression of 
Kl/KlO (normal) and K6/K16 (hyperproliferation- 
associated) keratins. Whether retinoids can reverse 
the differentiation process in a cell already commit- 
ted to differentiate terminally or whether retinoids 
suppress the choice of mitotically active basal cells 
to undergo a commitment to differentiate termi- 
nally awaits additional investigation. 
Retinoids are commonly used in the clinical treat- 
ment of many different skin disorders. The finding 
that retinoids can inhibit both the expression of 
K6/K16 and also the abnormal differentiation typi- 
cally associated with hyperproliferation might ex- 
plain some of the therapeutic effects of vitamin A 
and its analogues on the skin. However, retinoids 
can increase the population of basal-like cells with- 
out a loss in their proliferative potential, which is a 
worrisome side effect of the successful inhibition of 
abnormal differentiation. 
II. Divergence of Epidermis and Hair Follicle Path- 
ways of Differentiation During Embryogenesis. 
All epithelial components of the skin are derived 
from a single layer of cells, the embryonic basal 
layer. In the past year this laboratory has begun to 
elucidate the timing and appearance of biochemical 
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