Signal Transduction by the Epidermal Growth 
Factor Receptor 
^^^^ 
Roger J. Davis, Ph.D. — Assistant Investigator 
Dr. Davis is also Associate Professor in the Program in Molecular Medicine and the Department of Bio- 
chemistry and Molecular Biology at the University of Massachusetts Medical School. He received his un- 
dergraduate and graduate education at Cambridge University and was a postdoctoral fellow with Michael 
Czech at the University of Massachusetts. 
CELLULAR proliferation is a highly regulated 
process. During embryonic development, 
rapid cell growth is required to form the tissues 
of the body. In contrast, cellular proliferation in 
adults is slow, primarily serving to replace senes- 
cent cells. Adults, however, retain a limited ca- 
pacity for rapid grovvT;h — for example, during 
wound healing. Regulation of this proliferative 
capacity is critically important. Errors in growth 
control result in a variety of diseases, including 
cancer. 
The local production of protein growth factors 
is an important mechanism that can account for 
the control of cellular proliferation. Our research 
group is investigating the action of a family of 
peptides that includes epidermal grovvT:h factor 
(EGF) and transforming growth factor-a (TGF- 
a). These agents are synthesized as cell surface 
glycoproteins that are split to release small solu- 
ble peptides. Both the membrane-bound precur- 
sor and the diffusible peptides are biologically 
active and bind to specific receptor molecules 
located at the surface of responsive cells. Secre- 
tion of these peptide growth factors contributes 
to the rapid grovv^h of some tumors. 
The long-term goal of this laboratory is to un- 
derstand the molecular basis for the control of 
cellular proliferation by the EGF receptor. It is 
known that the binding of growth factors to this 
receptor at the cell surface triggers a complex 
series of chemical reactions that lead to DNA syn- 
thesis within the nucleus and to cell division. 
However, the molecular details of the signaling 
pathways utilized by the receptor are poorly 
understood. 
Regulation of EGF Receptor Function 
The EGF receptor is a glycoprotein consisting 
of an extracellular domain that binds growTih fac- 
tors, a membrane-spanning domain, and a cyto- 
plasmic domain. The cytoplasmic domain is an 
enzyme, tyrosine kinase, that causes the covalent 
attachment of phosphate to tyrosine components 
of substrate proteins (phosphorylation). The 
binding of EGF to the receptor's extracellular do- 
main causes an increase in the tyrosine kinase ac- 
tivity of the cytoplasmic domain. EGF also causes 
the receptor to aggregate and to associate tran- 
siently with intracellular regulatory molecules to 
form a signal transduction complex. We are study- 
ing these interactions and investigating the con- 
sequences of the phosphorylation process. 
The ability of the growth factor EGF to increase 
the tyrosine kinase activity of its receptor is 
blocked when cells are incubated with a tumor 
promoter or with other growth factors. Under 
these conditions, the EGF receptor is itself phos- 
phorylated at multiple serine and threonine resi- 
dues. We are investigating the significance of this 
phosphorylation. Our approach is to construct 
receptors with point mutations at the sites of 
phosphorylation, using recombinant DNA tech- 
nology. These studies have demonstrated that the 
phosphorylation of a single threonine residue 
blocks the ability of EGF to stimulate the recep- 
tor's tyrosine kinase activity. Phosphorylation 
also alters the internalization of the receptor. We 
are investigating the structural basis for the ef- 
fects of phosphorylation on the regulation of EGF 
receptor function. 
Signaling by the EGF Receptor 
A principal question that we must answer in 
order to understand the mechanism of signal 
transduction by the EGF receptor is how a signal 
that is initiated at the cell surface can be transmit- 
ted to the nucleus to cause DNA replication. One 
class of regulatory molecules that could account 
for this process is the protein kinases. We have 
recently identified a novel protein kinase activity 
that is acutely regulated by growth factors. This 
protein kinase is located in both the nuclear and 
cytosolic compartments of cells. Substrates that 
we have identified include the EGF receptor and 
the nuclear transcription factors expressed by the 
proto-oncogenes c-myc and cfun. We are inves- 
tigating the structure of this protein kinase by mo- 
lecular cloning, and examining the role of this 
enzyme during signal transduction. 
Tissue Specificity of Tumor Induction 
The gene for the EGF receptor is a frequent site 
of integration by avian leukosis viruses. Insertion 
of the virus causes the expression of a truncated 
EGF receptor. The formation of a virus containing 
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