Regulation of Gene Expression 
in Steroid Hormone Biosynthesis 
Keith L. Parker, M.D., Ph.D. — Assistant Investigator 
Dr. Parker is also Associate Professor of Medicine and Biochemistry at Duke University Medical Center. 
After attending Williams College, he earned his M.D. and Ph.D. degrees in genetics at Washington 
University, studying with Donald Shreffler. He served as intern and resident in internal medicine at 
Parkland Memorial Hospital, Dallas. He then moved to the Department of Genetics at Harvard Medical 
School, where he was a postdoctoral fellow with Jonathan Seidman. Dr. Parker 's next move was to the 
faculty of Duke University Medical Center. 
THE adrenal gland plays an essential role in the 
body's ability to respond to stress. Two dif- 
ferent parts of the adrenal gland, an outer cortex 
and an inner medulla, produce distinct compo- 
nents of this response. The medulla produces epi- 
nephrine and norepinephrine, which are re- 
leased very rapidly, preparing the organism for 
immediate physical activity. In contrast, the cor- 
tex produces steroid hormones, which are re- 
leased more slowly and exert prolonged effects. 
These adrenal steroids constitute two major 
classes: glucocorticoids, which are made in the 
inner zone of the cortex and control carbohydrate 
metabolism, and mineralocorticoids, which are 
made in the outer zone and regulate salt and 
water balance. Both classes of steroid hormones 
are formed from cholesterol by the sequential ac- 
tion of a related group of steroidogenic enzymes. 
One of these, the cholesterol side-chain cleavage 
enzyme, is expressed in all steroidogenic tissues. 
A second, 2 1 -hydroxylase, is expressed through- 
out the adrenal cortex. Finally, there are distinct 
forms of 1 1/3-hydroxylase: one form produces 
mineralocorticoids in the outer zone, and the 
other produces glucocorticoids in the inner 
zone. The physiological regulators of these two 
classes of adrenal steroids differ markedly, de- 
spite the shared role of the same enzymes in their 
biosynthesis. 
We are interested in defining the events that 
control the expression of the adrenal steroido- 
genic enzymes. These studies have addressed two 
major questions. First, what mechanisms direct 
the expression of these related genes within adre- 
nocortical cells? Second, what determines the 
functional differentiation of the adrenal cortex 
into mineralocorticoid- and glucocorticoid- 
producing zones? 
Our studies of gene regulation have focused on 
the 5'-flanking regions of these genes. This part, 
the promoter region, contains most sequences 
important in transcriptional regulation of other 
genes. These promoter analyses identified a pro- 
tein, steroidogenic factor 1, that plays a major 
role in regulating the expression of all three ste- 
roidogenic enzymes. We only found this protein 
in cells that made steroid hormones, suggesting 
that it contributes to the cell-selective expression 
of these genes. The global role of this protein in 
the expression of three distinct genes suggests 
that it coordinates the adrenocortical expression 
of a network of enzymes. 
We next used cow adrenal glands, providing 
much greater amounts of protein, and purified 
the bovine form of steroidogenic factor 1. The 
presently available amounts of purified protein 
should be sufficient to determine its amino acid 
sequence and to raise specific antibodies. The 
combination of specific antibodies and amino 
acid sequence data should allow us to clone the 
gene encoding this key regulatory protein. By 
comparing the primary structures of steroido- 
genic regulatory protein and the previously de- 
scribed transcriptional regulatory proteins (such 
as the steroid hormone receptor proteins), we 
may gain new insights into the mechanisms that 
regulate the adrenal steroidogenic enzymes. The 
specific probes and antibodies to steroidogenic 
factor 1 will further permit us to study the mecha- 
nisms that regulate its expression. These studies 
will provide new insights into the basis for tissue- 
specific differences in the production of steroid 
hormones and may identify important avenues for 
therapeutic intervention in clinical settings of 
abnormal adrenal steroidogenesis. 
In a related effort, we are trying to define the 
potential role of the steroidogenic enzymes in hy- 
pertension. Extremely prevalent, hypertension 
affects approximately 20 percent of the adult 
population and is a major risk factor for heart at- 
tacks and strokes, the leading causes of death in 
developed nations. Although the underlying de- 
fect is unknown in most cases, family studies indi- 
cate a significant genetic component. Certain an- 
imal models of hypertension have directly 
implicated disordered adrenal steroidogenesis as 
an important contributor to hypertension. We are 
therefore investigating in more detail the role of 
adrenal steroids in hypertension. 
Initial studies implicated a single 1 1(8- 
hydroxylase protein in the biosynthesis of both 
mineralocorticoids and glucocorticoids, but we 
317 
