on the cell surface which supply the cells
with cholesterol by binding to cholesterol-carrying LDL particles
in serum. The receptor-bound lipoprotein was taken into the cells
through a novel process that Goldstein and Brown named “receptor-mediated
endocytosis.” Together with a colleague, Richard Anderson
(now chairman of Cell Biology at UT Southwestern), Goldstein and
Brown showed that the key event was the clustering of receptors
into depressions on the cell surface called “coated pits.”
Every few minutes these pits sink below the surface of the cell,
and the cell membrane closes over them to form vesicles that migrate
through the cytoplasm. The coated vesicles, which contain receptor-bound
LDL, fuse with other membrane organelles and eventually they fuse
with lysosomes. This exposes the LDL to proteases and lipases that
release the cholesterol from the LDL particle so that it can be
used by the cell to make new membranes. The receptor dissociates
from the LDL and returns to the cell surface where it is re-used
in a process termed receptor recycling.
By 1982, Brown, Goldstein and their colleagues had purified
the LDL receptor from the adrenal glands of cows, and the following
year they cloned its gene. The gene sequence showed that the LDL
receptor was a mosaic composed of parts shared with various other
genes. Each of these parts was encoded by a discrete exon. This
observation provided one of the earliest demonstrations that evolution
progresses by duplicating exons and shuffling them among genes.
By sequencing receptor genes from subjects with homozygous FH,
Brown and Goldstein showed directly that these subjects had inherited
mutant LDL receptor genes from both parents.
Purifying and Cloning the LDL Receptor
A particularly informative mutation was found in patient JD. This
patient had an amino acid substitution in the tail of the receptor
that extends into the cytoplasm. The mutant receptor binds LDL
on the cell surface, but it does not cluster into coated pits
and it therefore cannot carry LDL into the cell. This mutation
disclosed the first sorting signal on a membrane protein –
i.e., a discrete signal that directs the protein to its proper
membrane location in the cell.
Dissecting Receptor Regulation and
Its Implications for Statin Therapy
From the earliest studies of the LDL receptor, the Brown/Goldstein
team recognized that the receptor was regulated. When cells are
deprived of cholesterol, the number of receptors increases. Conversely,
when cholesterol accumulates in cells, the receptor gene is silenced
and the number of receptors falls. In the body, the liver produces
the most LDL receptors and therefore it removes most LDL from
blood. Goldstein and Brown realized that the number of receptors
in the liver could be made to increase if the cholesterol content
of liver cells could be lowered. This can be achieved by eating
a diet that is low in cholesterol and saturated fats. It can also
be achieved by drugs that block the synthesis of cholesterol by
inhibiting the rate-determining enzyme 3-hydroxy-3-methylglutaryl
coenzyme A reductase (HMG CoA reductase). In 1975 a Japanese scientist,
Akira Endo, discovered potent HMG CoA reductase inhibitors as
natural products produced by certain molds. Brown and Goldstein
showed that these inhibitors could raise liver LDL receptors in
dogs, and this led to a profound fall in plasma LDL levels. The
HMG CoA reductase inhibitors were then tested on people, and they
were shown to produce a profound fall in plasma LDL levels, which
in turn leads to a potent protection from heart attacks. This
class of drugs, collectively called statins, are taken daily by
more than 30 million people worldwide. Their mechanism of action
can be traced directly to the work of Brown and Goldstein.
In a quest to understand the precise mechanism for cholesterol
regulation of the LDL receptor gene, Brown, Goldstein and their
colleagues set out on a long quest to discover the postulated
transcription factor that was regulated by cholesterol. This eventually
led to the discovery of Sterol Regulatory Element-Binding Proteins
(SREBPs) and their unique mechanism of regulation (see Current
Research).
