Past Research

In 1968 two young physicians-in-training, Michael Brown and Joseph Goldstein, were called to treat a pair of siblings, ages 6 and 8, who were dying of recurrent heart attacks. The cause was a genetically elevated level of cholesterol-carrying low density lipo-protein (LDL) in blood. The children were born with blood cholesterol levels over 1000 mg/dl, and they began having heart attacks before age 5. The LDL Receptor Pathway Figure 1 The children suffered from a rare genetic disorder called homozygous familial hypercholesterolemia (FH), which afflicts one in a million children. Doctors Brown and Goldstein determined to find the genetic cause of this defect, in part because they hoped that this understanding would provide insights into more subtle forms of hypercholesterolemia that are responsible for one-third of all deaths in the Western world.

Six year old girl with homozygous familial hypercholesterolemia

    In preparation for research careers, the two young physicians sought training in basic science – Goldstein mastered the emerging field of molecular biology with Marshall Nirenberg, who received the Nobel Prize for cracking the genetic code, and he subsequently studied human genetics with Arno Motulsky, a pioneer in the field. Brown learned biochemistry with Earl Stadtman, a founder of the field of enzyme regulation and a recipient of the US National Medal of Science. In 1972 Brown and Goldstein were reunited as junior faculty members at UT Southwestern in Dallas and began the work that would eventually earn them the Nobel Prize in Medicine or Physiology in 1985 and the National Medal of Science in 1988.

    In unraveling FH, Goldstein and Brown took the then-novel approach of studying human skin fibroblasts grown in long-term tissue culture. In 1974, they found that normal cells have high affinity receptors 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 (former 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.

    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.

    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).