Gregory A. Graf

Bio: Gregory A. Graf is an academic researcher from University of Kentucky. The author has contributed to research in topics: Cholesterol & Adipose tissue. The author has an hindex of 24, co-authored 48 publications receiving 4734 citations. Previous affiliations of Gregory A. Graf include University of Texas Southwestern Medical Center & Carlos III Health Institute.

Accumulation of Dietary Cholesterol in Sitosterolemia Caused by Mutations in Adjacent ABC Transporters

Abstract: In healthy individuals, acute changes in cholesterol intake produce modest changes in plasma cholesterol levels. A striking exception occurs in sitosterolemia, an autosomal recessive disorder characterized by increased intestinal absorption and decreased biliary excretion of dietary sterols, hypercholesterolemia, and premature coronary atherosclerosis. We identified seven different mutations in two adjacent, oppositely oriented genes that encode new members of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter family (six mutations in ABCG8 and one in ABCG5) in nine patients with sitosterolemia. The two genes are expressed at highest levels in liver and intestine and, in mice, cholesterol feeding up-regulates expressions of both genes. These data suggest that ABCG5 and ABCG8 normally cooperate to limit intestinal absorption and to promote biliary excretion of sterols, and that mutated forms of these transporters predispose to sterol accumulation and atherosclerosis.

Caveolins, liquid-ordered domains, and signal transduction.

Abstract: Caveolae were originally identified as flask-shaped invaginations of the plasma membrane in endothelial and epithelial cells (14). Prior to the development of biochemical methods for their purification, caveolae were thought to principally mediate the transcellular movement of molecules (101, 145). Recently, the development of novel purification procedures has greatly expanded our knowledge regarding the putative functions of caveolae in vivo. In this review, we seek to update the working definition of caveolae, describe the functional roles of the caveolin gene family, and summarize the evidence that supports a role for caveolae as mediators of a number of cellular signaling processes.

Coexpression of ATP-binding cassette proteins ABCG5 and ABCG8 permits their transport to the apical surface

Abstract: Mutations in either ATP-binding cassette (ABC) G5 or ABCG8 cause sitosterolemia, an autosomal recessive disorder of sterol trafficking. To determine the site of action of ABCG5 and ABCG8, we expressed recombinant, epitope-tagged mouse ABCG5 and ABCG8 in cultured cells. Both ABCG5 and ABCG8 underwent N-linked glycosylation. When either protein was expressed individually in cells, the N-linked sugars remained sensitive to Endoglycosidase H (Endo H). When ABCG5 and ABCG8 were coexpressed, the attached sugars were Endo H–resistant and neuraminidase-sensitive, indicating that the proteins were transported to the trans-Golgi complex. The mature, glycosylated forms of ABCG5 and ABCG8 coimmunoprecipitated, consistent with heterodimerization of these two proteins. The Endo H–sensitive forms of ABCG5 and ABCG8 were confined to the endoplasmic reticulum (ER), whereas the mature forms were present in non-ER fractions in cultured hepatocytes. Immunoelectron microscopy revealed ABCG5 and ABCG8 on the plasma membrane of these cells. In polarized WIF-B cells, recombinant ABCG5 localized to the apical (canalicular) membrane when coexpressed with ABCG8, but not when expressed alone. To our knowledge this is the first direct demonstration that trafficking of an ABC half-transporter to the cell surface requires the presence of its dimerization partner.

Lipid rafts and signal transduction

Abstract: Signal transduction is initiated by complex protein-protein interactions between ligands, receptors and kinases, to name only a few. It is now becoming clear that lipid micro-environments on the cell surface -- known as lipid rafts -- also take part in this process. Lipid rafts containing a given set of proteins can change their size and composition in response to intra- or extracellular stimuli. This favours specific protein-protein interactions, resulting in the activation of signalling cascades.

Seven-transmembrane receptors.

Abstract: Seven-transmembrane receptors, which constitute the largest, most ubiquitous and most versatile family of membrane receptors, are also the most common target of therapeutic drugs. Recent findings indicate that the classical models of G-protein coupling and activation of second-messenger-generating enzymes do not fully explain their remarkably diverse biological actions.

Nuclear receptors and lipid physiology: opening the X-files.

Abstract: Cholesterol, fatty acids, fat-soluble vitamins, and other lipids present in our diets are not only nutritionally important but serve as precursors for ligands that bind to receptors in the nucleus. To become biologically active, these lipids must first be absorbed by the intestine and transformed by metabolic enzymes before they are delivered to their sites of action in the body. Ultimately, the lipids must be eliminated to maintain a normal physiological state. The need to coordinate this entire lipid-based metabolic signaling cascade raises important questions regarding the mechanisms that govern these pathways. Specifically, what is the nature of communication between these bioactive lipids and their receptors, binding proteins, transporters, and metabolizing enzymes that links them physiologically and speaks to a higher level of metabolic control? Some general principles that govern the actions of this class of bioactive lipids and their nuclear receptors are considered here, and the scheme that emerges reveals a complex molecular script at work.