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Many of you in the audience are just beginning your careers as physician-scientists. And most of you, I suspect, are struggling with the question of how can I do something [...]

When mice are subjected to 60% calorie restriction for several days, they lose nearly all of their body fat. Although the animals lack energy stores, their livers produce enough glucose to maintain blood glucose at viable levels even after a 23-hour fast. This adaptation is mediated by a marked increase in plasma growth hormone (GH), which is elicited by an increase in plasma ghrelin, a GH secretagogue. In the absence of ghrelin, calorie-restricted mice develop hypoglycemia, owing to diminished glucose production. To determine the site of GH action, in the current study we used CRISPR/Cas9 and Cre recombinase technology to produce mice that lack GH receptors selectively in liver (L-Ghr-/- mice) or in adipose tissue (Fat-Ghr-/- mice). When subjected to calorie restriction and then fasted for 23 hours, the L-Ghr-/- mice, but not the Fat-Ghr-/- mice, developed hypoglycemia. The fall in blood glucose in L-Ghr-/- mice was correlated with a profound drop in hepatic triglycerides. Hypoglycemia was prevented by injection of lactate or octanoate, two sources of energy to support gluconeogenesis. Electron microscopy revealed extensive autophagy in livers of calorie-restricted control mice but not in L-Ghr-/- mice. We conclude that GH acts through its receptor in the liver to activate autophagy, preserve triglycerides, enhance gluconeogenesis, and prevent hypoglycemia in calorie-restricted mice, a model of famine. [ABSTRACT FROM AUTHOR]

The proteolytic cleavage of sterol regulatory element-binding proteins (SREBPs) is regulated by SREBP cleavage-activating protein (SCAP), which forms complexes with SREBPs in membranes of the endoplasmic reticulum (ER). In sterol-depleted cells, SCAP facilitates cleavage of SREBPs by Site-1 protease, thereby initiating release of active NH2-terminal fragments from the ER membrane so that they can enter the nucleus and activate gene expression. In sterol-overloaded cells, the activity of SCAP is blocked, SREBPs remain bound to membranes, and transcription of sterol-regulated genes declines. Here, we provide evidence that sterols act by inhibiting the cycling of SCAP between the ER and Golgi. We use glycosidases, glycosidase inhibitors, and a glycosylation-defective mutant cell line to demonstrate that the N-linked carbohydrates of SCAP are modified by Golgi enzymes in sterol-depleted cells. After modification, SCAP returns to the ER, as indicated by experiments that show that the Golgi-modified forms of SCAP cofractionate with ER membranes on density gradients. In sterol-overloaded cells, the Golgi modifications of SCAP do not occur, apparently because SCAP fails to leave the ER. Golgi modifications of SCAP are restored when sterol-overloaded cells are treated with brefeldin A, which causes Golgi enzymes to translocate to the ER. These studies suggest that sterols regulate the cleavage of SREBPs by modulating the ability of SCAP to transport SREBPs to a post-ER compartment that houses active Site-1 protease.

The integrity of cell membranes is maintained by a balance between the amount of cholesterol and the amounts of unsaturated and saturated fatty acids in phospholipids. This balance is maintained by membrane-bound transcription factors called sterol regulatory element-binding proteins (SREBPs) that activate genes encoding enzymes of cholesterol and fatty acid biosynthesis. To enhance transcription, the active [NH.sub.2]-terminal domains of SREBPs are released from endoplasmic reticulum membranes by two sequential cleavages. The first is catalyzed by Site-1 protease (S1P), a membrane-bound subtilisin-related serine protease that cleaves the hydrophilic loop of SREBP that projects into the endoplasmic reticulum lumen. The second cleavage, at Site-2, requires the action of S2P, a hydrophobic protein that appears to be a zinc metalloprotease. This cleavage is unusual because it occurs within a membrane-spanning domain of SREBP. Sterols block SREBP processing by inhibiting S1P. This response is mediated by SREBP cleavage-activating protein (SCAP), a regulatory protein that activates S1P and also serves as a sterol sensor, losing its activity when sterols overaccumulate in cells. These regulated proteolytic cleavage reactions are ultimately responsible for controlling the level of cholesterol in membranes, cells, and blood.

When the nuclear type of sterol regulatory element-binding protein-1c (nSREBP-1c/ADD1) is overexpressed in cultured 3T3-LI preadipocytes, adipocyte differentiation occurs. Transgenic mice with overexpression of nSREBP-1c in fat tissue have been produced under regulation of the adipocyte-specific aP2 enhancer/promoter. Disordered differentiation of fat tissue, insulin resistance of significance, diabetes mellitus and fatty liver from birth, with high plasma triglyceride levels later, were seen in a syndrome. Mice of this type have many of the same things seen in congenital generalized lipodystrophy (CGL).

Author(s): Joseph L Goldstein [1] Cataracts are the major cause of reversible blindness throughout the world, affecting about 20 million people. Until 1970, cataracts were the leading cause of blindness [...]

Author(s): Joseph L Goldstein [1] Few scientific events have been celebrated so grandiosely as this year's fiftieth anniversary of the discovery of the chemical structure of DNA. The centerpiece of [...]

Author(s): Iichiro Shimomura [1]; Robert E. Hammer [2]; Shinji Ikemoto [1]; Michael S. Brown [1]; Joseph L. Goldstein [1] Shimomura et al. reply -- Human lipodystrophy (also called lipoatrophic diabetes) [...]

Overexpression of Low Density Lipoprotein (LDL) Receptor Eliminates LDL from Plasma in Transgenic Mice HORMONES AND OTHER LIGANDS ARE KNOWN TO REGULATE their own receptors, but we have only recently [...]

Deletion in Cysteine-Rich Region of LDL Receptor Impedes Transport to Cell Surface in WHHL Rabbit RECEPTORS AND OTHER CELL SURFACE PROTEINS FOLLOW a complex path from their sites of synthesis [...]

A Receptor-Mediated Pathway for Cholesterol Homeostasis IN 1901, AFTER STUDYING A PATIENT WITH BLACK URINE, A physician named Archibald Garrod suggested that a single mutant gene can produce a discrete [...]

Cell surface receptors are multifunctional proteins with binding sites that face the external environment and effector sites that couple the binding to an intracellular event. Many receptors have an additional [...]

Approximately one person in every 500 inherits one mutant gene for the low density lipoprotein (LDL) receptor, a cell surface protein that carries the cholesterol transport protein LDL from plasma [...]