Immunofluorescence mapping demonstrates that the NG2 proteoglycan is invariably expressed by the mural cell component of mouse neovascular structures. This pattern is independent of the developmental mechanism responsible for formation of the vasculature (vasculogenesis or angiogenesis). Thus, NG2 is expressed in the embryonic heart by cardiomyocytes, in developing macrovasculature by smooth muscle cells, and in nascent microvessels by vascular pericytes. Due to the scarcity of proven markers for developing pericytes, NG2 is especially useful for identification of this cell type. The utility of NG2 as a pericyte marker is illustrated by two observations. First, pericytes are associated with endothelial tubes at an early point in microvessel development. This early interaction between pericytes and endothelial cells has important implications for the role of pericytes in the development and stabilization of microvascular tubes. Second, the pericyte to endothelial cell ratio in developing capillaries varies from tissue to tissue. Because the extent of pericyte investment is likely to affect the physical properties of the vessel in question, it is important to understand the mechanisms that control this process. Additional insight into these and other aspects of vascular morphogenesis should be possible through use of NG2 as a mural cell marker.

Due to their structural complexity, proteoglycans are highly interactive macromolecules that participate in a broad range of cell-cell and cell-matrix interactions, including regulation of cell adhesion, proliferation, motility, and differentiation (Refs.

No abstract

The human homologue of NG2, the human melanoma proteoglycan (HMP), is expressed on most human melanomas. To investigate the role of this proteoglycan in melanoma progression, we have attempted to identify functionally important molecular ligands for NG2. Immunohistochemical analysis of cell lines that endogenously express NG2/HMP suggests that NG2/HMP associates with CD44 and alpha4beta1 integrin, two molecules previously implicated in melanoma progression. Transfection of rat NG2 into the NG2-negative B16 mouse melanoma cell line also resulted in a highly colocalized pattern of expression between the transfected rat NG2 and the endogenously expressed mouse CD44 and alpha4beta1 integrin molecules. In functional assays, expression of NG2 decreased the adhesion of B16 melanoma cells to CD44 monoclonal antibodies, hyaluronic acid, the C-terminal 40-kDa fibronectin fragment, and the CS1 fibronectin peptide, suggesting that NG2 may negatively modulate CD44- and alpha4beta1-mediated binding events. Expression of NG2 increased the proliferation of melanoma cells in culture and increased tumorigenicity in vivo. Moreover, NG2 expression led to increased lung metastasis of B16F1 and B16F10 melanoma cells in experimental metastasis studies. Together, these studies demonstrate that NG2 is capable of modulating the adhesion, proliferation, and metastatic potential of melanoma cells.

؉ . We show that inhibition of the mNCE enhances mitochondrial oxidative metabolism and increases glucose-stimulated insulin secretion in rat islets and INS-1 cells. The benzothiazepine CGP37157 inhibited mNCE activity in INS-1 cells (50% inhibition at IC 50 ‫؍‬ 1.5 mol/l) and increased the glucose-induced rise in mitochondrial Ca 2؉ ([Ca 2؉ ] m ) 2.1 times. Cellular ATP content was increased by 13% in INS-1 cells and by 49% in rat islets by CGP37157 (1 mol/l). Krebs cycle flux was also stimulated by CGP37157 when glucose was present. Insulin secretion was increased in a glucosedependent manner by CGP37157 in both INS-1 cells and islets. In islets, CGP37157 increased insulin secretion dose dependently (half-maximal efficacy at EC 50 ‫؍‬ 0.06 mol/l) at 8 mmol/l glucose and shifted the glucose dose response curve to the left. In perifused islets, mNCE inhibition had no effect on insulin secretion at 2.8 mmol/l glucose but increased insulin secretion by 46% at 11 mmol/l glucose. The effects of CGP37157 could not be attributed to interactions with the plasma membrane sodium calcium exchanger, L-type calcium channels, ATP-sensitive K ؉ channels, or [Ca 2؉ ] m uniporter. In hyperglycemic clamp studies of Wistar rats, CGP37157 increased plasma insulin and C-peptide levels only during the hyperglycemic phase of the study. These results illustrate the potential utility of agents that affect mitochondrial metabolism as novel insulin secretagogues. Diabetes 52:965-973, 2003 M itochondrial oxidative metabolism plays an important role in the insulin secretory process in pancreatic ␤-cells. Stimulus-secretion coupling in the ␤-cell depends on the metabolism of glucose and the subsequent mitochondrial oxidative phosphorylation that generates ATP. ATP closes ATPsensitive K ϩ (K ATP ) channels, causing depolarization of the ␤-cell membrane, opening of voltage-dependent calcium channels, and Ca 2ϩ influx (1,2). Although two main processes, glycolysis and oxidative phosphorylation, are responsible for ATP synthesis during glucose metabolism, oxidative phosphorylation is the predominant pathway in the pancreatic ␤-cell (3,4). Insulin secretion induced by other secretagogues, such as leucine and glyceraldehydes, is also mediated by the production of ATP (5,6). The critical regulatory role of oxidative ATP production in glucose-stimulated insulin secretion (GSIS) is underscored by the observation that disrupting mitochondrial oxidative metabolism blocks nutrient-mediated insulin secretion. For example, inhibition of oxidative phosphorylation (7,8), blockade of NADH transport into mitochondria (9,10), or elimination of mitochondrial DNA from ␤-cells in vitro (11-15) or in vivo (16) all prevent GSIS. We tested the hypothesis that enhancing oxidative ATP production in response to glucose can increase insulin secretion. The normal feed-forward regulatory role of mitochondrial Ca 2ϩ ([Ca 2ϩ ] m ) in the ␤-cell was exploited to enhance oxidative metabolism.The influx of calcium into the cytoplasm of the ␤-cell in response to a nutrient l...

The human homologue of NG2, the human melanoma proteoglycan (HMP), is expressed on most human melanomas. To investigate the role of this proteoglycan in melanoma progression, we have attempted to identify functionally important molecular ligands for NG2. Immunohistochemical analysis of cell lines that endogenously express NG2/HMP suggests that NG2/HMP associates with CD44 and alpha4beta1 integrin, two molecules previously implicated in melanoma progression. Transfection of rat NG2 into the NG2-negative B16 mouse melanoma cell line also resulted in a highly colocalized pattern of expression between the transfected rat NG2 and the endogenously expressed mouse CD44 and alpha4beta1 integrin molecules. In functional assays, expression of NG2 decreased the adhesion of B16 melanoma cells to CD44 monoclonal antibodies, hyaluronic acid, the C-terminal 40-kDa fibronectin fragment, and the CS1 fibronectin peptide, suggesting that NG2 may negatively modulate CD44- and alpha4beta1-mediated binding events. Expression of NG2 increased the proliferation of melanoma cells in culture and increased tumorigenicity in vivo. Moreover, NG2 expression led to increased lung metastasis of B16F1 and B16F10 melanoma cells in experimental metastasis studies. Together, these studies demonstrate that NG2 is capable of modulating the adhesion, proliferation, and metastatic potential of melanoma cells.

The transmembrane proteoglycan NG2 is able to interact both with components of the extracellular matrix and with the actin cytoskeleton. An examination of the distribution of NG2 during cell spreading suggests that NG2 can associate with two distinct types of actin-containing cytoskeletal structures, depending on the nature of the stimulus derived from the substratum. On fibronectin-coated dishes, cell surface NG2 associates exclusively with stress fibers developing within the cell. On poly-L-lysine-coated dishes, cell surface NG2 is associated with radial processes extending from the cell periphery. Spreading on fibronectin/poly-L-lysine mixtures, as well as on matrix components such as laminin, tenascin, and type VI collagen, produces cells with mosaic characteristics, i.e., NG2 is associated with both types of structures. NG2-positive radial processes are distinct from a second population of radial structures that contain fascin. NG2-positive extensions appear to be individual self-contained units (filopodia), whereas fascin is associated with actin ribs within sheets of membrane (lamellipodia). NG2- and fascin-positive structures are often localized to opposite poles of spreading cells, suggesting a possible role for the two classes of cellular extensions in the establishment of cell polarity during morphogenesis or migration. Time lapse imaging confirms the presence of lamellipodia on the leading edges of migrating cells, while numerous filopodia are present on trailing edges.

To characterize tyrosine kinase activity in signaling ligand/receptor internalization, metabolic labeling and surface radioligand binding were used to follow the processing of both normal and tyrosine kinase-deficient human insulin receptors. The mutant receptor (A/K1018) has an alanine substituted for lysine 1018 in the ATP-binding domain. Rat 1 fibroblasts, expressing either normal human insulin receptors (HIRc) or A/K1018 receptors, were assayed to determine the insulin receptor half-life as well as internalization and down-regulation. Our results show that insulin greatly reduces the half-life of normal insulin receptors (from 9.9 to 5.7 h). The A/K1018 receptor had a much longer half-life (24 h), which was not reduced by the presence of saturating insulin concentrations. The A/K1018 receptor does not undergo down-regulation after long term insulin exposure, while HIRc cells showed a 34% decrease in insulin receptor number. This down-regulation is accounted for by the accelerated turnover rate of normal receptors in the presence of insulin. To confirm that the kinase activity is necessary for normal endocytosis, we also show that ATP depletion in HIRc cells resulted in significant decreases in receptor internalization and that tyrosine kinase-defective receptors also fail to internalize in a different cell type (rat Fao hepatocytes). Lastly, the complement of normal rat insulin receptors in cells expressing the kinase-defective receptors endocytose normally. We conclude that the defect in endocytosis observed in kinase-defective receptors is intrinsic to this receptor and not due to a dominant inhibitory effect on cellular endocytotic machinery.