Cell density-dependent expression of EDG family receptors and mesangial cell proliferation: role in lysophosphatidic acid-mediated cell growth.

Lysophosphatidic acid (LPA), a major member of the bioactive lysophospholipids in serum, possesses diverse physiological activities including cell proliferation. Recently, three endothelial differentiation gene (EDG) family receptors, including EDG-2 (LPA1), EDG-4 (LPA2), and EDG-7 (LPA3), have been identified as LPA receptors. The role of LPA and their receptors in mesangial cell physiology is not clearly understood. This study examined the expression profile of EDG receptors as a function of cell density and the participation of EDG receptors in human mesangial cell proliferation by LPA. We showed that mesangial cells express all three EDG family LPA receptors in a cell density-dependent manner. EDG-7 maximally expressed at sparse cell density and minimally expressed in dense cell population. The EDG-2 expression pattern was opposite to the EDG-7. No changes in EDG-4 expression as a function of cell density were noted. DNA synthetic rate was greater in sparse cell density compared with dense cell population and followed a similar pattern with EDG-7 expression. Comparative studies in sparse and dense cell density indicated that EDG-7 was positively associated, whereas EDG-2 was negatively associated with cell proliferation rate. LPA induced mesangial cell proliferation by 1.5- to 3.5-fold. Dioctanoylglycerol pyrophosphate, an antagonist for EDG-7, almost completely inhibited mesangial cell proliferation induced by LPA. We suggest that EDG-7 regulates LPA-mediated mesangial cell proliferation. Additionally, these data suggest that EDG-7 and EDG-2 LPA receptors play a diverse role as proliferative and antiproliferative, respectively, in mesangial cells. Regulation of EDG family receptors may be importantly linked to mesangial cell-proliferative processes.

Niacin noncompetitively inhibits DGAT2 but not DGAT1 activity in HepG2 cells.

Niacin is a widely used lipid-regulating agent in dyslipidemic patients. Previously, we have shown that niacin inhibits triacylglycerol synthesis. In this report, using HepG2 cells, we have examined the effect of niacin on the mRNA expression and microsomal activity of diacylglycerol acyltransferase 1 and 2 (DGAT1 and DGAT2), the last committed but distinctly different enzymes for triglyceride synthesis. Addition of niacin to the DGAT assay reaction mixture dose-dependently (0-3 mM) inhibited DGAT activity by 35-50%, and the IC(50) was found to be 0.1 mM. Enzyme kinetic studies showed apparent K(m) values of 8.3 microM and 100 microM using [(14)C]oleoyl-CoA and sn-1,2-dioleoylglycerol as substrates, respectively. A decrease in apparent V(max) was observed with niacin, whereas the apparent K(m) remained constant. A Lineweaver-Burk plot of DGAT inhibition by niacin showed a noncompetitive type of inhibition. Niacin selectively inhibited DGAT2 but not DGAT1 activity. Niacin inhibited overt DGAT activity. Niacin had no effect on the expression of DGAT1 and DGAT2 mRNA. These data suggest that niacin directly and noncompetitively inhibits DGAT2 but not DGAT1, resulting in decreased triglyceride synthesis and hepatic atherogenic lipoprotein secretion, thus indicating a major target site for its mechanism of action.

Changes in the expression of the NR2B subunit during aging in macaque monkeys.

Humans, non-human primates and rodents show declines in spatial memory abilities with increased age. Some of these declines in mice are related to changes in the expression of the epsilon2 (epsilon2) (NR2B) subunit of the N-methyl-D-aspartate receptor. The purpose of this study was to determine whether primates show changes during aging in the mRNA expression of the NR2B subunit. In situ hybridization was performed on tissue sections from three different ages of Rhesus monkeys (Macaca mulatta; 6-8, 10-12, and 24-26 years). There was a significant decrease in the mRNA expression of the NR2B subunit overall in the prefrontal cortex and in the caudate nucleus between young and old monkeys. There were no significant changes in NR2B mRNA expression in the hippocampus or the parahippocampal gyrus. The results in the prefrontal cortex, caudate and hippocampus were similar to those seen previously in C57BL/6 mice during aging, which suggests that mice may be useful as a model for primates to further examine the age-related changes in the expression of the NR2B subunit of the NMDA receptor in several important regions of the brain.

Age-related deficits in mice performing working memory tasks in a water maze.

This study determined whether mice exhibit spatial working memory deficits with increased age. C57BL/6JNia mice of 3 different ages were tested in the Morris water maze with 2 protocols designed to assess immediate and delayed working memory abilities. Young mice required multiple trials in order to show improvements in the working memory task. Deficits in immediate working memory were detected in both 10- and 24- to 26-month-old mice. Reference memory deficits and declines in performance in the delayed working memory task were only seen in 24- to 26-month-olds. This increased susceptibility of immediate working memory processes to the aging process in mice may be related to their need for more rehearsal in the water maze than other species.