Induction of murine macrophage growth by oxidized low density lipoprotein is mediated by granulocyte macrophage colony-stimulating factor

We have examined whether certain secreted factor(s) is involved in oxidized low density lipoprotein (Ox-LDL)-induced murine macrophage growth. An antibody against granulocyte-macrophage colony-stimulating factor (GM-CSF) effectively inhibited Ox-LDL-induced macrophage growth by >80%. Ox-LDL as well as phospholipase A2-treated acetylated LDL enhanced mRNA levels and protein release of GM-CSF from macrophages, while neither acetylated LDL nor lysophosphatidylcholine (lyso-PC) showed such effects. The maximal induction of GM-CSF by Ox-LDL was noted at 4 h, followed by a time-dependent decrease to a basal level within 24 h. Ox-LDL-induced macrophage growth was inhibited by 75% by replacement of the culture medium at 24 h by a fresh medium containing the same concentration of Ox-LDL, when GM-CSF had already returned to the basal level. Thus, a cytokine(s) other than GM-CSF is also expected to participate in Ox-LDL-induced macrophage growth in a later phase. The Ox-LDL-induced GM-CSF release was inhibited by calphostin C, a protein kinase C inhibitor, and was significantly reduced in macrophages from the knockout mice lacking class A, type I and type II macrophage scavenger receptors (MSR-AI/AII). These results taken together indicate that effective endocytosis of lyso-PC of Ox-LDL by macrophages through MSR-AI/AII and subsequent protein kinase C activation have led to GM-CSF release into the medium which may play a priming role in conjunction with other cytokines in Ox-LDL-induced macrophage growth.

Serotonin enhances the production of type IV collagen by human mesangial cells

BACKGROUND

The plasma concentration of 5-hydroxytryptamine (5-HT) in diabetic patients is higher than that in normal subjects. Since recent reports have demonstrated the presence of 5-HT2A receptor in glomerular mesangial cells, it is possible that 5-HT may be involved in the development of diabetic nephropathy through the 5-HT2A receptor in mesangial cells. Because expansion of the glomerular mesangial lesion is a characteristic feature of diabetic nephropathy, we examined the effect of 5-HT on the production of type IV collagen by human mesangial cells.

METHODS

Human mesangial cells were incubated with 5-HT with or without 5-HT receptor antagonists, protein kinase C (PKC) inhibitor or transforming growth factor-beta (TGF-beta) antibody. Type IV collagen mRNA and protein concentration in medium were measured by Northern blot analysis and enzyme-linked immunosorbent assay (ELISA), respectively. TGF-beta mRNA and bioactivity in the medium were measured by Northern blot analysis and bioassay using mink lung epithelial cells, respectively.

RESULTS

5-HT stimulated the production of type IV collagen by human mesangial cells, which was inhibited by ketanserin and sarpogrelate hydrochloride, 5-HT2A receptor antagonists, but not by ondansetron, a 5-HT3 receptor antagonist. 5-HT increased the bioactivities of both active and total TGF-beta. However, the 5-HT-enhanced production of type IV collagen was completely inhibited by an anti-TGF-beta antibody. Furthermore, a PKC inhibitor, calphostin C, inhibited the 5-HT-induced increase in type IV collagen secretion, and the activity of membrane PKC was increased by 5-HT. Phorbol ester activated type IV collagen production as well as active and total TGF-beta. Calphostin C completely inhibited the 5-HT-enhanced activity of active TGF-beta, but did not inhibit exogenous TGF-beta-induced increase in type IV collagen secretion.

CONCLUSIONS

Our results suggest that 5-HT-enhanced production of type IV collagen by human mesangial cells is mediated by activation of PKC and subsequent increase in active TGF-beta activity.

Transfection of inducible nitric oxide synthase gene causes apoptosis in vascular smooth muscle cells

BACKGROUND

Excess production of nitric oxide (NO) by inducible NO synthase (iNOS) has been implicated in a variety of physiological processes including vascular remodeling. To elucidate whether endogenous NO generated by iNOS is involved in the programmed cell death (apoptosis) of the vasculature, iNOS cDNA- expressing construct was transfected into rat and human vascular smooth muscle cells (VSMCs) by lipofection.

METHODS AND RESULTS

VSMCs transiently transfected with iNOS cDNA functionally expressed 130 kd iNOS protein with full catalytic activity to generate massive NO in proportion to the doses of cDNA used; its enzymatic activity as well as NO production was completely blocked by an NOS inhibitor, NG-monomethyl-L-arginine (LNMMA). Overexpression of iNOS led to a marked inhibition of DNA synthesis as well as induction of apoptosis in VSMCs. Evidence for apoptotic cell death was provided by internucleosomal DNA fragmentation by agarose gel electrophoresis, positive staining for TdT-mediated dUTP biotin nick end-labeling, and appearance of hypodiploid cells by flow cytometry analysis. Apoptosis after transfection with iNOS cDNA was abrogated by LNMMA. Transfection of iNOS cDNA caused accumulation of the tumor suppressor gene p53 but not of bcl-2, which was also blocked by LNMMA.

CONCLUSIONS

These results demonstrate that massive generation of endogenous NO derived from iNOS overexpression leads to a marked apoptosis in VSMCs, thus suggesting an important role of NO as a proapoptotic factor for VSMCs in the process of vascular remodeling.

Adrenomedullin as a novel growth-promoting factor for cultured vascular smooth muscle cells: role of tyrosine kinase-mediated mitogen-activated protein kinase activation

To examine whether adrenomedullin (AM), a novel vasodilator peptide, acts as a growth modulator in the vasculature, the effects of AM on protein tyrosine phosphorylation, mitogen-activated protein kinase (MAPK) activation, protooncogene expression, DNA synthesis, and cell proliferation were studied in cultured rat vascular smooth muscle cells (VSMC). AM and calcitonin gene-related peptide (CGRP), although weaker than AM, stimulated DNA synthesis and cell proliferation of quiescent VSMC, whose effects were inhibited by a CGRP receptor antagonist, CGRP-(8-37). AM induced a rapid increase in MAPK activity, followed by the expression of the immediate early protooncogene c-fos. AM-induced MAPK activation and cell proliferation were completely blocked by protein tyrosine kinase inhibitors (genistein and ST638). Moreover, AM rapidly induced tyrosine phosphorylation of several proteins (approximately 120, approximately 90, and approximately 50 kDa) and transiently increased association of a tyrosine-phosphorylated protein (approximately 120 kDa) and Shc with the glutathione-S-transferase-Grb2 fusion protein. A MAPK kinase inhibitor (PD98059) also reduced the AM-induced MAPK activation, c-fos messenger RNA expression, and cell proliferation. Although AM has been shown to induce vasodilation through cAMP production in VSMC, a cAMP antagonist (Rp-cAMP-thionate) and a protein kinase A inhibitor (KT5720) failed to block AM-induced MAPK activation and DNA synthesis. Moreover, 8-bromo-cAMP and forskolin did not affect the MAPK activity. AM had no effect on either the intracellular Ca2+ concentration or inositol 1,4,5-trisphosphate formation. In addition, a protein kinase C inhibitor (GF109203X) did not inhibit the AM-induced MAPK activation. These data suggest that in addition to its vasodilatory effect through the cAMP-dependent pathway, AM exerts its mitogenic activity via protein tyrosine kinase-mediated MAPK activation in quiescent rat VSMC.

Down-regulation of adenylate cyclase coupled to adrenomedullin receptor in vascular smooth muscle cells

Adrenomedullin activates receptor-mediated adenylate cyclase to cause vasorelaxation. To elucidate whether desensitization of adenylate cyclase coupled to vascular adrenomedullin receptors occurs, we studied the adenylate cyclase activity after treatment with rat adrenomedullin in cultured rat aortic vascular smooth muscle cells. Cyclic AMP (cAMP) generation induced by adrenomedullin was markedly decreased by pretreatment with adrenomedullin: a maximal reduction (approximately 80%) was induced after 2 h and persisted during 24 h. Desensitization was independent of protein kinase A, protein kinase C, protein tyrosine kinase or receptor sequestration, because pretreatment with either isoproterenol, forskolin, tetradecanoylphorbol acetate, cytochalasin D, or colchicine did not affect the adrenomedullin-stimulated cAMP response. Furthermore, preincubation with inhibitors for these protein kinases prior to pretreatment with adrenomedullin failed to affect the adrenomedullin-induced decrease in cAMP response following the second stimulation with adrenomedullin. The present results provide the evidence for the existence of desensitization of adenylate cyclase coupled to vascular adrenomedullin receptors.

Endothelin-B receptor-mediated suppression of endothelial apoptosis

Endothelin (ET)-1 is an endothelium-derived vasoconstrictor as well as a mitogen. We have recently described a novel role of ET-1 as a survival factor for rat endothelial cells from serum deprivation-induced apoptosis. The present study was designed to determine which receptor subtype (ETA or ETB) is responsible for and what intracellular mediators are involved in endothelial apoptosis. Apoptotic cell death was evaluated by nucleosomal ladders on agarose gel electrophoresis and immunohistochemical study using anti-single-stranded DNA antiserum. ET-1 and an ETB receptor agonist suppressed endothelial apoptosis, whose effects were abrogated by an ETB receptor antagonist but not by an ETA receptor antagonist. Addition of an ETB receptor antagonist or nonselective ETA/B receptor antagonists, but not an ETA receptor antagonist, enhanced the apoptotic events caused by serum deprivation, suggesting an autocrine/paracrine role of endogenous ET-1 in protecting against endothelial apoptosis. The effect of ET-1 in suppressing apoptosis was unaffected by any of the following reagents: a phospholipase C inhibitor (U73122), a tyrosine kinase inhibitor (ST638), an MEK inhibitor (PD98059), a phosphatidylinositol-3 kinase inhibitors (wortmannin, LY294002). Taken together, these results confirm a role for ET-1 as an autocrine/paracrine survival factor for rat endothelial cells, in which neither phospholipase C, tyrosine kinase, MAP kinase, nor phosphatidylinositol-3 kinase is involved in mediating the antiapoptotic effect of ET-1.

Creation and characterization of a mitochondrial DNA-depleted pancreatic beta-cell line: impaired insulin secretion induced by glucose, leucine, and sulfonylureas

It has been proposed that mitochondrial oxidative phosphorylation in pancreatic beta-cells plays an important role in insulin secretion. To examine the impact of mitochondrial dysfunction on insulin secretion, we created a MIN6 cell line that depleted mitochondrial DNA (mtDNA) by treatment with ethidium bromide (EtBr), and studied the response of the cell line to various secretagogues. MIN6 cells cultured with 0.5 microg/ml EtBr for over 2 months (termed MIN6 deltamt cells) revealed a marked (>90%) decrease in mtDNA content and a lack of mRNAs encoded by mtDNA. MIN6 deltamt cells showed the defects of cytochrome c oxidase activity, glucose- and leucine-induced increase in cellular ATP content, and respiratory chain-driven ATP synthesis, suggesting that MIN6 deltamt cells lost oxidative phosphorylation activity due to the selective disruption of the subunits of respiratory chain enzymes encoded by mtDNA. MIN6 deltamt cells also showed a decrease in glucose utilization, suggesting the impairment of the glycolytic pathway as well. After stimulation with glucose and leucine, MIN6 deltamt cells showed no response in insulin secretion or intracellular free Ca2+ concentration ([Ca2+]i). On the other hand, arginine stimulated insulin secretion and an increase in [Ca2+]i in MIN6 deltamt cells as in MIN6 cells. Glibenclamide also stimulated insulin secretion and an increase in [Ca2+]i in both types of cells, but the responses of MIN6 deltamt cells were significantly lower than those of MIN6 cells. These results suggest the importance of ATP production in insulin secretion and an increase in [Ca2+]i, both induced by glucose and leucine. Moreover, mitochondrial function turns out to be not essential but important for the activation of sulfonylurea-induced insulin secretion.

Molecular cloning, functional expression and tissue distribution of rat acyl-coenzyme A:cholesterol acyltransferase

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an enzyme catalyzing the intracellular formation of cholesteryl esters from free cholesterol and fatty acyl-CoA. In the present study, we cloned rat ACAT cDNA and determined its tissue distribution. Rat ACAT cDNA, having a coding region of 1635 bp with its deduced protein sequence of 545 amino acids and two typical motifs such as signature sequences and leucine heptad motif, showed 83, 92 and 90% identity with human, mouse, and hamster ACAT, respectively. Expression of rat ACAT cDNA in A293 cells and CHO cells resulted in a 3.0 to 3.5-fold increase in the enzyme activity. Among twelve tissues examined, ACAT activity was highest in adrenal followed by liver and intestine while that of aorta was extremely low. The mRNA level was also the highest in adrenal among four tissues examined. However, in contrast to its high ACAT activity, the liver mRNA level was extremely low (adrenal >> intestine > aorta >> liver). Consistent with mRNA levels, immunohistochemical analyses with a specific ACAT antibody detected significant ACAT signals in adrenal and intestine but a negligible signal in liver. These results indicate that adrenal most abundantly expresses ACAT in rat. Furthermore, rat liver showed a high ACAT activity but an extremely low ACAT mRNA and negligible immunohistochemical reactivity, suggesting the presence of a structurally different ACAT protein(s) in rat liver.

Bradykinin potentiates insulin-stimulated glucose uptake and enhances insulin signal through the bradykinin B2 receptor in dog skeletal muscle and rat L6 myoblasts

Previously we demonstrated that bradykinin infusion could increase glucose uptake into dog peripheral tissues, and that bradykinin could potentiate insulin-induced glucose uptake through glucose transporter 4 (GLUT4) translocation in dog adipocytes. However, skeletal muscle is the predominant tissue for insulin-mediated glucose disposal. The aim of this study was to determine how bradykinin affected insulin-stimulated glucose uptake in dog skeletal muscle and myotubes transformed from rat L6 myoblasts. The bradykinin receptor binding studies revealed that dog skeletal muscle and rat L6 myoblasts possessed significant numbers of bradykinin receptors (Kd = 88 and 76 pmol/l, Bmax = 82.5 and 20 fmol/mg protein respectively). An RT-PCR (reverse transcriptase-polymerase chain reaction) amplification showed mRNA specific for bradykinin B2 receptor in both cells. Bradykinin significantly increased 2-deoxyglucose uptake in isolated muscle and L6 myoblasts in the presence of insulin (10(-7) mol/l) in a dose-dependent manner, but not in the absence of insulin. Bradykinin also enhanced insulin-stimulated GLUT4 translocation, and insulin-induced phosphorylation of insulin receptor beta subunit and insulin receptor substrate-1 (IRS-1) without affecting the binding affinities or numbers of cell surface insulin receptors in both cells. It is concluded that bradykinin could potentiate the insulin-induced glucose uptake through GLUT4 translocation in dog skeletal muscle and rat L6 myoblasts. This effect could be explained by the potency of bradykinin to upregulate the insulin receptor tyrosine kinase activity which stimulates phosphorylation of IRS-1, followed by an increase in GLUT4 translocation.

Endothelin-1 is a potent survival factor for c-Myc-dependent apoptosis

Many vertebrate cells are resistant to apoptotic stimuli, whose variety and the mechanisms involved are not fully understood. Endothelin-1 is an endothelium-derived vasoactive peptide that mediates many physiological functions, such as vasoconstriction and cell proliferation. Deregulated expression of c-Myc induces apoptosis in serum-deprived fibroblasts. Using a panel of isogenic fibroblast cell lines with differential c-myc expression levels, we demonstrate that low doses of endothelin-1 protect fibroblasts against serum deprivation-induced apoptosis, which occurs through a c-Myc-dependent process. The endothelin-1-induced cell survival was mediated by the ET(A) receptor and was not linked to the ability of endothelin-1 to induce cell proliferation. The survival function of endothelin-1 was abrogated by inhibiting the mitogen-activated protein kinase pathway. These results demonstrate a hitherto unappreciated role of endothelin-1 as a potent survival factor for c-Myc-dependent apoptosis, a process mediated by the ET(A) receptor and the mitogen-activated protein kinase pathway.

Abnormal FSH hypersecretion as an endocrinological manifestation of POEMS syndrome

Two patients with POEMS syndrome had high basal levels of FSH in the absence of primary hypogonadism. They were fertile eugonadal men with normal serum testosterone and estradiol levels. Provocation with LH-releasing hormone revealed blunted response of FSH secretion, but normal response of LH secretion in both patients, and one had pituitary microadenoma on brain MRI. This report adds primary FSH hypersecretion in the absence of primary testicular failure to the list of endocrinopathy of POEMS syndrome.

Closed-loop subcutaneous insulin infusion algorithm with a short-acting insulin analog for long-term clinical application of a wearable artificial endocrine pancreas

Considering the management and safety of the insulin delivery route when a wearable artificial endocrine pancreas is applied to ambulatory diabetic patients on a long-term basis, we developed a s.c. insulin infusion algorithm by analyzing the dynamics of a s.c. injected short-acting insulin analog (Insulin Lispro) by a three-compartment model. Principally the insulin infusion algorithm was developed as a transfer function with the first-order delay in both proportional and derivative actions to blood glucose concentrations. The parameters for this algorithm were calculated to simulate a physiological plasma insulin profile as closely as possible. By applying this algorithm with regular insulin, diabetic patients showed a 2 h postprandial hyperglycemia and a delayed hyperinsulinemia, followed by hypoglycemic episodes 4-5 h after oral glucose load, just as observed in the computer simulation study. However, using Insulin Lispro, a near-physiological glycemic control (postprandial blood glucose of 153.1 +/- 8.3 mg/100 ml at 60 min and 90.3 +/- 7.1 mg/100 ml at 180 min, respectively) could be achieved without showing any delayed hyperinsulinemia or hypoglycemia. Daily glycemic excursions were also controlled near-physiologically and although the daily insulin requirement (731.7 +/- 160.5 mU/kg/day) was slightly higher, it was not significantly different from that with i.v. insulin infusion (622.3 +/- 142.6 mU/kg/day). These results indicate that the application of s.c. insulin infusion algorithm with Insulin Lispro is feasible for long-term glycemic control with a wearable artificial endocrine pancreas in ambulatory diabetic patients.

Enhanced, simplified glucose sensors: long-term clinical application of wearable artificial endocrine pancreas

At present, 2 major problems should be solved before long-term application of the wearable artificial endocrine pancreas, the development of a reliable and stable glucose monitoring system and the development of a subcutaneous insulin infusion algorithm. With either a miniaturized extracorporeal glucose monitoring system based on microdialysis sampling method or a ferrocene-mediated needle-type glucose sensor covered with highly biocompatible membrane, poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate) (poly[MPC-co-BMA]) membrane, subcutaneous glucose concentrations could be monitored for 7 days without any in vivo calibrations, followed by 14 days with one point calibration. Considering the management and safety of the insulin delivery route, subcutaneous insulin infusion is obligatory. With the subcutaneous insulin infusion algorithm using a short acting insulin analogue (Insulin Lispro), near physiological glycemic control could be established in diabetic patients without showing any delayed hyperinsulinemia or hypoglycemia. The wearable artificial endocrine pancreas is now recognized as an excellent therapeutic tool for regulating blood glucose excursions physiologically in ambulatory diabetic patients on a long-term basis.

Human beta-migrating very low density lipoprotein induces foam cell formation in human mesangial cells

To elucidate the mechanism of foam cell formation in the mesangial region of a kidney observed in a familial type III hyperlipoproteinemic patient presenting with diabetes mellitus and nephrotic syndrome, we have examined, in the present study, the effect of human beta-VLDL (apo E2/E2) on foam cell formation in human mesangial cells, since an increase in beta-VLDL is a characteristic feature of this patient. Human beta-VLDL (apo E2/E2) induced foam cell formation in human mesangial cells. The binding of [125I]LDL to human mesangial cells was inhibited completely by both LDL and beta-VLDL. On the other hand, the binding of [125I]beta-VLDL was completely inhibited by beta-VLDL, but partially by LDL. The LDL receptor, but not the VLDL receptor was down-regulated by accumulation of cholesteryl esters. These results suggest that human beta-VLDL (apo E2/E2)-induced foam cell formation in mesangial cells is mediated through both the LDL receptor pathway and the beta-VLDL specific pathway, in which the VLDL receptor is one of the candidates.

Biphasic regulation of the preproendothelin-1 gene by c-myc

Endothelin-1 (ET-1), a potent vasoconstrictive/mitogenic peptide originally isolated from vascular endothelium, stimulates the expression of immediate early response genes such as c-myc. The c-myc protooncogene participates in regulating the cascade of events that follow mitogenic stimulation of quiescent cells. Using a panel of isogenic fibroblast cell lines with differential c-myc expression levels (obtained by disrupting one c-myc gene copy with targeted homologous recombination and subsequently stably transfecting the heterozygous cells with an exogenous c-myc transgene), we demonstrate that c-Myc protein regulates ET-1 gene transcription in a biphasic fashion: as an activator at low concentrations and as a repressor at high concentrations. Using rat endothelial cells treated with antisense c-myc oligodeoxynucleotides, we also show that c-myc regulates ET-1 synthesis and secretion in a biphasic manner. The present report, therefore, demonstrates the existence of a signal transduction pathway that regulates the synthesis and secretion of ET-1 via the immediate early transcription factor, c-Myc.

Pioglitazone (AD-4833) ameliorates insulin resistance in patients with NIDDM. AD-4833 Glucose Clamp Study Group, Japan

We evaluated the effect of pioglitazone, a thiazolidinedione compound, on insulin-stimulated glucose disposal (Rd) and its efficacy on carbohydrate and lipid metabolism in patients with non-insulin-dependent diabetes mellitus (NIDDM). Twenty NIDDM subjects (mean age 58.2+/-9.4 year, body mass index (BMI) 23.9+/-3.4 kg/ m2 (mean+/-S.D.], three with diet alone, 17 with sulfonylureas [SU]) participated in this trial from five diabetes clinics. Euglycemic (5.3 mmol/liter) hyperinsulinemic (insulin infusion rate 9 micromoles x kg[-1] x min[-1]) clamp studies were performed before and after oral administration of pioglitazone (30 mg/day) for 87+/-10 days. The Rd significantly improved from 5.5+/-2.5 to 8.3+/-3.1 mg x kg(-1) x min(-1). Fasting plasma glucose (FPG) level significantly decreased from 11.0+/-2.0 mmol/liter to 8.9+/-1.1 mmol/liter with a significant improvement in the hemoglobin A1c level from 9.2+/-1.8% to 8.3+/-1.5%. Fasting serum insulin and C peptide levels decreased from 83+/-36 pmol/liter and 0.62+/-0.21 nmol/liter to 66+/-29 pmol/liter and 0.58+/-0.25 nmol/liter, respectively. Fasting serum triglyceride and free fatty acids levels significantly decreased with concomitant increase of fasting serum HDL-cholesterol levels from 1.2+/-0.2 to 1.5+/-0.3 mmol/liter. The change in Rd between before and after pioglitazone administration correlated with baseline values of FPG (rho=0.633), serum insulin (rho=0.653), BMI (rho=0.456), Rd (rho 0.558) and 1,5-AG (rho=-0.522). These data indicate that pioglitazone enhances the insulin action in NIDDM patients on diet alone or SU, and thereby improves both plasma glucose level and lipid profiles.

Two intracellular signaling pathways for activation of protein kinase C are involved in oxidized low-density lipoprotein-induced macrophage growth

Recent studies demonstrated that oxidized LDL (Ox-LDL) induces macrophage growth in vitro. The present study was undertaken to elucidate the intracellular signaling pathways for macrophage growth. Ox-LDL initiated a rapid and transient rise in intracellular free calcium ion and induced activation of membrane protein kinase C (PKC). Pertussis toxin completely inhibited the Ox-LDL-induced rise in free calcium ion and significantly inhibited macrophage growth by 50%. Moreover, PKC inhibitors calphostin C and H-7 significantly inhibited Ox-LDL-induced macrophage growth by 80%. On the other hand, phospholipase A2-treated acetylated LDL did not induce a rise in calcium but significantly activated PKC and led to significant macrophage growth that was significantly inhibited by calphostin C by 90%. These results suggest the presence of two intracellular signaling pathways for activation of PKC, a rise in calcium that was mediated by pertussis toxin-sensitive G protein and the internalization of lysophosphatidylcholine through the scavenger receptors. These two pathways may play an important role in Ox-LDL-induced macrophage growth.

Beta-very low density lipoprotein induces triglyceride accumulation through receptor mediated endocytotic pathway in 3T3-L1 adipocytes

To elucidate the mechanism of triglyceride (TG) accumulation in adipocytes induced by TG-rich lipoproteins, we examined the effect of beta-very low density lipoprotein (beta-VLDL) on TG accumulation in 3T3-L1 adipocytes. Beta-VLDL did not induce TG accumulation in 3T3-L1 preadipocytes but in 3T3-L1 adipocytes. TG accumulation was significantly inhibited by cytochalasin B, an inhibitor of receptor mediated endocytosis. In contrast, cytochalasin B did not inhibit free fatty acid induced TG accumulation in adipocytes. The binding of [125I]beta-VLDL to preadipocytes was inhibited completely by both beta-VLDL and LDL. In sharp contrast, the binding of [125I]beta-VLDL to adipocytes was inhibited completely by beta-VLDL, but partially by LDL. The VLDL receptor mRNA was only expressed in adipocytes. These results suggest that beta-VLDL induced TG accumulation in adipocytes may be mediated through the VLDL receptor pathway.

Endothelin-1 as an autocrine/paracrine apoptosis survival factor for endothelial cells

Endothelin-1 (ET-1), an endothelium-derived vasoactive peptide, functions as a potent vasoconstrictor as well as mitogen. We show here a novel role for ET-1 as an apoptosis survival factor for cultured rat endothelial cells. When we rendered endothelial cells obtained from rat aorta quiescent by serum starvation, significant portions of cultured cells underwent apoptotic death as demonstrated by nucleosomal laddering on agarose gel electrophoresis, flow cytometry analysis with FACS, and the TdT-mediated dUTP biotin nick-end labeling (TUNEL) method. ET-1 dose-dependently (10[-12] to 10[-6] mol/L) suppressed the apoptosis induced by serum starvation. The ET(B) receptor antagonist (BQ788; 10[-6] mol/L) and ET(A/B) receptor antagonists (PD142893 and PD145065; 10[-6] mol/L), but not the ET(A) receptor antagonist (BQ123; 10[-6] mol/L), blocked the apoptosis protective effect of 10[-7] mol/L ET-1. Nonimmune rabbit serum reduced the apoptotic event induced by serum deprivation, whereas neutralization of endogenous ET-1 by polyclonal anti-ET-1 antiserum abrogated this protective effect. The ET(B) receptor antagonist (BQ788; 10[-8] to 10[-6] mol/L), but not the ET(A) receptor antagonist (BQ123; 10[-8] to 10[-6] mol/L), significantly inhibited proliferation of endothelial cells. These data suggest that ET-1, as well as mitogen, functions as an apoptosis survival factor for endothelial cells in an autocrine/paracrine manner via the ET(B) receptor.