Fatty acid-related modulations of membrane fluidity in cells: detection and implications.

Metabolic homeostasis of fatty acids is complex and well-regulated in all organisms. The biosynthesis of saturated fatty acids (SFA) in mammals provides substrates for ß-oxidation and ATP production. Monounsaturated fatty acids (MUFA) are products of desaturases that introduce a methylene group in cis geometry in SFA. Polyunsaturated fatty acids (n-6 and n-3 PUFA) are products of elongation and desaturation of the essential linoleic acid and α-linolenic acid, respectively. The liver processes dietary fatty acids and exports them in lipoproteins for distribution and storage in peripheral tissues. The three types of fatty acids are integrated in membrane phospholipids and determine their biophysical properties and functions. This study was aimed at investigating effects of fatty acids on membrane biophysical properties under varying nutritional and pathological conditions, by integrating lipidomic analysis of membrane phospholipids with functional two-photon microscopy (fTPM) of cellular membranes. This approach was applied to two case studies: first, pancreatic beta-cells, to investigate hormetic and detrimental effects of lipids. Second, red blood cells extracted from a genetic mouse model defective in lipoproteins, to understand the role of lipids in hepatic diseases and metabolic syndrome and their effect on circulating cells.

Perturbations in the HDL metabolic pathway predispose to the development of osteoarthritis in mice following long-term exposure to western-type diet.

OBJECTIVE: Recent data suggest that obesity and related metabolic aberrations are associated with osteoarthritis (OA) development, a phenomenon that is attributed at least in part to the consumption of lipid-rich diets. To date, the molecular mechanisms that govern the lipid-OA connection remain largely unknown. Given the important role of high-density lipoprotein (HDL) in plasma and tissue lipid metabolism, the main purpose of the present study was to investigate the role of HDL metabolism in the pathobiology of OA. METHODS: We used apolipoprotein A-I (apoA-I)(-/-) mice that lack classical apoA-I containing HDL, LCAT(-/-) mice that have only immature HDL and relatively reduced HDL-cholesterol levels and control C57BL/6 mice. Mice were placed on chow or western-type (WTD) and monitored for 24 weeks. Knee joints were removed and articular cartilage was isolated for further analyses. RESULTS: The LCAT(-/-) mice were significantly more sensitive to the development of diet-induced obesity compared to the C57BL/6 and apoA-I(-/-) mice. Morphological, biochemical and molecular analyses revealed that the LCAT(-/-) obese mice developed OA, while the C57BL/6 mice that were fed WTD did not. Notably, apoA-I(-/-) mice that received WTD also developed OA although their body-weight gain was similar to their wild-type counterparts. Interestingly, bone marrow from LCAT(-/-) and apoA-I(-/-) mice contained significantly increased number of adipocytes, compared to the other groups. CONCLUSIONS: Our findings suggest that perturbations in HDL metabolism predispose to OA following chronic insult with WTD and raise the challenging possibility that HDL has a causative relation to OA in patients with metabolic syndrome.

Adenovirus mediated expression "in vivo" of the chemokine receptor CXCR1.

A major hurdle in the structural analysis of membrane proteins is the expression of a functional and homogeneous form of the protein. Except for rhodopsin, most G protein-coupled receptors (GPCRs) are endogenously expressed at very low levels. Heterologous expression of GPCRs in bacteria, yeast, insect cells or mammalian cell lines often yields proteins with large amounts of misfolded proteins and heterogeneous posttranslational modifications. Here, we report a novel mammalian "in vivo" system for the expression of the chemokine receptor CXCR1. This receptor was expressed in liver of mice infected with adenovirus encoding CXCR1. Liver plasma membranes from infected mice displayed high-levels of (125)I-labeled human interleukin-8 (IL-8) binding. The pharmacological profile of the recombinant CXCR1 expressed "in vivo" was similar to those expressed in neutrophils. We found that the incorporation of the detergent solubilized CXCR1 into phospholipid vesicles in the presence of Gi/Go proteins is required for the reconstitution of (125)I-IL-8 binding. On the basis of the presence of the several endogenous His residues and glycosylation moieties in CXCR1 we fractionated the detergent-solubilized plasma membranes by employing Ni- and Concanavalin A-based chromatography. Fractions enriched with CXCR1 were monitored by (125)I-IL-8-bound to the receptor and Western blots with anti-CXCR1 antibodies. This robust expression system could be readily applied for the expression of GPCRs and other eukaryotic membrane proteins.

The amino-terminal 1-185 domain of apoE promotes the clearance of lipoprotein remnants in vivo. The carboxy-terminal domain is required for induction of hyperlipidemia in normal and apoE-deficient mice.

Apolipoprotein E (apoE) promotes receptor-mediated catabolism of apoE-containing lipoprotein remnants. Impairments in remnant clearance are associated with type III hyperlipoproteinemia and premature atherosclerosis. In humans, apoE plasma levels correlate with plasma triglyceride levels, suggesting that excess apoE may also affect plasma triglyceride levels. We have used adenovirus-mediated gene transfer in mice to map the domains of apoE required for cholesterol and triglyceride clearance, in vivo. Adenovirus expressing apoE3 and apoE4 at doses of (1-2) x 10(9) pfu increased plasma cholesterol and triglyceride levels in normal C57BL6 mice and failed to normalize the high cholesterol levels of apoE-deficient mice due to induction of hypertriglyceridemia. In contrast, an adenovirus expressing the truncated apoE 1-185 form normalized the cholesterol levels of E(-)(/)(-) mice and did not cause hypertriglyceridemia. Northern blot analysis of hepatic RNA from mice expressing the full-length and the truncated apoE forms showed comparable steady-state apoE mRNA levels of the full-length apoE forms that cause hyperlipidemia and the truncated apoE forms that do not cause hyperlipidemia. The findings suggest that the amino-terminal residues 1-185 of apoE are sufficient for the clearance of apoE-containing lipoprotein remnants by the liver, whereas domains of the carboxy-terminal one-third of apoE are required for apoE-induced hyperlipidemia.

Domains of apolipoprotein E contributing to triglyceride and cholesterol homeostasis in vivo. Carboxyl-terminal region 203-299 promotes hepatic very low density lipoprotein-triglyceride secretion.

Apolipoprotein (apo) E has been implicated in cholesterol and triglyceride homeostasis in humans. At physiological concentration apoE promotes efficient clearance of apoE-containing lipoprotein remnants. However, high apoE plasma levels correlate with high plasma triglyceride levels. We have used adenovirus-mediated gene transfer in apoE-deficient mice (E(-)/-) to define the domains of apoE required for cholesterol and triglyceride homeostasis in vivo. A dose of 2 x 10(9) plaque-forming units of apoE4-expressing adenovirus reduced slightly the cholesterol levels of E(-)/- mice and resulted in severe hypertriglyceridemia, due to accumulation of cholesterol and triglyceride-rich very low density lipoprotein particles in plasma. In contrast, the truncated form apoE4-202 resulted in a 90% reduction in the plasma cholesterol levels but did not alter plasma triglyceride levels in the E(-)/- mice. ApoE secretion by cell cultures, as well as the steady-state hepatic mRNA levels in individual mice expressing apoE4 or apoE4-202, were similar. In contrast, very low density lipoprotein-triglyceride secretion in mice expressing apoE4, but not apoE4-202, was increased 10-fold, as compared with mice infected with a control adenovirus. The findings suggest that the amino-terminal 1-202 region of apoE4 contains the domains required for the in vivo clearance of lipoprotein remnants. Furthermore, the carboxyl-terminal 203-299 residues of apoE promote hepatic very low density lipoprotein-triglyceride secretion and contribute to apoE-induced hypertriglyceridemia.

Type V collagen regulates the assembly of collagen fibrils in cultures of bovine vascular smooth muscle cells.

Vascular smooth muscle cells (SMCs), the major cellular constituent of the medial layer of an artery, synthesize the majority of connective tissue proteins, including fibrillar collagen types I, III, and V/XI. Proper collagen synthesis and deposition, which are important for the integrity of the arterial wall, require the antioxidant vitamin C. Vitamin C serves as cofactor for the enzymes prolyl and lysyl hydroxylase, which are responsible for the proper hydroxylation of collagen. Here, the role of type V collagen in the assembly of collagen fibrils in the extracellular matrix (ECM) of cultured vascular SMCs was investigated. Treatment of SMCs with vitamin C resulted in a dramatic induction in the levels of the cell-layer associated pepsin-resistant type V collagen, whereas only a minor induction in the levels of types I and III collagen was detected. Of note, the deposition of type V collagen was accompanied by the formation of striated collagen fibrils in the ECM. Immunohistochemistry demonstrated that type V collagen, but not type I collagen, became masked as collagen fibrils matured. Furthermore, the relative ratio of type V to type I collagen decreased as the ECM matured as a function of days in culture, and this decrease was accompanied by an increase in the diameter of collagen fibrils. Together these results suggest that the masking of type V collagen is caused by its internalization on continuous deposition of type I collagen on the exterior of the fibril. Furthermore, they suggest that type V collagen acts as framework for the initial assembly of collagen molecules into heterotypic fibrils, regulating the diameter and architecture of these fibrils.

B-Myb represses trans-activation of the Col5A2 collagen promoter indirectly via inhibition of binding of factors interacting with positive elements within the first exon.

B-myb, a member of the myb gene family, was originally isolated based on its high homology with c-myb in the DNA-binding domain. Previously we showed that B-myb is expressed in bovine vascular smooth muscle cells (SMCs) in a cell cycle-dependent fashion, and inhibits type I collagen gene promoter activity. Here, we have explored its role in regulation of another fibrillar collagen gene, Col5A2, encoding the (alpha2 chain of type V collagen. Ectopic expression of B-Myb decreased alpha 2(V) promoter activity and endogenous alpha 2(V) collagen mRNA levels. The responsive region of the alpha 2(V) collagen gene was localized to a fragment including 100 bp of basal promoter and 150 bp of exon 1 sequences, which contained two CRE-like elements. Binding to these elements increased upon deprivation of serum-growth factors, when expression of the Col5A2 gene is elevated, leading us to test their role despite the failure of excess unlabelled CRE oligonucleotide from the somatostatin gene to successfully compete for binding. Mutation of the elements significantly decreased the basal level of alpha2(V) collagen promoter activity and ablated inhibition by B-Myb. Furthermore, addition of B-Myb-glutathionine S-transferase fusion protein inhibited complex formation. Thus, these results confirm a major role for B-Myb in mediating intracellular signals controlling collagen gene expression in vascular SMCs. A model of indirect repression of the Col5A2 gene by B-Myb, via interaction with a positively-acting matrix regulatory factor, termed MRF-V, is discussed.

Basic fibroblast growth factor-induced decrease in type I collagen gene transcription is mediated by B-myb.

Basic fibroblast growth factor (bFGF), a member of the fibroblast growth factor family, potently induces increased vascular smooth muscle cell (SMC) proliferation and decreased expression of type I collagen. Recently, our laboratory demonstrated that, in bovine vascular SMCs, expression of B-myb, a member of the myb gene family, is dependent upon cellular growth state and that B-myb decreases alpha1(I) collagen promoter activity in transient transfection assays. Nuclear run-off analysis indicated that the decrease in alpha1(I) collagen mRNA level seen upon bFGF treatment was due to a decline in the rate of alpha1(I) procollagen gene transcription. Thus, we investigated the potential role of B-Myb in the down-regulation of type I collagen gene expression by bFGF. Using Northern blot analysis, we found that bFGF treatment of bovine aortic SMCs caused an increase in B-myb mRNA levels. Ectopic expression of B-myb decreased endogenous alpha1(I) collagen mRNA levels. Importantly, introduction of a B-myb antisense oligonucleotide prevented the drop in the alpha1(I) collagen mRNA levels seen upon treatment with bFGF. Together, these results indicate that B-myb mediates signals leading to the decreased rate of alpha1(I) collagen gene transcription caused by bFGF.

Synchronization of cultured vascular smooth muscle cells following reversal of quiescence induced by treatment with the antioxidant N-acetylcysteine.

Smooth muscle cell (SMC) proliferation plays an important role in the pathogenesis of vascular diseases such as atherosclerosis and postangioplasty restenosis. Recently we demonstrated the thiol antioxidant N-acetylcysteine (NAC) inhibits constitutive NF-kappa B/Rel activity and growth of vascular SMCs. Here we show that treatment of human and bovine aortic SMC with the thiol antioxidant NAC causes cells to exit the cell cycle and remain quiescent as determined by a greatly reduced incorporation of [3H]thymidine and G0/G1 DNA content. Removal of NAC from the culture medium stimulates SMCs to synchronously reenter the cell cycle as judged by induction of cyclin D1 and B-myb gene expression during mid and late G1 phase, respectively, and induction of histone gene expression and [3H]thymidine incorporation during S phase. The time course of cyclin D1, B-myb, and histone gene expression after NAC removal was similar to that of serum-deprived cells induced to resume cell cycle progression by the addition of fetal bovine serum to the culture medium. Taken together, these results indicate that NAC treatment causes SMCs to enter a reversible G0 quiescent, growth-arrested state. Thus, NAC provides an important new method for synchronizing SMCs in culture.

Basic fibroblast growth factor decreases type V/XI collagen expression in cultured bovine aortic smooth muscle cells.

Vascular smooth muscle cells (SMCs), the major cellular constituent of an artery, synthesize the bulk of fibrillar collagens, including type V/XI, which regulates heterotypic collagen fibril assembly. Basic fibroblast growth factor (bFGF) is a heparin-binding polypeptide growth factor that has been implicated in important events during the development of atherosclerosis, such as early intimal SMC proliferation. Here we have investigated the effects of bFGF on aortic SMC expression of type V/XI collagen. Treatment of exponentially growing or serum-deprived subconfluent cultures of bovine aortic SMCs with bFGF decreased the steady-state levels of the mRNAs for collagen type V/XI, including alpha 1(V), alpha 2(V), and alpha 1(XI). The effect of bFGF was time dependent with a two- and a fourfold decrease in alpha 2(V) mRNA observed after treatment for 24 and 48 h, respectively. This decrease resulted from a drop in the rate of alpha 2(V) gene transcription; no change was observed in the stability of the alpha 2(V) mRNA. Furthermore, accumulation of collagen protein decreased upon bFGF treatment. As expected, treatment with bFGF increased the rate of proliferation of serum-deprived SMCs, as judged by DNA content in the cultures, thymidine incorporation, and steady-state mRNA levels of the S-phase-expressed histone H3.2. These results suggest that bFGF plays an important role in the regulation of collagen fibril structure, with potential implications for the development and organization of an atherosclerotic lesion.

A-myb is expressed in bovine vascular smooth muscle cells during the late G1-to-S phase transition and cooperates with c-myc to mediate progression to S phase.

The Myb family of transcription factors is defined by homology within the DNA binding domain and includes c-Myb, A-Myb, and B-Myb. The protein products of the myb genes all bind the Myb-binding site (MBS) [YG(A/G)C(A/C/G)GTT(G/A)]. A-myb has been found to display a limited pattern of expression. Here we report that bovine aortic smooth muscle cells (SMCs) express A-myb. Sequence analysis of isolated bovine A-myb cDNA clones spanning the entire coding region indicated extensive homology with the human gene, including the putative transactivation domain. Expression of A-myb was cell cycle dependent; levels of A-myb RNA increased in the late G1-to-S phase transition following serum stimulation of serum-deprived quiescent SMC cultures and peaked in S phase. Nuclear run-on analysis revealed that an increased rate of transcription can account for most of the increase in A-myb RNA levels. Treatment of SMC cultures with 5,6-dichlorobenzimidazole riboside, a selective inhibitor of RNA polymerase II, indicated an approximate 4-h half-life for A-myb mRNA during the S phase of the cell cycle. Expression of A-myb by SMCs was stimulated by basic fibroblast growth factor, in a cell density-dependent fashion. Cotransfection of a human A-myb expression vector activated a multimerized MBS element-driven reporter construct approximately 30-fold in SMCs. The activity of c-myb and c-myc promoters, which both contain multiple MBS elements, were similarly transactivated, approximately 30- and 50-fold, respectively, upon cotransfection with human A-myb. Lastly, A-myb RNA levels could be increased by a combination of phorbol ester plus insulin-like growth factor 1. To test the role of myb family members in progression through the cell cycle, we comicroinjected c-myc and myb expression vectors into serum-deprived quiescent SMCs. The combination of c-myc and either A-myb or c-myb but not B-myb synergistically led to entry into S phase, whereas microinjection of any vector alone had little effect on S phase entry. Thus, these results suggest that A-myb is a potent transactivator in bovine SMCs and that its expression induces progression into S phase of the cell cycle.