Increased collagen gene expression in vascular smooth muscle cells cultured in serum or isoleucine deprived medium

B-Myb Represses Elastin Gene Expression in Aortic Smooth Muscle Cells

B-Myb represses collagen gene transcription in vascular smooth muscle cells (SMCs) in vitro and in vivo. Here we sought to determine whether elastin is similarly repressed by B-Myb. Levels of tropoelastin mRNA and protein were lower in aortas and isolated SMCs of adult transgenic mice expressing the human B-myb gene, driven by the basal cytomegalovirus promoter, compared with age-matched wild type (WT) animals. However, the vessel wall architecture and levels of insoluble elastin revealed no differences. Since elastin deposition occurs early in development, microarray analysis was performed using nontransgenic mice. Aortic levels of tropoelastin mRNA were low during embryonal growth and increased substantially in neonates, whereas B-myb levels varied inversely. Tropoelastin mRNA expression in aortas of 6-day-old neonatal transgenic and WT animals was comparable. Recently, we demonstrated that cyclin A-Cdk2 prevents B-Myb-mediated repression of collagen promoter activity. Cyclin A2 levels were higher in neonatal versus adult WT or transgenic mouse aortas. Ectopic cyclin A expression reversed the ability of B-Myb to repress elastin gene promoter activity in adult SMCs. These results demonstrate for the first time that B-Myb represses SMC elastin gene expression and that cyclin A plays a role in the developmental regulation of elastin gene expression in the aorta. Furthermore, the findings provide additional insight into the mechanism of B-myb-mediated resistance to femoral artery injury.

Regulation of heparan sulfate proteoglycan nuclear localization by fibronectin

Heparan sulfate proteoglycans (HSPG) regulate multiple cellular processes and mediate the cellular uptake of numerous molecules. While heparan sulphate glycosaminoglycan chains are known to modulate receptor binding of several heparin-binding proteins, here we show that distinct extracellular matrices direct HSPG to the nucleus. We analyzed HSPG localization in primary corneal fibroblasts, cultured on fibronectin or collagen type I matrices, using confocal laser scanning microscopy and cell fractionation. Image analysis revealed that the nuclear localization of HSPG core proteins was greater when cells were cultured on fibronectin versus collagen. Matrices containing the heparin-binding domain of fibronectin, but not the integrin-activating domain, demonstrated increased nuclear staining of core proteins. Furthermore, activation of protein kinase C with phorbol 12-myristate 13-acetate inhibited nuclear targeting of HSPG in cells on fibronectin, whereas inhibition of protein kinase C with Ro-31-8220 greatly enhanced nuclear localization of HSPG in cells on both collagen and fibronectin. We propose a matrix-dependent mechanism for nuclear localization of cell surface HSPG involving protein kinase C-mediated signaling. Nuclear localization of HSPG might play important roles in regulating nuclear function.

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.

Serum increases the CD13 receptor expression, reduces the transduction of fluid-mechanical forces, and alters the metabolism of HL60 cells cultured in agitated bioreactors

The effects of serum medium concentration on the CD13 receptor surface content and mRNA levels of HL60 (human promyelocytic leukemia) cells were examined using flow cytometry and Northern blotting. Increasing the serum concentration from 2.5% to 10% and from 5% to 10% increased the CD13 receptor surface content of HL60 cells by 100% and 25%, respectively, in spinner flasks agitated at 60 rpm. In bioreactors at 80 rpm, increasing the serum concentration from 2.5% to 10% and from 5% to 10% increased the CD13 receptor surface content by 60% and 35%, respectively. This increase in CD13 receptor surface content was correlated with a 30% and a 20% increase in CD13 mRNA levels. Increasing serum concentrations also increased the average HL60 cell size under non-damaging conditions (60 rpm in spinner flasks, 80 rpm in bioreactors). Under conditions of agitation at 300 rpm in 2 L bioreactors, increasing serum concentrations (2.5% vs. 10%, 5% vs. 10%) allowed for higher HL60 apparent growth rates, but decreased the CD13 receptor surface content and mRNA levels. In view of our earlier findings on the effects of agitation on the CD13 antigen, these data suggest that serum reduces the transduction of mechanical forces that affect CD13 expression. At 300 rpm, HL60 cells cultured in 10% serum exhibited glucose consumption and lactate production rates that were approximately 50% and 60% lower than the values of cells cultured in 5% and 2.5% serum, respectively.

Effect of high-glucose concentrations on the expression of collagens and fibronectin by fibroblasts in culture

Extracellular matrix macromolecules such as collagen and fibronectin are progressively altered during aging and age-related diseases like diabetes. We investigated the effect of high-glucose concentration (mimicking diabetic conditions) and the influence of in vitro cell aging [comparing 4th-passage fibroblasts (P4) to 15th-passage fibroblasts (P15)] on collagen and fibronectin synthesis. Fibroblasts were incubated at postconfluency with radiolabeled precursors, [3H] proline for collagen, [35S] methionine for fibronectin. We report that in control conditions (5 mM glucose) collagen III production increased with in vitro cell aging. High glucose concentrations (10 and 15 mM) increased specifically collagen III synthesis both at the mRNA and protein levels, without alteration of collagen I production in P4 and P15 cells. Fibronectin synthesis was also increased both during in vitro cell aging and in high glucose-treated P4 fibroblasts. Taken together, these data suggest similarities between changes of phenotypic expression of collagen and fibronectin induced by in vitro cell aging and conditions imitating diabetes.

Collagen biosynthesis by neointimal smooth muscle cells cultured from rabbit aortic explants 15 weeks after de-endothelialization

Extracellular matrix (ECM) accumulation in arterial neointima, developed in response to de-endothelialization, is a prolonged process. In this study, we examined the relationship between increased collagen accumulation and synthetic activity of neointimal smooth muscle cells (SMCs) derived from aortic explants fifteen weeks after balloon catheter injury. Freshly confluent SMCs, derived either from normal aorta or from aortic neointima, were used in this study. The newly synthesized collagen was analysed by measuring [3H]-proline incorporation; and the mRNA expression for two major types of collagen, collagen type I and type III, was studied by Northern blot analysis. Our results indicated a three fold increase in protein (collagen) synthesis by neointimal SMCs. At the same time, the steady-state mRNA for procollagen I and procollagen III was elevated five and three times, respectively. These data indicate that persistent synthesis contributes to collagen accumulation in the arterial neointima and both transcriptional and post-transcriptional regulation take part in this process.

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.

B-Myb expression in vascular smooth muscle cells occurs in a cell cycle-dependent fashion and down-regulates promoter activity of type I collagen genes

The members of the Myb family of transcription factors are defined by homology in the DNA-binding domain; all bind the Myb-binding site (MBS) sequence (YG(A/G)C(A/C/G)GTT(G/A)). Here we report that cultured bovine vascular smooth muscle cells (SMCs) express B-myb. Levels of B-myb RNA found in exponential growth were reduced dramatically in serum-deprived quiescent SMCs; B-myb mRNA levels increased in the cell cycle during the late G1 to S phase transition following restimulation with serum, epidermal growth factor, or phorbol ester plus insulin-like growth factor-1. Changes in the rate of B-myb gene transcription could account for part of the observed increase following serum addition. Treatment of SMC cultures with actinomycin D indicated a >4-h half-life for B-myb mRNA during the S phase of the cell cycle. Cotransfection of either a bovine or human B-myb expression vector down-regulated the activity of a multimerized MBS element-driven reporter construct in SMCs. Putative MBS elements were detected upstream of the promoters of the two chains of type I collagen, which we have found to be expressed inversely with growth state of the SMC (Kindy, M. S., Chang, C.-J., and Sonenshein, G. E. (1988) J. Biol. Chem. 263, 11426-11430). In cotransfection experiments, B-myb expression down-regulated the promoter activity of alpha1(I) and alpha2(I) collagen constructs an average of 92 and 82%, respectively. Thus, B-myb represents a potential link in the observed inverse relationship between collagen gene expression and growth of vascular SMCs.

Derivatized dextrans modulate collagen synthesis in aortic smooth muscle cells

The effect of specifically derivatized dextrans, with or without antiproliferative activity on smooth muscle cells (SMC), was investigated on type I and type III collagen biosynthesis and mRNA levels in post-confluent SMC cultures. Our results indicate that dextran derivatives decreased total protein and collagen synthesis independently of their antiproliferative activities. However, the most substituted dextran, the one exhibiting the strongest antiproliferative activity towards SMC, was the most active in modulating type III collagen expression. In addition, only the two dextran derivatives bearing benzylamide groups inhibited collagen excretion.

Aortic smooth muscle cells in a three-dimensional collagen lattice culture. Evidence for posttranslational regulation of collagen synthesis

Aortic smooth muscle cells were cultivated as monolayers on plastic or within collagen lattices with low- and high-serum supplementation, and the expression of mRNAs specific for pro alpha 1 (I) and pro alpha 1 (III) collagen were studied by slot blot hybridization. The steady-state levels of pro alpha 1 (I) and pro alpha 1 (III) collagen mRNA of cells within collagen lattices were found to be higher than those grown on plastic, although the production of collagen was lower. The degradation of pro alpha 1 (I) and pro alpha 1 (III) collagen mRNAs as revealed in the presence of actinomycin D was not affected by culturing the cells within a collagen lattice. In vitro translation assays of mRNAs of monolayer- and lattice-cultured cells showed no differences in translatability. These data suggest the involvement of posttranslational control of collagen production in collagen lattice-cultured smooth muscle cells.

Responsiveness of aortic smooth muscle cells to soluble growth mediators is influenced by cell-matrix contact

Excessive proliferation and overexpression of collagens by smooth muscle cells (SMCs) are important features of atherogenesis. To understand the role of the extracellular matrix in the regulation of these processes, we examined proliferation and protein/collagen synthesis of SMCs in contact with a collagen matrix. Adult pig SMCs were isolated from the aortic media by collagenase digestion, subcultured as monolayers, and then embedded into a three-dimensional network of type I collagen, ie, a collagen lattice. Cells were subsequently exposed to growth-promoting media, and their behavior was observed in comparison with monolayer cultures on plastic. Treatment of monolayers with increasing concentrations of fetal calf serum resulted in activation of the cell cycle, onset of cell proliferation, and increased protein/collagen synthesis. In contrast, similar treatment of collagen lattice-cultured SMCs failed to influence cell proliferation and protein/collagen synthesis. However, stimulation of proliferation of lattice-cultured SMCs by platelet-derived growth factor-A/B was feasible; nevertheless, the rate of proliferation was modest compared with monolayers. In addition, the onset of proliferation was accompanied by a decrease in collagen synthesis of the cells. Thus, a collagenous matrix appears to suppress the responsiveness of SMCs to soluble growth mediators. It is speculated that interactions between SMCs and the extracellular matrix may modify proliferation and protein/collagen synthesis of cells not only in vitro but also in vivo during atherogenesis by making and breaking binding sites between extracellular collagen and matrix receptors.