Coordinate expression of alpha-tropomyosin and caldesmon isoforms in association with phenotypic modulation of smooth muscle cells

Clinical and Molecular Insights into Gastrointestinal Dysfunction in Myotonic Dystrophy Types 1 & 2

Myotonic dystrophy (DM) is a highly variable, multisystemic disorder that clinically affects one in 8000 individuals. While research has predominantly focused on the symptoms and pathological mechanisms affecting striated muscle and brain, DM patient surveys have identified a high prevalence for gastrointestinal (GI) symptoms amongst affected individuals. Clinical studies have identified chronic and progressive dysfunction of the esophagus, stomach, liver and gallbladder, small and large intestine, and rectum and anal sphincters. Despite the high incidence of GI dysmotility in DM, little is known regarding the pathological mechanisms leading to GI dysfunction. In this review, we summarize results from clinical and molecular analyses of GI dysfunction in both genetic forms of DM, DM type 1 (DM1) and DM type 2 (DM2). Based on current knowledge of DM primary pathological mechanisms in other affected tissues and GI tissue studies, we suggest that misregulation of alternative splicing in smooth muscle resulting from the dysregulation of RNA binding proteins muscleblind-like and CUGBP-elav-like is likely to contribute to GI dysfunction in DM. We propose that a combinatorial approach using clinical and molecular analysis of DM GI tissues and model organisms that recapitulate DM GI manifestations will provide important insight into defects impacting DM GI motility.

Angiotensin II-Induced Long Non-Coding RNA Alivec Regulates Chondrogenesis in Vascular Smooth Muscle Cells

Long non-coding RNAs (lncRNAs) play key roles in Angiotensin II (AngII) signaling but their role in chondrogenic transformation of vascular smooth muscle cells (VSMCs) is unknown. We describe a novel AngII-induced lncRNA Alivec (Angiotensin II-induced lncRNA in VSMCs eliciting chondrogenic phenotype) implicated in VSMC chondrogenesis. In rat VSMCs, Alivec and the nearby gene Acan, a chondrogenic marker, were induced by growth factors AngII and PDGF and the inflammatory cytokine TNF-α. AngII co-regulated Alivec and Acan through the activation of AngII type1 receptor signaling and Sox9, a master transcriptional regulator of chondrogenesis. Alivec knockdown with GapmeR antisense-oligonucleotides attenuated the expression of AngII-induced chondrogenic marker genes, including Acan, and inhibited the chondrogenic phenotype of VSMCs. Conversely, Alivec overexpression upregulated these genes and promoted chondrogenic transformation. RNA-pulldown coupled to mass-spectrometry identified Tropomyosin-3-alpha and hnRNPA2B1 proteins as Alivec-binding proteins in VSMCs. Furthermore, male rats with AngII-driven hypertension showed increased aortic expression of Alivec and Acan. A putative human ortholog ALIVEC, was induced by AngII in human VSMCs, and this locus was found to harbor the quantitative trait loci affecting blood pressure. Together, these findings suggest that AngII-regulated lncRNA Alivec functions, at least in part, to mediate the AngII-induced chondrogenic transformation of VSMCs implicated in vascular dysfunction and hypertension.

Flow shear stress enhances intracellular Ca2+ signaling in pulmonary artery smooth muscle cells from patients with pulmonary arterial hypertension

An increase in cytosolic Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary arterial smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and an important stimulus for pulmonary arterial medial hypertrophy in patients with idiopathic pulmonary arterial hypertension (IPAH). Vascular smooth muscle cells (SMC) sense the blood flow shear stress through interstitial fluid driven by pressure or direct exposure to blood flow in case of endothelial injury. Mechanical stimulus can increase [Ca(2+)]cyt. Here we report that flow shear stress raised [Ca(2+)]cyt in PASMC, while the shear stress-mediated rise in [Ca(2+)]cyt and the protein expression level of TRPM7 and TRPV4 channels were significantly greater in IPAH-PASMC than in normal PASMC. Blockade of TRPM7 by 2-APB or TRPV4 by Ruthenium red inhibited shear stress-induced rise in [Ca(2+)]cyt in normal and IPAH-PASMC, while activation of TRPM7 by bradykinin or TRPV4 by 4αPDD induced greater increase in [Ca(2+)]cyt in IPAH-PASMC than in normal PASMC. The bradykinin-mediated activation of TRPM7 also led to a greater increase in [Mg(2+)]cyt in IPAH-PASMC than in normal PASMC. Knockdown of TRPM7 and TRPV4 by siRNA significantly attenuated the shear stress-mediated [Ca(2+)]cyt increases in normal and IPAH-PASMC. In conclusion, upregulated mechanosensitive channels (e.g., TRPM7, TRPV4, TRPC6) contribute to the enhanced [Ca(2+)]cyt increase induced by shear stress in PASMC from IPAH patients. Blockade of the mechanosensitive cation channels may represent a novel therapeutic approach for relieving elevated [Ca(2+)]cyt in PASMC and thereby inhibiting sustained pulmonary vasoconstriction and pulmonary vascular remodeling in patients with IPAH.

Arp5 is a key regulator of myocardin in smooth muscle cells

Arp5 suppresses myocardin activity through both direct binding to myocardin and binding to SRF to prevent transcriptional activation of myogenic genes by the myocardin–SRF complex.

Myocardin (Myocd) and Myocd-related transcription factors (MRTFs) are robust coactivators of serum response factor (SRF). RPEL motifs are monomeric globular actin (G-actin) binding elements that regulate MRTF localization and activity. However, the function of the RPEL motif in Myocd is largely unknown because of its low affinity for G-actin. Here, we demonstrated that the Myocd RPEL motif bound to actin-related protein 5 (Arp5) instead of conventional actin, resulting in a significant suppression of Myocd activity. In addition, Arp5 bound to a DNA binding domain of SRF via its C-terminal sequence and prevented the association of the Myocd–SRF complex with the promoter regions of smooth muscle genes. Well-differentiated smooth muscle cells mainly expressed a specific splicing variant of arp5; therefore, the protein level of Arp5 was markedly reduced by partial messenger RNA decay and translational suppression. In dedifferentiated smooth muscle cells, Arp5 knockdown restored the differentiated phenotype via Myocd activation. Thus, Arp5 is a key regulator of Myocd activity.

Bladder smooth muscle organ culture preparation maintains the contractile phenotype

Smooth muscle cells, when subjected to culture, modulate from a contractile to a secretory phenotype. This has hampered the use of cell culture for molecular techniques to study the regulation of smooth muscle biology. The goal of this study was to develop a new organ culture model of bladder smooth muscle (BSM) that would maintain the contractile phenotype and aid in the study of BSM biology. Our results showed that strips of BSM subjected to up to 9 days of organ culture maintained their contractile phenotype, including the ability to achieve near-control levels of force with a temporal profile similar to that of noncultured tissues. The technical aspects of our organ culture preparation that were responsible, in part, for the maintenance of the contractile phenotype were a slight longitudinal stretch during culture and subjection of the strips to daily contraction-relaxation. The tissues contained viable cells throughout the cross section of the strips. There was an increase in extracellular collagenous matrix, resulting in a leftward shift in the passive length-tension relationship. There were no significant changes in the content of smooth muscle-specific α-actin, calponin, h-caldesmon, total myosin heavy chain, protein kinase G, Rho kinase-I, or the ratio of SM1 to SM2 myosin isoforms. Moreover the organ cultured tissues maintained functional voltage-gated calcium channels and large-conductance calcium-activated potassium channels. Therefore, we propose that this novel BSM organ culture model maintains the contractile phenotype and will be a valuable tool for the use in cellular/molecular biology studies of bladder myocytes.

Actin stress fibers--assembly, dynamics and biological roles

Actin filaments assemble into diverse protrusive and contractile structures to provide force for a number of vital cellular processes. Stress fibers are contractile actomyosin bundles found in many cultured non-muscle cells, where they have a central role in cell adhesion and morphogenesis. Focal-adhesion-anchored stress fibers also have an important role in mechanotransduction. In animal tissues, stress fibers are especially abundant in endothelial cells, myofibroblasts and epithelial cells. Importantly, recent live-cell imaging studies have provided new information regarding the mechanisms of stress fiber assembly and how their contractility is regulated in cells. In addition, these studies might elucidate the general mechanisms by which contractile actomyosin arrays, including muscle cell myofibrils and cytokinetic contractile ring, can be generated in cells. In this Commentary, we discuss recent findings concerning the physiological roles of stress fibers and the mechanism by which these structures are generated in cells.

New insights into the regulation of the actin cytoskeleton by tropomyosin

The actin cytoskeleton is regulated by a variety of actin-binding proteins including those constituting the tropomyosin family. Tropomyosins are coiled-coil dimers that bind along the length of actin filaments. In muscles, tropomyosin regulates the interaction of actin-containing thin filaments with myosin-containing thick filaments to allow contraction. In nonmuscle cells where multiple tropomyosin isoforms are expressed, tropomyosins participate in a number of cellular events involving the cytoskeleton. This chapter reviews the current state of the literature regarding tropomyosin structure and function and discusses the evidence that tropomyosins play a role in regulating actin assembly.

Growth inhibition and differentiation of cultured smooth muscle cells depend on cellular crossbridges across the tubular lumen of type I collagen matrix honeycombs

Although rabbit vascular smooth muscle cells (SMCs) showed a differentiated phenotype in three-dimensional type I collagen matrices (honeycombs, diameter of pores=200-500 microm), mouse vascular SMCs proliferated in honeycombs having the same pore size. Here we investigated the relationship between pore sizes of honeycombs and differentiation of SMCs using various pore sizes of honeycombs. Rabbit SMCs (length: 200+/-32 microm) and mouse SMCs (49+/-10 microm) formed crossbridges in honeycombs with 200-300 microm and less than 200 microm of pores, respectively. Both SMCs spread on the inner wall but did not form crossbridges in honeycombs with larger pores. [(3)H]Thymidine incorporation and cell number of both SMCs were decreased when the crossbridges were formed in honeycombs. Because proliferation inhibition and crossbridge formation were observed in the culture of rabbit and mouse SMCs using 200-300 microm and less than 200 microm pore sized honeycombs, respectively, these data suggested that forming crossbridges was important for the inhibition of proliferation of SMCs. Rabbit SMCs differentiation was accompanied by the expression of caldesmon heavy chain when cultured in honeycombs having less than 300 microm pores. Proliferation of mouse SMCs stopped in honeycombs having less than 200 microm pores, but caldesmon heavy chain was not detected despite the expression of its mRNA. Proliferation of SMCs stopped on plates when cells reached confluent state, however, caldesmon heavy chain was not expressed. These data suggested that an appropriate structure and suitable honeycomb pore size are important for the differentiation of SMCs.

Bone marrow stem cells for urologic tissue engineering

OBJECTIVES

Experiments in rats and dogs have demonstrated the potential of bone marrow-derived mesenchymal stem cells (MSCs) for urinary tract tissue engineering. However, the small graft size in rats and a failure to identify the MSCs in engineered tissues made it difficult to assess the true potential of these cells. Our goals were to characterize MSCs from pigs, determine their ability to differentiate into smooth muscle cells (SMCs) and use them in an autologous augmentation cystoplasty.

METHODS

MSCs were isolated from pigs and analyzed for common markers of MSCs by flow cytometry. SMC differentiation was determined by immunoblotting. MSCs were isolated, genetically labeled, expanded in vitro, seeded onto small intestinal submucosa (SIS) and used for autologous bladder augmentation.

RESULTS

Porcine MSCs are morphologically and immunophenotypically similar to human MSCs. Culturing MSCs at low density enhances proliferation rates. MSCs consistently differentiate into mature SMCs in vitro when maintained at confluence. Labeled MSCs grew on SIS over one week in vitro and survived a 2-week implantation as an autologous bladder augment in vivo. Some label-positive cells with SMC morphology were detected, but most SMCs were negative. Notably, many cells with a urothelial morphology stained positively.

CONCLUSIONS

Porcine MSCs have similar properties to MSCs from other species and consistently undergo differentiation into mature SMC in vitro under specific culture conditions. Labeled MSCs within SIS may assist tissue regeneration in augmentation cystoplasty but may not significantly incorporate into smooth muscle bundles.

Caldesmon and the regulation of cytoskeletal functions

Caldesmon (CaD) is an extraordinary actin-binding protein, because in addition to actin, it also bindsmyosin, calmodulin and tropomyosin. As a component of the smoothmuscle and nonmuscle contractile apparatus CaD inhibits the actomyosin ATPase activity and its inhibitory action is modulated by both Ca2+ and phosphorylation. The multiplicity of binding partners and diverse biochemical properties suggest CaD is a potent and versatile regulatory protein both in contractility and cell motility. However, after decades ofinvestigation in numerous laboratories, hard evidence is still lacking to unequivocally identify its in vivo functions, although indirect evidence is mounting to support an important role in connection with the actin cytoskeleton. This chapter reviews the highlights of the past findings and summarizes the current views on this protein, with emphasis of its interaction with tropomyosin.

Regulatory role of vitamins E and C on extracellular matrix components of the vascular system

The protective effect of vitamins E (alpha-tocopherol) and C (L-ascorbic acid) in the prevention of cardiovascular disease (CVD) has been shown in a number of situations but a secure correlation is not universally accepted. Under certain conditions, both, L-ascorbic acid and alpha-tocopherol can exhibit antioxidant properties and thus may reduce the formation of oxidized small molecules, proteins and lipids, which are a possible cause of cellular de-regulation. However, non-antioxidant effects have also been suggested to play a role in the prevention of atherosclerosis. Vitamin E and C can modulate signal transduction and gene expression and thus affect many cellular reactions such as the proliferation of smooth muscle cells, the expression of cell adhesion and extracellular matrix molecules, the production of O(2)(-) by NADPH-oxidase, the aggregation of platelets and the inflammatory response. Vitamins E and C may modulate the extracellular matrix environment by affecting VSMC differentiation and the expression of connective tissue proteins involved in vascular remodeling as well as the maintenance of vascular wall integrity. This review summarizes individually the molecular activities of vitamins E and C on the cells within the connective tissue of the vasculature, which are centrally involved in the maintenance of an intact vascular wall as well as in the repair of atherosclerotic lesions during disease development.

The functional consequence of RhoA knockdown by RNA interference in rat cerebral arteries

Uridine triphosphate (UTP) constricts cerebral arteries by activating transduction pathways that increase cytosolic [Ca(2+)] and myofilament Ca(2+) sensitivity. The signaling proteins that comprise these pathways remain uncertain with recent studies implicating a role for several G proteins. To start clarifying which G proteins enable UTP-induced vasoconstriction, a small interfering RNA (siRNA) approach was developed to knock down specified targets in rat cerebral arteries. siRNA directed against G(q) and RhoA was introduced into isolated cerebral arteries using reverse permeabilization. Following a defined period of organ culture, arteries were assayed for contractile function, mRNA levels, and protein expression. Targeted siRNA reduced RhoA or G(q) mRNA expression by 60-70%, which correlated with a reduction in RhoA but not G(q) protein expression. UTP-induced constriction was abolished in RhoA-depleted arteries, but this was not due to a reduction in myosin light chain phosphorylation. UTP-induced actin polymerization was attenuated in RhoA-depleted arteries, which would explain the loss of agonist-induced constriction. In summary, this study illustrates that siRNA approaches can be effectively used on intact arteries to induce targeted knockdown given that the protein turnover rate is sufficiently high. It also demonstrates that the principal role of RhoA in agonist-induced constriction is to facilitate the formation of F-actin, the physical structure to which phosphorylated myosin binds to elicit arterial constriction.

Transcription Enhancer Factor-1-dependent Expression of the α-Tropomyosin Gene in the Three Muscle Cell Types

In vertebrates, the actin-binding proteins tropomyosins are encoded by four distinct genes that are expressed in a complex pattern during development and muscle differentiation. In this study, we have characterized the transcriptional machinery of the alpha-tropomyosin (alpha-Tm) gene in muscle cells. Promoter analysis revealed that a 284-bp proximal promoter region of the Xenopus laevis alpha-Tm gene is sufficient for maximal activity in the three muscle cell types. The transcriptional activity of this promoter in the three muscle cell types depends on both distinct and common cis-regulatory sequences. We have identified a 30-bp conserved sequence unique to all vertebrate alpha-Tm genes that contains an MCAT site that is critical for expression of the gene in all muscle cell types. This site can bind transcription enhancer factor-1 (TEF-1) present in muscle cells both in vitro and in vivo. In serum-deprived differentiated smooth muscle cells, TEF-1 was redistributed to the nucleus, and this correlated with increased activity of the alpha-Tm promoter. Overexpression of TEF-1 mRNA in Xenopus embryonic cells led to activation of both the endogenous alpha-Tm gene and the exogenous 284-bp promoter. Finally, we show that, in transgenic embryos and juveniles, an intact MCAT sequence is required for correct temporal and spatial expression of the 284-bp gene promoter. This study represents the first analysis of the transcriptional regulation of the alpha-Tm gene in vivo and highlights a common TEF-1-dependent regulatory mechanism necessary for expression of the gene in the three muscle lineages.

Resting smooth muscle cells as a model for studying vascular cell activation

Vascular smooth muscle (VSM) cells constitute the main structural components of tunica media. Under physiological conditions, these cells display a contractile phenotype and a low proliferative activity. However, they may also acquire a synthetic phenotype and become predominantly proliferative if stimulated under certain stress conditions. This capacity plays a major role in the inception and progression of such cardiovascular diseases as atherosclerosis, hypertension and restenosis. Porcine coronary smooth muscle (PCSM) cells exhibit a synthetic phenotype (ON cells) under standard culturing conditions, but they can be reverted to a contractile phenotype (OFF cells) in a serum-free medium. However, OFF cells can also re-acquire a synthetic phenotype (OFF/ON cells) upon serum administration. In the present study, proliferative and contractile behaviors were characterized by expression of specific differentiation markers. Taken together, these results demonstrate that porcine vascular smooth muscle cells can retain their phenotypic plasticity in culture, and thus mimic in vitro their in vivo differentiation states. OFF cells may thus provide a suitable model system in studying the mechanism(s) by which either known or unknown serum factors may trigger vascular smooth muscle activation. In the present study, this possibility was actually tested by exposing OFF cells to fetal bovine serum (FBS), PDGF-BB and IGF-I. Data show that only FBS could induce a synthetic phenotype in OFF cells, while both PDGF-BB and IGF-I failed to induce any VSM activation.

In vitro growth of human urinary tract smooth muscle cells on laminin and collagen type I-coated membranes under static and dynamic conditions

This study investigates in vitro growth of human urinary tract smooth muscle cells under static conditions and mechanical stimulation. The cells were cultured on collagen type I- and laminin-coated silicon membranes. Using a Flexcell device for mechanical stimulation, a cyclic strain of 0-20% was applied in a strain-stress-time model (stretch, 104 min relaxation, 15 s), imitating physiological bladder filling and voiding. Cell proliferation and alpha-actin, calponin, and caldesmon phenotype marker expression were analyzed. Nonstretched cells showed significant better growth on laminin during the first 8 days, thereafter becoming comparable to cells grown on collagen type I. Cyclic strain significantly reduced cell growth on both surfaces; however, better growth was observed on laminin. Neither the type of surface nor mechanical stimulation influenced the expression pattern of phenotype markers; alpha-actin was predominantly expressed. Coating with the extracellular matrix protein laminin improved in vitro growth of human urinary tract smooth muscle cells.

Roles of Hemodynamic Forces in Vascular Cell Differentiation

The pulsatile nature of blood flow is a key stimulus for the modulation of vascular cell differentiation. Within the vascular media, physiologic stress is manifested as cyclic strain, while in the lumen, cells are subjected to shear stress. These two respective biomechanical forces influence the phenotype and degree of differentiation or proliferation of smooth muscle cells and endothelial cells within the human vasculature. Elucidation of the effect of these mechanical forces on cellular differentiation has led to a surge of research into this area because of the implications for both the treatment of atherosclerotic disease and the future of vascular tissue engineering. The use of mechanical force to directly control vascular cell differentiation may be utilized as an invaluable engineering tool in the future. However, an understanding of the role of hemodynamics in vascular cell differentiation and proliferation is critical before application can be realized. Thus, this review will provide a current perspective on the latest research and controversy behind the role of hemodynamic forces for vascular cell differentiation and phenotype modulation. Furthermore, this review will illustrate the application of hemodynamic force for vascular tissue engineering and explicate future directions for research.

Fluvastatin Prevents Vascular Hyperplasia by Inhibiting Phenotype Modulation and Proliferation Through Extracellular Signal-Regulated Kinase 1 and 2 and p38 Mitogen-Activated Protein Kinase Inactivation in Organ-Cultured Artery

OBJECTIVE

We examined the inhibitory mechanisms of fluvastatin on FBS-induced vascular hypertrophy assessed by organ-cultured rat tail artery.

METHODS AND RESULTS

After 5 days of culture with 10% FBS, hyperplastic morphological changes in the media layer were induced. Treatment with 1 mumol/L fluvastatin significantly inhibited these changes. In the FBS-cultured arteries, the protein expression ratio of alpha-actin/beta-actin was significantly decreased, indicating the change to synthetic phenotype. Fluvastatin restored the decreased expression ratio, and the addition of mevalonate (100 mumol/L) suppressed this recovery. In accordance with the synthetic morphological changes, the absolute force of contractions induced by stimuli was decreased. Fluvastatin treatment also restored the decreased contractility, and the addition of mevalonate suppressed this recovery. In the arteries cultured with FBS, extracellular signal-regulated kinase 1 and 2 (ERK1/2) and p38 mitogen-activated protein kinase (p38MAPK) phosphorylation were significantly increased. Fluvastatin inhibited these phosphorylations, and mevalonate prevented the action of fluvastatin.

CONCLUSIONS

These results suggest that fluvastatin inhibits vascular smooth muscle phenotype modulation to synthetic phenotype and proliferation by inhibiting the local metabolic pathway of cholesterol in smooth muscle cells, which inhibits hyperplastic changes in the vascular wall. The antihyperplastic actions by statins may be induced by inhibiting the ERK1/2 and p38MAPK activities, possibly through inhibition of prenylated Ras. We examined the inhibitory mechanisms of fluvastatin on FBS-induced vascular hypertrophy assessed by organ-cultured artery. Results suggest that fluvastatin inhibits vascular smooth muscle phenotype modulation and proliferation by inhibiting the ERK1/2 and p38MAPK activities through depletion of mevalonate in smooth muscle cells, resulting in inhibiting vascular hyperplastic changes.

Stretch of the vascular wall induces smooth muscle differentiation by promoting actin polymerization

Stretch of the vascular wall by the intraluminal blood pressure stimulates protein synthesis and contributes to the maintenance of the smooth muscle contractile phenotype. The expression of most smooth muscle specific genes has been shown to be regulated by serum response factor and stimulated by increased actin polymerization. Hence we hypothesized that stretch-induced differentiation is promoted by actin polymerization. Intact mouse portal veins were cultured under longitudinal stress and compared with unstretched controls. In unstretched veins the rates of synthesis of several proteins associated with the contractile/cytoskeletal system (alpha-actin, calponin, SM22alpha, tropomyosin, and desmin) were dramatically lower than in stretched veins, whereas other proteins (beta-actin and heat shock proteins) were synthesized at similar rates. The cytoskeletal proteins gamma-actin and vimentin were weakly stretch-sensitive. Inhibition of Rho-associated kinase by culture of stretched veins with Y-27632 produced similar but weaker effects compared with the absence of mechanical stress. Induction of actin polymerization by jasplakinolide increased SM22alpha synthesis in unstretched veins to the level in stretched veins. Stretch stimulated Rho activity and phosphorylation of the actin-severing protein cofilin-2, although both effects were slow in onset (Rho-GTP, >15 min; cofilin-P, >1 h). Cofilin-2 phosphorylation of stretched veins was inhibited by Y-27632. The F/G-actin ratio after 24 h of culture was significantly greater in stretched than in unstretched veins, as shown by both ultracentrifugation and confocal imaging with phalloidin/DNase I labeling. The results show that stretch of the vascular wall stimulates increased actin polymerization, activating synthesis of smooth muscle-specific proteins. The effect is partially, but probably not completely, mediated via Rho-associated kinase and cofilin downstream of Rho.

Characterization of a mammalian smooth muscle cell line that has retained transcriptional and posttranscriptional potencies

Unlike skeletal and cardiac muscle cells that differentiate irreversibly, smooth muscle cells (SMCs) retain a high degree of plasticity. During the so-called phenotypic modulation, SMCs can undergo transition between a contractile phenotype and a highly proliferative synthetic phenotype, as apparent from the extinction of numerous smooth muscle (SM) markers when they are passaged in culture. It would be very useful to have an SMC line that can be indefinitely propagated for the cellular and molecular analysis of the mechanisms that underlie the control of SM differentiation. This report describes an immortalized rabbit aorta SMC-derived cell line (U8A4) that has conserved differentiated properties through multiple subcultures. U8A4 cells can grow in the absence of serum and express the SMC markers studied, including SM alpha-actin, SM calponin, SM22alpha, SM alpha-tropomyosin (alpha-TM), SM myosin heavy chain (SM-MHC), and myocardin. U8A4 cells can activate SMC-restricted promoters like those of SM22alpha, SM calponin, and SM-MHC genes as efficiently as described previously for rat SMC lines (PAC1, A7r5, and A10). These cells can also process exogenous alpha-TM transcripts according to an SM-specific pattern. These results demonstrate that the U8A4 cell line constitutes a good alternative model to existing SMC lines that could facilitate the study of the transcriptional and posttranscriptional regulatory mechanisms underlying SMC differentiation.

Estrogen receptors in atherosclerotic human aorta: inhibition of human vascular smooth muscle cell proliferation by estrogens

Estrogen has been postulated to exert direct anti-atherogenic effects via binding to estrogen receptors (ERs) in vascular smooth muscle cells (VSMCs). Therefore, we believe it is important to examine the status of ER expression in the human cardiovascular system and its disorders. In this study, we first evaluated the relative abundance of messenger RNA (mRNA) of both ER subtypes (ERalpha and ERbeta) in the human aorta using reverse transcription followed by quantitative polymerase chain reaction (RT-qPCR). We then examined the immunolocalization of both ERs in VSMCs of human atherosclerotic lesions. In order to examine which ER subtype was associated with the anti-atherogenic effects of estrogen, we examined the effects of estrogen in two VSMC cell lines, one positive only for ERalpha and the other positive only for ERbeta. The relative abundance of mRNAs for both ERs was higher in female aorta with a mild degree of atherosclerosis than in female aorta with a severe degree of atherosclerosis (P < 0.05). In addition, the number of ERalpha and/or ERbeta double positive cells in the neointima was higher in female aorta with a mild degree of atherosclerosis than in female aorta with severe atherosclerosis (P < 0.05). Our in vitro study found that estradiol was able to significantly inhibit the proliferation of ERalpha positive VSMCs but not ERbeta positive VSMCs (P < 0.05). Moreover, estradiol was found to significantly suppress proliferating cell nuclear antigen (PCNA) mRNA levels in ERalpha positive VSMCs compared to that of ERbeta positive VSMCs, consistent with the findings of cell proliferation. Results from this study suggest that estrogens can inhibit the proliferation of VSMCs through ERalpha, especially in pre-menopausal women. Our study also indicates that decreased levels of ER, especially ERalpha, in the female atherosclerotic neointima may be associated with progression of atherosclerotic changes.

Unzipping the role of myosin light chain phosphatase in smooth muscle cell relaxation

Recently, it has been hypothesized that myosin light chain (MLC) phosphatase is activated by cGMP-dependent protein kinase (PKG) via a leucine zipper-leucine zipper (LZ-LZ) interaction through the C-terminal LZ in the myosin-binding subunit (MBS) of MLC phosphatase and the N-terminal LZ of PKG (Surks, H. K., Mochizuki, N., Kasai, Y., Georgescu, S. P., Tang, K. M., Ito, M., Lincoln, T. M., and Mendelsohn, M. E. (1999) Science 286, 1583-1587). Alternative splicing of a 3'-exon produces a LZ+ or LZ- MBS, and the sensitivity to cGMP-mediated smooth muscle relaxation correlates with the relative expression of LZ+/LZ- MBS isoforms (Khatri, J. J., Joyce, K. M., Brozovich, F. V., and Fisher, S. A. (2001) J. Biol. Chem. 276, 37250 -37257). In the present study, we determined the effect of LZ+/LZ- MBS isoforms on cGMP-induced MLC20 dephosphorylation. Four avian smooth muscle MBS-recombinant adenoviruses were prepared and transfected into cultured embryonic chicken gizzard smooth muscle cells. The expressed exogenous MBS isoforms were shown to replace the endogenous isoform in the MLC phosphatase holoenzyme. The interaction of type I PKG (PKGI) with the MBS did not depend on the presence of cGMP or the MBS LZ. However, direct activation of PKGI by 8-bromo-cGMP produced a dose-dependent decrease in MLC20 phosphorylation (p<0.05) only in smooth muscle cells expressing a LZ+ MBS. These results suggest that the activation of MLC phosphatase by PKGI requires a LZ+ MBS, but the binding of PKGI to the MBS is not mediated by a LZ-LZ interaction. Thus, the relative expression of LZ+/LZ- MBS isoforms could explain differences in tissue sensitivity to NO-mediated vasodilatation.

Tropomyosin 4 expression is enhanced in dedifferentiating smooth muscle cells in vitro and during atherogenesis

Dedifferentiation of smooth muscle cells (SMC) from the contractile to the synthetic phenotype is a key event in atherosclerosis. A comparable phenotypic change from the contractile to the synthetic state is rapidly incurred when SMC are grown in culture. To identify genes that characterize the contractile and synthetic phenotypes, we performed differential display reverse transcription polymerase chain reactions on RNA from porcine arterial contractile SMC obtained directly from medial tissues and from SMC made synthetic by cell culturing. One of the differentially expressed cDNAs we identified encoded tropomyosin 4 (TM4). Whereas basal levels of TM4 existed in contractile SMC, the amount of TM4 transcripts strongly increased in synthetic SMC (33% vs. 86-106%; p < 0.005). Induction of foam cell formation had no additional enhancing effect on the expression of TM4 in cultivated SMC. We also tested whether TM4 expression was correspondingly enhanced during atherogenesis. The number of TM4-expressing SMC increased with plaque development as demonstrated by simultaneous in situ hybridization and immunohistochemistry. We compared the localization patterns of myosin heavy chain isoforms in normal arteries and lesions of increasing severity and determined that TM4 expression was relegated mainly to SMC of the synthetic phenotype in the media and intima during atherogenesis. The present study demonstrates that upregulation of TM4 mRNA is a relevant marker of dedifferentiation in vascular SMC.

Bladder smooth muscle cell phenotypic changes and implication of expression of contractile proteins (especially caldesmon) in rats after partial outlet obstruction

BACKGROUND

The purpose of the present study was to investigate morphological changes in bladder smooth muscle of rats with partial outlet obstruction. We investigated smooth muscle cell phenotypic changes and implication of synthetic phenotype in contractility decrease and bladder compliance after bladder outlet obstruction.

METHODS

Partial bladder outlet obstruction was introduced in female rats. Bladder were removed at 1, 3, 6, 10 and 20 weeks after the obstruction. Temporal pattern of changes in bladder mass, light microscopic pathogenesis and phenotypic expression of the bladder smooth muscle cells in the electron micrographs were investigated. Expression of contractile protein was also investigated by the immunoblotting method.

RESULTS

Marked increase in bladder mass with marked thickening of smooth muscle layer was observed at 1 week after obstruction. The ratio of myocytes exhibiting contractile and synthetic phenotypes was almost constant until 6 weeks after the obstruction, but thereafter, synthetic phenotypes gradually increased and the ratio (synthetic/contractile phenotype) was 1.5-fold at 20 weeks after the obstruction. Caldesmon was most markedly expressed after the obstruction among contractile proteins examined by the immunoblotting method.

CONCLUSION

Phenotypic changes were confirmed in bladder smooth muscle, and the decrease of the ratio of contractile phenotype was observed after long-term obstruction of the bladder outlet. Among the contractile proteins in the bladder smooth muscle cell, caldesmon was considered a reliable marker for predicting the pathogenetic conditions of the bladder.

Differential gene expression in vascular smooth muscle cells in primary atherosclerosis and in stent stenosis in humans

OBJECTIVE

We sought to identify differentially expressed genes in human in stent stenosis (ISS) to provide insights into the mechanism of disease.

METHODS AND RESULTS

Using representation difference analysis, we examined differential gene expression between cultured normal human medial vascular smooth muscle cells (VSMCs) and cells from primary atherosclerotic plaques or ISS sites. Specific groups of genes were overexpressed in ISS and plaque VSMCs, including cell cycle regulatory proteins and cell matrix and contractile proteins. Differential expression was validated by virtual Northern analysis, reverse transcriptase-polymerase chain reaction, in situ hybridization, and immunohistochemistry. All ISS genes were expressed by normal intima and had even higher expression in primary plaque VSMCs.

CONCLUSIONS

ISS VSMCs have a stable gene expression profile reflecting an intimal pattern, intermediate between normal medial and primary plaque VSMCs. Differential expression profiling may identify markers of disease that are overexpressed in ISS and also help elucidate the origin of the ISS lesion.

A triad of serum response factor and the GATA and NK families governs the transcription of smooth and cardiac muscle genes

Serum response factor and the (CC(A/T)(6)GG) (CArG) box interact to promote the transcription of c-fos and muscle genes; this tissue-specific activity may require co-regulators for serum response factor. The alpha(1) integrin promoter contains two cis-elements besides the CArG box: a TAAT sequence, a consensus binding site for homeoproteins, and a GATA-binding box. As a candidate TAAT-binding factor, we cloned an NK family homeobox gene, Nkx-3.2, which is expressed mainly in smooth muscle tissues and skeletal structures. Nkx-3.2, serum response factor, and GATA-6 were co-expressed only in the medial smooth muscle layer of arteries. These three transcription factors formed a complex with their corresponding cis-elements and cooperatively transactivated smooth muscle genes, including alpha(1) integrin, SM22alpha, and caldesmon. Cardiac muscle-specific members of the NK and GATA families exist, and the triad of Nkx-2.5, serum response factor, and GATA-4 also transactivated the cardiac atrial natriuretic factor gene, which contains a CArG-like box, a GATA-binding box, and an NK-binding element. Our findings demonstrate that smooth and cardiac muscle have a shared transcriptional machinery and that the GATA and NK families confer muscle specificity on the serum response factor/CArG interaction.

Smooth muscle myosin heavy chain and caldesmon expression in the anterior vaginal wall of women with and without pelvic organ prolapse

OBJECTIVE

The aim of this study was to quantify the expression of smooth muscle myosin heavy chain (SM-MHC) and caldesmon in the anterior vaginal wall of women with and without pelvic organ prolapse.

STUDY DESIGN

Immunoblot analysis was conducted on protein extracts from the vaginal muscularis of 15 women with and 11 women without pelvic organ prolapse by using specific antibodies for SM-MHC, nonmuscle MHC-B, and caldesmon. The fraction of muscularis containing smooth muscle was determined by morphometric analysis of histologic cross sections. Reverse transcriptase-polymerase chain reaction was used to amplify SM-MHC isoforms produced by alternative splicing in the myosin head.

RESULTS

Whereas the expression of SM-MHC was increased modestly (2-fold), expression of smooth muscle caldesmon was increased 6- to 7-fold in vaginal muscularis from women with prolapse. The relative distribution of SM-MHC isoforms was similar in both groups.

CONCLUSIONS

Caldesmon is increased substantially in vaginal smooth muscle of women with pelvic organ prolapse. Caldesmon inhibits actin-activated myosin magnesium adenosine triphosphatase activity and inhibits the maintenance of contractile force. Thus, this disproportionate increase in caldesmon, relative to myosin, may result in inhibition of vaginal smooth muscle contractility and force maintenance.

Transcriptional Activation of β-Tropomyosin Mediated by Serum Response Factor and a Novel Barx Homologue, Barx1b, in Smooth Muscle Cells

Tropomyosin (TM) is a regulatory protein of actomyosin system. Muscle type-specific expression of TM isoforms is generated from different genes and by alternative splicing. beta-TM isoforms in chicken skeletal and smooth muscles are encoded by a single gene and transcribed from the same promoter. We previously reported a smooth muscle cell (SMC) phenotype-dependent change in beta-TM expression (Kashiwada, K., Nishida, W., Hayashi, K., Ozawa, K., Yamanaka, Y., Saga, H., Yamashita, T., Tohyama, M., Shimada, S., Sato, K., and Sobue, K. (1997) J. Biol. Chem. 272, 15396-15404), and identified beta-TM as an SMC-differentiation marker. Here, we characterized the transcriptional machinery of the beta-TM gene in SMCs. Promoter and gel mobility shift analyses revealed an obligatory role for serum response factor and its interaction with the CArG box sequence in the SMC-specific transcription of the beta-TM gene in differentiated SMCs. We further isolated a novel homologue of the Barx homeoprotein family, Barx1b, from chicken gizzard. Barx1b was exclusively localized to SMCs of the upper digestive organs and their attached arteries and to craniofacial structures. Serum response factor and Barx1b bound each other directly, coordinately transactivated the beta-TM gene in differentiated SMCs and heterologous cells, and formed a ternary complex with a CArG probe. Taken together, these results suggest that SRF and Barx1b are coordinately involved in the SMC-specific transcription of the beta-TM gene in the upper digestive organs and their attached arteries.

Changes in Ca2+ signaling and contractile protein isoforms in smooth muscle cells from guinea pig ileum during culture

Single smooth muscle cells (SMCs) isolated from guinea pig ileum using collagenase and papain were cultured on laminin-coated dishes in MEM containing fetal calf serum. Temporal changes in intracellular calcium ion concentration in response to carbachol and to ATP were investigated using fluo-3/AM and fluorescence microscopy. It was observed that carbachol caused an increased intracellular calcium ion in freshly isolated single SMCs but a reduced or negative response of cultured SMCs before confluence. On the other hand, ATP was observed to cause an increase in the calcium ion content of SMCs throughout the culture. SDS-PAGE and Western blot analyses revealed changes in the expression of contractile proteins as follows. l-Caldesmon and non-muscle type myosin heavy chain (NMHC) (considered to be marker molecules for dedifferentiation in smooth muscle cells) and non-muscle type tropomyosin were not observed in freshly isolated single SMCs. l-Caldesmon and NMHC appeared in the cultured SMCs within 2 days and the tropomyosin isoform was observed 6 days following seeding. Simultaneously, smooth muscle type myosin heavy chain (SMHC) decreased strikingly and the 41 kDa tropomyosin monomer was lost. The content of alpha-actin decreased gradually to a minimum on day 6 when non-muscle type tropomyosin appeared, and the cells began to proliferate rapidly. These results suggest that the loss of contractility in cultured smooth muscle cells is more closely related to changes in contractile protein profiles than to receptor-mediated signal transduction and that in addition to NMHC and l-caldesmon, non-muscle type tropomyosin may be useful as a marker molecule for de-differentiation of smooth muscle cells.

Mitogen-induced up-regulation of non-smooth muscle isoform of alpha-tropomyosin in rat aortic smooth muscle cells

Correlation between the expression of the alpha-tropomyosin isoforms and cell growth was investigated in rat aortic smooth muscle cells. The levels of exon 1a, exons 1a+2a (smooth muscle type) and exons 1a+2b (non-smooth muscle type) were determined by reverse transcription-polymerase chain reaction (RT-PCR). When the cells were cultured, the level of exons 1a+2b transiently increased while reaching a maximum at 3-5 days. When the serum-deprived confluent cells were stimulated with 3-20% serum for 1.5 h, the level of exons 1a+2b increased by about twofold. The 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) but not 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimi de (GF 109203X) inhibited this up-regulation. Phorbol-12, 13-dibutyrate (PDB) mimicked the effect of serum. The DNA synthesis as determined by the incorporation of 5-bromo-2'-deoxy-uridine (BrdU) was not enhanced by the 1.5 h stimulation with serum or phorbol ester. The up-regulation of non-smooth muscle isoform of alpha-tropomyosin occurred during G(0)/G(1) transition before entering S phase. Protein phosphorylation is suggested to be involved in the up-regulation. However, the responsible kinase(s) remain to be elucidated.

Molecular mechanism of phenotypic modulation of smooth muscle cells

Phenotypic modulation of smooth muscle cells is closely associated with vasculogenesis, enterogenesis and some diseases such as atherosclerosis, hypertension and leiomyogenic tumorigenicity. During phenotypic modulation, smooth muscle cells change their morphology, cell function and biochemical characteristics. Recent studies have focused on the regulation mechanism of smooth muscle cell-specific genes at the levels of transcription and/or alternative splicing in a phenotype-dependent manner. Typical examples of such genes include caldesmon, alpha-tropomyosin, myosin heavy chain, SM22, calponin and alpha 1 integrin. Cell adhesion molecules and growth factors/cytokines also play a critical role for controlling phenotype of smooth muscle cells via signal transduction pathways such as phosphoinositide 3-kinase and mitogen-activated protein kinases.

TGF-beta superfamily members do not promote smooth muscle-specific alternative splicing, a late marker of vascular smooth muscle cell differentiation

Smooth muscle (SM) specific alternate splicing of a number of genes is a late marker of the differentiated vascular smooth muscle cell (VSMC) phenotype and is one of the first differentiation characteristics to be lost during de-differentiation and in disease. An understanding of how this aspect of VSMC phenotype is regulated may provide insights into the earliest events of the atherosclerotic process. TGF-beta1 is a potent regulator of VSMC differentiation and can induce expression of SM-specific contractile proteins in both pluripotent stem cells and de-differentiated VSMCs. The purpose of this study was to test the hypothesis that members of the TGFbeta-superfamily can also effect SM-specific alternative splicing. Firstly, we established that SM-specific splicing of alpha-tropomyosin, vinculin and SM-myosin heavy chain (MHC) increases during rat fetal/neonatal development and is decreased in VSMCs following balloon-induced carotid injury in the rat. Treatment of cultured rat VSMCs with TGFbeta-superfamily members resulted in a significant reduction in the ratio of SM to non-muscle (NM) alpha-tropomyosin, but did not effect SM-specific alternative splicing of vinculin or SM-MHC. Treatment of pluripotent C3H10T1/2 cells with TGF-beta1, which increased SM differentiation marker expression, did not increase SM-specific alpha-tropomyosin splicing. Taken together, these results demonstrate differential regulation of SM-specific alternative splicing and indicate that although TGF-beta1 promotes VSMC differentiation marker expression, TGF-beta1 cannot act as the sole trigger of VSMC differentiation.

Maturational differences between vascular and bladder smooth muscle during ovine development

Maturation rates of vascular and visceral smooth muscle (SM) during ovine development were compared by quantifying contractile protein, myosin heavy chain (MHC) isoform contents, and contractile properties of aortas and bladders from female fetal (n = 19) and postnatal (n = 21) sheep. Actin, myosin, and protein contents rose progressively throughout development in both tissues (P </= 0.003); however, expression patterns differed. During the last trimester, i. e., 101-130 days (term approximately 145 days), bladder actin and MHC contents were approximately twofold greater (P < 0.04) than those in the aorta. Although the fractional content of 204-kDa SM1 MHC in the bladder decreased from 74 +/- 3% at midgestation to 48 +/- 2% 3 mo postnatal, the aorta exhibited an increase from 30 +/- 2% to 65 +/- 2%. Bladder MHC (MHC-B) migrating at 200 kDa contained only SM2 throughout development. In contrast, 200-kDa MHC in the aorta was predominantly nonmuscle MHC-B at midgestation, which was gradually replaced by SM2 as development progressed. Along with its early expression of SM2, bladder muscle obtained maximal stress generating capacity (1.7 x 10(5) N/m(2)) by term gestation, whereas the aorta exhibited no contractions until after birth. We conclude that whereas aortic SM maturation is delayed until after birth, bladder SM matures biochemically and functionally during prenatal development, thus supporting early requirements for micturition.

Phenotype-dependent expression of cadherin 6B in vascular and visceral smooth muscle cells

We used mRNA subtraction of differentiated and dedifferentiated smooth muscle cells (SMCs) to reveal the molecular mechanisms underlying the phenotypic modulation of SMCs. With this approach, we found that a 10 kb mRNA encoding a homotypic cell adhesion molecule, cadherin 6B, was strongly expressed in differentiated vascular and visceral SMCs, but not in the dedifferentiated SMCs derived from them. In vivo, cadherin 6B was expressed in vascular and visceral SMCs, in addition to brain, spinal cord, retina and kidney, at a late stage of chicken embryonic development. These results suggest that cadherin 6B is a novel molecular marker for vascular and visceral SMC phenotypes and is involved in the late differentiation of SMCs.

A myosin phosphatase targeting subunit isoform transition defines a smooth muscle developmental phenotypic switch

Smooth muscle myosin phosphatase dephosphorylates the regulatory myosin light chain and thus mediates smooth muscle relaxation. The activity of this myosin phosphatase is dependent upon its myosin-targeting subunit (MYPT1). Isoforms of MYPT1 have been identified, but how they are generated and their relationship to smooth muscle phenotypes is not clear. Cloning of the middle section of chicken and rat MYPT1 genes revealed that each gene gave rise to isoforms by cassette-type alternative splicing of exons. In chicken, a 123-nucleotide exon was included or excluded from the mature mRNA, whereas in rat two exons immediately downstream were alternative. MYPT1 isoforms lacking the alternative exon were only detected in mature chicken smooth muscle tissues that display phasic contractile properties, but the isoform ratios were variable. The patterns of expression of rat MYPT1 mRNA isoforms were more complex, with three major and two minor isoforms present in all smooth muscle tissues at varying stoichiometries. Isoform switching was identified in the developing chicken gizzard, in which the exon-skipped isoform replaced the exon-included isoform around the time of hatching. This isoform switch occurred after transitions in myosin heavy chain and myosin light chain (MLC(17)) isoforms and correlated with a severalfold increase in the rate of relaxation. The developmental switch of MYPT1 isoforms is a good model for determining the mechanisms and significance of alternative splicing in smooth muscle.

Versican/PG-M isoforms in vascular smooth muscle cells

The expression of increased amounts of proteoglycans in the extracellular matrix may play a role in vascular stenosis and lipid retention. The large chondroitin sulfate proteoglycan versican is synthesized by vascular smooth muscle cells (SMCs), accumulates during human atherosclerosis and restenosis, and has been shown to bind LDLs. We recently demonstrated that adult rat aortic SMCs express several versican mRNAs. Four versican splice variants, V0, V1, V2, and V3, have recently been described, which differ dramatically in length. These variants differ in the extent of modification by glycosaminoglycan chains, and V3 may lack glycosaminoglycan chains. In this study, we characterized versican RNAs from rat SMCs by cloning, sequencing, and hybridization with domain-specific probes. DNA sequence was obtained for the V3 isoform, and for a truncated V0 isoform. By hybridization of polyadenylated RNA with domain-specific probes, we determined that the V0, V1, and V3 isoforms are present in vascular SMCs. We confirmed the presence of the V3 isoform in polyadenylated RNA and in RT-PCR products by hybridization with an oligonucleotide that spans the splice junction between the hyaluronan-binding domain and the epidermal growth factor-like domain. In addition, a novel splice variant was cloned by PCR amplification from both rat and human SMC RNA. This appears to be an incompletely spliced variant, retaining the final intron. PCR analysis shows that this intron can be retained in both V1 and V3 isoforms. The predicted translation product of this variant would have a different carboxy-terminus than previously described versican isoforms.

Changes in the balance of phosphoinositide 3-kinase/protein kinase B (Akt) and the mitogen-activated protein kinases (ERK/p38MAPK) determine a phenotype of visceral and vascular smooth muscle cells

The molecular mechanisms behind phenotypic modulation of smooth muscle cells (SMCs) remain unclear. In our recent paper, we reported the establishment of novel culture system of gizzard SMCs (Hayashi, K., H. Saga, Y. Chimori, K. Kimura, Y. Yamanaka, and K. Sobue. 1998. J. Biol. Chem. 273: 28860-28867), in which insulin-like growth factor-I (IGF-I) was the most potent for maintaining the differentiated SMC phenotype, and IGF-I triggered the phosphoinositide 3-kinase (PI3-K) and protein kinase B (PKB(Akt)) pathway. Here, we investigated the signaling pathways involved in de-differentiation of gizzard SMCs induced by PDGF-BB, bFGF, and EGF. In contrast to the IGF-I-triggered pathway, PDGF-BB, bFGF, and EGF coordinately activated ERK and p38MAPK pathways. Further, the forced expression of active forms of MEK1 and MKK6, which are the upstream kinases of ERK and p38MAPK, respectively, induced de-differentiation even when SMCs were stimulated with IGF-I. Among three growth factors, PDGF-BB only triggered the PI3-K/PKB(Akt) pathway in addition to the ERK and p38MAPK pathways. When the ERK and p38MAPK pathways were simultaneously blocked by their specific inhibitors or an active form of either PI3-K or PKB(Akt) was transfected, PDGF-BB in turn initiated to maintain the differentiated SMC phenotype. We applied these findings to vascular SMCs, and demonstrated the possibility that the same signaling pathways might be involved in regulating the vascular SMC phenotype. These results suggest that changes in the balance between the PI3-K/PKB(Akt) pathway and the ERK and p38MAPK pathways would determine phenotypes of visceral and vascular SMCs. We further reported that SMCs cotransfected with active forms of MEK1 and MKK6 secreted a nondialyzable, heat-labile protein factor(s) which induced de-differentiation of surrounding normal SMCs.

Phenotype-dependent expression of alpha-smooth muscle actin in visceral smooth muscle cells

Alpha-Smooth muscle actin is one of the molecular markers for a phenotype of vascular smooth muscle cells, because the actin is a major isoform expressed in vascular smooth muscle cells and its expression is upregulated during differentiation. Here, we first demonstrate that the phenotype-dependent expression of this actin in visceral smooth muscles is quite opposite to that in vascular smooth muscles. This actin isoform is not expressed in adult chicken visceral smooth muscles including gizzard, trachea, and intestine except for the inner layer of intestinal muscle layers, whereas its expression is clearly detected in these visceral smooth muscles at early stages of the embryo (10-day-old embryo) and is developmentally downregulated. In cultured gizzard smooth muscle cells maintaining a differentiated phenotype, alpha-smooth muscle actin is not detected while its expression dramatically increases during serum-induced dedifferentiation. Promoter analysis reveals that a sequence (-238 to -219) in the promoter region of this actin gene acts as a novel negative cis-element. In conclusion, the phenotype-dependent expression of alpha-smooth muscle actin would be regulated by the sum of the cooperative contributions of the negative element and well-characterized positive elements, purine-rich motif, and CArG boxes and their respective transacting factors.

Transcriptional down-regulation of the rabbit pulmonary artery endothelin B receptor during phenotypic modulation

1. We confirmed that endothelium-independent contraction of the rabbit pulmonary artery (RPA) is mediated through both an endothelin A (ET(A)R) and endothelin B (ET(B2)R) receptor. 2. The response of endothelium-denuded RPA rings to endothelin-1 (ET-1, pD2 = 7.84 +/- 0.03) was only partially inhibited by BQ123 (10 microM), an ET(A)R antagonist. 3. Pretreatment with 1 nM sarafotoxin S6c (S6c), an ET(B)R agonist, desensitized the ET(B2)R and significantly attenuated the response to ET-3 (pD2 = 7.40 +/- 0.02 before, <6.50 after S6c). 4. Pretreatment with S6c had little effect on the response to ET-1, but BQ123 (10 microM) caused a parallel shift to the right of the residual ETAR-mediated response to ET-1 (pD2 = 7.84 +/- 0.03 before S6c, 7.93 +/- 0.03 after S6c, 6.81 +/- 0.05 after BQ123). 5. Binding of radiolabelled ET-1 to early passage cultures of RPA vascular smooth muscle cells (VSMC) displayed two patterns of competitive displacement characteristic of the ET(A)R (BQ123 pIC50 = 8.73 +/- 0.05) or ET(B2)R (S6c pIC50 = 10.15). 6. Competitive displacement experiments using membranes from late passage VSMC confirmed only the presence of the ET(A)R (ET-1 pIC50 = 9.3, BQ123 pIC50 = 8.0, S6c pIC50 < 6.0). 7. The ET(A)R was functionally active and coupled to rises in intracellular calcium which exhibited prolonged homologous desensitization. 8. Using a reverse transcriptase polymerase chain reaction for the rabbit ET(B2)R, we demonstrated the absence of mRNA expression in phenotypically modified VSMC. 9. We conclude that the ET(B2)R expressed by VSMC which mediates contraction of RPA is rapidly down-regulated at the transcriptional level during phenotypic modulation in vitro.

Differentiated phenotype of smooth muscle cells depends on signaling pathways through insulin-like growth factors and phosphatidylinositol 3-kinase

Under conventional culture conditions, smooth muscle cells display their phenotypic modulation from a differentiated to a dedifferentiated state. Here, we established a primary culture system of smooth muscle cells maintaining a differentiated phenotype, as characterized by expression of smooth muscle-specific marker genes such as h-caldesmon and calponin, cell morphology, and ligand-induced contractility. Laminin retarded the progression of dedifferentiation of smooth muscle cells. Insulin-like growth factors (IGF-I and IGF-II) and insulin markedly prolonged the differentiated phenotype, with IGF-I being the more potent. In contrast, serum, epidermal growth factor, transforming growth factors, and platelet-derived growth factors potently induced dedifferentiation compared with angiotensin II, arginine-vasopressin, and basic fibroblast growth factor. Using the present culture system, we investigated signaling pathways regulating a phenotype of smooth muscle cells. In cultured cells, IGF-I specifically activated phosphatidylinositol 3-kinase (PI3-kinase) and its downstream target, protein kinase B, but not mitogen-activated protein kinases. Specific inhibitors of PI3-kinase (wortmannin and LY294002) induced dedifferentiation of smooth muscle cells even when they were cultured on laminin under IGF-I-stimulated conditions. The sole effect of laminin to retard the dedifferentiation was completely blocked by anti-IGF-I antibody, and laminin promoted the endogenous expression of IGF-I in cultured cells. The reduced promoter activity of the caldesmon gene induced by platelet-derived growth factor BB was overcome by the forced expression of the constitutive active form of PI3-kinase p110alpha catalytic subunit. These findings suggest that an IGF-I signaling pathway through PI3-kinase plays a critical role in maintaining a differentiated phenotype of smooth muscle cells.

Tropomyosin isoform diversity and neuronal morphogenesis

Tropomyosins (Tm) are a large family of isoforms obtained from multiple genes and by extensive alternative splicing. They bind in the alpha-helical groove of the actin filament and are therefore core components of this extensive cytoskeletal system. In non-muscle cells the Tm isoforms have been implicated in a diversity of processes including cytokinesis, vesicle transport, motility, morphogenesis and cell transformation. Using immunohistochemical localization in cultured primary cortical neurons with an antibody that potentially identifies all non-muscle TM5 gene isoforms compared with one that specifically identifies a subset of isoforms, the possibility was raised that there were considerably more isoforms derived from this gene than the four previously described. Using polymerase chain reaction (PCR) analysis we have now shown that the rat brain generates at least 10 mRNA isoforms using multiple combinations of terminal exons and two internal exons. There is extensive developmental regulation of these isoforms in the brain and there appears to be a switch in the preferential use of the two internal exons 6a to 6b from the embryonic to the adult isoforms. Specific isoforms using alternate carboxyl-terminal exons are differentially localized within the adult rat cerebellum. It is suggested that the tightly regulated spatial and temporal expression of Tm isoforms plays an important role in the development and maintenance of specific neuronal compartments. This may be achieved by isoforms providing unique structural properties to actin-based filaments within functionally distinct neuronal domains.

Expression of peroxisome proliferator-activated receptor gamma (PPARgamma) in rat aortic smooth muscle cells

Peroxisome proliferator-activated receptor gamma (PPARgamma), a member of nuclear receptors, is expressed at a high level in adipose tissue and plays an important role in adipocyte differentiation. In the present study, we identified the expression of PPARgamma in rat aortic smooth muscle cells (RASMC) using reverse transcription-polymerase chain reaction and gel mobility shift assay. In addition, to investigate whether PPARgamma in RASMC is functional or not, we examined the effect of two specific ligands for PPARgamma, a thiazolidinedione anti-diabetic agent, troglitazone, and 15-deoxy-Delta12,14-prostaglandin J2, on the transcriptional activity of PPAR responsive element (PPRE). A significant increase in the activity of PPRE by addition of these ligands was found. These results suggest that in RASMC, target genes for PPARgamma may be activated by specific ligands for PPARgamma through PPRE in their promoters. In conclusion, PPARgamma is expressed and functional in vascular smooth muscle cells.

c-Myc gene single-strand binding protein-1, MSSP-1, suppresses transcription of alpha-smooth muscle actin gene in chicken visceral smooth muscle cells

The expression of alpha-smooth muscle actin is coordinately regulated by positive and negative cis- elements in the promoter region. Although cis -elements and trans -acting factors involved in the positive regulation of the alpha-smooth muscle (alpha-SM) actin gene have been well characterized, details of negative regulation remain unclear. In functional analyses using cultured gizzard smooth muscle cells, we identified a sequence ranging from -238 to -219 in the promoter region as a novel negative element. Mutation and deletion analyses further revealed that a sequence, TATCTTA (-228 to -222), is essential for negative regulation. Gel shift assay and Southwestern blotting indicated that a nuclear protein factor specifically interacts with single- or double-strand DNA including this sequence, and the protein factor displays a highly potent binding to the sense strand DNA. cDNA cloning and gel shift analysis using anti-MSSP-1 antibodies revealed that this protein factor is a chicken homolog of human MSSP-1 (c- myc gene single-strand binding protein-1). In fact, overexpression of MSSP-1 in cultured smooth muscle cells suppresses the promoter activity. These results suggest a novel function of MSSP-1 regarding the transcriptional regulation of alpha-sm actin gene.

Functional involvement of serum response factor in the transcriptional regulation of caldesmon gene

A 22-bp fragment including the CArG element (CArG1) is essential for the transcription of the caldesmon gene. In this study, we investigated the effects of serum response factor (SRF) on the functional regulation of caldesmon promoter in smooth muscle cells. Gel supershift assay revealed that SRF was one component of the CArG1-protein complex. Dominant-negative mutants of SRF suppressed the promoter activity of caldesmon, whereas wild-type SRF overcame this suppression. These results suggest that SRF functions as a core activating factor of the caldesmon promoter. Furthermore, fractionation of smooth muscle cells' nuclear extracts using DNA affinity paramagnetic particles suggests that SRF transactivates the caldesmon promoter in concert with additional factors in the flow-through fraction recruited to the CArG element.

Smooth muscle cell phenotype-dependent transcriptional regulation of the alpha1 integrin gene

The expressional regulation of chicken alpha1 integrin in smooth muscle cells was studied. The alpha1 integrin mRNA was expressed developmentally and was distributed dominantly in vascular and visceral smooth muscles in chick embryos. In a primary culture of smooth muscle cells, alpha1 integrin expression was dramatically down-regulated during serum-induced dedifferentiation. Promoter analyses revealed that the 5'-upstream region (-516 to +281) was sufficient for transcriptional activation in differentiated smooth muscle cells but not in dedifferentiated smooth muscle cells or chick embryo fibroblasts. Like other alpha integrin promoters, the promoter region of the alpha1 integrin gene lacks TATA and CCAAT boxes and contains binding sites for AP1 and AP2. The essential difference from other alpha integrin promoters is the presence of a CArG box-like motif. Deletion and site-directed mutation analyses revealed that the CArG box-like motif was an essential cis-element for transcriptional activation in differentiated smooth muscle cells, whereas the binding sites for AP1 and AP2 were not. Using specific antibodies, a nuclear protein factor specifically bound to the CArG box-like motif was identified as serum response factor. These results indicate that alpha1 integrin expression in smooth muscle cells is regulated transcriptionally in a phenotype-dependent manner and that serum response factor binding plays a crucial role in this regulation.