NF- kappa B independent suppression of endothelial vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 gene expression by inhibition of flavin binding proteins and superoxide production

Oxidation-reduction (redox) coupled mechanisms play an important role in the regulation of cell surface adhesion molecule expression. In endothelial cells membrane-bound NADH/NADPH oxidase is a significant source of intracellular superoxide (O(2)(-)) production. We explored the role of flavin containing proteins such as NADH/NADPH oxidase in the induction of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) gene expression in human aortic endothelial cells (HAECs) and human dermal microvascular endothelial cells (HMECs). Treatment of HAECs by tumor necrosis factor- alpha (TNF- alpha, 100 U/ml) for 1 h induced a 31% increase in O(2)(-)production within 5 min as determined by lucigenin chemiluminescence analysis of whole cells (n=4, P<0.05). Pretreatment with the NADH/NADPH oxidase inhibitor diphenylene iodonium (DPI, 40 microm) for 1 h inhibited O(2)(-)production. DPI also inhibited TNF and LPS-induced VCAM-1 and ICAM-1 cell surface expression and TNF- alpha, LPS, or IL-1 beta induced VCAM-1 and ICAM-1 mRNA accumulation. However, DPI did not inhibit TNF- alpha -induced activation of nuclear NF- kappa B-like binding activity in HAECs and HMECs. Furthermore, DPI did not inhibit TNF- alpha induced transactivation of NF- kappa B-driven VCAM-1 and HIV-LTR promoter gene constructs in transiently transfected HMECs. These data suggest that flavin binding proteins such as NADH/NADPH oxidase can regulate VCAM-1 gene expression independent of NF- kappa B. Furthermore, intracellular O(2)(-)generation is not necessary for NF- kappa B activation or for transactivation of NF- kappa B driven promoters.

Dithiocarbamates: effects on lipid hydroperoxides and vascular inflammatory gene expression

Dithiocarbamates are a well-defined family of antioxidants that may have therapeutic uses such as in treatment of inflammation and atherosclerosis. A critical event in the pathogenesis of atherosclerosis is the infiltration of inflammatory cells into the vessel wall. Vascular cell adhesion molecule-1 (VCAM-1) plays a pivotal role in this process by mediating leukocyte binding to endothelial cells. VCAM-1 expression is stimulated by oxidized polyunsaturated fatty acids such as 13-hydroperoxy-octadecadienoic acid (13-HPODE), and this lipid hydroperoxide has been proposed to be a second messenger for induction of VCAM-1 gene expression. Pyrrolidine dithiocarbamate (PDTC) markedly represses cytokine-induced VCAM-1 gene expression in cultured human endothelial cells; however, its effects on the oxidative second messenger pathway are unknown. Using a lipoxygenase (LO) inhibition assay in tandem with a colorimetric assay for lipid peroxides, we determined that PDTC does not inhibit the enzymatic oxidation of linoleic acid to 13-HPODE by LO, but directly interacts with and chemically reduces 13-HPODE. We hypothesize that dithiocarbamates may intercept the oxidative second-messenger-induced expression of VCAM-1 and other redox-regulated genes important in inflammation and atherosclerosis.

Decreased secretion of ApoB follows inhibition of ApoB-MTP binding by a novel antagonist

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are essential for the efficient assembly of triglyceride-rich lipoproteins. Evidence has been presented for physical interactions between these proteins. To study the importance of apoB-MTP binding in apoB secretion, we have identified a compound, AGI-S17, that inhibited (60-70% at 40 microM) the binding of various apoB peptides to MTP but not to an anti-apoB monoclonal antibody, 1D1, whose epitope overlaps with an MTP binding site in apoB. AGI-S17 had no significant effect on the lipid transfer activity of the purified MTP. In contrast, another antagonist, BMS-200150, did not affect apoB-MTP binding but inhibited MTP's lipid transfer activity. The differential effects of these inhibitors suggest two functionally independent, apoB binding and lipid transfer, domains in MTP. AGI-S17 was then used to study its effect on the lipid transfer and apoB binding activities of MTP in HepG2 cells. AGI-S17 had no effect on cellular lipid transfer activities, but it inhibited coimmunoprecipitation of apoB with MTP. These studies indicate that AGI-S17 inhibits apoB-MTP binding but has no effect on MTP's lipid transfer activity. Experiments were then performed to study the effect of inhibition of apoB-MTP binding on apoB secretion in HepG2 cells. AGI-S17 (40 microM) did not affect cell protein levels but decreased the total mass of apoB secreted by 70-85%. Similarly, AGI-S17 inhibited the secretion of nascent apoB by 60-80%, but did not affect albumin secretion. These studies indicate that AGI-S17 decreases apoB secretion most likely by inhibiting apoB-MTP interactions. Thus, the binding of MTP to apoB may be important for the assembly and secretion of apoB-containing lipoproteins and can be a potential target for the development of lipid-lowering drugs. It is proposed that the apoB binding may represent MTP's chaperone activity that assists in the transfer from the membrane to the lumen of the endoplasmic reticulum and in the net lipidation of nascent apoB, and may be essential for lipoprotein assembly and secretion.

Oxidative stress as a regulator of gene expression in the vasculature

Oxidative stress and the production of intracellular reactive oxygen species (ROS) have been implicated in the pathogenesis of a variety of diseases. In excess, ROS and their byproducts that are capable of causing oxidative damage may be cytotoxic to cells. However, it is now well established that moderate amounts of ROS play a role in signal transduction processes such as cell growth and posttranslational modification of proteins. Oxidants, antioxidants, and other determinants of the intracellular reduction-oxidation (redox) state play an important role in the regulation of gene expression. Recent insights into the etiology and pathogenesis of atherosclerosis suggest that this disease may be viewed as an inflammatory disease linked to an abnormality in oxidation-mediated signals in the vasculature. In this review, we summarize the evidence supporting the notion that oxidative stress and the production of ROS function as physiological regulators of vascular gene expression mediated via specific redox-sensitive signal transduction pathways and transcriptional regulatory networks. Elucidating, at the molecular level, the regulatory processes involved in redox-sensitive vascular gene expression represents a foundation not only for understanding the pathogenesis of atherosclerosis and other inflammatory diseases but also for the development of novel therapeutic treatment strategies.

Angiotensin II induces vascular cell adhesion molecule-1 expression in rat vasculature: A potential link between the renin-angiotensin system and atherosclerosis

BACKGROUND

Cardiovascular ischemic events may occur more frequently in hypertensive patients with activated renin-angiotensin systems. We tested the hypothesis that angiotensin II (Ang II) may contribute to atherosclerosis by increasing expression of vascular inflammatory genes such as vascular cell adhesion molecule-1 (VCAM-1).

METHODS AND RESULTS

Rats infused with norepinephrine or Ang II for 6 days developed similar hypertensive responses, but only Ang II-treated rats exhibited significant increases in aortic VCAM-1 protein and mRNA expression. Oral losartan treatment (50 mg. kg(-1). d(-1)) inhibited Ang II-induced hypertension and aortic VCAM-1 mRNA expression. Ang II treatment significantly increased VCAM-1 mRNA expression in cultured rat aortic smooth muscle cells (RASMCs). Ang II also induced nuclear NF-kappaB-like binding activity and transactivated an NF-kappaB-driven VCAM-1 promoter. Losartan and proteasome inhibitors blocked Ang II-induced NF-kappaB activation and VCAM-1 mRNA accumulation. IkappaB-alpha overexpression in RASMCs inhibited Ang II-induced VCAM-1 promoter transactivation.

CONCLUSIONS

Ang II may contribute to atherogenesis by activation of VCAM-1 through proteasome dependent, NF-kappaB-like transcriptional mechanisms.

Hyperglycemia-induced activation of nuclear transcription factor kappaB in vascular smooth muscle cells

The transcriptional nuclear factor (NF)-kappaB can be activated by diverse stimuli such as cytokines, mitogens, oxidative stress, and lipids, leading to the transactivation of several genes that play important roles in the development of atherosclerosis. Because oxidative stress may play a key role in the pathogenesis of diabetic vascular disease, we have examined whether culture of porcine vascular smooth muscle cells (PVSMCs) under high glucose (HG) conditions (25 mmol/l) to simulate the diabetic state can lead to the activation of NF-kappaB, and also whether cytokine- or growth factor-induced NF-kappaB activation is altered by HG culture. We observed that PVSMCs cultured in HG showed significantly greater activation of NF-kappaB in the basal state compared with cells cultured in normal glucose (NG) (5.5 mmol/l). Treatment of the cells with cytokines, such as tumor necrosis factor (TNF)-alpha and interleukin-1beta, or with growth factors, such as platelet-derived growth factor, insulin-like growth factor-I, and epidermal growth factor, all led to NF-kappaB activation in cells cultured in both NG and HG. However, their effects were markedly greater in HG. The augmented TNF-alpha-induced NF-kappaB activation in HG was associated with increased TNF-alpha-mediated transcriptional activation of the vascular cell adhesion molecule-1 promoter. Immunoblotting with an antibody to the p65 subunit of NF-kappaB indicated that the levels of this protein were higher in the nuclear extracts from cells cultured in HG compared with NG. Cells cultured in HG also produced significantly greater amounts of the reactive oxygen species superoxide. HG-induced NF-kappaB activation was inhibited by a protein kinase C inhibitor, calphostin C. These results suggest that hyperglycemia-induced activation of NF-kappaB in VSMCs may be a key mechanism for the accelerated vascular disease observed in diabetes.

Angiotensin II induces monocyte chemoattractant protein-1 gene expression in rat vascular smooth muscle cells

Monocyte infiltration into the vessel wall, a key initial step in the process of atherosclerosis, is mediated in part by monocyte chemoattractant protein-1 (MCP-1). Hypertension, particularly in the presence of an activated renin-angiotensin system, is a major risk factor for the development of atherosclerosis. To investigate a potential molecular basis for a link between hypertension and atherosclerosis, we studied the effects of angiotensin II (Ang II) on MCP-1 gene expression in rat aortic smooth muscle cells. Rat smooth muscle cells treated with Ang II exhibited a dose-dependent increase in MCP-1 mRNA accumulation that was prevented by the AT1 receptor antagonist losartan. Ang II also activated MCP-1 gene transcription. Inhibition of NADH/NADPH oxidase, which generates superoxide and H2O2, with diphenylene iodonium or apocynin decreased Ang II-induced MCP-1 mRNA accumulation. Induction of MCP-1 gene expression by Ang II was inhibited by catalase, suggesting a second messenger role for H2O2. The tyrosine kinase inhibitor genistein and the mitogen-activated protein kinase kinase inhibitor PD098059 inhibited Ang II-induced MCP-1 gene expression, consistent with a mitogen-activated protein kinase-dependent signaling mechanism. Ang II may thus promote atherogenesis by direct activation of MCP-1 gene expression in vascular smooth muscle cells.

The study of the influence of flow on vascular endothelial biology

It is now recognized that the mechanical environment of a cell has an influence on its structure and function. For the vascular endothelial cell that resides at the interface of the flowing blood and the underlying vessel wall, there is mounting evidence of the importance of flow and the associated wall shear stress in the regulation of endothelial biology. Not only is it a sensitive regulator of endothelial structure and function, but different flow environments will influence endothelial cell biology differently. Furthermore, there may be an interaction between the chemical environment of a cell and its mechanical environment. This is illustrated by the inhibition by steady laminar shear stress of the cytokine induction of VCAM-1. Results also are presented in which flow studies have been conducted using a co-culture model of the vessel wall. These experiments provide evidence of a quiescent endothelium; however, much more needs to be done to engineer the cell culture environment to make it more physiologic.

Oscillatory shear stress stimulates adhesion molecule expression in cultured human endothelium

Low and oscillatory shear stresses are major features of the hemodynamic environment of sites opposite arterial flow dividers that are predisposed to atherosclerosis. Atherosclerosis is a focal inflammatory disease characterized initially by the recruitment of mononuclear cells into the arterial wall. The specific characteristics of the hemodynamic environment that facilitate the generation of arterial inflammatory responses in the presence of, for example, hyperlipidemia are unknown. We show here that prolonged oscillatory shear stress induces expression of endothelial cell leukocyte adhesion molecules, which are centrally important in mediating leukocyte localization into the arterial wall. Vascular cell adhesion molecule-1 was upregulated an average 9-fold relative to endothelial monolayers in static culture. Intercellular adhesion molecule-1 and E-selectin exhibited 11-fold and 7.5-fold increases, respectively. Upregulation of these adhesion molecules was associated with enhanced monocyte adherence. Cytokine stimulation of surface vascular cell adhesion molecule-1 was maximally induced after 6 and 8 hours of cytokine incubation. Oscillatory shear stress for these time periods elicited respective vascular cell adhesion molecule-1 levels of 16% and 30% relative to those observed for cytokine stimulation. Surface intercellular adhesion molecule-1 induction by cytokine stimulation for 24 hours was found to be approximately five times the level detected after 24 hours of oscillatory shear stress. Experiments performed in the presence of the antioxidant N-acetylcysteine demonstrated that the expression of vascular cell adhesion molecule-1 could be almost totally abolished, whereas that of intercellular adhesion molecule-1 was typically reduced by approximately 70%. These results imply that oscillatory shear stress per se is sufficient to stimulate mononuclear leukocyte adhesion and, presumptively, migration into the arterial wall. These results further indicate that atherosclerotic lesion initiation is likely related, at least in part, to unique signals generated by oscillatory shear stress and that the mechanism of upregulation is, to some extent, redox sensitive.

Role of activating protein-1 in the regulation of the vascular cell adhesion molecule-1 gene expression by tumor necrosis factor-alpha

Endothelial cell surface expression of VCAM-1 is one of the initial steps in the pathogenesis of atherosclerosis. The inflammatory response transcription factor nuclear factor (NF)-kappaB plays an important role in the regulation of VCAM-1 expression by various stimuli including tumor necrosis factor (TNF)-alpha. Other transcription factors may modulate this response through interaction with NF-kappaB factors. Since c-Fos/c-Jun (activating protein-1 (AP-1)) are expressed in vascular endothelium during proinflammatory conditions, we investigated the role of AP-1 proteins in the expression of VCAM-1 by TNF-alpha in SV40 immortalized human microvascular endothelial cells (HMEC). TNF-alpha induced expression of both early protooncogenes, c-fos and c-jun. The ability of TNF-alpha to activate the kappaB-motif (kappaL-kappaR)-dependent VCAM-1 promoter-chloramphenicol acetyltransferase (CAT) reporter gene lacking a consensus AP-1 element was markedly inhibited by co-transfection of the expression vector encoding c-fos ribozyme, which decreases the level of c-fos by degrading c-fos mRNA, or c-fos or c-jun oligonucleotides. Conversely, co-transfection of c-Fos and c-Jun encoding expression vectors potentiated the p65/NF-kappaB-mediated transactivation of the VCAM-1 promoter-CAT reporter gene. Furthermore the c-Fos encoding expression vector potentiated by 2-fold the transactivation activity of a chimeric transcriptional factor Gal/p65 (containing the transactivation domain of p65 and the DNA binding domain of the yeast transcriptional factor Gal-4). Consistent with the promoter studies, curcumin and NDGA, inhibitors of AP-1 activation, markedly inhibited the ability of TNF-alpha to activate the expression of VCAM-1 mRNA levels at concentrations that did not inhibit the activation of NF-kappaB. In gel mobility supershift assays, the antibodies to c-Fos or c-Jun inhibited the binding of TNF-alpha-activated nuclear NF-kappaB to the kappaL-kappaR, suggesting that both c-Fos and c-Jun interacted with NF-kappaB. These results suggest that AP-1 proteins may mediate the effect of TNF-alpha in the regulation of VCAM-1 expression through interaction with NF-kappaB factors in endothelial cells.

E-selectin gene expression in vascular smooth muscle cells. Evidence for a tissue-specific repressor protein

E-Selectin is an inducible, endothelium-specific, cell surface adhesion molecule that mediates inflammatory responses in the vasculature. Nonendothelial cell types such as cultured human aortic smooth muscle cells (HASMCs) lack the ability to express E-selectin. We tested the hypothesis that HASMCs express a negative regulatory factor that inhibits E-selectin gene expression. E-Selectin mRNA and gene transcription were not detected in HASMCs after treatment with tumor necrosis factor-alpha (TNF-alpha) by Northern and nuclear runoff analyses, respectively. However, both E-selectin mRNA and gene transcription were dramatically induced by TNF-alpha in the same cells pretreated with the protein synthesis inhibitor cycloheximide. Lipopolysaccharide demonstrated similar effects. Furthermore, E-selectin was detected on the cell surface of HASMCs after washing out of cycloheximide. Cycloheximide pretreatment enabled immortalized human dermal microvascular endothelial cells that have lost the ability to express E-selectin to induce both E-selectin mRNA and gene transcription in response to TNF-alpha. Induction of E-selectin mRNA by lipopolysaccharide or TNF-alpha in cycloheximide-treated HASMCs was inhibited by the antioxidant pyrrolidinedithiocarbamate and the serine protease inhibitor N alpha-L-tosyl-L-phenylalanine chloromethyl ketone, suggesting that a nuclear factor-kappa B-like mechanism may play an important role in E-selectin gene expression in HASMCs. These data strongly suggest that a labile repressor protein(s) plays an important role in inhibiting E-selectin gene expression in HASMCs likely at the level of gene transcription. Except for this repressor, HASMCs and endothelial cells may share similar regulatory mechanisms for controlling E-selectin expression.

Generation of a polyclonal antibody against lipid peroxide-modified proteins

A specific polyclonal antibody against the lipid peroxide (LOOH)-modified rabbit serum albumin (RSA) was generated in rabbits. The antibody selectively recognized the modified protein in a concentration-dependent manner and did not cross react with aldehyde-modified proteins or proteins directly oxidized with the free radical generator 2,2'-azobis (2-amidinopropane) hydrochloride (AAPH). Oxidized low-density lipoprotein (Ox-LDL), but not native LDL, was also recognized by the antibody in a concentration-dependent manner. The antibody also cross reacted with several other proteins modified by LOOH suggesting that the antibody is directed towards a common epitope and not towards the protein sequence. Western blot analysis of normal human plasma showed that at least three different proteins are recognized by the antibody. RAW cells, preincubated with LOOH, were immunostained with the antibody and the antigenic epitopes were present intracellularly, while controls lacking in the primary antibodies failed to show immunoreactivity. Atherosclerotic arteries from cholesterol-fed monkeys and human atherosclerotic lesions were also immunostained by the antibody. The immunoreactivity was co-localized in areas rich in foam cell macrophages. These results suggest that LOOH-modified proteins present an unique antigenic epitope that may represent a primary product of interaction of LOOH with proteins.

Antioxidant-sensitive regulation of inflammatory-response genes in Kaposi's sarcoma cells

Kaposi's sarcoma (KS) is a multifocal vascular lesion characterized by abnormal proliferation of endothelial-like KS cells linked to a pronounced leukocyte infiltration. KS lesions contain novel herpes-like DNA sequences, KSHV, hypothesized to originate from the viral pathogen for KS. Using cultured KS cells that retain the KSHV sequences, diverse signals, including tumor necrosis factor alpha, interleukin (IL) 1 beta, polyinosinic acid/polycytidylic acid and lipopolysaccharide, induced the expression of the cytokine IL-6 and cellular adhesion molecules involved in leukocyte recruitment, including vascular adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1). The thiol-antioxidant pyrrolidine dithiocarbamate (PDTC) selectively inhibited > 90% of the activation of nuclear factor kappa B-like DNA binding activity in KS cells. PDTC also reduced by > 85% induced levels of VCAM-1 and IL-6 at the mRNA, protein, and functional levels in KS cells. In contrast, PDTC did not inhibit the induced expression of either ICAM-1 or E-selectin. These studies show that PDTC differentially modulates the expression of inflammatory response genes in KS cells that contain KSHV, suggesting that reduction-oxidation-sensitive events are involved in the regulation of these genes. These studies also suggest that thiol-antioxidants such as PDTC may play a potentially therapeutic role in the treatment of KS by preventing induction of specific inflammatory response genes that may be involved in the pathogenesis of KS.

Nitric oxide regulates vascular cell adhesion molecule 1 gene expression and redox-sensitive transcriptional events in human vascular endothelial cells

Decreased nitric oxide (NO) activity, the formation of reactive oxygen species, and increased endothelial expression of the redox-sensitive vascular cell adhesion molecule 1 (VCAM-1) gene in the vessel wall are early and characteristic features of atherosclerosis. To explore whether these phenomena are functionally interrelated, we tested the hypothesis that redox-sensitive VCAM-1 gene expression is regulated by a NO-sensitive mechanism. In early passaged human umbilical vein endothelial cells and human dermal microvascular endothelial cells, the NO donor diethylamine-NO (DETA-NO, 100 microM) reduced VCAM-1 gene expression induced by the cytokine tumor necrosis factor alpha (TNF-alpha, 100 units/ml) at the cell surface level by 65% and intracellular adhesion molecule 1 (ICAM-1) gene expression by 35%. E-selectin gene expression was not affected. No effect on expression of cell adhesion molecules was observed with DETA alone. Moreover, DETA-NO suppressed TNF-alpha-induced mRNA accumulation of VCAM-1 and TNF-alpha-mediated transcriptional activation of the human VCAM-1 promoter. Conversely, treatment with NG-monomethyl-L-arginine (L-NMMA, 1 mM), an inhibitor of NO synthesis, augmented cytokine induction of VCAM-1 and ICAM-1 mRNA accumulation. By gel mobility shift analysis, DETA-NO inhibited TNF-alpha activation of DNA binding protein activity to the VCAM-1 NF-kappa B like binding sites. Peroxy-fatty acids such as 13-hydroperoxydodecanoeic acid (linoleyl hydroperoxide) may serve as an intracellular signal for NF-kappa B activation. Using thin layer chromatography, DETA-NO (100 microM) suppressed formation of this metabolite, suggesting that DETA-NO modifies the reactivity of oxygen intermediates in the vascular endothelium. Through this mechanism, NO may function as an immunomodulator of the vessel wall and thus mediate inflammatory events involved in the pathogenesis of atherosclerosis.

Activation of the double-stranded-RNA-activated protein kinase and induction of vascular cell adhesion molecule-1 by poly (I).poly (C) in endothelial cells

Double-stranded RNA (dsRNA) induces the vascular cell adhesion molecule VCAM-1 to high levels of expression in human umbilical vein endothelial (HUVE) cells. Although VCAM-1 is also induced by the cytokine interleukin 1 beta (IL-1 beta), activation of the dsRNA-activated protein kinase (PKR) occurs only in response to incubation with dsRNA but not with IL-1 beta. Incubation of HUVE cells with the synthetic dsRNA, poly (I).poly (C), activates PKR with increased autophosphorylation, increased phosphorylation of the translation factor eIF2 alpha, and increased activation of the transcription factor NF-kappa B. Promoter analysis in HUVE cells using a VCAM-1 promoter linked to CAT reporter gene demonstrates that poly (I).poly (C) responsiveness resides in the minimal VCAM-1 promoter that contains two NF-kappa B sites, and deletion of the NF-kappa B sites eliminates basal and poly (I).poly (C)-induced CAT activity, supporting the importance of NF-kappa B in the poly (I).poly (C)-mediated induction of VCAM-1. In vitro studies using purified reagents demonstrate that PKR is capable of phosphorylating I kappa B alpha (the inhibitory subunit of NF-kappa B) in a dsRNA-dependent manner. This suggests that phosphorylation of I kappa B alpha by PKR could be an initial step in the activation of NF-kappa B by dsRNA. NF-kappa B is also activated by IL-1 beta in HUVE cells, but this activation occurs without increased PKR autophosphorylation or eIF2 alpha phosphorylation. Poly (I).poly (C) induces VCAM-1 mRNA levels that are dramatically higher and sustained longer than levels induced by IL-1 beta. Although phosphorylation of eIF2 alpha interferes with protein translation, sufficient VCAM-1 mRNA translation occurs in response to poly (I).poly (C) to yield VCAM-1 protein levels that are similar to levels that are induced by IL-1 beta. This suggests that the higher, sustained VCAM-1 mRNA levels that occur in response to incubation with poly (I).poly (C) compensate for the partial translational block resulting from increased eIF2 alpha phosphorylation. These studies indicate that transcriptional and translational regulatory events that occur in response to activation of PKR by dsRNA are important in the regulation of VCAM-1 gene expression in HUVE cells.

Cell type-specific transactivation of the VCAM-1 promoter through an NF-kappa B enhancer motif

Cytokine activation of vascular cell adhesion molecule-1 (VCAM-1) gene expression by endothelial cells is an important feature in a variety of vascular inflammatory responses. Cytokines transcriptionally activate the VCAM-1 promoter in endothelial cells at least in part through two closely linked NF-kappa B enhancer motifs, kappa L-kappa R (positions -77 and -63). However, cytokine activation of the dimeric NF-kappa B transcriptional factor (p50+p65 subunits) occurs in almost all cell types, whereas VCAM-1 gene expression exhibits a cell type-specific pattern of expression. Tumor necrosis factor-alpha markedly transactivated a transiently transfected minimal kappa L-kappa R motif-driven VCAM-1 promoter, p85VCAMCAT, in passaged human vascular endothelial cells but not in the human epithelial cell line, HeLa suggesting that cell type-specific factors may function through the kappa L-kappa R motif. Both cell types exhibited similar inductions of NF-kappa DNA binding activity and transcriptional activity. However, co-transfection of HeLa cells with p65 and p50 expression vectors demonstrated that the minimal VCAM-1 promoter was effectively transactivated by p65 alone but that additional co-expression of p50 blocked this activity. Furthermore, cytokine activation of the minimal VCAM-1 promoter in HeLa cells was recovered by inhibition of p50 expression using antisense oligonucleotide. These studies suggest that the NF-kappa B(p50+p65 heterodimer) does not support transactivation of the VCAM-1 promoter with the p50 subunit potentially playing a significant inhibitory role in suppressing cytokine activation of VCAM-1. In addition, p65 associated transcriptional factors other than NF-kappa B may serve as positive, cytokine-inducible, cell type-specific regulators of VCAM-1 gene expression.

Modified low density lipoprotein and its constituents augment cytokine-activated vascular cell adhesion molecule-1 gene expression in human vascular endothelial cells

Early features in the pathogenesis of atherosclerosis include accumulation of oxidized LDL (oxLDL) and endothelial expression of the vascular adhesion molecule VCAM-1. Because antioxidants inhibit endothelial VCAM-1 expression, we tested the hypothesis that oxLDL functions as a prooxidant signal in atherogenesis to augment VCAM-1 activation by inflammatory signals. Cultured human aortic endothelial cells (HAECs) or human umbilical vein endothelial cells (HUVECs) were incubated with unmodified LDL, oxLDL, or glycated LDL for 48 h. No change in VCAM-1, intercellular cell adhesion molecule-1 (ICAM-1), or E-selectin expression from control was observed by ELISA. However, dose-response and time course studies demonstrated that oxLDL enhanced VCAM-1 expression induced by the cytokin tumor necrosis factor alpha (TNF alpha) 63% in HAECs and 45% in HUVECs over unmodified LDL or control. Using flow cytometry analysis, oxLDL augmented TNF alpha-induced VCAM-1 expression in a uniform HAEC population. oxLDL had no effect on E-selection induction. oxLDL augmented TNF alpha-induced ICAM-1 expression 44% in HAECs but not in HUVECs. Glycated LDL augmented TNF alpha-induced VCAM-1 expression 35% in HAECs but not HUVECs. Similar results were obtained with 13-HPODE or lysophosphatidylcholine, significant components of oxLDL. 13-HPODE augmented TNF alpha-induced mRNA accumulation and transcriptional activation of VCAM-1 in HAECs. These results suggest that as long-term regulatory signals, specific oxidized fatty acid and phospholipid components of oxLDL augment the ability of vascular endothelial cells to express cytokine-mediated VCAM-1. These studies link oxidant signals conferred by oxLDL to oxidation-sensitive regulatory mechanisms controlling the expression of endothelial cell adhesion molecules involved in early atherosclerosis.

Regulation of Na+,K(+)-ATPase alpha-subunit isoforms in rat tissues during hypertension

We investigated the regulation of the protein expression of the alpha isozymes of Na+,K(+)-ATPase in reference to the enzyme activity in the heart, brain and skeletal muscle of rats during deoxycorticosterone acetate (DOCA)-salt hypertension. Treatment of rats with DOCA and salt for 28 days produced a significant increase in systolic blood pressure compared to the control groups which remained normotensive. Rats treated with DOCA expressed greater amounts of the immunoreactive alpha-1 isoform than untreated controls in whole heart membranes. However, the DOCA-induced increase in the alpha-1 isoform did not occur during DOCA-salt hypertension. There was a parallel change in the enzyme activity of the Na+,K(+)-ATPase and the protein expression of the alpha-1 isoform as a result of these treatments. We have also demonstrated that the hearts of DOCA-salt hypertensive rats expressed less of the alpha-2 isoform compared to the controls. We could not detect any alteration in the alpha-1 and alpha-2 isoforms of the skeletal muscle and alpha-1, alpha-2 and alpha-3 isoforms of the whole brain Na+,K(+)-ATPase during salt or DOCA treatments alone or DOCA-salt hypertension. Furthermore, the Na+,K(+)-ATPase activity was unaltered in these tissues during these treatments. In conclusion, cardiac Na+,K(+)-ATPase alpha-subunit protein expression appears to be regulated during DOCA-salt hypertension. In the skeletal muscle and brain, tissues not subjected directly to increased pressure, this regulation of the Na+,K(+)-ATPase was not apparent.

Evidence for the regulation of Na+, K(+)-ATPase alpha 1 gene expression through the interaction of aldosterone and cAMP-inducible transcriptional factors

Mineralocorticoid hormones such as aldosterone modulate cellular ion homeostasis at least in part through the regulation of Na+, K(+)-ATPase (NAKA) gene expression. While aldosterone acts at the transcriptional level through its ligand-inducible mineralocorticoid receptor (MR), tissue specific and other transcriptional factors may interact with the MR to modulate this regulatory response. cAMP also regulates NAKA alpha 1 gene expression which at the transcriptional level is mediated, in part, through a cAMP response element (CRE) present on a highly conserved, 48 base pair enhancer region, the PUC-1 core, of the rat NAKA alpha 1 subunit gene promoter. We have tested the hypothesis that the MR interacts with cAMP induced transcriptional factors to modulate the NAKA alpha 1 gene expression. In transient transfection studies a PUC-1 core attached to an enhancerless SV40 promoter driven reporter gene (pB1CAT) was induced by 8-bromo-cAMP in HeLa cells. Co-transfected MR expression vector inhibited the 8-bromo-cAMP inducible activity of pB1CAT. DNA binding studies suggested that the PUC-1 core binds both CREB/ATF proteins as well as the glucocorticoid hormone class of steroid receptors. These results suggest that the MR suppresses cAMP-mediated activation of PUC-1 core driven CAT activity possibly through a direct interaction with CREB/ATF transcriptional factors. This in turn suggests that the interaction of two distinct signal transduction systems, aldosterone and cAMP, may define the mineralocorticoid responsiveness of the Na+, K(+)-ATPase alpha 1 gene.

Na pump defects in chronic uremia cannot be attributed to changes in Na-K-ATPase mRNA or protein

We have found abnormalities in Na-K-adenosine-triphosphatase (Na-K-ATPase) function in different tissues of rats with chronic renal failure (CRF). A potential mechanism for these findings is a change in Na-K-ATPase alpha- and/or beta-gene expression. To evaluate this possibility, we compared CRF with pair-fed, sham-operated rats to determine whether chronic uremia changes the expression of Na-K-ATPase alpha 1-, alpha 2-, beta 1-, and beta 2-isoform mRNAs or protein in different types of skeletal muscle, heart, liver, adipose, and kidney tissue. In CRF rats, alpha 1-mRNA in heart tended to be higher and beta 2-mRNA was lower in fat and kidney. There were no other statistically significant differences in isoform mRNAs in tissues of CRF compared with the control rats. Western blot analysis revealed a 38% increase in alpha 1-protein in adipocytes and a 61% decrease in kidney of CRF rats but no significant differences in the amounts of isoform protein in other tissues. Thus, in uremia, posttranslational events or inhibitors of the enzyme are more likely causes of defects in Na-K-ATPase than changes in mRNA or protein abundance.

Expression of Na-K-ATPase alpha- and beta-subunit mRNA and protein isoforms in the rat nephron

Na-K-ATPase is a heterodimeric complex composed of an alpha-catalytic and a glycosylated beta-subunit. Previous studies using in situ hybridization and Northern blot analysis to determine alpha- and beta-subunit mRNA isoform expression in the rat kidney have given conflicting results. This heterogeneity may be due to detection of alpha 2- or alpha 3-isoforms arising from nonrenal epithelial sources such as peripheral nerves or vascular smooth muscle. To address this possibility, we investigated alpha-subunit mRNA isoform expression in different nephron segments using tubule microdissection and reverse transcription-polymerase chain reaction amplification. Southern blot analysis of polymerase chain reaction products using isoform-specific primers and probes detected the expression of alpha 1- and alpha 3-mRNA isoforms in whole kidney, cortical collecting ducts (CCD), and proximal tubule S2 subsegments (S2). No evidence for alpha 2 was found in kidney or microdissected CCD or S2. To determine whether alpha 1- or alpha 3-mRNA in the CCD or S2 is translated into protein, Western blot analysis of total protein from microdissected S2, CCD, medullary thick ascending limb (MTAL), or outer medullary collecting duct outer stripe (OMCDos) was performed. Total protein was separated according to size by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, then probed using polyclonal antibodies specific for the alpha 1-, alpha 2-, alpha 3-, beta 1-, and beta 2-protein isoforms. Rat brain was used as a positive control and demonstrated that the antibodies could detect a single 97- and 35-kDa band for the alpha- and beta-isoforms, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue specific membrane association of alpha 1T, a truncated form of the alpha 1 subunit of the Na pump

We have assessed the Na pump alpha-subunit isoform content utilizing site directed antibodies in two vascular smooth muscle (VSM) preparations known to contain functional Na pump sites, VSM microsomal fractions (Na+, K(+)-ATPase) and intact primary confluent cells (ouabain inhibited 86Rb uptake). A comparison of isoform content was made with kidney microsomes. Both VSM and kidney microsomes contained a full length alpha 1 subunit (approximately 100 kDa) as well as a truncated subunit, alpha 1T (approximately 66 kDa). SDS treatment of VSM microsomes effected an increase in Na+, K(+)-ATPase and a retention of alpha 1T. SDS treated kidney microsomes retained the alpha 1 isoform and Na+, K(+)-ATPase. Confluent VSM cells showed no detectable alpha 1, only alpha 1T. In the absence of detectable full length alpha 1, the alpha 1T protein may represent a functional Na pump component in canine VSM.

Antioxidants and atherosclerosis: a molecular perspective

Current models of atherogenesis link abnormalities in the oxidative state of the vascular wall with interactions with the immune system, leading to a cycle of localized inflammatory and growth responses that result in the characteristics of the mature atherosclerotic lesion. The oxidative modification of LDL may be an important manifestation and mediator of this process, although the degree to which this contributes to atherogenesis has not been directly assessed. Another important mechanism may involve the linkage of the oxidative state of the vascular endothelial cell, through specific transcriptional regulatory factors, to control the expression of a gene involved in this disease process. This further expands the idea of oxidative stress as an important regulatory signal in the pathogenesis of atherosclerosis and provides important paradigms for the development of novel therapeutic treatment regimens, drug design, and diagnostic assessments of disease state.

Regulation of Na-K-ATPase gene expression by aldosterone in vascular smooth muscle cells

Na-K-adenosinetriphosphatase (ATPase) activity profoundly influences vascular cell excitability, contractility, and volume regulation. The recent finding of mineralocorticoid hormone receptors in vascular tissue suggests the possibility that Na-K-ATPase gene expression in vascular tissue is regulated by the mineralocorticoid aldosterone. In this study, we investigated Na-K-ATPase gene expression by aldosterone in cultured rat vascular smooth muscle cells (VSMC). Na-K-ATPase alpha 1- and beta 1-isoform mRNAs, but not alpha 2- and alpha 3-isoform mRNAs, were expressed in cultured rat VSMC. Aldosterone caused a 2.3-fold increase in the alpha 1 mRNA and a 4.7-fold increase in the beta 1 mRNA accumulation with peak elevations at 24 and 6 h, respectively. Aldosterone induced the alpha 1 mRNA expression at physiological concentrations (half-maximum effective concentration = 2-3 nM), consistent with the binding of aldosterone to mineralocorticoid hormone receptors. The augmented alpha 1 mRNA expression by aldosterone was associated with a twofold increase in the alpha 1-subunit protein accumulation. Pretreatment of VSMC with cycloheximide caused a 10-fold increase in the alpha 1 mRNA expression, and the aldosterone-mediated alpha 1 mRNA accumulation was not observed in the presence of cycloheximide. Transfection experiments with the luciferase reporter gene revealed that aldosterone response sequences are located within the 5'-flanking regions of the alpha 1-isoform gene. These data demonstrate that the mineralocorticoid aldosterone directly stimulates Na-K-ATPase gene expression and protein accumulation in VSMC.

Vascular cell adhesion molecule-1 (VCAM-1) gene transcription and expression are regulated through an antioxidant-sensitive mechanism in human vascular endothelial cells

Oxidative stress and expression of the vascular cell adhesion molecule-1 (VCAM-1) on vascular endothelial cells are early features in the pathogenesis of atherosclerosis and other inflammatory diseases. Regulation of VCAM-1 gene expression may be coupled to oxidative stress through specific reduction-oxidation (redox) sensitive transcriptional or posttranscriptional regulatory factors. In cultured human umbilical vein endothelial (HUVE) cells, the cytokine interleukin 1 beta (IL-1 beta) activated VCAM-1 gene expression through a mechanism that was repressed approximately 90% by the antioxidants pyrrolidine dithiocarbamate (PDTC) and N-acetylcysteine (NAC). Furthermore, PDTC selectively inhibited the induction of VCAM-1, but not intercellular adhesion molecule-1 (ICAM-1), mRNA and protein accumulation by the cytokine tumor necrosis factor-alpha (TNF alpha) as well as the noncytokines bacterial endotoxin lipopolysaccharide (LPS) and double-stranded RNA, poly(I:C) (PIC). PDTC also markedly attenuated TNF alpha induction of VCAM-1-mediated cellular adhesion. In a distinct pattern, PDTC partially inhibited E-selectin gene expression in response to TNF alpha but not to LPS, IL-1 beta, or PIC. TNF alpha and LPS-mediated transcriptional activation of the human VCAM-1 promoter through NF-kappa B-like DNA enhancer elements and associated NF-kappa B-like DNA binding proteins was inhibited by PDTC. These studies suggest a molecular linkage between an antioxidant sensitive transcriptional regulatory mechanism and VCAM-1 gene expression that expands on the notion of oxidative stress as an important regulatory signal in the pathogenesis of atherosclerosis.

Heterogeneity of protein kinase C-mediated rapid regulation of Na/K-ATPase in kidney epithelial cells

Na/K-ATPase in renal epithelium is expressed at the basolateral surface and thus is critical for vectorial solute transport. One potential mode of regulation of Na/K-ATPase involves the intracellular effector protein kinase C (PKC). In kidney cell lines, activation of PKC by the phorbol ester phorbol 12,13-dibutyrate (PDBu) (1 microM) inhibited Na/K-ATPase transport activity in OK cells (Vmax decreased 42%; p < 0.02), but not in LLC-PK1 cells. By immunoblot, both cell types expressed detectable levels of PKC alpha and PKC sigma. In response to PDBu, PKC alpha translocated from the cytosol to the membrane fractions of both cell lines. Phorbol ester treatment increased incorporation of 32PO4 in multiple substrates in both cell types, but a approximately 109-kDa substrate with neutral pI was detected only in the OK cell. Anti-LEAVE, directed against a highly conserved sequence in the H4-H5 loop of all known alpha isoforms of Na/K-ATPase, recognized a approximately 109-kDa membrane protein from both cell lines. Anti-LEAVE also identified a protein that comigrated with the large phosphoprotein which was only present in OK cells. Following 32PO4 loading and PDBu treatment, anti-LEAVE immunoprecipitated a approximately 109-kDa phosphoprotein in OK but not LLC-PK1 cells. These data support the notion that PKC is capable of phosphorylating the alpha subunit and inhibiting Na/K-ATPase transport activity in intact renal cells. Furthermore, they suggest that some forms of Na/K-ATPase in the kidney are not susceptible to PKC phosphorylation and that this heterogeneity may contribute to response diversity.

Digitalis and the Na+,K(+)-ATPase

Recent clinical, physiological, biochemical, and molecular biology studies strongly suggest that digitalis glycosides function in a complex manner through differential binding to and inactivation of multiple distinct Na+,K(+)-ATPase isoforms that are differentially expressed and regulated throughout the cardiovascular system. The alpha 1 isoform predominates in the ventricular: myocardium, whereas the alpha 2 and alpha 3 isoforms may localize to the conducting system structures. The peripheral vasculature also potentially expresses three digitalis receptors, as do neurons in the central nervous system. It is likely that similar heterogeneity exists in the autonomic nervous system as well as in the cardiopulmonary baroreceptor structures. Therefore, differential regulation of these isoforms, by either genetic predisposition or hormones, could dissociate contractile from conduction function and play a role in determining the degree, if any, of therapeutic response to digitalis glycosides. Similarly, genetic polymorphism of the alpha subunits has been observed in humans and rats and may play an important functional role in the ion transport function in a strain of hypertensive rats. Genetic differences in the regulation or structure and function of each isoform could confer allele-specific functional and pharmacological features such as predisposition to digitalis toxicity. Alterations in the degree and type of Na+,K(+)-ATPase isoforms expressed during cardiac hypertrophy and cardiac development may mediate increases or decreases in cardiac sensitivity to digitalis glycosides. This unexpected complexity of the digitalis receptor raises new questions about the role of digitalis glycosides in the treatment of congestive heart failure.

Vascular smooth muscle expresses a truncated Na+, K(+)-ATPase alpha-1 subunit isoform

By regulating transmembrane Na+ and K+ concentrations and membrane potential, the Na+,K(+)-ATPase plays an important role in regulating cardiac, skeletal, and smooth muscle function. A high degree of amino acid sequence and structural identity characterizes the three Mr 100,000 Na+,K(+)-ATPase alpha subunit isoforms expressed in cardiac and skeletal muscle. Strikingly, vascular smooth muscle utilizes alternative RNA processing of the alpha-1 gene to express a structurally distinct Mr approximately 65,000 isoform, alpha 1-T (truncated). Analysis of both its mRNA and protein structure reveals that alpha-1-T represents a major, evolutionarily conserved, truncated Na+,K(+)-ATPase isoform expressed in vascular smooth muscle. This demonstrates an unexpected complexity in the regulation of vascular smooth muscle Na+,K(+)-ATPase gene expression and suggests that a structurally novel, truncated alpha subunit may play a role in vascular smooth muscle active ion transport.

Disproportionate alpha- and beta-mRNA sodium pump subunit content in canine vascular smooth muscle

Total RNA isolated from canine saphenous vein smooth muscle cells was examined for Na+ pump alpha- and beta-mRNA content by three increasingly sensitive methods: Northern analysis, RNase protection assay, and polymerase chain reaction. Northern analysis clearly showed the presence of beta-mRNA. An alpha 1 signal was not demonstrable using either Northern analysis with a 1.5-kb cDNA probe or RNase protection. However, an "alpha 1-like" transcript was detected using polymerase chain reaction with two sets of primers constructed to selected areas of the rat alpha 1-cDNA. These data suggest that beta-mRNA in canine blood vessels is in large excess of the alpha 1-mRNA. This discrepancy may be reflected in the relatively low number of Na+ pump sites in this tissue.

Aldosterone-mediated regulation of Na+, K(+)-ATPase gene expression in adult and neonatal rat cardiocytes

By altering the Na+/K+ electrochemical gradient, Na+,K(+)-ATPase activity profoundly influences cardiac cell excitability and contractility. The recent finding of mineralocorticoid hormone receptors in the heart implies that Na+,K(+)-ATPase gene expression, and hence cardiac function, is regulated by aldosterone, a corticosteroid hormone associated with certain forms of hypertension and classically involved in regulating Na+,K(+)-ATPase gene expression and transepithelial Na+ transport in tissues such as the kidney. The regulation by aldosterone of the major cardiac Na+,K(+)-ATPase isoform genes, alpha-1 and beta-1, were studied in adult and neonatal rat ventricular cardiocytes grown in defined serum-free media. In both cell types, aldosterone-induced a rapid and sustained 3-fold induction in alpha-1 mRNA accumulation within 6 h. beta-1 mRNA was similarly induced. alpha-1 mRNA induction occurred over the physiological range with an EC50 of 1-2 nM, consistent with binding of aldosterone to the high affinity mineralocorticoid hormone receptor. In adult cardiocytes, this was associated with a 36% increase in alpha subunit protein accumulation and an increase in Na(+)-K(+)-ATPase transport activity. Aldosterone did not alter the 3-h half-life of alpha-1 mRNA, indicating an induction of alpha-1 mRNA synthesis. Aldosterone-dependent alpha-1 mRNA accumulation was not blocked by the protein synthesis inhibitor cycloheximide, whereas amiloride inhibited both an aldosterone-dependent increase in intracellular Na+ [Na+]i) and alpha-1 mRNA accumulation. This demonstrates that aldosterone directly stimulates Na+,K(+)-ATPase alpha-1 subunit mRNA synthesis and protein accumulation in cardiac cells throughout development and suggests that the heart is a mineralocorticoid-responsive organ. An early increase in [Na+]i may be a proximal event in the mediation of the hormone effect.

Augmented Na,K-ATPase gene expression in spontaneously hypertensive rat hearts

Abnormalities in cardiovascular Na,K-ATPase ion-transport function and regulation may play an important role in the pathogenesis of hypertension. However, it is not known whether these abnormalities are secondary to the effects of hypertension, such as increased pressure, or reflect an intrinsic abnormality in Na,K-ATPase gene expression and regulation. A genetic model of hypertension was used to address this issue. Na,K-ATPase alpha subunit gene expression in hearts was compared between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Pre-hypertensive, 4-week old SHR hearts exhibited an approximately 4 fold elevation in alpha 1 and 8 fold elevation in alpha 2 mRNA levels compared with age-matched WKY hearts. These SHR mRNA levels remained almost equivalent throughout the development of hypertension at 8 and 16 weeks of age. WKY alpha 1 and alpha 2 mRNA levels exhibited a progressive increase during the same time period. The neonatal alpha 3 mRNA isoform was detected only in pre-hypertensive (4-week) SHR hearts. We conclude that cardiac Na,K-ATPase alpha subunit gene expression is significantly altered in SHR even before the onset of hypertension. These findings suggest that an abnormality in cardiac Na,K-ATPase gene expression constitutes an early, if not primary, event in spontaneous hypertension.

Properties of adult rat ventricular cells in long-term culture

Adult mammalian cardiac myocytes in long-term culture offer advantages over intact heart tissue and freshly isolated ventricular cell preparations for a variety of experimental studies. To characterize this preparation in detail, we have examined the physiological properties of isolated adult rat ventricular cells maintained in culture for 10-14 days. Adult rat myocytes in longterm culture contracted spontaneously, with electrical coupling of adjacent cells, at 1-3 Hz. Most myocytes showed myofibrils with well-developed mitochondria and transverse tubular systems. They showed predominantly the V1 type myosin heavy chain isoform. In standard physiological superfusion media (pH 7.35), the intracellular pH of cultured cells measured with 2',7'-bis-carboxyethyl-carboxyfluorescein (BCECF) was 7.26 +/- 0.03, and was regulated by an amiloride-sensitive Na-H exchanger. The time-averaged free intracellular Ca2+ level of cultured adult rat myocytes measured with fura-2 at an extracellular Ca2+ level of 1 mM was 99.0 +/- 16.8 nM. Ouabain, Bay k 8644 or isoproterenol caused a significant rise in time-averaged intracellular [Ca2+], while the dihydropyridine Ca channel blocker nifedipine induced a decrease in intracellular [Ca2+]. Measurements of contractile state with an optical-video system demonstrated that ouabain. Bay k 8644, isoproterenol, or elevated extracellular [Ca2+] increased the amplitude of cell motion and the rates of both shortening and relaxation, while nifedipine lowered them. Microelectrode impalements indicated a resting potential of -75 +/- 1 mV and an action potential amplitude of 100 +/- 2 mV. Exposure of cultured adult rat cardiocytes to the thyroid hormone triiodothyronine (10 nM) for 48 hours resulted in a 2-fold increase in NaK-ATPase alpha-1 catalytic subunit mRNA accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Intricate combinatorial patterns of exon splicing generate multiple regulated troponin T isoforms from a single gene

Mechanisms of alternative RNA splicing, important in the generation of protein diversity, are common but incompletely understood. Among the contractile proteins, troponin T exists in several isoforms, shown to be derived in part from a novel pattern of differential RNA splicing in the 3' region of the rat skeletal fast troponin T gene. In fact, this gene has a previously unsuspected capacity to encode multiple isoforms. The isolation of four distinct but related cDNAs from this gene, which share discontinuous subsegments of sequence identity in their 5' regions, and the determination of the genomic sequence, demonstrate that small exons with characteristic split codon structure are differentially spliced in intricate combinatorial patterns to generate a minimum of 10, and potentially 64, distinct troponin T mRNAs, encoding different isoforms, in a developmentally regulated and tissue-specific manner. At least two of these mRNAs are spliced from structurally identical primary transcripts, necessitating control by trans-acting factors.

A novel mechanism of alternative RNA splicing for the developmentally regulated generation of troponin T isoforms from a single gene

Troponin T (TnT) is a major regulatory protein of the striated muscle that exhibits developmental and tissue-specific structural heterogeneity. The molecular basis for this heterogeneity was studied at the level of TnT structural gene organization and RNA expression. Two tissue-specific and developmentally regulated TnT mRNAs, alpha and beta, are derived from a single fast skeletal muscle TnT gene. Although otherwise structurally identical from amino acid 70 to the end of the 3' untranslated region, the alpha and beta TnT mRNAs differ by a small internal oligonucleotide coding for amino acids 229 to 242. These isoform-specific oligopeptides, both spanning the same internal portion of the TnT protein, are encoded by two distinct and adjacent miniexons in the TnT gene. Alternative and mutually exclusive splicing of these two miniexons results in the incorporation of either exon into the mature TnT mRNA and argues persuasively against a processive scanning model of RNA splice site selection.

Transcriptional and cell cycle-mediated regulation of myosin heavy chain gene expression during muscle cell differentiation

The molecular mechanisms regulating the induction of myosin heavy chain (MHC) gene expression during muscle cell differentiation were studied using a MHC cDNA recombinant plasmid. During in vitro L6E9 cell myogenesis, cytoplasmic MHC mRNA content/cell nucleus increases a minimum of 500-fold during the first 6 days of differentiation. Two independent parameters regulating MHC mRNA accumulation were directly measured. (i) Intrinsic (chemical) MHC mRNA stability (t1/2 = 55-60 h) is the same in both myotubes and when first detected in myoblasts. This suggests that the intrinsic stability of the MHC mRNA molecule does not change during myogenesis. (ii) The rate of MHC gene transcription and MHC mRNA synthesis increases approximately 100-fold during myogenesis but is insufficient to account for the entire MHC mRNA accumulation. An additional independent parameter was found to profoundly affect MHC mRNA accumulation. Withdrawal from the cell cycle, as occurs during terminal myogenic differentiation, increases the final accumulation of stable mRNAs, such as MHC, by increasing mRNA effective stability. During L6E9 myogenesis, the transition from mitotically active myoblasts (doubling time = 16 h) to postmitotic myotubes results in a 4-5-fold increase in the effective stability of cytoplasmic MHC mRNA. This cell cycle-mediated effect, combined with the induction of MHC mRNA synthesis, completely accounts for MHC mRNA accumulation. A parallel effect occurs in the total cytoplasmic poly(A)+ mRNA population. The rates of synthesis of each of the two major stability components (t1/2 = 5 and 50 h, respectively) are equally increased 2-3-fold during myogenesis. However, the composition of the cytoplasmic mRNA population changes due to the preferential accumulation of stable mRNAs. We conclude that both transcriptional and cell cycle-mediated regulation of MHC gene expression is necessary, but either alone is not sufficient to produce the differentiated muscle cell phenotype.

Reversibility of muscle differentiation in the absence of commitment: analysis of a myogenic cell line temperature-sensitive for commitment

The interrelationship between commitment (irreversible withdrawal from the cell cycle) and muscle-specific gene expression was analyzed with the myogenic cell line ts 3b-2, which is temperature sensitive for commitment and cell fusion. The rates of synthesis and levels of accumulation of muscle-specific mRNAs and proteins in the ts 3b-2 cells at permissive and nonpermissive temperatures are comparable, indicating that neither commitment nor cell fusion is required for induction of muscle-specific gene expression. In the absence of commitment, the cells are reversibly withdrawn from the cell cycle during gene induction, and expression of the muscle-specific genes is deinduced upon the switch to growth-stimulating conditions. The deinduction reflects coordinate and preferential cessation of muscle-specific mRNA synthesis, coupled with destabilization of the muscle-specific mRNAs in the cytoplasm, without effect on constitutively expressed housekeeping protein genes. The phenotype of the ts 3b-2 cells demonstrates that commitment and muscle-specific gene expression are both required, but alone are insufficient, to produce the terminally differentiated muscle phenotype.

Cytoplasmic processing of myosin heavy chain messenger RNA: evidence provided by using a recombinant DNA plasmid

A recombinant DNA plasmid, designated pMHC25, has been constructed that contains structural gene sequences for rat skeletal muscle myosin heavy chain (MHC). The identity of the MHC sequence insert in pMHC25 was determined by muscle-tissue specificity, inhibition of MHC protein synthesis in vitro by hybrid-arrested translation, purification of mRNA that directs the synthesis of MHC protein in vitro, and hybridization to a 33S cytoplasmic mRNA found only in differentiated muscle cells. pMHC25-DNA-excess filter hybridizations were used to show that more than 90% of the newly synthesized MHC mRNA that appears in the cytoplasm of differentiated L6E9 myotubes contains a long 3' poly(A) tail. In contrast, 90% of the MHC mRNA that accumulates in the cytoplasm of these same cells during myogenic differentiation lacks this long 3' poly(A) tail. These results suggest the occurrence of a posttranscriptional event in differentiated L6E9 myotubes that involves the cytoplasmic processing of poly(A)+ MHC mRNA to poly(A)- or poly(A)-short MHC mRNA.