Expression of the c-myb proto-oncogene in bovine vascular smooth muscle cells

Pentosan Polysulfate Affords Pleotropic Protection to Multiple Cells and Tissues

Pentosan polysulfate (PPS), a small semi-synthetic highly sulfated heparan sulfate (HS)-like molecule, shares many of the interactive properties of HS. The aim of this review was to outline the potential of PPS as an interventional therapeutic protective agent in physiological processes affecting pathological tissues. PPS is a multifunctional molecule with diverse therapeutic actions against many disease processes. PPS has been used for decades in the treatment of interstitial cystitis and painful bowel disease, it has tissue-protective properties as a protease inhibitor in cartilage, tendon and IVD, and it has been used as a cell-directive component in bioscaffolds in tissue engineering applications. PPS regulates complement activation, coagulation, fibrinolysis and thrombocytopenia, and it promotes the synthesis of hyaluronan. Nerve growth factor production in osteocytes is inhibited by PPS, reducing bone pain in osteoarthritis and rheumatoid arthritis (OA/RA). PPS also removes fatty compounds from lipid-engorged subchondral blood vessels in OA/RA cartilage, reducing joint pain. PPS regulates cytokine and inflammatory mediator production and is also an anti-tumor agent that promotes the proliferation and differentiation of mesenchymal stem cells and the development of progenitor cell lineages that have proven to be useful in strategies designed to effect repair of the degenerate intervertebral disc (IVD) and OA cartilage. PPS stimulates proteoglycan synthesis by chondrocytes in the presence or absence of interleukin (IL)-1, and stimulates hyaluronan production by synoviocytes. PPS is thus a multifunctional tissue-protective molecule of potential therapeutic application for a diverse range of disease processes.

CDK9 inhibitors selectively target estrogen receptor-positive breast cancer cells through combined inhibition of MYB and MCL-1 expression

Our previous studies showed that MYB is required for proliferation of, and confers protection against apoptosis on, estrogen receptor-positive (ER(+ve)) breast cancer cells, which are almost invariably also MYB(+ve). We have also shown that MYB expression in ER(+ve) breast cancer cells is regulated at the level of transcriptional elongation and as such, is suppressed by CDK9i. Here we examined the effects of CDK9i on breast cancer cells and the involvement of MYB in these effects. ER(+ve) breast cancer cell lines including MCF-7 were much more sensitive (> 10 times) to killing by CDK9i than ER(-ve)/MYB(-ve) cells. Moreover, surviving cells showed a block at the G2/M phase of the cell cycle. Importantly, ectopic MYB expression conferred resistance to apoptosis induction, cell killing and G2/M accumulation. Expression of relevant MYB target genes including BCL2 and CCNB1 was suppressed by CDK9 inhibition, and this too was reversed by ectopic MYB expression. Nevertheless, inhibition of BCL2 alone either by MYB knockdown or by ABT-199 treatment was insufficient for significant induction of apoptosis. Further studies implied that suppression of MCL-1, a well-documented target of CDK9 inhibition, was additionally required for apoptosis induction, while maximal levels of apoptosis induced by CDK9i are likely to also involve inhibition of BCL2L1 expression. Taken together these data suggest that MYB regulation of BCL2 underlies the heightened sensitivity of ER(+ve) compared to ER(-ve) breast cancer cells to CDK9 inhibition, and that these compounds represent a potential therapeutic for ER(+ve) breast cancers and possibly other MYB-dependent cancers.

Involvement of phosphatidylserine externalization in the down-regulation of c-myb expression in differentiating C2C12 cells

Analysis of c-myb gene down-regulation in differentiating C212 cells revealed that in proliferating cells, c-myb expression is high and ceases as the proliferation rate decreases. However, a low level of c-myb mRNA was detected in confluent non-proliferating differentiating cells for an extended period of time before it declined to an undetectable level. The time course of c-myb gene silencing in differentiating cells correlated with exposition of phosphatidylserine (PS) on the cell surface. Moreover, the interaction of exposed PS with exogenously added annexin V perturbed PS-mediated cell signaling and transiently up-regulated the declining c-myb expression. We, therefore, suggest that cell surface-exposed PS, which plays a role in the process of myotube formation, is also involved in the down-regulation of c-myb expression.

Somatostatin modulates voltage-dependent Ca2+ channels in GH3 cells via a specific G(o) splice variant

In rat pituitary GH3 cells Ca2+ current through L-type channels is reduced by somatostatin. This modulation of channel activity by somatostatin receptors is mediated by a guanine nucleotide-binding regulatory protein (G protein). It is sensitive to pertussis toxin, indicating the involvement of a G(o)- or Gi-type G protein in this pathway. The identity of this G protein was determined by suppressing the expression of endogenous G proteins individually via intranuclear injection of antisense oligonucleotides. This method was applied to GH3 cells to screen several G protein alpha, beta and gamma subunits for their roles in the defined signal transduction pathway. The loss of somatostatin's modulating activity on the voltage-dependent Ca2+ channel after oligonucleotide injection revealed the involvement of G(o) alpha 2 beta 1 gamma 3 to the exclusion of other closely related subtypes.

Role of Pim-1 in Smooth Muscle Cell Proliferation

The proliferation of vascular smooth muscle cells (VSMCs) and alterations of their phenotype are implicated in the pathogenesis of atherosclerosis. Arterial wall injury induces the expression of proto-oncogenes, leading to the proliferation of VSMCs. In particular, c-Myc and c-Myb play a central role in cell cycle progression and are essential for VSMC replication. The protooncogene Pim-1 cooperates with c-Myc and enhances the transcriptional activity of c-Myb in hematopoietic cells, suggesting that Pim-1 is involved in cell cycle regulation. The aim of this study was to examine the possible involvement of Pim-1 in VSMC proliferation. Pim-1 was substantially induced in neointimal VSMCs of balloon-injured rat carotid arteries, and in vivo infection with a dominant negative Pim-1-expressing adenovirus (Ad-DN-Pim-1) markedly suppressed neointima formation and cell cycle progression in the balloon-injured arteries. In cultured VSMCs, treatment with serum or H(2)O(2) induced Pim-1 expression, and H(2)O(2)- or serum-stimulated cell cycle progression and DNA synthesis were almost completely inhibited by DN-Pim-1 overexpression. Furthermore, we performed immunohisto-chemical staining for Pim-1 in human thoracic aortas and coronary arteries obtained from six individuals at autopsy and found that Pim-1-positive cells are observed predominantly in the thickened intima of the aortas and coronary arteries. To the best of our knowledge, this is the first report showing Pim-1 expression in rodent and human arterial walls. To summarize, Pim-1 expression was observed in the neointima of balloon-injured rat carotid arteries and in human thoracic aortas and coronary arteries showing intimal thickening, and the specific inhibition of Pim-1 function markedly suppressed neointima formation after balloon injury and the proliferation of cultured VSMCs, suggesting that Pim-1 plays a role in VSMC proliferation.

c-Myb Protein Interacts with Rcd-1, a Component of the CCR4 Transcription Mediator Complex

Transcriptional initiation of eukaryotic genes depends on the cooperative interaction of various transcription factors. Using the yeast two-hybrid assay, we have identified the murine Rcd-1 protein as a cofactor of the c-myb proto-oncogene product. Rcd-1 is evolutionarily conserved among many species, and moreover the yeast homologue CAF40 is part of the carbon catabolite repressor protein transcriptional mediator thought to be involved in the negative regulation of genes transcribed by RNA polymerase II. Rcd-1 is located mainly in the nucleus, and it interacts with c-Myb both in vitro and in vivo. The activation of the myeloid c-myb-specific mim-1 promoter is repressed by Rcd-1. Interestingly, rcd-1 is an erythropoietin regulated gene, which also represses the action of the AP-1 transcription factor on its target genes.

Positive and negative determinants of target gene specificity in myb transcription factors

The A-Myb and c-Myb transcription factors share a highly conserved DNA-binding domain and activate the same promoters in reporter gene assays. However, the two proteins have distinct biological activities, and expressing them individually in human cells leads to the activation of distinct sets of endogenous genes, suggesting that each protein has a unique transcriptional specificity. Here, the structural and functional features of the Myb proteins were compared, using assays of endogenous gene expression to measure changes in specificity. When the Myb proteins were tested in different cell types, they activated unique and nearly nonoverlapping sets of genes in each cellular context. Deletion and domain swap experiments identified small, discreet positive and negative elements in A-Myb and c-Myb that were required for the regulation of specific genes, such as DHRS2, DSIPI, and mim-1. The results suggest that individual functional elements in the transcriptional activation domains are responsible for activating specific cellular genes in a context-specific manner. The results also have important implications for interpreting results from reporter gene assays, which fail to detect the differences in activity identified through endogenous gene assays, and fusion protein constructs that alter the transcriptional activation domains and the activities of the Myb proteins.

RelB-p50 NF-kappa B complexes are selectively induced by cytomegalovirus immediate-early protein 1: differential regulation of Bcl-x(L) promoter activity by NF-kappa B family members

The NF-kappa B/Rel family has been implicated in control of transcription of the Bcl-x(L) gene, a target which mediates cell survival signals. The cytomegalovirus (CMV) immediate-early protein 1 (IE1) was previously shown to induce NF-kappa B activity. Here, we report that in both vascular smooth muscle cells (SMCs) and NIH 3T3 cells, surprisingly, IE1 failed to induce Bcl-x(L) promoter activity, although it induced activity of E8-CAT, a reporter construct driven by two copies of the NF-kappa B element upstream of the c-myc promoter (upstream regulatory element [URE]). Thus, the subunit nature of the NF-kappa B/Rel factors induced by IE1 was examined using immunofluorescence and immunoblotting. IE1 was found to selectively induce nuclear RelB and p50 in SMCs and NIH 3T3 cells. An increase in RelB protein mediated by IE1 could, in part, be related to an increase in steady-state relB mRNA levels. Consistent with this subunit identification, IE1 was unable to induce E8-CAT activity in relB(-/-) murine embryonic fibroblast cells. In cotransfection analysis of SMCs and NIH 3T3 cells, RelB and p50 proteins failed to induce Bcl-x(L) promoter activity while inducing E8-CAT. Furthermore, the NF-kappa B element of the Bcl-x(L) promoter only weakly bound RelB-p50 complexes compared to the URE NF-kappa B element. Overall, these findings demonstrate in SMCs and NIH 3T3 cells that the CMV IE1 protein selectively induces RelB and p50, which fail to activate the Bcl-x(L) promoter, indicating a strong specificity of binding and activity for the RelB member of the NF-kappa B family. Furthermore, our results implicate RelB in CMV infection of cells such as vascular SMCs.

Localization of c-Myb and induction of apoptosis by antisense oligonucleotide c-Myb after angioplasty of porcine coronary arteries

Previous studies have shown that inhibition of the proto-oncogene c-myb inhibits neointimal formation in various animal models. However, the temporal and spatial expression of c-Myb in the vessel wall after injury is not known, and the mechanism of action of antisense oligonucleotide (AS-ODN-c-myb) inhibition remains unclear. One potential effect of cell cycle dysregulation by inhibition of c-myb is an increase in the rates of apoptosis. In this study, c-Myb expression after percutaneous transluminal coronary angioplasty (PTCA) injury and induction of apoptosis after AS-ODN-c-myb treatment were determined. Immunohistochemistry and cellular phenotyping were used to localize c-Myb expression in porcine coronary arteries at various time intervals after PTCA. In vitro, the effects of AS-ODN-c-myb on the apoptosis of porcine vascular smooth muscle cells (PVSMCs) and endothelial cells were determined by using a cell-death ELISA and time-lapse video microscopy. In vivo, local delivery of AS-ODN-c-myb was performed after PTCA of pig coronary arteries, and apoptosis was quantified at 6 hours. c-Myb is induced in pig coronary arteries after angioplasty, with maximal expression in inflammatory cells at 18 hours and in vascular smooth muscle cells at 3 to 7 days. In vitro, AS-ODN-c-myb enhanced PVSMCs (6.8+/-0.8% [P=<0.001] versus 0.5% serum) but not endothelial cell apoptosis (1.4+/-0.5% [P=NS] versus 0.5% serum). In vivo, 6 hours after porcine coronary angioplasty and delivery of AS-ODN-c-myb, the proportion of apoptotic cells within the media was 4.2+/-0.8% (PTCA alone), 2.3+/-0.2% (PTCA+vehicle), and 9.0+/-1.1% (PTCA+AS-ODN-c-myb; P<0.05 versus PTCA alone and P<0.01 versus PTCA+saline). c-Myb is expressed after PTCA of pig coronary arteries, and AS-ODN-c-myb induces apoptosis of PVSMCs in vitro and medial cells in vivo.

[Cellular and molecular biology of atherosclerotic lesions]

The association of atherosclerosis with the most common risk factors including elevation of low density lipoprotein (LDL) levels, diabetes, hypertension and cigarette smoking, led to the hypothesis of "response to injury" to explain how the lesions develop. According to this hypothesis, one of the earliest events in atherogenesis is the accumulation of LDL in the arterial wall where they undergo oxidation. These LDL impair endothelial function, and thus, all the antiatherogenic properties of the endothelium. In addition, macrophages and smooth muscle cells take up these LDL, through different receptors, and become foam cells. The accumulation of foam cells in the arterial wall contributes to lesion development. Therefore, lesion development involves the activation of endothelial cells, as well as smooth muscle cells and monocytes/macrophages. In this activation different growth factors (PDGF, EGF, etc.), cytokines (IL-1b, TNFa, etc.) and the modified LDL themselves, play an important role. Through several signal transduction pathways these molecules activate transcription factors, such as the nuclear factor kappa B (NF-kB) or protooncogenes such as c-fos, c-myc, that regulate the expression of genes involved in the inflammatory/proliferative response of the lesions.

c-Myb-binding sites mediate G(1)/S-associated repression of the plasma membrane Ca(2+)-ATPase-1 promoter

We demonstrate that two Myb-binding sites of the mouse plasma membrane Ca(2+)-ATPase-1 (PMCA1) promoter are required for G(1)/S cell cycle stage-associated repression of PMCA1 promoter activity. Nuclear run-on experiments revealed G(1)/S-associated repression of PMCA1 transcription. Ribonuclease protection assays revealed two transcription initiation sites between two Myb-binding sites in the PMCA1 promoter. Gel shift assays showed that c-Myb can bind to wild-type but not point mutated Myb binding sequences of the PMCA1 promoter. Transient transfection assays using cell cycle-synchronized vascular smooth muscle cells (VSMC) and PMCA1 promoter-luciferase constructs showed a 2-fold decrease in reporter activity at G(1)/S as compared with G(0). Overexpression of wild-type c-Myb severely repressed PMCA1 promoter activity at both G(0) and G(1)/S while co-transfection of a dominant negative c-Myb, or a construct encoding an anti-c-Myb neutralizing antibody, completely abolished the repression seen at G(1)/S. Single nucleotide substitutions in the first, second, or both Myb-binding sites alleviated the G(1)/S-associated repression of PMCA1 promoter activity in transformed rat VSMC and primary mouse VSMC cultures. We conclude that c-Myb mediates G(1)/S-associated transcriptional repression of the PMCA1 Ca(2+) pump in rodent VSMC by direct binding to the PMCA1 promoter.

JNK and c-Jun but not ERK and c-Fos are associated with sustained neointima-formation after balloon injury

BACKGROUND

Formation of neointima after balloon angioplasty is regulated via inducible transcription factors (ITF), such as c-Jun and c-Fos, depending on mitogen activated protein (MAP) kinases, which have been shown to be activated after balloon injury. The precise localization of activated MAP-kinases and concomitant expression of transcription factors in the vessel wall remains to be elucidated. We have now studied the localization and time-dependent expression of MAP-kinases together with corresponding ITFs in the rat carotid angioplasty model.

DESIGN

Animals were sacrificed at 0. 5, 6 and 24 h and 3, 5, 7, 14 and 28 days after injury. Cryocut sections were stained using antibodies directed against c-Jun, phosphorylated c-Jun, c-Fos, c-Jun amino-terminal kinase (JNK), extracellular signal related kinase (ERK), von Willebrand factor, ki67 antigen, and alpha-actin.

RESULTS

c-Jun expression was strongly induced in smooth muscle cells (SMC) 30 min after injury and remained upregulated for 24 h, thereafter dropping to basal levels at day 3. Re-expression was observed at day 7 and 14 but not day 28. Expression patterns of JNK and phosphorylated c-Jun were highly congruent to that of c-Jun. In contrast, c-Fos expression was restricted to 30 min and, less pronounced than c-Jun and JNK, was visible after 7 days. Also, its expression was congruent with the presence of ERK.

CONCLUSIONS

These findings demonstrate a clear association between MAP-kinases and their transcription factor substrates in vivo with a predominant association of JNK and c-Jun with sustained SMC proliferation.

v-Myb can transform and regulate the differentiation of melanocyte precursors

The activity of the c-Myb transcription factor is essential for the development of definitive multi- and uni-lineage progenitors of the haemopoietic system. Reflecting this requirement, c-Myb has been oncogenically activated by transduction in the E26 avian retrovirus which elicits an acute leukaemia by transforming haemopoietic progenitors. Here, we report the novel finding that Myb in cooperation with EGF receptor signalling can be used to generate clonally expanded populations of transformed cells which have the phenotype of melanocyte precursors. Through the use of a conditional temperature sensitive mutant of Myb, we show that in the transformed cells Myb regulates commitment to melanocyte differentiation and possibly proliferation. These results add to our understanding of the roles of c-Myb beyond the haemopoietic system and to our knowledge and means of investigating the importance of transcription factors in the melanocyte lineage.

Tissue-selective expression of dominant-negative proteins for the regulation of vascular smooth muscle cell proliferation

The transcription factors c-myb and c-myc are essential for vascular smooth muscle cell (VSMC) replication and are rapidly induced following mitogenic stimulation of quiescent VSMCs in vitro and in vivo following balloon catheter injury. Consequently, interference with c-myb and c-myc function provides a possible avenue for the prevention of VSMC proliferation associated with intimal hyperplasia. We have carried out studies focused on the inhibition of VSMC proliferation using dominant-negative gene constructs incorporating the DNA-binding domains of the c-myb or c-myc genes fused to the repressor domain of the Drosophila engrailed gene. Transient transfection of rat, rabbit and human vascular SMCs results in a dramatic inhibition of proliferation for at least 72 h after transfection. Furthermore, this inhibition of cellular proliferation was found to be due, at least in part, to the induction of apoptosis. Coupling expression of the chimeric dominant-negative proteins to transcriptional regulatory elements of the human vascular smooth muscle alpha-actin gene allows specific targeting of vascular smooth muscle cells.

Myb targeted therapeutics for the treatment of human malignancies

For the past several years, we have been engaged in developing a therapeutically effective strategy for disrupting gene function with reverse complementary, or so called 'antisense', oligodeoxynucleotides (ODN). This pursuit has focused on finding appropriate diseases in which to apply this approach, and suitable gene targets. Of the genes that we have targeted for disruption using the antisense ODN strategy (Clevenger et al., 1995; Gewirtz and Calabretta, 1988; Ratajczak et al., 1992c; Small et al., 1994) one that has been of particular interest, and one where therapeutically motivated disruptions are now in clinical trial, is the myb gene (reviewed in Lyon et al., 1994). These trials involve treatment of human leukemias. These diseases are a logical choice for developing oncogene targeted therapies because of easy access to tissues, and the abundance of knowledge about the cell and molecular biology of these diseases. Nevertheless, as will be touched on below, other malignancies have also been examined as models for Myb targeted therapy with some surprisingly encouraging results. Finally, while we have focused our efforts on the ODN strategy, I will allude briefly to other strategies for disrupting Myb function with therapeutic intent.

Myb-dependent regulation of thrombospondin 2 expression. Role of mRNA stability

The nuclear transcription factor c-Myb, which is highly expressed in hematopoietic cells, has been shown to be functional in NIH 3T3 cells: cells that do not possess detectable levels of c-Myb. To identify endogenous target genes of c-Myb in fibroblasts, RNA isolated from NIH 3T3 cells stably transfected with a full-length or a dominant negative c-myb construct (GREMyb and GREMEn, respectively) was subjected to differential display analysis. 5'-Rapid amplification of cDNA ends of a selected band, sequencing, and a nucleotide homology search led to the identification of thrombospondin 2 (TSP 2) as the gene product repressed in GREMyb and induced in GREMEn cells. The pattern of TSP 2 expression during the cell cycle was consistent with c-myb-dependent regulation. The possibility that the identified transcript was TSP 1, a homologous product known to be repressed by v-Src, c-Jun, and v-Myc, was ruled out by using a TSP 2-specific DNA probe and by showing a distinct pattern of regulation of TSP 1 and TSP 2 expression. Nuclear run-on and TSP 2 promoter-reporter (chloramphenicol acetyltransferase) assays showed similar transcriptional levels in GREMyb and NIH 3T3 cells. However, mRNA stability studies showed a much shorter TSP 2 mRNA half-life in GREMyb compared with wild type NIH 3T3 cells, suggesting that c-myb affects TSP 2 expression via a post-transcriptional mechanism. The implications of a protooncogene-mediated suppression of TSP expression are discussed.

A comparison of guanosine-quartet inhibitory effects versus cytidine homopolymer inhibitory effects on rat neointimal formation

Phosphorothioate oligodeoxynucleotides (PS oligos) manifest antisense and G-quartet aptameric inhibitory effects on vascular smooth muscle cell (SMC) proliferation. PS oligo cytidine homopolymers also have nonsequence-specific, non-G-quartet inhibitory effects on in vitro and in vivo SMC proliferation. In this study, we compared the effects of S-dC18 and S-dC28, 18-mer and 28-mer cytidine homopolymers, respectively, which lack guanosines, with those of ZK10, a G-tetrad forming compound, on in vitro SMC proliferation and in vivo neointimal formation. ZK10 significantly inhibited in vitro human aortic SMC proliferation. At the same molar concentration, ZK10 had significantly greater inhibitory potency on SMC proliferation than either S-dC18, S-dC28, or 7DG-ZK10, which is a modified ZK10 with ten 7-deaza guanosine substitutions. ZK10 was significantly more potent than S-dC18 and S-dC28 in inhibiting PDGF-induced in vitro SMC migration. S-dC18, S-dC28, and ZK10 treatment significantly reduced the intima/media area ratio after rat carotid artery balloon injury compared with the values of the control groups. ZK10 was a more potent inhibitor of neointimal formation than the same chain length S-dC18. ZK10 formed higher-order structures, as shown on gel electrophoresis, in contrast to S-dC28 and 7DG-ZK10. Therefore, the 18-mer ZK10 has comparable in vivo SMC inhibitory effects to the 28-mer S-dC28, a fact that may have ramifications for the development of optimal PS oligos to inhibit angioplasty restenosis.

Inhibitory nonsequence-specific effects of cytidine homopolymers on in vivo neointimal formation

Phosphorothioate oligodeoxynucleotides (PS oligos) manifest both antisense and G-quartet aptameric inhibitory effects on vascular smooth muscle cell (SMC) proliferation. In this study, we examined the effects of three cytidine (S-dC) homopolymers lacking any guanosines of various chain length-S-dC28, S-dC18 and S-dC12-on in vitro SMC proliferation and in vivo neointimal formation. S-dC18 significantly inhibited human vascular SMC proliferation, although it had only half the potency as the same dose of S-dC28. Furthermore, S-dC12 at the same concentrations as S-dC18 did not significantly inhibit vascular SMC proliferation. S-dC28 and S-dC18 inhibited PDGF-induced in vitro SMC migration, whereas D-dC12 had no significant effect on PDGF-induced in vitro SMC migration. We determined the effects of S-dC28, S-dC18, and S-dC12 on neointimal SMC formation in the rat carotid balloon injury model. Rat carotid artery neointimal formation after balloon injury was significantly attenuated by S-dC28 treatment compared with the control group and by S-dC18 treatment compared with the control group. S-dC28 and S-dC18 treatment significantly reduced the intima/media area ratio compared with the values of the control groups. However, S-dC12 did not significantly inhibit neointimal formation. We investigated the time course of the inhibitory effects of S-dC28 on rat carotid artery neointimal formation. S-dC28 significantly inhibited rat carotid artery intimal area and intima/media area ratio at 4 weeks and 8 weeks. Fluoresceinated S-dC28 (FITC-S-dC28) was found to be present throughout the rat carotid arterial wall within 6 hours after balloon injury. Taken together, the potent non-G-quartet, nonsequence-specific inhibitory effects of S-dC compounds on in vitro SMC proliferation and in vivo neointimal formation in the rat carotid balloon injury model are chain length dependent and long lasting.

Local delivery of c-myb antisense oligonucleotides during balloon angioplasty

Intraluminal delivery of antisense oligonucleotides to c-myb was assessed following balloon angioplasty in swine peripheral arteries. Successful delivery and intramural persistence of oligonucleotide for over 24 h were demonstrated following angioplasty with hydrogel balloons coated with 32P-labeled antisense. Delivery of fluorescein-labeled antisense demonstrated further localization within the arterial media and intracellularly. Preliminary in vitro studies demonstrated the feasibility of inhibition of porcine lymphocyte proliferation using the murine antisense to c-myb. Twelve iliac or carotid arteries underwent angioplasty with antisense-coated balloons, while the contralateral vessels underwent angioplasty with the same-sized balloons coated with the complementary sense strand. Six to seven days later, dilated arterial segments were surgically isolated. In 10 of 12 vessel pairs, antisense-treated vessels demonstrated less cellular proliferation than did contralateral sense-treated vessels, as assessed by quantitative immunohistochemical staining of proliferating cell nuclear antigen, and smooth muscle cell proliferation was reduced 18% in antisense-treated vessels compared to the contralateral sense-treated vessels (PCNA-positive nuclear area: 7.7 +/- 4.9% vs. 9.3 +/- 5.2%, P < 0.04)-intraluminal delivery of antisense oligonucleotides to c-myb is feasible with a catheter-based system and may reduce smooth muscle cell proliferation following arterial injury.

Regulation of vascular smooth muscle cell proliferation by plasma membrane Ca(2+)-ATPase

We have previously shown that reductions in c-Myb-dependent transcription inhibit cell cycle progression and decrease intracellular Ca2+ concentrations in vascular smooth muscle cells (VSMC). We now report that these effects are largely mediated by a 4- to 10-fold increased rate of La(3+)-sensitive 45Ca extrusion, which is associated with 2- to 4-fold increased levels of plasma membrane Ca(2+)-ATPase 1 (PMCA1) mRNA and protein. PMCA4 mRNA, present at much lower concentrations, undergoes similar changes during suppression of c-Myb activity. We also report that PMCA1 expression is regulated during VSMC cell cycle progression, such that levels of PMCA1 are 40% lower at the G1/S interface than at G0. Moreover, transient overexpression of PMCA1a in VSMC elevates the 45Ca efflux rate by approximately 2-fold, decreases resting and peak thapsigargin-releasable Ca2+ concentrations at G1/S by 43% (68 nM) and 52% (160 nM), respectively, and reduces the rate of cell proliferation by over 2.5-fold. These data define a mechanism for c-Myb-dependent Ca2+ homeostasis and support a critical role for PMCA in the regulation of VSMC growth.

c-Myb-dependent cell cycle progression and Ca2+ storage in cultured vascular smooth muscle cells

Considerable controversy surrounds the role of the c-myb proto-oncogene in vascular smooth muscle cells (VSMCs). Previous investigations using antisense approaches have suggested a relationship between c-myb expression, cell cycle progression, and cytoplasmic Ca2+ concentration ([Ca2+]cyt). However, the ability of certain antisense oligonucleotides to bind and inactivate growth factors allows alternative explanations. To define more specifically the role of c-Myb in cultured VSMCs (SVE and A10 cell lines), we have generated stable cell clones expressing a dominant-negative c-Myb lacking critical elements of the DNA binding domain (delta5-SVE) and transiently transfected cell populations (GRE-MEn-SVE and GRE-MEn-A10) expressing a glucocorticoid-inducible chimeric protein that targets the Drosophila Engrailed repressor domain to c-Myb-responsive promoters. The delta5-SVE clones and GRE-MEn cell populations exhibit a 60% reduction in mean intracellular c-Myb activity, as measured by cotransfection assays with a c-Myb-responsive reporter, a 42% decrease in the mean S phase entry of growth-arrested (G[0]) cells after serum stimulation, and a 36% inhibition of mean cell proliferation over 4 days. These cells also display 28% (34-nmol/L) and 30% (42-nmol/L) reductions in mean [Ca2+]cyt at G(0) and at the G1/S interface, respectively, as well as significant reductions in the peak [Ca2+]cyt responses to thapsigargin (5 micromol/L) and caffeine (10 mmol/L). These latter reductions in operationally defined Ca2+ pools were observed both at different stages of the cell cycle and after transient induction of the dominant-interfering construct, suggesting that c-Myb regulates these releasable Ca2+ stores independent of its effects on cell cycle progression.

The effect of oligonucleotides to c-myb on vascular smooth muscle cell proliferation and neointima formation after porcine coronary angioplasty

Proto-oncogenes, including c-myb, are expressed early after vascular injury. The application of antisense oligodeoxynucleotides (AS-ODNs) against these genes inhibits cell proliferation and neointima formation in small animals and in peripheral arteries. The aim of this study was to investigate the specificity of action of AS-ODN-c-myb in vitro and to assess its effect, when delivered locally, on neointima formation after percutaneous transluminal coronary angioplasty (PTCA) in porcine coronary arteries. AS-ODN-c-myb inhibited the proliferation of vascular smooth muscle cells (VSMCs) in vitro in a dose-dependent manner. There was a corresponding reduction in steady state levels of c-myb mRNA and protein. Expression of another early gene, c-fos, was unaffected. S1 nuclease analysis demonstrated intact full-length AS-ODN-c-myb retrieved from VSMCs in culture after 12 hours. A range of ODNs, related and unrelated to c-myb, with and without a GGGG sequence, inhibited VSMC proliferation. Phosphorothioated AS-ODN-c-myb was 30 times less potent than unphosphorothioated AS-ODN-c-myb. PTCA induced porcine coronary artery neointima formation. c-myb mRNA was maximally induced 18 hours after injury. Unmodified AS-ODN-c-myb, sense-ODN-c-myb, saline, or nothing was delivered immediately after balloon dilatation via a double-skinned porous balloon (Transport, SciMed). Fluorescence-labeled AS-ODN-c-myb was deposited throughout the vessel wall. Mean maximum intima/media cross-sectional area 4 weeks after PTCA was reduced with AS-ODN-c-myb by 79% compared with saline (P < .05), 82% compared with sense-ODN-c-myb, and 63% compared with nothing (P < .10). Conclusions are as follows: (1) c-myb is expressed in VSMCs after vascular injury. (2) AS-ODN-c-myb is retained intact in VSMCs, reducing their proliferation in vitro in dose-dependent fashion, with reduction in c-myb mRNA and protein, whereas sense-ODN-c-myb is not. (3) A range of ODNs can reduce VSMC proliferation by a non-sequence-specific mechanism. (4) Phosphorothioate protection of antisense molecules may reduce their efficacy. (5) Local delivery of unmodified AS-ODN-c-myb via the Transport catheter reduces neointima formation after porcine PTCA. (6) Local delivery of fluid may exacerbate neointimal thickening.

Optimizing the cell efficacy of synthetic ribozymes. Site selection and chemical modifications of ribozymes targeting the proto-oncogene c-myb

Expression of the proto-oncogene c-myb is necessary for proliferation of vascular smooth muscle cells. We have developed synthetic hammerhead ribozymes that recognize and cleave c-myb RNA, thereby inhibiting cell proliferation. Herein, we describe a method for the selection of hammerhead ribozyme cleavage sites and optimization of chemical modifications that maximize cell efficacy. In vitro assays were used to determine the relative accessibility of the ribozyme target sites for binding and cleavage. Several ribozymes thus identified showed efficacy in inhibiting smooth muscle cell proliferation relative to catalytically inactive controls. A combination of modifications including several phosphorothioate linkages at the 5'-end of the ribozyme and an extensively modified catalytic core resulted in substantially increased cell efficacy. A variety of different 2'-modifications at positions U4 and U7 that confer nuclease resistance gave comparable levels of cell efficacy. The lengths of the ribozyme binding arms were varied; optimal cell efficacy was observed with relatively short sequences (13-15 total nucleotides). These synthetic ribozymes have potential as therapeutics for hyperproliferative disorders such as restenosis and cancer. The chemical motifs that give optimal ribozyme activity in smooth muscle cell assays may be applicable to other cell types and other molecular targets.

Differential regulation of c-Myb-induced transcription activation by a phosphorylation site in the negative regulatory domain

The c-myb protooncogene encodes a highly conserved 75-89-kDa transcription factor that contains three functional domains, an amino-terminal DNA binding domain (DBD), a central acidic transactivation domain, and a carboxyl-terminal negative regulatory domain (NRD). Two acute transforming retroviruses, avian myeloblastosis virus and the E26 leukemia virus, transduced portions of c-myb and encode Myb proteins that are truncated in both the DBD and the NRD. Several conserved potential sites for phosphorylation by proline-directed serine/threonine protein kinases reside in or near the NRD, suggesting that phosphorylation might play a role in regulating c-Myb. We have previously demonstrated that serine 528, located in the NRD, is a target for p42(mapk) in vitro. Serine 528 is phosphorylated in vivo in several cell lines, and substitution of serine 528 to alanine (S528A) resulted in an increased ability of Myb to transactivate a synthetic promoter containing five copies of the mim-1A Myb-responsive element and a minimal herpes tk promoter. We have tested the ability of S528A Myb to transactivate a series of cellular target promoters and report that the serine to alanine substitution increased the ability of Myb to activate transcription from the CD34 promoter but not the c-myc or mim-1 promoters. This suggests that phosphorylation of serine 528 may differentially regulate c-Myb activity at different promoters. The DNA binding and multimerization activities of c-Myb appear to be unaffected by the S528A substitution, suggesting that phosphorylation of serine 528 may mediate its effect on the transcription transactivating activity of c-Myb by regulating interactions with other proteins.

Inhibition of smooth muscle cell proliferation and migration in vitro by antisense oligonucleotide to c-myb

PURPOSE

Smooth muscle cell (SMC) migration and proliferation are prominent features of intimal hyperplasia. Previous studies have shown that inhibition of c-myb inhibits arterial SMC proliferation. Our goal was to evaluate the effect of an antisense oligonucleotide targeted to c-myb on the proliferation and migration of SMC explanted from synthetic vascular grafts.

METHODS

SMCs were enzymatically removed from aortas and Dacron grafts explanted from dogs (n = 5). For proliferation studies, quiescent SMCs were incubated with either 0.0, 0.5, 5.0, or 10.0 microM antisense (GTGTCGGGGTCTCCGGGC) or sense (GCCCGGAGACCCCGACAC) oligonucleotides to c-myb. Proliferation was measured after 24 hours by incorporation of [3H]thymidine. Migration was assessed 24 hours after a razor injury.

RESULTS

Antisense to c-myb consistently inhibited proliferation and migration of both native aortic and graft SMCs in a dose-dependent fashion. At a concentration of 10 microM antisense oligonucleotide, aortic and graft SMC proliferation rates were 32% +/- 20% and 56% +/- 9% of control samples, respectively. At 25 microM antisense, the number of migrating aortic and graft SMCs decreased to 41.9% +/- 26.8% and 51.9% +/- 34.1% of control samples, respectively.

CONCLUSIONS

Our results suggest that antisense oligonucleotides to c-myb may be useful in the inhibition of SMC proliferation and migration associated with development of intimal hyperplasia.

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

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

Binding site analysis of c-Myb: screening of potential binding sites by using the mutation matrix derived from systematic binding affinity measurements

The c-Myb oncoprotein is known to bind to multiple sites in the promoters of target genes. We have developed a protocol to screen the binding site of c-Myb by using the systematic binding data derived form measurements of binding affinity for oligonucleotide containing a known Myb-binding site and its complete single mutants. We first applied the method to predict the binding affinity for the known binding sites and compared with available experimental data. The predicated binding sites agree with many putative binding sites of known target promoters. However, there are some binding sites not predicated by the analysis. These sequences deviate from the consensus sequence derived from the binding analyses. In the light of the structure of Myb-DNA complex, these results indicate that different DNA-binding modes may be used by c-Myb to recognize different classes of binding sites. We also screened the sequence database for potential Myb-binding sites, and found sequences of several promoters that have not been identified experimentally but could be the target for c-Myb.

Smooth muscle cell proliferation is proportional to the degree of balloon injury in a rat model of angioplasty

BACKGROUND

A variable degree of smooth muscle cell (SMC) proliferation after balloon injury has been reported in previous rat studies. It is unknown whether balloon injury induces c-fos expression and whether it is related to the degree of vascular injury in vivo. Therefore, we tested the hypothesis that proportional increases in neointimal formation and c-fos expression might be present after different degrees of balloon dilation.

METHODS AND RESULTS

Angioplasty of the carotid artery was performed with a balloon catheter. Vascular injury was evaluated at 0, 0.5, 1.0, 1.5, and 2 atm (n = 6 for all). In 40 additional rats, total RNA dot blots were performed to assess the effect of various degrees of balloon injury on c-fos expression. SMC proliferation proportional to the increases of inflation pressure was found between 0 and 2 atm with neointimal areas of 0.002 +/- 0.002, 0.069 +/- 0.014, 0.128 +/- 0.043, 0.190 +/- 0.010, and 0.255 +/- 0.041 mm2, respectively. When the degree of SMC proliferation (neointima and neointima/media ratio) was plotted against balloon inflation pressure, a linear relation was observed (r = .733, P < .001 and r = .755, P < .001, respectively). An increase in c-fos expression proportional to the degree of injury was found 30 minutes after injury.

CONCLUSIONS

Neointimal proliferation produced by balloon injury is related to balloon inflation pressure, supporting the concept of an SMC proliferative response proportional to the degree of injury. The increase in SMC proliferation is associated with a proportional increase in the early expression of the c-fos nuclear proto-oncogene.

c-Myb repression of c-erbB-2 transcription by direct binding to the c-erbB-2 promoter

The c-myb proto-oncogene product (c-Myb) is a transcriptional activator that can bind to the specific DNA sequences. Although c-Myb also represses an artificial promoter containing the Myb binding sites, natural target genes transcriptionally repressed by c-Myb have not been identified. We have found that the human c-erbB-2 promoter activity is repressed by c-Myb or B-Myb in a chloramphenicol acetyltransferase co-transfection assay. Domain analyses of c-Myb suggested that Myb represses the c-erbB-2 promoter activity by competing with positive regulators of the c-erbB-2 promoter. In in vitro transcription assays, Myb proteins containing only the DNA binding domain could repress c-erbB-2 promoter activity. Two Myb binding sites in the c-erbB-2 promoter were critical for transcriptional repression by c-Myb. One of the two Myb binding sites overlaps the TATA box, and DNase I footprint analyses indicated that c-Myb can compete with TFIID. These results suggest that Myb-induced trans-repression of the c-erbB-2 promoter partly involves competition between Myb and TFIID.

c-myb affects intracellular calcium handling in vascular smooth muscle cells

The protooncogene c-myb is responsible for elevating intracellular calcium concentration ([Ca2+]i) at the G1/S interface in vascular smooth muscle cells (VSMC). However, the molecular components of this pathway are undefined, and the biological effects of increased levels of divalent cation are unknown. We have demonstrated that growth-arrested c-myb-transfected VSMC, compared with wild type VSMC, exhibit a fourfold increased number of insulin-like growth factor I (IGF-I) receptors, increased amount of secreted IGF-I activity, and a twofold increased level of [Ca2+]. The c-myb transfected cells, compared with wild type cells, also possess a twofold increased rate of calcium influx and a twofold decreased rate of calcium efflux. The elevated calcium influx rate of transfected cells is decreased to that of wild type cells with IGF-I neutralizing antibody, whereas the decreased calcium efflux rate of transfected cells is increased to that of wild type cells with antisense c-myb oligonucleotides. Proliferating wild type VSMC exhibit an increased calcium influx rate in late G1, which is dependent on production of augmented amounts of IGF-I activity but not increased levels of IGF-I receptors. The wild type VSMC also show a decreased calcium efflux rate at the same point in the cell cycle, which is dependent on expression of c-myb. The treatment of wild type cells with antisense c-myb or IGF-I receptor oligonucleotides induces a late G1 block in cell proliferation, which can be overcome by exposure to the calcium ionophore, 4-bromo-A-27318, in amounts sufficient to raise [Ca2+]i to levels observed at the G1/S interface. We conclude that IGF-I/IGF-I receptors and c-myb are involved in control of [Ca2+]i at the G1/S interface by separately regulating the rates of calcium influx and efflux and that elevated levels of divalent cation are necessary for progression of VSMC into the S phase of the cell cycle.

Effects of antisense c-myb oligonucleotides on vascular smooth muscle cell proliferation and response to vessel wall injury

The process of restenosis after arterial balloon dilatation has been demonstrated to involve smooth muscle cell hyperplasia. Initial reports with antisense oligonucleotides directed against the proto-oncogene c-myb suggest marked in vitro specificity and in vivo efficacy. In the present study, we sought to confirm and extend the hypothesis that antisense to c-myb results in a specific antiproliferative effect with a comprehensive assessment by using different oligonucleotide preparations, different species, and tissue and cellular uptake experiments. Phosphorothioate-protected oligonucleotides representing the appropriate sequence for antisense to c-myb and multiple controls were used to inhibit proliferation of platelet-derived growth factor- and fetal bovine serum-stimulated rat, dog, and human aortic smooth muscle cells in vitro and neointimal proliferation in the rat carotid injury model. In vitro experiments using identical culture conditions in rat, dog, and human aortic smooth muscle cells failed to show specificity as well as consistency in growth inhibitory effects that could be attributed to an antisense mechanism. Proliferation of smooth muscle cell growth in culture was consistently inhibited with oligomers containing a contiguous 4-guanosine residue motif. In vivo, the rat carotid injury neointimal hyperplasia was similar for antisense c-myb (0.095 +/- 0.009 mm2) and sense c-myb (0.090 +/- 0.009 mm2). Fluorescent-labeled oligonucleotides were present in tissue after local delivery via pluronic gel, and their activity rapidly declined over a 72-hour period. Our findings point to the potential nonspecificity and lack of consistency of the antisense oligonucleotide to c-myb in vitro and in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Members of the nuclear factor kappa B family transactivate the murine c-myb gene

Expression of the c-myb proto-oncogene is primarily detected in normal tissue and tumor cell lines of immature hematopoietic origin, and the down-regulation of c-myb expression is associated with hematopoietic maturation. Cell lines that represent mature, differentiated hematopoietic cell types contain 10-100-fold less c-myb mRNA than immature hematopoietic cell types. Differences in steady-state c-myb mRNA levels appear to be primarily maintained by a conditional block to transcription elongation that occurs in the first intron of the gene. The block to transcription elongation has been mapped, using nuclear run-on analysis, to a region of DNA sequence that is highly conserved between mouse and man. Two sets of DNA-protein interactions, flanking the site of the block to transcription elongation, were detected that exhibited DNA-binding activities that strongly correlated with low steady-state c-myb mRNA levels. Several criteria demonstrated that members of the nuclear factor kappa B (NF-kappa B) family of transcription factors were involved in the DNA-protein interactions identified in these two sets. Surprisingly, cotransfection experiments demonstrated that coexpression of members of the NF-kappa B family, specifically p50 with p65 and p65 with c-Rel, transactivated a c-myb/chloramphenicol acetyltransferase reporter construct that contained 5'-flanking sequences, exon I, intron I, and exon II of the c-myb gene. Transactivation by these heterodimer combinations was dependent on regions of the c-myb first intron containing the NF-kappa B-binding sites. These findings suggest that NF-kappa B family members may be involved in either modifying the efficiency of transcription attenuation or acting as an enhancer-like activity to increase transcription initiation. Thus, the regulation of c-myb transcription may be quite complex, and members of the NF-kappa B family likely play an important role in this regulation.

Human A-myb gene encodes a transcriptional activator containing the negative regulatory domains

The myb gene family has three members, c-myb, A-myb, and B-myb. A-myb mRNA is mainly expressed in testis and peripheral blood leukocytes. A-Myb can activate transcription from the promoter containing Myb-binding sites in all cells examined. In addition to the two domains (a DNA-binding domain and a transcriptional activation domain), two negative regulatory domains have been identified in A-Myb. These results indicate that A-Myb functions as a transcriptional activator mainly in testis and peripheral blood cells, and the regulatory mechanism of A-Myb activity is similar to that of c-Myb.

The role of growth factors in vascular cell development and differentiation

The control of vascular growth and differentiation is a complex system of activity and interaction between positive and negative modulators of these processes. A number of important stimulators and inhibitors of both smooth muscle cells and endothelial cells have now been purified and biochemically characterized. Imbalances in the activity of these factors can result in serious pathologies. In this chapter, we briefly discuss the biology of blood vessel development and growth, review the current literature which describes these stimulators and inhibitors, and discuss current therapeutic strategies designed around these growth modulators.

Mithramycin inhibits myointimal proliferation after balloon injury of the rat carotid artery in vivo

BACKGROUND

Smooth muscle proliferation and extracellular matrix formation in the subintimal region of blood vessels that have been subjected to intimal injury are responsible for restenosis following balloon angioplasty of the coronary arteries and for accelerated atherosclerosis in a variety of other pathophysiological states. The immediate early-response gene c-myc is overexpressed in proliferating vascular smooth muscle cells in vitro, and c-myc antisense oligomers have been shown to reduce c-myc expression and to inhibit proliferation of vascular smooth muscle cells in culture. Mithramycin is a commercially available G-C-specific DNA binding drug that selectively inhibits transcription of genes, such as c-myc, that have G-C-rich promoter sequences. This study tested the hypothesis that mithramycin inhibits transcription of the c-myc proto-oncogene and prevents myointimal proliferation after balloon injury of the rat carotid artery in vivo.

METHODS AND RESULTS

Ten-week-old male Sprague-Dawley rats received mithramycin (150 micrograms/kg IP) or distilled H2O 1 hour before and 1 hour after balloon injury of the right common carotid artery. After 2 weeks, the rats were killed by overdose of pentobarbital, and the injured right and uninjured control left arteries were pressure-fixed and subjected to morphological analysis for evaluation of the degree of myointimal thickening. Separate groups of rats were killed at 2 and 6 hours after vascular injury, and total RNA from injured and control vessels of mithramycin- and vehicle-treated rats was subjected to Northern analysis for assessment of steady-state c-myc mRNA levels. The areas of neointima and the ratios of neointimal to medial area were significantly less in mithramycin-treated than in control rats (0.6 +/- 0.1 versus 1.2 +/- 0.1 mm2, P < .01 and 95 +/- 16% versus 190 +/- 14%, P < .01). Lumen size was significantly greater in mithramycin-treated than in control rats (1.5 +/- 0.1 versus 0.8 +/- 0.1 mm2, P < .01). Steady-state c-myc mRNA levels were increased 10-fold and 2-fold (compared with undamaged carotid arteries) at 2 and 6 hours after balloon injury, respectively; mithramycin treatment reduced c-myc mRNA levels at 2 and 6 hours by 66% and 53%, respectively.

CONCLUSIONS

These results support the hypothesis that systemic administration of mithramycin immediately (1 hour before and after intervention effectively inhibits transcription of the c-myc proto-oncogene and prevents myointimal proliferation after balloon injury of the rat carotid artery in vivo. Because mithramycin has been shown to be well tolerated by humans and to effectively inhibit transcription of c-myc in proliferating human cells, this agent may be useful in the prevention of coronary restenosis.

Downregulation of c-myc expression by antisense oligonucleotides inhibits proliferation of human smooth muscle cells

BACKGROUND

Proliferation of smooth muscle cells (SMCs) plays an important role in vascular pathobiology, being involved in the development of coronary restenosis and atherosclerosis. The activation of nuclear proto-oncogenes appears to be a final common pathway onto which various mitogenic signals coverage. Accordingly, we attempted to determine whether the activation of the c-myc nuclear proto-oncogene is essential for human SMC proliferation and explored the possibility of inhibiting their growth using antisense oligonucleotides directed against c-myc messenger RNA (mRNA).

METHODS AND RESULTS

Proliferation of human SMCs was associated with an increase in c-myc mRNA expression after growth stimulation. Using 15-mer phosphorothioate oligonucleotides (oligomers), we tested their growth-inhibitory effect in SMCs in vitro. Antisense oligomers directed against the translation initiation region of the human c-myc gene exhibited a significant antiproliferative effect, whereas sense and mismatched oligomers did not inhibit the growth. The growth-inhibitory effect of c-myc antisense oligomers was dose dependent and preventable by an excess of sense oligomers. Furthermore, growth inhibition of SMCs treated with c-myc antisense oligomers was associated with a marked decrease in the c-myc mRNA level. Phosphorothioate oligomers remained stable in medium containing 20% serum and were detectable in SMCs as early as 1 hour after cell exposure. Intact oligomers rapidly accumulated intracellularly and persisted within human SMCs for at least 16 hours.

CONCLUSIONS

c-myc antisense oligomers reduced c-myc expression and produced a significant growth inhibition of human SMCs, indicating an important role of c-myc gene activation in the process of SMC proliferation. Furthermore, extracellular stability and rapid cellular uptake provide the basis for future studies assessing the therapeutic role of the c-myc antisense approach in reducing SMC proliferation in the process of vascular restenosis.

Cell cycle-dependent gene expression in V point-arrested BALB/c-3T3 cells

Density-arrested BALB/c-3T3 cells stimulated to proliferate in an amino acid-deficient medium arrest in mid-G1 at a point termed the V point. Cells released from V point arrest require 6 hr to traverse late G1 and enter S phase. As data presented here show that mRNA synthesis is needed for 2-3 hr after release of cells from the V point, after which inhibition of mRNA synthesis does not prevent entry into S phase, we used this mid-G1 arrest protocol to analyze gene expression in late G1. We found that although stimulation of cells in amino acid-deficient medium did not inhibit the induction of genes expressed in early G1, genes normally expressed in late G1 were expressed only after release from the V point. The expression of late G1 genes in cells released from the V point was temporally similar, in respect to G1 location, as was seen in stimulation of quiescent G0 cells. As this protocol effectively divides gene expression into early (pre-V point) and late (post-V point) categories, it should be useful in studies of growth factor-modulated events that regulate traverse of late G1 and commitment to DNA synthesis. In addition, we used c-myb antisense oligonucleotides to show that c-myb expression, which occurs in late G1, is required for BALB/c-3T3 fibroblasts to traverse late G1 and initiate DNA synthesis.

Heparin inhibits the progression phase of subcultured endothelial cell proliferation in rat aorta

The anti-proliferative effects of heparin on subcultured endothelial cells (EC) of rat aorta were investigated to determine whether heparin inhibits the competence phase or the progression phase in the cell cycle using the starting time and the rate of proliferation. Fetal bovine serum (FBS)-stimulated EC proliferation increased to 1.5-times in cell number, but decreased to 0.5-times in 3H-thymidine incorporation, compared with the proliferation of rat primary cultured smooth muscle cells (SMC). The FBS-effects on EC proliferation were attributed to the progression phase rather than the competence phase. Heparin (1, 10 or 100 micrograms/ml) significantly inhibited the proliferation of FBS (5%)-stimulated EC, and the extent of inhibition was the same in both cell number and 3H-thymidine incorporation. In the 3H-thymidine incorporation every 3 hr, heparin reduced the rate of incorporation into G0-arrested EC, but did not affect the starting time of DNA synthesis. When the index of competence phase, starting time, was plotted against that of progression phase, rate of proliferation, the inhibition of heparin was attributed to the progression phase. These results demonstrate that heparin selectively inhibits the progression phase in subcultured EC of rat aorta.