Nucleolin promotes Ang II‐induced phenotypic transformation of vascular smooth muscle cells by regulating EGF and PDGF‐BB

RNA‐binding properties of nucleolin play a fundamental role in regulating cell growth and proliferation. We have previously shown that nucleolin plays an important regulatory role in the phenotypic transformation of vascular smooth muscle cells (VSMCs) induced by angiotensin II (Ang II). In the present study, we aimed to investigate the molecular mechanism of nucleolin‐mediated phenotypic transformation of VSMCs induced by Ang II. Epidermal growth factor (EGF) and platelet‐derived growth factor (PDGF) inhibitors were used to observe the effect of Ang II on phenotypic transformation of VSMCs. The regulatory role of nucleolin in the phenotypic transformation of VSMCs was identified by nucleolin gene mutation, gene overexpression and RNA interference technology. Moreover, we elucidated the molecular mechanism underlying the regulatory effect of nucleolin on phenotypic transformation of VSMCs. EGF and PDGF‐BB played an important role in the phenotypic transformation of VSMCs induced by Ang II. Nucleolin exerted a positive regulatory effect on the expression and secretion of EGF and PDGF‐BB. In addition, nucleolin could bind to the 5′ untranslated region (UTR) of EGF and PDGF‐BB mRNA, and such binding up‐regulated the stability and expression of EGF and PDGF‐BB mRNA, promoting Ang II‐induced phenotypic transformation of VSMCs.     

血管紧张素Ⅱ对培养心肌细胞c-fos表达和蛋白质合成的作用

本实验在培养新生大鼠心肌细胞上,探讨血管紧张素Ⅱ对心肌细胞ｃ－ｆｏｓ ｍＲＮＡ表达和蛋白质合成的影响。结果显示,血管紧张素Ⅱ能诱导ｃ－ｆｏｓ ｍＲＮＡ的表达,增加蛋白质含量,并呈量－效关系,还能加速＾３Ｈ－亮氨酸的掺入速度。上述这些作用可血管紧张素Ⅱ受体拮抗ｓａｒａｌａｓｉｎ所阻断,提示这些作用是受体介导的。     

Platelet-derived growth factor (PDGF)-induced phospholipase C-mediated hydrolysis of phosphoinositides in vascular smooth muscle cells--different sensitivity of PDGF- and angiotensin II-induced phospholipase C reactions to protein kinase C-activating phorbol esters

In cultured rabbit vascular smooth muscle cells (VSMC), platelet-derived growth factor (PDGF), a potent mitogen for VSMC, induced the dose- and time-dependent formation of inositol mono-, bis- and trisphosphates (IP1, IP2 and IP3, respectively). The doses of PDGF necessary for these reactions were similar to those for DNA synthesis. The maximal level of IP1 was comparable to, and those of IP2 and IP3 were about half of those induced by angiotensin II, a potent vasoconstrictor. However, the time courses of the PDGF-induced reactions were slower than those of the angiotensin II-induced ones. Moreover, protein kinase C-activating phorbol esters inhibited the angiotensin II-induced reactions, but did not the PDGF-induced ones. These results indicate that PDGF induces the phospholipase C reactions in VSMC but suggest that the signaling mechanism of PDGF to the phospholipase C is different from that of angiotensin II.     

Characterization of epidermal growth factor receptors in cultured vascular smooth muscle cells of rat aorta

Characteristics of specific receptors for epidermal growth factor (EGF) and its effect on cellular proliferation and synthesis of DNA and protein were studied in cultured vascular smooth muscle cells (VSMC) from rat aorta. Binding studies using 125I-EGF revealed the presence of high affinity binding sites for EGF on VSMC in culture: the apparent dissociation constant was approximately 2.5 X 10(-10)M and the maximal binding capacity was approximately 67,000 sites/cell. EGF stimulated cellular proliferation and incorporation of [3H]thymidine and [3H]leucine into the cells in a dose-dependent fashion; the approximate half-maximal stimulation was induced with 1.5 X 10(-10)M. Platelet-derived growth factor (PDGF) had an additive effect with EGF on DNA synthesis by VSMC. Preincubation of VSMC with unlabeled EGF resulted in a substantial reduction in the number of receptors without changing the affinity, suggesting receptor "down-regulation" mechanism. These data indicate that rat aortic VSMCs have specific receptors for EGF, and suggest that EGF, in addition to PDGF, is also involved in the cell growth of VSMC.     

Angiotensin II-induced vascular smooth muscle cell hypertrophy: PDGF A-chain mediates the increase in cell size

We report here that angiotensin II-mediated hypertrophy of vascular smooth muscle cells (VSMC) exhibits PDGF A-chain-and -pathways. Secretion of PDGF A-chain is required for the increase in cell size, but not for the increase in protein synthesis. Angiotensin II stimulates a hypertrophic growth response in VSMC characterized by increases in cell size and protein synthesis, but not cell number. Because angiotensin II-stimulated VSMC hypertrophy has been associated with increased PDGF A-chain expression, we studied its role in the hypertrophic response by inhibiting PDGF A-chain expression with hydrocortisone or anti-PDGF antibody. Hydrocortisone (1 μM for 48 h) inhibited basal protein synthesis by 47%, but angiotensin II-stimulated protein synthesis was enhanced (111% increase after hydrocortisone treatment vs. 25% increase in control). In contrast, hypertrophy, as measured by cell size, was completely inhibited. Although hydrocortisone had no effect on early growth signals stimulated by angiotensin II (e.g., activation of protein kinase C, stimulation of Na⁺/H⁺ exchange, and c-fos and c-myc expression), it significantly decreased angiotensin II-stimulated secretion of PDGF-like material into the medium from 0.4 to 0.1 ng/ml/24 h (p < 0.01). However, the time course for PDGF secretion (maximal at 16–24 h) was significantly slower than the time course for angiotensin II-stimulated protein synthesis (maximal at 4–12 h). To block the action of PDGF A-chain selectively, VSMC were treated with anti-PDGF A-chain antibody. The antibody completely inhibited the angiotensin II-stimulated increase in cell size, but it had no significant effect on protein synthesis at early times (<8 h). These findings demonstrate two pathways involved in angiotensin II-stimulated VSMC hypertrophy: an increase in cell size dependent on PDGF A-chain and an increase in protein synthesis independent of PDGF A-chain. © 1993 Wiley-Liss, Inc.     

Effects of angiotensins on cellular hypertrophy and c-fos expression in cultured arterial smooth muscle cells.

An increase in cell size and protein content was observed when quiescent arterial smooth muscle cells in culture were incubated with either angiotensin II or III. These effects were inhibited by the specific angiotensin type-1 receptor antagonist losartan (DuP753) but not by CGP42112A. In parallel, a transient and dose-dependent induction of c-fos was demonstrated not only with angiotensins II and III but also with angiotensin I. Both angiotensins II and III exerted their maximal effect at 1 microM, while angiotensin I needed a tenfold-higher concentration to exert an identical effect. As for hypertrophy, losartan also inhibits angiotensin-induced c-fos expression, suggesting that this gene may be involved into the hypertrophic process. Angiotensin-I-mediated c-fos induction is partially inhibited by the angiotensin-converting enzyme inhibitors captopril and trandolaprilate; given that an angiotensin-converting enzyme activity was detected in these smooth muscle cell cultures, these results suggest that angiotensin-I-induced c-fos expression is mediated in part via angiotensin-I conversion to angiotensin II, but also by other unidentified pathway(s). Angiotensin I could essentially induce smooth muscle cell hypertrophy by indirect mechanisms, while angiotensins II and III act directly on smooth muscle cells.     

Verapamil blocks basal and angiotensin II-induced RNA synthesis of rat aortic vascular smooth muscle cells.

We evaluated VER effect on RNA synthesis of quiescent and angiotensin II (AII)- stimulated cultured rat aortic vascular smooth muscle cells (VSMC). In a dose-dependent manner, VER decreased [3H]uridine uptake by quiescent VSMCs (ED50 7 x 10(-6)M), an effect that was shared by other calcium antagonists, but to a variable degree. VER caused a significant effect within 3 hours and attained a maximal effect at 7 hours. In addition VER caused a 22 +/- 2% decrease in [3H]uridine uptake by VSMCs stimulated with 10% fetal bovine serum, while it completely abolished [3H]uridine uptake by VSMCs induced by AII. We conclude that VER decreases basal and inhibits AII-induced increase in mRNA synthesis of VSMCs. These data may explain in part how VER causes a decrease in vascular resistance and alters the vasoconstrictor effect of AII.     

Effects of epidermal growth factor and platelet-derived growth factor on c-fos and c-myc mRNA levels in normal human fibroblasts

The mRNA levels of two proto-oncogenes, c-fos and c-myc, were determined in human foreskin fibroblasts exposed to epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) in a serum-free, defined medium (MCDB 104). Untreated, quiescent cells were found to have low or undetectable levels of c-fos and c-myc mRNA. Within 10 min after the addition of EGF or PDGF the c-fos mRNA level increased, reached a peak at 30 min, and then declined to the control level after 60 min. The level of c-myc mRNA increased somewhat later and peaked after 8 h in cultures treated with either of the growth factors. The c-myc mRNA level remained elevated throughout the 24 h of investigation. The concentrations of EGF and PDGF required for a maximal effect on c-fos or c-myc expression were found to be similar to those that give maximal effect on cell proliferation. Both c-fos and c-myc mRNA expression were super-induced by the addition of cycloheximide. The addition of neutralizing PDGF antibodies to cultures that had received PDGF 4 h earlier inhibited the subsequent increase in the c-myc mRNA level, indicating that the effect of PDGF on c-myc expression is not caused by a "hit and run," mechanism. Density-inhibited cells responded to EGF and PDGF by an increase in c-fos and c-myc mRNA levels in the absence of any mitogenic response. The present results conform to the view that the c-fos and c-myc proto-oncogenes may be important (or necessary) but not sufficient for the initiation of DNA synthesis. Moreover, the finding that both EGF and PDGF increase c-fos and c-myc expression supports our previous suggestion that these two growth factors may in part act via a common intracellular pathway in the prereplicative phase of human fibroblasts.     

Cell cycle dependent growth factor regulation of gene expression

The expression of the proto-oncogenes c-fos and c-myc is a rapid response of G0-arrested fibroblasts to serum and peptide growth factors; however, the role of the c-fos and c-myc gene products in subsequent cell cycle transit is not understood. We examined the expression of c-fos and c-myc mRNA in Balb/c 3T3 murine fibroblasts in response to platelet-derived growth factor (PDGF) and platelet-poor plasma, using arrest points associated with density dependent growth inhibition or metabolic inhibition to synchronize cells in S phase of the cell cycle. The expression of c-fos and c-myc mRNA in Balb/c 3T3 cells was differentially regulated with respect to growth factor dependence and cell cycle dependence. c-fos expression was induced in the presence of PDGF and was unaffected by plasma. The induction of c-fos expression in response to PDGF was cell cycle independent, occurring in cells transiting S phase and G2 as well as in G0 arrest. In contrast, c-myc expression was both growth factor and cell cycle dependent. In G0 arrested cells, c-myc expression was PDGF-dependent and plasma-independent, and PDGF was required for maintenance of elevated c-myc levels during G1 transit. In cells transiting S phase, c-myc mRNA was induced in response to PDGF, but was also plasma-dependent in S phase cells that had been "primed" by exposure to PDGF during S phase.     

Angiotensin II induces c-fos mRNA in aortic smooth muscle. Role of Ca2+ mobilization and protein kinase C activation

Vasoconstrictors such as angiotensin II (Ang II) play an important role in the pathogenesis of hypertension. These agonists may be responsible for the abnormal vascular smooth muscle cell (VSMC) growth seen in hypertension, either indirectly as a consequence of elevating blood pressure or directly as a result of receptor-mediated effects on VSMC growth. To investigate whether Ang II might directly initiate or modulate some of the "early" genetic programs associated with growth in VSMC, the expression of the proto-oncogene c-fos was studied in cultured rat aortic VSMC. Ang II rapidly induced the accumulation of c-fos mRNA, with maximal levels occurring at approximately 30 min. Induction of c-fos mRNA by Ang II was concentration-dependent, with a maximal response at 100 nM. Ang II induction of c-fos mRNA was blocked by its competitive inhibitor, [sarcosine 1,isoleucine 8]angiotensin II. Induction of c-fos mRNA was not dependent upon Ang II-stimulated intracellular alkalinization or activation of Na+/H+ exchange, but was dependent upon mobilization of intracellular Ca2+ and protein kinase C activation. Epidermal growth factor, a VSMC mitogen, also induced c-fos mRNA in VSMC, but by a mechanism different from that of Ang II. These results demonstrate that the vasoconstrictor hormone Ang II induces in VSMC one of the earliest genes, c-fos, associated with the proliferative response.     

Endothelin stimulates c-fos and c-myc expression and proliferation of vascular smooth muscle cells

Recently, a potent vasoconstrictor peptide, endothelin (EDT), was isolated from vascular endothelial cells. We examined its effect on rat vascular smooth muscle cells (VSMCs). EDT induced the elevation of intracellular calcium, which was dependent on extracellular calcium and inhibited by a calcium-channel antagonist in a competitive manner. EDT caused a rapid and transient increase in the c-fos and c-myc mRNA levels and stimulated the DNA synthesis of VSMCs in a dose-dependent manner. This effect of EDT on the proliferation of VSMCs might be related to the development of atherosclerosis.     

Interrelationships of platelet-derived growth factor isoform-induced changes in c-fos expression, intracellular free calcium, and mitogenesis

Both increases in c-fos proto-oncogene expression and intracellular free calcium ([Ca2+]i) have been implicated as necessary components of the signal transduction pathway by which platelet-derived growth factor (PDGF) stimulates DNA synthesis in cultured BALB/c3T3 fibroblasts. To determine the interrelationship between PDGF-induced increases in c-fos proto-oncogene expression and [Ca2+]i, purified, recombinant BB and AA homodimeric isoforms of PDGF were used to evaluate the dose-response relationships and mechanisms of growth factor-induced changes in these two parameters as well as DNA synthesis. Concentration-dependent increases in [Ca2+]i, c-fos expression, and [3H]thymidine incorporation were observed with both BB and AA PDGF isoforms. BB PDGF was consistently more potent and efficacious than the AA isoform in eliciting a given response. The [Ca2+]i dependency of PDGF-induced increases in c-fos expression and DNA synthesis was determined by pretreatment of cells with agents that inhibit increases in [Ca2+]i: BAPTA, Quin-2, and TMB-8. Under these conditions, PDGF-induced DNA synthesis was blocked, whereas c-fos expression was enhanced. Conversely, in cells made deficient in protein kinase C (PKC) activity by prolonged treatment with phorbol ester, BB and AA PDGF-induced c-fos expression was inhibited by 75-80%, while PDGF-induced increases in [Ca2+]i and DNA synthesis were unaffected or enhanced. Additionally, the PKC-independent component of PDGF-stimulated c-fos expression was found to be independent of increases in [Ca2+]i. These data suggest that 1) both BB and AA PDGF isoforms elicit alterations in [Ca2+]i and c-fos proto-oncogene expression through the same or similar mechanisms in BALB/c3T3 fibroblasts, 2) PDGF-stimulated increases in [Ca2+]i are not required for c-fos expression, and 3) distinct pathways regulate PDGF-induced c-fos expression and mitogenesis, with c-fos expression being substantially PKC-dependent yet [Ca2+]i-independent, while mitogenesis is [Ca2+]i-dependent yet PKC-independent.     

Induction of cyclic AMP synthesis by forskolin is followed by a reduction in the expression of c-myc messenger RNA and inhibition of 3H-thymidine incorporation in human fibroblasts

We have studied the effect of increased intracellular levels of cyclic AMP on the growth response to platelet-derived growth factor (PDGF) of human foreskin fibroblasts in culture. It was found that forskolin, a potent stimulator of adenylate cyclase activity, inhibits the stimulatory effect of PDGF on 3H-thymidine incorporation with a dose dependence similar to that observed with regard to cyclic AMP formation. A time-course study indicated that forskolin has no effect on ongoing DNA synthesis but affects events in the prereplicative phase. The cell-cycle block induced by forskolin was found to be reversible; after removal of the drug, DNA synthesis was initiated after a lag period, similar to that of the prereplicative phase of control cells. Forskolin had no effect on PDGF binding, receptor autophosphorylation, or c-fos mRNA expression. However, a reduction in PDGF-induced c-myc mRNA expression was observed in cultures given forskolin. Forskolin was also found to have a marked stimulatory effect on the expression of interferon-beta 2 mRNA expression. However, we were unable to demonstrate that the growth-inhibitory effect of forskolin is mediated by interferon-beta. In conclusion, an increase in cAMP levels leads to a reversible inhibition of PDGF-induced DNA synthesis in human fibroblasts, which may be related to an inhibition of c-myc mRNA expression.     

Effects of calcium-antagonists and calmodulin inhibitors on DNA synthesis in cultured rat vascular smooth muscle cells

To investigate the role of intracellular Ca2+ in the mechanism of cellular proliferation of vascular smooth muscle cells (VSMC), the effects of Ca2+-antagonists and calmodulin (CaM) inhibitors on DNA synthesis stimulated by serum-derived growth factors were studied in cultured VSMCs derived from rat aorta. DNA synthesis assessed by incorporation of [3H]thymidine into the cells was significantly stimulated by epidermal growth factor (EGF), platelet-derived growth factor (PDGF) or fetal bovine serum (FBS), of which the effects were dose-dependently inhibited by a variety of Ca2+-antagonists, such as verapamil, diltiazem and nicardipine. Trifluoperazine and W-7, both specific CaM inhibitors, similarly inhibited DNA synthesis stimulated by EGF, PDGF or FBS in a dose-dependent manner, whereas W-5, a less specific CaM inhibitor, was minimally effective. These data suggest that the Ca2+-CaM system plays an important role in the mechanism of growth factor-induced DNA synthesis in VSMCs.     

The antiproliferative effect of interferon and the mitogenic activity of growth factors are independent cell cycle events : Studies with vascular smooth muscle cells and endothelial cells

We studied the antagonistic effects of interferon (IFN) and growth factors in G0/G1-arrested normal bovine aortic smooth muscle cells (SMC) which were stimulated by serum, or purified platelet derived growth factor (PDGF), supplemented with plasma-derived serum (PDS). The growth response, measured as [3H]thymidine incorporation into DNA, was dependent on the concentration of the mitogen. Human IFN alpha, recombinant human IFN alpha 2, or a crude bovine-IFN preparation prepared from virus-infected bovine aortic endothelial cells, inhibited SMC growth induced by either serum or PDGF with PDS. The extent of IFN inhibition was inversely related to the concentration of the mitogenic stimulus. We also investigated whether IFN inhibited the early events in G1 phase, stimulated by the competence factor PDGF, or the progression of the cell into the S phase induced by PDS. The results indicated that IFN inhibited these two stages of the G1 phase independently. In addition, we investigated the antiproliferative effect of IFN on bovine aortic endothelial cells (BAEC), which do not respond to PDGF but to the mitogenic activity of fibroblast growth factor (FGF). IFN inhibited the mitogenic activity of FGF in a dose-dependent manner. The results indicate that the anti-proliferative activity of IFN and the mitogenic effects of different growth factors are independent.