Immunoelectron Microscopic Localization of Plasminogen Activator Inhibitor Type 1 (PAI-1) in Smooth Muscle Cells from Morphologically Normal and Atherosclerotic Human Arteries

Vascular wall fibrinolytic system proteins are believed to play a pivotal role in atherogenesis. Tissue-type plasminogen activator (t-PA) and urokinase plasminogen activator (u-PA) influence persistence of luminal thrombi and proteolysis of extracellular matrix, respectively. The major physiologic inhibitor of t-PA and u-PA is plasminogen activator inhibitor type 1 (PAI-1). All three of these fibrinolytic system proteins have been detected in vascular endothelial cells, smooth muscle cells, and macrophages by light microscopic immunohistochemistry. This study was undertaken to delineate, by immunoelectron microscopy, the loci of PAI-1 in smooth muscle cells from intact morphologically normal and atherosclerotic human arteries as well as in isolated and cultured smooth muscle cells from arteries. In intact vessels, PAI-1 immunoreactivity was associated with contractile filaments in cells in both normal and atherosclerotic tissues. Lipid-laden smooth muscle cells in atherosclerotic vessels were mainly of the synthetic phenotype and displayed lesser amounts of PAI-1 associated with rough endoplasmic reticulum and contractile filaments. Isolated smooth muscle cells exhibited either a contractile or synthetic phenotype. In the cells with a contractile phenotype, PAI-1 was associated with the contractile elements, whereas in the cells with a synthetic phenotype, the PAI-1 was associated predominantly with elements of the endoplasmic reticulum. Because PAI-1 is associated predominantly with contractile filaments in smooth muscle cells, the net amount of immunodetectable PAI-1 appears to be greater in contractile compared with synthetic phenotype cells.     

Immunoelectron Microscopic Localization of Plasminogen Activator Inhibitor Type 1 (PAI-1) in Smooth Muscle Cells from Morphologically Normal and Atherosclerotic Human Arteries

Vascular wall fibrinolytic system proteins are believed to play a pivotal role in atherogenesis. Tissue-type plasminogen activator (t-PA) and urokinase plasminogen activator (u-PA) influence persistence of luminal thrombi and proteolysis of extracellular matrix, respectively. The major physiologic inhibitor of t-PA and u-PA is plasminogen activator inhibitor type 1 (PAI-1). All three of these fibrinolytic system proteins have been detected in vascular endothelial cells, smooth muscle cells, and macrophages by light microscopic immunohistochemistry. This study was undertaken to delineate, by immunoelectron microscopy, the loci of PAI-1 in smooth muscle cells from intact morphologically normal and atherosclerotic human arteries as well as in isolated and cultured smooth muscle cells from arteries. In intact vessels, PAI-1 immunoreactivity was associated with contractile filaments in cells in both normal and atherosclerotic tissues. Lipid-laden smooth muscle cells in atherosclerotic vessels were mainly of the synthetic phenotype and displayed lesser amounts of PAI-1 associated with rough endoplasmic reticulum and contractile filaments. Isolated smooth muscle cells exhibited either a contractile or synthetic phenotype. In the cells with a contractile phenotype, PAI-1 was associated with the contractile elements, whereas in the cells with a synthetic phenotype, the PAI-1 was associated predominantly with elements of the endoplasmic reticulum. Because PAI-1 is associated predominantly with contractile filaments in smooth muscle cells, the net amount of immunodetectable PAI-1 appears to be greater in contractile compared with synthetic phenotype cells.     

PAI-1反义RNA对人主动脉平滑肌细胞中血管内皮生长因子的影响

目的 探讨纤溶酶原激活物抑制剂－1（plasminogen activator inhibitor-1,PAI-1)反义RNA对离体培养的主动脉平滑肌细胞（smooth muscle cell,smc)PAI-1表达的作用及对血管内皮生长因子（vscular endothelial growth factor,VEGF)表达的影响。方法 PCR扩增PAI－1第2外显子,将PCR产物纯化克隆后连入真核细胞表达载体pcDNA3．1,构建PAI－1反应RNA重组质粒。将pcDNA3.1-反应PAI－1重组质粒转染SMC中。通过免疫组化、Western印迹、ELISA检测细胞中PAI－1表达的改变;通过免疫荧光技术观察细胞中PAI－1表达量的变化对VEGF的影响。 结果 转染后第3天、细胞中PAI－1含量最低,VEGF的表达也减少。第5天、PAI－1含量逐渐高,VEGF也相应增加。第7天、PAI－1含量接近于正常,VEGF也增至正常水平。结论 反义PAI－1RNA能有效阻断SMC中PAI－1的蛋白合成,同时抑制细胞中VEGF的表达。     

Active proliferation of different cell types, including lymphocytes, in human atherosclerotic plaques.

Cell proliferation, an important mechanism of atherosclerotic plaque growth, occurs among smooth muscle, inflammatory cell, and other cell types. We have identified different topographical patterns of cell proliferation in human carotid plaques, based on cell type. Cell proliferation was determined with an antibody to the proliferating cell nuclear antigen (PCNA), combined with cell type-specific antibodies. Despite low levels of overall proliferative activity, the intima displayed more proliferative activity than the underlying media (1.61 +/- 0.35% in intima versus 0.05 +/- 0.03% in media; P < 0.01). The preponderant proliferative cell type in the intima was the monocyte/macrophage (46.0% of PCNA-positive cells), with a minority being smooth muscle alpha-actin-positive (9.7%), microvascular endothelial (14.3%), and T cells (13.1%). Smooth muscle cells were the dominant proliferating cell type in the media (44.4% of PCNA-positive cells versus 20% endothelial cells, 13.0% monocyte/macrophages, and 14.3% T cells). Within the plaque, foam-cell-rich regions mostly displayed proliferation among macrophages (66.5%), whereas in vascularized fields PCNA positivity was almost equally shared by endothelial cells (23.8%), monocyte/macrophages (26.3%), smooth muscle alpha-actin-positive cells (14.0%), and to a lesser extent, T cells (8.2%). Logistic and linear regression analyses also demonstrated that location in foam-cell-rich regions was a significant predictor of proliferation only among monocyte/macrophages, whereas location in vascularized regions was a good predictor of PCNA positivity among both inflammatory and noninflammatory cells. These different patterns of cell type proliferation suggest possibly different distributions of putative responsible growth regulatory factors in human atherosclerosis.     

Genetic loss of Gas6 induces plaque stability in experimental atherosclerosis

The growth arrest-specific gene 6 (Gas6) plays a role in pro-atherogenic processes such as endothelial and leukocyte activation, smooth muscle cell migration and thrombosis, but its role in atherosclerosis remains uninvestigated. Here, we report that Gas6 is expressed in all stages of human and mouse atherosclerosis, in plaque endothelial cells, smooth muscle cells and macrophages. Gas6 expression is most abundant in lesions containing high amounts of macrophages, ie thin fibrous cap atheroma and ruptured plaque. Genetic loss of Gas6 does not affect the number and size of initial and advanced plaques in ApoE(-/-) mice, but alters its plaque composition. Compared to Gas6(+/+): ApoE(-/-) mice, initial and advanced plaques of Gas6(-/-): ApoE(-/-) mice contained more smooth muscle cells and more collagen and developed smaller lipid cores, while the expression of TGFbeta was increased. In addition, fewer macrophages were found in advanced plaques of Gas6(-/-): ApoE(-/-) mice. Hence, loss of Gas6 promotes the formation of more stable atherosclerotic lesions by increasing plaque fibrosis and by attenuating plaque inflammation. These findings identify a role for Gas6 in plaque composition and stability.     

Effect of chronic nicotine delivery on the proliferation rate of endothelial and smooth muscle cells in experimentally induced vascular wall plaques

To study the effect of nicotine, cholesterol feeding, and their combination on endothelial and smooth muscle cells in vascular wall plaques an experimental method was established which allows the immunohistochemical detection and quantification of the fractions of endothelial and smooth muscle cells in DNA synthesis under the effect of these stimuli. For this purpose standardized fibromuscular plaques were produced by electrostimulation in the common carotid arteries of rabbits. The animals received either nicotine via implanted osmotic minipumps or a cholesterol diet or both. Plaque size was determined at the end of the experiments after 7 or 14 days as well as the fraction of endothelial and smooth muscle cells in DNA synthesis during exposure to bromodeoxyuridine (BrdU). The BrdU labeling index of endothelial cells clearly increased under chronic nicotine administration for either 7 days or 14 days compared to controls. The combination of nicotine and cholesterol diet led to a more significant increase. In contrast, the BrdU labeling index of smooth muscle cells was not increased under nicotine delivery. The combination of nicotine and cholesterol, however, led to a significant increase of the BrdU labeling index of smooth muscle cells in the plaques compared to cholesterol feeding. Measurement of the plaque size revealed no difference between controls and nicotine-treated animals after 14 days of nicotine delivery, whereas the combination of cholesterol and nicotine produced increased plaque formation compared to a group of animals which received a cholesterol diet alone.Abbreviations BrdU bromodeoxyuridine - EC endothelial cell - HDL high-density lipoprotein - LDL low-density lipoprotein - SMC smooth muscle cell     

Cysteinyl leukotriene signaling through perinuclear CysLT1 receptors on vascular smooth muscle cells transduces nuclear calcium signaling and alterations of gene expression

Leukotrienes are pro-inflammatory mediators that are locally produced in coronary atherosclerotic plaques. The response induced by cysteinyl leukotrienes (CysLT) in human coronary arteries may be altered under pathological conditions, such as atherosclerosis. The aim of the present study was to elucidate cysteinyl leukotriene signaling in vascular smooth muscle cells (SMCs) and the effects of inflammation on this process. Immunohistochemical analysis of human carotid endarterectomy samples revealed that the CysLT(1) leukotriene receptor was expressed in areas that also stained positive for α-smooth muscle actin. In human coronary artery smooth muscle cells, lipopolysaccharide significantly upregulated the CysLT(1) receptor and significantly enhanced the changes in intracellular calcium induced by leukotriene C(4) (LTC(4)). In these cells, the CysLT(1) receptor exhibited a perinuclear expression, and LTC(4) stimulation predominantly enhanced nuclear calcium increase, which was significantly inhibited by the CysLT(1) receptor antagonist MK-571. Microarray analysis revealed, among a number of significantly upregulated genes after 24?h stimulation of human coronary artery smooth muscle cells with LTC(4), a 5-fold increase in mRNA levels for plasminogen activator inhibitor (PAI)-2. The LTC(4)-induced increase in PAI-2 expression was confirmed by real-time quantitative PCR and ELISA and was inhibited by the CysLT(1) receptor antagonist MK-571 and by calcium chelators. In summary, pro-inflammatory stimulation of vascular SMCs upregulated a perinuclear CysLT(1) receptor expression coupled to nuclear calcium signaling and changes in gene expression, such as upregulation of PAI-2. Taken together, these findings suggest a role of nuclear CysLT(1) receptor signaling in vascular SMCs inducing gene expression patterns associated with atherosclerosis.     

Smad protein and TGF-beta signaling in vascular smooth muscle cells

Transforming growth factor-beta1 (TGF-beta1) plays a role in vascular remodeling by stimulating vascular smooth muscle cell (SMC) growth and matrix-protein synthesis at sites of vascular injury. Smad proteins have been shown to mediate intracellular signaling of this growth factor. We investigated the expression and phosphorylation of Smads in cultured rat aortic smooth muscle cells. In addition, we evaluated the effects of overexpression of Smad proteins on TGF-beta signal transduction by adenovirus-mediated gene transfer. In rat SMC, Smad1, Smad2, Smad3, Smad4 and Smad5 were detected by immunoprecipitation. Using antisera against phosphorylated Smad2, we showed that TGF-beta1-induced Smad2 phosphorylation in a concentration- and time-dependent manner. Using adenovirus-mediated transfection method, we demonstrated that overexpression of Smad2 or Smad4 was associated with an increased production of TGF-beta1-induced plasminogen activator inhibitor-1 (PAI-1). However, the most prominent expression of PAI-1 was observed upon cotransfection of both Smad2 and Smad4. Both the proliferative effect of TGF-beta1 under serum-free conditions and its anti-proliferative effect under serum-rich conditions were suppressed by the adenovirus-mediated overexpression of Smad7. These results indicated that Smads proteins were expressed in vascular SMC and that they mediated TGF-beta signaling in those cells.     

Distribution of plasminogen activator inhibitor (PAI-1) in tissues.

Extracts of human tissue were analysed for plasminogen activator inhibitor (PAI-1) antigen and activity. PAI-1 was localised in tissues by an immunochemical method, using monoclonal antibodies. PAI-1 occurred throughout the body; its concentration and activity differed considerably from organ to organ. Extracts of liver and spleen had the greatest abundance of PAI-1, but the activity of the inhibitor was much higher in liver than in spleen: the liver may be a source of plasma PAI-1. Immunochemical staining for PAI-1 was observed in endothelium, platelets and their precursor cells, the megakaryocytes, and locations central to the process of haemostasis. PAI-1 also occurred in neutrophil polymorphs and macrophages, cells important in inflammatory and immune processes, but not in lymphocytes. Other cell types, in particular, vascular smooth muscle cells and mesangial cells, also stained positively for PAI-1 and such cells seem to represent an important reservoir of PAI-1.     

人参皂甙对内毒素致血管内皮细胞表达TF和PAI-1的影响

目的:研究人参皂甙对内毒素脂多糖(LPS)致血管内皮细胞组织因子(TF)和纤溶酶原激活物抑制剂1(PAI-1)表达的影响,探讨人参皂甙对心血管疾病的保健及防治机制。方法:用酶消化法培养人脐静脉内皮细胞(HUVEC);ELISA方法检测HUVEC条件培养液PAI-1蛋白量;用一步凝固法检测HUVECTF活性;Northernblot检测HUVECTF和PAI-1的mRNA表达。结果:LPS能使HUVECPAI-1蛋白和TF活性及其mRNA表达显著增强,人参皂甙则明显抑制LPS对HUVEC的这种作用。结论:通过拮抗LPS致HUVECTF和PAI-1的表达,人参皂甙可维持血管内皮功能稳定,而发挥其心血管疾病的保健与防治作用。     

mmLDL对人脐静脉内皮细胞PAI-1表达的影响及其转录调控机制

目的:探讨弱氧化修饰低密度脂蛋白(mmLDL)对人脐静脉内皮细胞(HUVEC)PAI-1活性和mRNA表达的影响及其转录调控机制。方法:人脐静脉内皮细胞的培养和鉴定。用发色底物法测定PAI-1活性。Northern印迹分析法检测PAI-1mRNA的水平。采用基因重组技术构建含不同长度PAI-1 5'上游序列的荧光素酶报告基因质粒, 瞬时转染进入内皮细胞, 并检测荧光素酶的表达情况。 利用PCR和测序技术, 对构建质粒上AP-1元件进行定点突变。Western blot印迹杂交检测内皮细胞核内激活蛋白-1(AP-1)蛋白水平。结果:50 mg/L mmLDL诱导HUVECs PAI-1活性和mRNA表达量明显增高, 同时提高核内AP-1蛋白水平。mmLDL显著诱导构建质粒pGL3-PAI-1-1509/+90和pGL3-PAI-1-823/+90的荧光素酶活性, 但对质粒pGL3-PAI-1-553/+90和pGL3-PAI-1-47/+90诱导作用不明显。当PAI-1 5'上游序列的3个AP-1元件突变后, mmLDL的诱导作用明显降低。结论:(1)mmLDL增强血管内皮细胞 PAI-1活性与mRNA表达;(2)PAI-1活性提高与其mRNA表达增加呈正相关;(3) PAI－1 5'上游序列中3个AP-1元件在mmLDL对PAI－1诱导中具有重要调控作用。     

Levels of tissue-type plasminogen activator and plasminogen activator inhibitor 1 in disseminated intravascular coagulation syndrome

Since disseminated intravascular coagulation (DIC) may directly reflect the abnormal regulation of the fibrinolytic system by endothelial cells, we have measured the levels of tissue-type plasminogen activator (t-PA), type 1 PA inhibitor (PAI-1) and t-PA . PAI-1 complex which is formed as a result of interaction on the two factors, in the plasma of patients with DIC (n = 51) and healthy controls (n = 42). Antigens of t-PA, PAI-1 and t-PA . PAI-1 complex were significantly increased in the DIC plasma (36.4 +/- 25.1, 106.8 +/- 54.7 and 46.6 +/- 34.5 ng/ml, respectively) compared with those in normal plasma (8.5 +/- 4.3, 54.4 +/- 21.2 and 8.6 +/- 3.5 ng/ml, respectively). The molar ratio of t-PA to PAI-1 was much higher in the DIC plasma (1:3) than in normal plasma (1:6), which caused enhancement of the whole fibrinolytic activity in the DIC plasma. These changes resulted in significant consumption of plasminogen, alpha 2-plasmin inhibitor (alpha 2-PI) and a significant increase of plasmin . alpha 2-PI complex (PPI) and D-dimer. These results suggest that t-PA and its specific inhibitor PAI-1 both of which are secreted from endothelial cells into blood, play an important role on the progress of DIC.     

Arterial macrophages and regenerating endothelial cells express P-selectin in atherosclerosis-prone apolipoprotein E-deficient mice

P-selectin expression has been reported in platelets, endothelial cells, and vascular smooth muscle cells in response to vascular injury. Here, we report P-selectin expression on macrophages in the arterial wall after carotid denudation injury and spontaneous atherosclerosis in atherosclerosis-prone apoE-deficient (apoE(-/-)) mice. Double-immunofluorescence staining revealed robust P-selectin expression in macrophage-rich regions of both denudation-induced carotid neointimal lesions and innominate atherosclerotic plaques. Co-localization of P-selectin with macrophages was verified at the single cell level using double immunostaining plus 4,6-diamidino-2-phenylindole (for nuclei) counterstaining. No platelet staining was seen in association with the macrophage staining, excluding platelet contamination. Furthermore, P-selectin mRNA expression was readily detectable in macrophage-rich plaques of atherosclerotic innominate arteries and blood monocyte-derived macrophages from apoE(-/-) mice. Strong P-selectin expression was also seen in the areas of regenerated endothelium after arterial injury. In addition, co-localization of P-selectin with vascular smooth muscle cells was readily observed in denudation-injured carotid arteries at 7 and 14 days. We conclude that macrophages in carotid injury-induced neointimal lesions and spontaneous atherosclerotic plaques of the innominate artery acquire the ability to express P-selectin, as does regenerating endothelium. These findings provide a potential new paradigm in macrophage-mediated vascular inflammation, atherosclerosis, and neointimal hyperplasia after arterial injury.     

Role of plasminogen activator inhibitor in the reciprocal regulation of bovine aortic endothelial and smooth muscle cell migration by TGF-beta 1.

Vascular endothelial and smooth muscle cells exhibit reciprocal migratory responses after transforming growth factor (TGF)-beta 1 treatment. Endothelial cells exhibit a decreased migratory rate and smooth muscle cells exhibit an increased migratory rate. Previous studies have demonstrated increases in extracellular matrix and integrin synthesis and expression in response to TGF-beta 1. In this report, we illustrate the roles of plasminogen activator inhibitor in modulating the migratory rates in these two cell types. Endothelial cells appear to require a proteolytic phenotype for rapid migration, whereas vascular smooth muscle cells appear to require an anti-proteolytic phenotype. Modulation of proteinase/anti-proteinase activity ratios was accomplished via TGF-beta 1 induction, addition of exogenous plasminogen activator inhibitor, addition of anti-catalytic antibodies directed against urokinase plasminogen activator, overexpression of plasminogen activator inhibitor utilizing stable transfectants, and the use of vitronectin as a substratum. The reciprocal migratory behaviors exhibited by these two vascular cell types in response to TGF-beta 1 is discussed in the context that these two vascular cell types utilize distinct adhesive and signaling pathways in their interactions with extracellular matrix components and responsiveness to proteolytic activity.     

冠状动脉斑块中炎性细胞和平滑肌细胞的定量及其与斑块稳定 …

目的 探讨稳定及不稳定斑块中巨细胞、Ｔ淋巴细胞及平滑肌细胞数量的差异及其与斑块稳定性的关系。方法 将急性心肌梗死死亡病例的尸检冠状动脉完整剥离后固定、脱钙、连续取材并常规切片及ＨＥ染色,按有无脂质中心及其大小将斑块分为稳定斑块（斑块无或仅有较小的脂质中心）及不稳定斑块（脂质中心超过斑块的４０％）。选取２种斑块各１６３个,用ＣＤ６８、肌动蛋白、ＵＣＨＬ－１抗体分别对巨噬细胞、平滑肌细胞、Ｔ淋巴细胞进     

Effects of black cohosh on the plasminogen activator system in vascular smooth muscle cells

Objective

The rhizome of the Cimicifuga racemosa plant (commonly known as black cohosh) has been used for menopausal complaints. Studies regarding the cardiovascular effects of black cohosh are lacking. We investigated the effect of black cohosh on the plasminogen activator system in cultured vascular smooth muscle cells (VSMCs).

Methods

VSMCs were isolated from rat aortae. Expression of plasminogen activator inhibitor type 1 (PAI-1) and tissue-type plasminogen activator (t-PA) proteins were evaluated by Western blot analysis and enzyme-linked immunosorbent assay, respectively. The activities of PAI-1 and t-PA in the conditioned media were assessed by fibrin overlay zymography. A 40% 2-propanol extract of black cohosh was used.

Results

Black cohosh extract (BcEx) stimulated the protein expression of PAI-1, but it did not affect that of t-PA. Vitamin E, a potent antioxidant, inhibited the BcEx-induced increase in PAI-1 expression, while ICI 182,780, an estrogen receptor antagonist, had no effect. Fibrin overlay zymography revealed that BcEx increased the activity of PAI-1 in the conditioned media, while concurrently decreasing that of free t-PA by inducing a binding to PAI-1.

Conclusions

BcEx induces PAI-1 protein expression in the VSMCs likely via an oxidant mechanism. It also stimulates the enzyme activity of PAI-1 and reduces that of free t-PA. These findings suggest that black cohosh might exert a negative influence on fibrinolysis.     

Tissue plasminogen activator induced by dengue virus infection of human endothelial cells

Dengue hemorrhagic fever and dengue shock syndrome (DHF/DSS) are severe complications of dengue virus (DV) infection. However, the pathogenesis of hemorrhage induced by dengue virus infection is poorly understood. Since endothelial cells play a pivotal role in the regulation of hemostasis, we studied the effect of DV infection on the production of tissue plasminogen activator (tPA) and plasminogen activator inhibitor 1 (PAI-1) in vitro using both primary isolated endothelial cells, human umbilical cord veins cells, and a human microvascular endothelial cell line. DV infection significantly induced the secretion of tPA but not PAI-1 of human endothelial cells. In addition, tPA mRNA of endothelial cells was induced by DV as demonstrated by RT-PCR. Antibody against IL-6 but not control antibody inhibited DV-induced tPA production of endothelial cells. Furthermore, a good correlation between sera levels of IL-6 and tPA was found in DHF but not DF patients. These results suggest that IL-6 can regulate DV-induced tPA production of endothelial cells, which may play important roles in the pathogenic development of DHF/DSS.     

冠状动脉斑块中炎性细胞和平滑肌细胞的定量及其与斑块稳定性的关系

目的　探讨稳定及不稳定斑块中巨噬细胞、Ｔ淋巴细胞及平滑肌细胞数量的差异及其与斑块稳定性的关系。方法　将急性心肌梗死死亡病例的尸检冠状动脉完整剥离后固定、脱钙、连续取材并常规切片及ＨＥ染色,按有无脂质中心及其大小将斑块分为稳定斑块（ 斑块无或仅有较小的脂质中心） 及不稳定斑块（ 脂质中心超过斑块的４０ ％ ）。选取２ 种斑块各１６３ 个,用ＣＤ６８ 、肌动蛋白、ＵＣＨＬ－ １ 抗体分别对巨噬细胞、平滑肌细胞、Ｔ淋巴细胞进行免疫组化染色,并通过计算机图像分析系统对２ 种斑块纤维帽中的３ 种细胞进行定量。结果　不稳定斑块纤维帽中巨噬细胞及Ｔ淋巴细胞的面积百分比分别为８９．６％ 和３２．５％ ,稳定斑块分别为１８．３％ 和７．２ ％ ,差异均具有显著性（ Ｐ＜０．０１） 。而平滑肌细胞在稳定斑块纤维帽中的面积百分比明显大于不稳定斑块,分别为６８．４ ％ 和２１ ．８％ ,差异具有显著性（Ｐ＜ ０．０５） 。结论　稳定斑块与不稳定斑块不仅脂质中心大小不同,其纤维帽中炎细胞和平滑肌细胞的数量也不同,粥样坏死中心越大,其表面纤维帽中的巨噬细胞、Ｔ淋巴细胞就越多,而平滑肌细胞则越少。脂质中心大小和纤维帽中炎细胞数量是决定斑块稳定性的主要因素。