[CD137 signaling promotes the formation of plaque calcification via inhibiting the fusion of autophagy and lysosomal in Apo E(-/-) mice]

Objective: To investigate whether CD137 signaling promoted the formation of atherosclerotic plaque calcification by inhibiting the fusion of autophagosome and lysosome. Methods: (1) In vivo, CD137 agonist antibody and anti-CD137 antibody were used to stimulate and inhibit the CD137 signaling, respectively. Fifteen Apo E(-/-) mice were randomly divided into three groups: control group (intraperitoneal injection of IgG2b 200 µg) , CD137 agonist group (intraperitoneal injection of CD137 agonist antibody 200 µg) , anti-CD137 group (pretreatment with 200 µg anti-CD137 antibody for 24 hours, then injection of CD137 agonist antibody) . (2) In vitro, primary culture of mouse aortic VSMCs obtained through adherence methods for tissues explants. The cells was divided into three groups: control group, agonist-CD137 group (CD137 agonist antibody 10 μg/ml) , and anti-CD137 group (pretreatment with 10 μg/ml anti-CD137 antibody for 60 minutes, then incubated with 10 μg/ml CD137 agonist antibody) . Von kossa staining was used to detect the calcification in the cell and plaque. Immunohistochemical staining was used to observe the expression of LC3B, Beclin 1 and p62 which are associated with autophagy. The levels of autophagy related protein (LC3) , Beclin 1, p62, and the expression of Runx2 and bone morphogenetic protein 2, which is associated with osteogenic differentiation in the VSMCs, were determined by Western blot. The autophagy flow of each group was detected by fluorescence microscopy. The autophagy was observed by transmission electron microscope in vivo and in vitro. Results: (1) In vivo, the calcified plaque area in CD137 agonist group was significantly larger than that in the control group (3.01%±0.45% vs. 0.27%±0.06%, P<0.01) , and calcified plaque area in anti-CD137 group was significantly smaller compared with that in the CD137 agonist group (1.23%±0.39% vs. 3.01%±0.45%, P<0.05) . Immunohistochemical staining showed that the expression of early autophagy marker protein LC3B and Beclin 1 were significantly upregulated in CD137 agonist group and anti-CD137 group than in control group, and the highest expression was observed in CD137 agonist group (P<0.05) . The expression of advanced autophagy marker protein p62 was higher in the CD137 agonist group than in the anti-CD137 group (P<0.05) . (2) In vitro, the ratio of autophagy related protein LC3 Ⅱ/Ⅰ and p62 protein expression were significantly higher in CD137 agonist group and anti-CD137 group than in control group (P<0.01) , while the expression of p62 protein was significantly higher in CD137 agonist group than that in anti-CD137 group (P<0.05) . In the cell calcification inducing experiment, the expression of BMP-2 and Runx2 protein was significantly higher in CD137 agonist group than that in control group (P<0.01) , but the levels of BMP-2 and Runx2 protein were lower in anti-CD137 group than in CD137 agonist group (P<0.05) . Conclusion: Our results indicate that activation of CD137 signaling can promote the formation of atherosclerotic plaque calcification by inhibiting the fusion of autophagosome and lysosome.

[CD137 induces vascular muscle cells phenotype transformation through activating nuclear factor of activated T-cells 1 signaling]

Objective: To investigate whether CD137 induces primary vascular muscle cells (VSMCs) phenotype transformation through activating nuclear factor of activated T-cells 1(NFATc1) signaling. Methods: VSMCs were obtained from aorta of C57BL/6J mice (8 weeks, male) through tissue-piece inoculating. Cells were divided into control group, CD137 agonist group (treated with CD137L recombinant protein) and anti-CD137 group (treated with anti-CD137 antibody). In si-RNA transfection assay, cells were divided into si-control group and si-NFATc1 group which were transfected with control or si-NFATc1 sequence respectively. The levels of NFATc1 and other phenotype related protein such as α-smooth muscle actin (α-SMA), smooth muscle myosin heavy chain (SM-MHC), vimentin were detected by Q-PCR and Western blot. Nuclear protein expression and activity of NFATc1 were detected by immunofluorescence and Western blot. Transwell assay was performed to measure the migration of VSMCs. Results: According to Western blot, the expression of NFATc1 and vimentin was significantly upregulated (5.07±0.36 vs. 1.00±0.00, P<0.05; 3.23±0.27 vs. 1.00±0.00, P<0.05) while α-SMA and SM-MHC expressions was significantly downregulated (0.73±0.15 vs. 1.00±0.00, P<0.05; 0.45±0.05 vs. 1.00±0.00, P<0.05) in CD137 agonist group compare to control group. Compared with CD137 agonist group, the expression of NFATc1 and vimentin was significantly downregulated (1.56±0.27 vs. 5.07±0.36, P<0.05; 1.21±0.17 vs. 3.23±0.27, P<0.05), but the levels of α-SMA and SM-MHC were significantly upregulated (2.01±0.43 vs. 0.73±0.15, P<0.05; 2.85 ±0.32 vs. 0.45±0.05, P<0.05) in anti-CD137 group. Compared with si-con group, the expression of SM-MHC and α-SMA was significantly upregulated while the expression of vimentin was significantly downregulated in si-NFATc1 group. Transwell assay results demonstrated that migration cell numbers was significantly higher in CD137L group compared with control group(3.85±0.31 vs. 1.00±0.00, P<0.05), this effect was significantly attenuated by inhibiting NFATc1. Conclusion: CD137 could induce VSMC phenotype transformation through activating NFATc1 signaling.

[Impact of CD137-CD137L signaling mediated exocytosis of autophagosome within vascular smooth muscle cells on the formation of atherosclerotic calcification]

Objective: To explore whether CD137-CD137L signaling mediated exocytosis of autophagosome within vascular smooth muscle cells (VSMCs) could influence the formation of atherosclerotic calcification. Methods: Fifteen 8-week-old male ApoE(-/-)(C57BL/6J-KO) mice fed with high fat diet for 5 weeks were randomly divided into three groups by using stochastic indicator method as follows: control group, n=5; agonist-CD137 group: agonist-CD137 antibody 200 μg/2 weeks for 4 weeks, ip, n=5; anti-CD137 group: 200 μg anti-CD137 antibody+ 200 μg agonist-CD137 antibody/2 weeks for 4 weeks, ip, n=5. Von Kossa staining was applied to observe the calcification of the thoracic aortic atherosclerotic plaque in each group. Immunohistochemistry was used to detect the expression of LC3 and Beclin1 which were the autophage markers of early-to-mid stage; Western blot was adopted to quantify protein level of microtubule-associated proteins 1 light chain 3B(LC3B) and mammalian ortholog of the yeast autophagy-related gene 6 (Beclin1). Transmission electron microscope (TME) was used to observe the formation of autophagosome in plaque. C57BL/6J mouse VSMCs were cultured by using tissue piece inoculation method. Groups of in vitro studies were the same as in vivo study: control group, agonist-CD137 group, anti-CD137 group, the agonist-CD137 groups was treated with agonist-CD137 antibody (10 μg/ml) and anti-CD137 group was treated with anti-CD137 antibody (10 μg/ml) for 30 minutes, followed by agonist-CD137 antibody (10 μg/ml). Von Kossa staining and osteogenesis phenotypic alkaline phosphatase (ALP) activity detection were adopted to observe calcification in VSMCs. Autophagosomes were separated from the supernatant of the agonist-CD137 group with density gradient centrifugation method. VSMCs were divided into two groups: positive group (containing complete medium with above autophagosomes to a final concentration 15 μg/ml) and negative group (only complete medium) after being pretreated with mixed inflammatory cytokines (IL-1β、IFN-γ and TNF-α, final concentration was 25 ng/ml respectively) for 24 hours and calcium deposition and osteogenesis phenotypic marker bone morphogenetic protein 2(BMP2) were then detected. Results: (1) Compared with the control group, activation of the CD137-CD137L signal significantly increased the formation of calcification area in thoracic aortic atherosclerotic plaque of ApoE(-/-) mice((1.82±0.15)×10(4) μm(2) vs. (0.34±0.08)×10(4) μm(2,) P<0.01), this effect was significantly attenuated by inhibiting this signal ((0.83±0.30)×10(4) μm(2) vs. (1.82±0.15)×10(4) μm(2,) P<0.05); positive autophagy makers LC3B and Beclin1 were detected in both agonist-CD137 group and anti-CD137 groups and the expression of LC3B and Beclin1 was substantially higher in anti-CD137 group. Western blot analysis indicated that the expression of LC3B and Beclin1 in agonist-CD137 group was significantly upregulated compared with the control group (0.17±0.01 vs. 0.03±0.08, P<0.05, and 0.12±0.02 vs. 0.06±0.02, P<0.05), which could be significantly downregulated in anti-CD137 group (0.28±0.09 vs. 0.17±0.01, P<0.05 and 0.17±0.02 vs. 0.12±0.02, P<0.05). TME showed that the number (QTY /HP) of autophagosome of agonist-CD137 group and anti-CD137 group in plaque were both increased (14.67±2.52 vs. 3.67±1.53, P<0.01, and 15.33±2.08 vs. 3.67±1.53, P<0.01), while in the agonist-CD137 group, the number of extracellular autophagosome within thoracic aortic atherosclerotic plaque of ApoE(-/-) mice increased more substantially (5.33±1.53 vs. 1.33±0.58, P<0.01). (2) In vitro study showed that activating CD137-CD137L signal could promote calcium deposition in extracellular matrix and the activity of osteogenesis phenotypic ALP((6.73±0.02) μmol/mg protein vs. (1.07±0.03) μmol/mg protein, P<0.05), and ((563.20±0.72) U/mg protein vs. (117.50±0.64) U/mg protein, P<0.05), while these effects were significantly blunted in anti-CD137 group ((1.94±0.05) μmol/mg protein vs. (6.73±0.02) μmol/mg protein, P<0.05, and (236.10±0.14) U/mg protein vs. (563.20±0.72) U/mg protein, P<0.05). TME showed that the number of intracellular autophagosome in agonist-CD137 group and anti-CD137 group was both significantly higher than in control group ((21.65±1.34) μg/ml vs. (8.32±1.58) μg/ml, P<0.01, and (15.42±1.65) μg/ml vs. (8.32±1.58) μg/ml, P<0.05). After the density gradient centrifugation, exocytotic autophagosome in the medium of agonist-CD137 group was markedly higher than in control group ((14.67±1.53) μg/ml vs. (2.33±1.15) μg/ml, P<0.01). (3) Compared with the control group, autophagosomes isolated from culture supernatant (final concentration: 15 μg/ml) could significantly stimulate calcium deposition((2.30±0.10) μmol/mg protein vs. (0.15±0.40) μmol/mg protein, P<0.05) and enhance the expression of bone morphogenetic protein 2 (2.10±0.04 vs. 0.30±0.01, P<0.05). Conclusion: CD137-CD137L signaling could mediate exocytosis of autophagosome within VSMCs, thus influence the formation of atherosclerotic calcification.