hPMSC transplantation alleviated CCl4-injured liver fibrosis in mice
hPMSCs exhibited characteristics typical of MSCs. They had obvious fibroblastic morphology and were positive for the known cell surface markers CD105, CD73, CD90, and CD166, but negative for CD11b, CD34, and CD45. hPMSCs also exhibited low immunogenicity, with low or almost no expression of HLA-DR (Supplemental Fig. 1A-B). Additionally, hPMSCs showed multipotency when cultured under osteogenic, adipogenic, and chondrogenic differentiation conditions (Supplemental Fig. 1C).
We next tested the efficiency of hPMSC engraftment in experimental LF by CCl4 administration, as shown in the modeling process (Supplemental Fig. 2A). After CCl4 treatment for 2 weeks, normal liver lobules were destroyed and the fibrous connective tissue in the portal area was significantly increased. Four weeks later, the fibrous tissue further increased, extending to adjacent liver lobules, and dividing the liver tissue. Six weeks later, the increased collagen fibers formed linear fibrous septa, and pseudolobules formed, according to Sirius Red staining, Masson staining, and α-SMA staining (Supplemental Fig. 2B and 2C). AST, ALT, ALB and hepatic hydroxyproline content were also elevated in CCl4-treated mice, with a rising trend consistent with histopathological examinations (Supplemental Fig. 2D).
Time-point and cell-dose are two important parameters for cell-based therapy. In previous studies, MSCs were usually transplanted in vivo at the 4th week or 6th week after CCl4 administration (24, 25). They reflect the mild to moderate stage of LF and the severe stage of LF, respectively. In this study, these two time points were chosen as treatment 1 (T1) and treatment 2 (T2) respectively, and a comparative study was conducted, as shown in Figure. 1A. Two doses of hPMSCs, including conventional cell doses (2 × 106 cells/kg) and high cell doses (5 × 106 cells/kg) were analyzed as well, mice from fibrosis group received CCl4 followed by PBS injection were served as control (Fig. 1A). Compared with the fibrosis group, biochemical parameters of liver function, including ALT, AST, and ALB levels were reduced in all hPMSC treatment groups, especially in the T1 groups (Fig. 1B). Moreover, we found that the level of hydroxyproline, the main component in collagen tissue, was also reduced in all hPMSC treatment groups (Fig. 1B).
Histopathological examination using Sirius Red staining and Masson staining was performed to quantify the degree of LF. Compared to the fibrosis group, the fibrous area of liver tissue was significantly reduced in the hPMSC treatment groups (Fig. 2C). In addition, immunostaining showed α-SMA expression was decreased in hPMSC treatment groups, (Fig. 2D). Furthermore, the expression of fibrosis-related genes, including Acta2, Col1a1, and Vimentin was decreased upon hPMSC treatment, and downregulation of these genes was greater in T1 groups compared with T2 groups, as determined by qRT-PCR analysis. (Fig. 1E). These results suggest that hPMSC treatment could improve liver function and alleviate LF in CCl4-treated mice.
Compared with the T2 groups, the therapeutic effects of hPMSC transplantation were more profound in T1 groups according to the results of the blood biochemical indices, collagen area, and fibrosis-related genes. Moreover, doubling the cell doses did not further improve the therapeutic effect of hPMSCs, indicating that conventional cell doses (2 × 106 cells/kg) are sufficient to play a therapeutic role in experimental LF.
hPMSCs Transplantation Has an Anti-inflammatory Effect in CCl4-Injured Mice Liver
Inflammation is vital to the initiation and progression of LF (1). To investigate the potential anti-inflammatory effects of hPMSCs in vivo, we examined the differences in intrahepatic macrophages and inflammatory cytokines in liver tissue isolated from different mice. Compared to normal liver tissue, a large number of infiltrating macrophages (F4/80+) were found in fibrotic livers according to immunohistochemical staining, while the number of macrophages reduced with hPMSC treatment (Fig. 2A). To explore the source of infiltrating macrophages, we then examined the proportion of mononuclear/macrophage cells using FACS analysis. The results showed that there was no statistical difference in the proportion of monocyte-derived macrophages (MoMF, CD11bhighF4/80low) (28) from different liver tissues. Interestingly, the proportion of Kupffer cells (CD11blowF4/80high) (28) was 17.44 ± 3.18% in the hPMSC group, which was significantly lower than that in the fibrosis group (34.64 ± 2.12%) (Fig. 4B-C). These results suggested that hPMSC treatment could suppress the infiltration of macrophages, mainly by reducing the number of Kupffer cells.
We then detected the expression of inflammatory cytokines in the serum of mice by ELISA. As expected, the expression levels of inflammatory factors, including IL-6 and TNF-α, were lower in the hPMSC treatment group than in the LF group (Fig. 4D). However, there was no significant change (P = 0.07) in the expression level of IFN-γ, which may be related to the alleviation of inflammation in mice after treatment. These results were further confirmed by qRT-PCR analysis (Fig. 4E). Our findings indicated that hPMSC treatment contributes to the improvement of CCl4-injured mouse liver, at least in part through anti-inflammatory processes
hPMSCs inhibit TGF-β1 -induced HSCs activation in vitro.
HSC activation is an indispensable component in the initiation and progression of LF (3). We then investigated whether hPMSCs could regulate the activity of the HSCs in vitro. In the presence of TGF-β1, the number of activated HSCs that expressed the myogenic marker α-SMA was markedly increased, as determined by western blot analyses (Fig. 3A; Supplemental Fig. 3). Activated HSCs became elongated, with a dendritic-like shape, compared with unactivated HSCs (Fig. 3B). Activated HSCs were then cultured with secretomes in a gradient ratio of 10%, 20% and 40%. The source of the secretomes was the culture supernatant from hPMSCs, which was concentrated 15-fold in advance. The use of secretomes, but not hPMSCs was aimed at reducing the interference of cell components. Unactivated and activated HSCs without extra treatment were used as controls (Fig. 3A). Interestingly, the expression of α-SMA protein in activated HSCs was downregulated after treatment with 10% hPMSC secretomes, accompanied by the partial recovery of cell morphology. Furthermore, this inhibitory effect was more pronounced as the concentration of secretomes was increased to 40%, which indicated that the hPMSC secretomes inhibited HSC activation in a concentration-dependent manner (Fig. 3A and Fig. 1B). Additionally, to exclude the effect of MSC medium composition, MSC complete medium was also concentrated and tested in the same manner. Compared with the hPMSC supernatant, the concentrated MSC medium exhibited limited effects on the inhibition of HSC activation (Fig. 3A and Fig. 3B).
The suppression of HSC activation was also confirmed by qRT-PCR analysis, which revealed a downregulation of fibrosis-related genes in the hPMSC secretomes group compared with unactivated HSCs. In particular, Acta2, the α-SMA coding gene, was significantly downregulated compared with the activated HSCs, while the expression of Timp1, an anti-fibrotic gene was increased with hPMSC treatment (Fig. 3C). These results demonstrate that HSC activation can be inhibited by treatment with hPMSC secretomes.
Cav1 is a potential target for hPMSC treatment in liver fibrosis
To further investigate the mechanism of relieving LF with hPMSC treatment, we performed RNA sequencing (RNA-seq) analysis of liver tissues obtained from normal C57 mice (Control group), mouse models with hepatic fibrosis (Fibrosis group), and hPMSC-treated fibrosis mice (hPMSCs group). The RNAseq raw expression files and details have been deposited in NCBI GEO under accession nos. SRR12777460, SRR12777461, and SRR12777462. RNA-Seq analyses showed that the gene expression profiles of the hPMSC group was more closely resembled those seen in control liver tissues, and were significantly different from those fibrosis group (Fig. 4A). Furthermore, the genes included in three key functional clusters, including fibrosis, cytoskeleton, and inflammation-related factors were analyzed. The results revealed a significant change in the expression of these genes in fibrotic liver tissues, and they can be restored after hPMSC treatment (Fig. 4B-D).
In addition, the top 10 GO biological process terms are listed in Fig. 4E. The inclusion of the terms related to TGF-β signaling pathway, SMAD protein signal transduction and SMAD protein phosphorylation in this list suggests that regulation of TGF-β/Smad signaling may be a potential mechanism in the treatment of LF with hPMSCs. We also performed qRT-PCR analyses to check the expression of ten important fibrosis-related genes (Fig. 4F). Among them, Cav1 revealed the most significant differences between fibrotic group and hPMSCs group. Importantly, previous studies have shown that Cav1 can participates in regulating TGF-β/Smad signaling pathway in many situations (29), indicating that Cav1 may be a potential target for hPMSC treatment in LF.
To further confirm the above findings, we performed RNA-seq analyses on different cells, including activated HSCs (TGF-β1 group), unactivated HSCs (blank group), and activated HSCs, which were then treated with hPMSC secretomes (hPMSCs group) and medium (medium group), respectively. The RNAseq raw expression files and details have been deposited in NCBI GEO under accession nos. SRR12806194, SRR12806195, SRR12806196, and SRR12806197. The gene expression profiles of the hPMSC group was more closely resembled those seen in blank group, and were significantly different from those TGF-β1 group and medium (Fig. 5A). Furthermore, the expression of fibrosis-related genes as well as the ECM-associated genes similar to findings in liver tissue. It appears that, hPMSC secretomes can restore the expression of genes that had changed in the TGF-β1 group or medium group to some extent (Fig. 5B). In addition, the top GO biological process terms related to the proteinaceous ECM, and ECM suggest that hPMSC secretomes may reduce the formation of ECM by inhibiting the activation of HSCs, a key factor in alleviating LF (Fig. 5C). We also performed qRT-PCR analyses to check the expression of ten important fibrosis-related genes (Fig. 5D). The expression of Cav1 was significantly upregulated in activated HSCs when cultured with hPMSC secretomes, further supporting the finding of vivo analysis, indicating that Cav1 might be a potential target for hPMSC treatment in LF.
Downregulation of Cav1 is associated with activation of HSCs
To test this hypothesis, we investigated the effect of Cav1 on HSC activation. We measured the expression of Cav1 and α-SMA in liver tissues by immunofluorescence staining. Results showed that Cav1 was expressed at a low level both in normal liver tissues and in fibrotic liver tissues, where α-SMA was greatly upregulated after CCl4 administration. However, after treatment with hPMSCs, the expression of Cav1 was upregulated compared with that in fibrotic liver tissues, accompanied by the reduction of α-SMA in the hepatic lobular margin (Fig. 6A). To further illustrate the relationship between Cav1 downregulation and HSC activation, we also tested the expression of Cav1 and α-SMA in activated HSCs. Compared to unactivated HSCs, α-SMA was significantly increased while Cav1 was decreased in activated HSCs. After treatment with hPMSC secretomes, α-SMA levels were greatly attenuated, while Cav1 levels were partially restored in activated HSCs (Fig. 6B). These data demonstrated the involvement of Cav1 in HSC activation.
We then carried out loss-of-function experiments by transfecting an siRNA targeting human Cav1, which effectively reduced Cav1 expression in HSCs. Cav1 was not knocked down in siRNA negative control (siRNA NC)-transfected HSCs. Untransfected HSCs served as control cells (blank). HSC mRNAs from different groups were collected and subjected to qRT-PCR testing. Compared to control cells, the expression of pro-fibrotic genes, such as Acta2, and Col1a1 were upregulated by 1.5 − 3-fold in Cav1-silenced HSCs, accompanied by the attenuation of Timp1, an antifibrotic gene (Fig. 6C), indicating a vital role of Cav1 in HSC activation and collagen production. To explore the molecular mechanism of the antifibrotic effects of Cav1, we detected the regulation of Smad activation by Cav1 in HSCs using loss-of-function experiments. It is noteworthy that Smad genes, including Smad2 and Smad4, which are related to the TGF-β signaling pathway, were upregulated in Cav1-silenced HSCs, but not in negative control group and control cells (Fig. 6D). These data reveal the involvement of the TGF-β/Smad signaling pathway in Cav1-mediated HSC activation.
hPMSCs inhibit HSCs activation by restoring Cav1 function
To explore the interplay between hPMSC and Cav1 in HSC activation, we prepared siRNA-transfected HSCs, cells were activated by TGF-β1 and then treated with hPMSC secretomes (columns 3–6). Untransfected but activated HSCs served as control (column 2). Additionally, unactivated HSCs were also tested (column 1). The detailed operation is shown in Supplemental Table S5. As shown in Fig. 7A, the expression of Cav1 was reduced in HSCs in the presence of TGF-β1. It is noteworthy that the decreased Cav1 in activated HSCs can be upregulated by hPMSC secretomes. Knockdown of Cav1 in activated HSCs, however, alleviated the effect of hPMSC secretomes on the upregulation of Cav1 (Fig. 7A). Furthermore, the relative expression of the pro-fibrotic genes Acta2, Col1a1, and Desmin were also measured and normalized to β-actin. The data showed that the trend of changes in pro-fibrotic gene expression in different groups was opposite to that of Cav1 (Fig. 7A). These results indicate that hPMSC secretomes inhibit HSC activation by restoring Cav1 function in activated HSCs.
We then collected protein from HSCs of different groups and validated the results by western blot analysis. Consistent with the results from the qRT-PCR assay, Cav1 was reduced in activated HSCs that were induced by TGF-β1 (Fig. 7B). In contrast, hPMSC treatment upregulated Cav1 expression in activated HSCs and inhibited HSC activation, as indicated by the reduction of α-SMA production. However, Cav1 knockdown in activated HSCs attenuated the inhibitory effect of hPMSCs in activated HSCs (Fig. 7B). Importantly, after hPMSC treatment, HSCs showed reduced TGF-β/Smad signaling, as reflected by a significantly smaller amount of phosphorylation of Smad2 and α-SMA expression compared to that in the activated HSCs. However, knockdown of Cav1 by siRNA increased Smad-2 phosphorylation and α-SMA expression in activated HSCs even after treatment with hPMSC secretomes (Fig. 7B). In summary, hPMSC treatment restored the function of Cav1, and elevated Cav1 was sufficient to inhibit HSC activation and collagen production, partly by regulating the TGF-β/Smad signaling pathway.