Our data demonstrate for the first time that administration of an agonistic Tie2 monoclonal antibody ameliorates renal tubulointerstitial fibrosis, dampens renal inflammation, and promotes the growth of peritubular capillaries after FA-induced renal injury in a mouse model. The potential protective effect of this antibody on endothelial cells was confirmed through in vitro experiments on HUVECs with LPS-induced injury, and the findings were in line with previous studies that have reported the protective effect of Ang-1/Tie2 activation on endothelial cells[15–18].
Recently, Carota et al. suggested that activation of Tie2 by inhibition of vascular endothelial protein tyrosine phosphatase reduces the expression of pro-inflammatory and pro-fibrotic gene targets in diabetic nephropathy. Rübig et al. also provided evidence that the in vivo activation of Tie2 by PEGylated VT, a drug-like Tie2 receptor agonist, can counteract microvascular endothelial barrier dysfunction, improve renal recovery, and reduce mortality in ischemic acute kidney injury. Here, we used a reliable model of CKD to study the impact of agonistic Tie2 monoclonal antibody administration on kidney fibrosis. In accordance with these previous findings, we found that agonistic Tie2 monoclonal antibody treatment prevented FA-induced renal vascular inflammation, improved the density of peritubular capillaries, and significantly attenuated interstitial fibrosis.
In the present study, we observed that the accumulation of FSP-1-positive cells and the interstitial fibrotic area were significantly decreased by agonistic Tie2 monoclonal antibody treatment. The findings indicate that agonistic Tie2 monoclonal antibodies have an anti-fibrotic effect. Consistent with these findings, the in vitro protein expression level of α-SMA, a putative marker of myofibroblasts, was increased in HUVECs with LPS-induced injury, while it was markedly repressed by treatment with the agonistic Tie2 monoclonal antibody. In addition, the anti-inflammatory effect of the agonistic Tie2 monoclonal antibody was demonstrated by downregulation of VCAM-1 expression. In accordance with these findings, there is increasing evidence that Tie2 activation is associated with anti-inflammatory effects in endothelial cells[34–36], and that Tie2 agonists inhibit endothelial VCAM-1 expression. In addition, in the present study, we examined LPS-induced apoptosis in endothelial cells as evidenced by elevated levels of the proapoptotic gene Bax. Interestingly, agonistic Tie2 monoclonal antibody administration enhanced endothelial cell survival in response to apoptotic injuries[2.38]. Hence, agonistic Tie2 monoclonal antibody inhibited inflammation, rescued apoptosis, and suppressed fibrosis. The preservation of peritubular capillaries by agonistic Tie2 monoclonal antibody treatment is a potential future research direction to further explore the mechanism of Tie2 monoclonal antibodies in the treatment of CKD or improving renal fibrosis.
There is increasing evidence to show that peritubular capillaries play an important role in CKD and are a key regulator of CKD progression. Peritubular capillary rarefaction is found not only in diabetic nephropathy and hypertensive nephropathy, but also in IgA nephropathy, congenital nephrotic syndrome, lupus nephritis, and polycystic kidney disease. Thus, protecting peritubular capillaries is a crucial approach to alleviating renal fibrosis[46–49]. In the present study, we observed that agonistic Tie2 monoclonal antibody administration in FAN mice is associated with an increase in the density of peritubular capillaries. This effect might be due to a Tie2 activation-induced elevation in endothelial cell number that directly enhances endothelial cell proliferation or prevents endothelial cells apoptosis . We were unable to explore the mechanisms in detail in the present study, so more research is needed to explore the signaling pathway involved in renal tubulointerstitial fibrosis and how it is affected by the administration of an agonistic Tie2 monoclonal antibody.