The down-regulation of XBP1, an unfolded protein response effector, promotes acute kidney injury to chronic kidney disease transition.

BACKGROUND

The activation of the unfolded protein response (UPR) is closely linked to the pathogenesis of renal injuries. However, the role of XBP1, a crucial regulator of adaptive UPR, remains unclear during the transition from acute kidney injury (AKI) to chronic kidney disease (CKD).

METHODS

We characterized XBP1 expressions in different mouse models of kidney injuries, including unilateral ischemia-reperfusion injury (UIRI), unilateral ureteral obstruction, and adenine-induced CKD, followed by generating proximal tubular XBP1 conditional knockout (XBP1) mice for examining the influences of XBP1. Human proximal tubular epithelial cells (HK-2) were silenced of XBP1 to conduct proteomic analysis and investigate the underlying mechanism.

RESULTS

We showed a tripartite activation of UPR in injured kidneys. XBP1 expressions were attenuated after AKI and inversely correlated with the severity of post-AKI renal fibrosis. XBP1 mice exhibited more severe renal fibrosis in the UIRI model than wide-type littermates. Silencing XBP1 induced HK-2 cell cycle arrest in G2M phase, inhibited cell proliferation, and promoted TGF-β1 secretion. Proteomic analysis identified TNF receptor associated protein 1 (Trap1) as the potential downstream target transcriptionally regulated by XBP1s. Trap1 overexpression can alleviate silencing XBP1 induced profibrotic factor expressions and cell cycle arrest.

CONCLUSION

The loss of XBP1 in kidney injury was profibrotic, and the process was mediated by autocrine and paracrine regulations in combination. The present study identified the XBP1-Trap1 axis as an instrumental mechanism responsible for post-AKI fibrosis, which is a novel regulatory pathway.