Objective: Aristolochic acid (AA)-induced CKD mouse model was used for in vivo examination. Mouse plasma and kidney tissue lysates were collected at 5 and 28 days after AA administration. In addition, prospectively collected paired samples of plasma and urine from 70 patients (60 patients distributed evenly from CKD stages 1 to 5 and 10 healthy controls) were prepared. After excluding hemolytic samples (n=16) and outliers (n=5; Dixon’s Q-test), 49 patients (6 controls) were finally enrolled for cProteasome activity tests, which employ hydrolysis of fluorogenic proteasome substrate, suc-LLVY-AMC. Methods: The mean cProteasome activity was significantly elevated in AA group (4676.25 [control] vs. 5411.8 [in AA-treated for 5 days] vs. 8197.5 [in AA 28 days] in relative fluorescence units [RFU]). In kidney tissues, proteasome subunit (PSMD1, PSMC2 and α3) and ER stress protein (Phospho-IRE1a, HSF1, BiP and CHOP) were up-regulated at AA 28 days than controls, while global autophagy flux appeared unchanged. There was a significant positive correlation between serum creatinine and cProteasome activity (R2=0.38, P-value = 0.001). Divided into three groups, prominent and largely stage-dependent differences in median cProteasome activity were observed. (Figure1A) cProteasome activity was not correlated with spot urine protein creatinine and was not detected in the urine samples. Results: The cProteasome level, its activity, and ER stress response appeared to be implicated in kidney fibrosis. While the origin and pathophysiological role remains to be further determined, our data open a novel perspectives on cProteasome not only as biomarker of CKD but also potential therapeutic target for renal-fibrosis. Conclusions: Objective: The ubiquitin-proteasome system is the vital pathway of cellular protein degradation. While the association between the chronic kidney disease (CKD) and the intracellular 26S proteasome, ~2.5 MDa protease holoenzyme in the eukaryotes, was suggested, the role of circulating proteasomes (cProteasome) in the plasma has not been characterized in the context of kidney fibrosis yet. Methods: Aristolochic acid (AA)-induced CKD mouse model was used for in vivo examination. Mouse plasma and kidney tissue lysates were collected at 5 and 28 days after AA administration. In addition, prospectively collected paired samples of plasma and urine from 70 patients (60 patients distributed evenly from CKD stages 1 to 5 and 10 healthy controls) were prepared. After excluding hemolytic samples (n=16) and outliers (n=5; Dixon’s Q-test), 49 patients (6 controls) were finally enrolled for cProteasome activity tests, which employ hydrolysis of fluorogenic proteasome substrate, suc-LLVY-AMC. Results: The mean cProteasome activity was significantly elevated in AA group (4676.25 [control] vs. 5411.8 [in AA-treated for 5 days] vs. 8197.5 [in AA 28 days] in relative fluorescence units [RFU]). In kidney tissues, proteasome subunit (PSMD1, PSMC2 and α3) and ER stress protein (Phospho-IRE1a, HSF1, BiP and CHOP) were up-regulated at AA 28 days than controls, while global autophagy flux appeared unchanged. There was a significant positive correlation between serum creatinine and cProteasome activity (R2=0.38, P-value = 0.001). Divided into three groups, prominent and largely stage-dependent differences in median cProteasome activity were observed. (Figure1A) cProteasome activity was not correlated with spot urine protein creatinine and was not detected in the urine samples. Conclusions: The cProteasome level, its activity, and ER stress response appeared to be implicated in kidney fibrosis. While the origin and pathophysiological role remains to be further determined, our data open a novel perspectives on cProteasome not only as biomarker of CKD but also potential therapeutic target for renal-fibrosis.