Abstract:
Mucin (MUC) is a large molecular weight glycoprotein containing numerous O-linked glycans. To date, 19 human mucin genes have been identified. In gallstone and hepatolithiasis, mucins are up-regulated and their role in lithogenesis has been demonstrated. Expression and lithogenic function of mucins in kidney stone disease have not been explored. This study aimed to investigate the expression of MUC1 and MUC2 in renal tissues of patients with nephrolithiasis and to examine the role of urinary mucins in calcium oxalate (CaOx) crystal aggregation. Renal biopsy and 24-hour urine specimens were obtained from 31 stone patients who underwent open stone surgery. Control renal tissues (n=7) were taken from non-cancerous and cancerous portions of nephrectomy patients with localized renal tumors. Control 24-hour urine samples were collected from 19 healthy subjects. Intrarenal expression of MUC1 and MUC2 transcripts were measured by real time RT-PCR. 8-hydroxydeoxyguanosine (8-OHdG), an oxidative DNA damage marker was determined in renal tissues. Urine specimens were analyzed for creatinine, oxalate, citrate, N-acetyl gluosaminidase (NAG) activity and 8-OHdG. Urinary mucins were partially purified from pooled urine samples of healthy and renal stone subjects using CL-2B gel filtration column. MUC1-positive fractions were pooled and named HP1 (for healthy pool, fractions 27-43), NLP1 (for nephrolithiasis pool, fractions 5-17) and NLP2 (fractions 24-40). The lithogenic activity of each pooled fraction was examined by in vitro aggregation assays. Urinary citrates significantly lower than healthy subjects. Level of urinary 8-OHdG and NAG activity in nephrolithiatic group was significantly higher than in healthy group. In nephrolithiasis, MUC1 was positive in all samples whereas MUC2-positive tissues were accounted for 64.52% (20/31). Expression level of MUC1 was higher than MUC2. MUC1 trended to be overexpressed in nephrolithiasis tissues compared to non-cancerouse renal tissue controls. Association between Oxidative DNA damage and the expression of MUC1 and MUC2 was not observed. CaOx crystal aggregation assay revealed that NLP1 had an aggregation coefficient significantly higher than HP1 and NLP2. Total Sialic acid content in NLP1 was lower than HP1 and NLP2. In conclusion, MUC1 trended to be up-regulated in stone-containing renal tissues. NLP1 had a very large size but low sialic acid content. It promoted the aggregation of CaOx crystals. The presence of large and low-negative-charge mucins in the urine of nephrolithiasis patients may enhance the growth of stone.