Monocyte Mitochondrial Function in Calcium Oxalate Stone Formers

Abstract

Objective: To investigate whether mitochondrial function is altered in circulating immune cells from calcium oxalate (CaOx) stone formers compared to healthy subjects.

Materials and methods: Adult healthy subjects (n = 18) and CaOx stone formers (n = 12) were included in a pilot study. Data collection included demographic and clinical values from electronic medical records. Bioenergetic function was assessed in monocytes, lymphocytes, and platelets isolated from blood samples using the Seahorse XF96 Analyzer. Plasma interleukin-6 (IL-6) was measured using enzyme-linked immunosorbent assay.

Results: All participants were age matched (44.5 ± 3.0 years for healthy subjects vs 42.3 ± 4.8 years for CaOx stone formers, P = .6905). CaOx stone formers did not have urinary tract infection, ureteral stones, or obstructing renal stones. Monocyte mitochondrial function was decreased in CaOx stone formers compared to healthy subjects. Specifically, mitochondrial maximal respiration (P = .0011) and reserve capacity (P < .0001) were significantly lower. In contrast, lymphocyte and platelet mitochondrial function was similar between the 2 groups. The bioenergetic health index, an integrated value of mitochondrial function, was significantly lower in monocytes from CaOx stone formers compared to healthy subjects (P = .0041). Lastly, plasma IL-6 levels were significantly increased (P = .0324).

Conclusion: The present pilot study shows that CaOx stone formers have decreased monocyte mitochondrial function. Plasma IL-6 was also increased in this cohort. These data suggest that impaired monocyte mitochondrial function and inflammation may be linked to CaOx kidney stone formation. Further studies are needed to confirm these findings in a larger cohort of patients.

Figure 1

Mitochondrial function in monocytes from calcium oxalate (CaOx) stone formers and healthy subjects (HS). (A) Illustration of the mitochondrial assay used to measure oxygen consumption rate (OCR) over time using the mitochondrial stress test. (B) OCR profiles of monocytes from healthy subjects (black circle) and CaOx stone formers (white circle) using the mitochondrial stress test. Indices of mitochondrial function—(C) maximal respiration and (D) reserve capacity were calculated from all study participants based on their respective bioenergetic profile. (E) The BHI was calculated using the equation: (ATP-linked OCR x Reserve Capacity)/(Proton Leak x Non-mitochondrial OCR). The indices are represented in a box plot with lower 25th percentile, median, upper 75th percentile, and whiskers drawn at 1.5 × interquartile range. Results are n=18 healthy subjects and n=11 CaOx stone formers. *p<0.05 compared to healthy subject monocytes.

Figure 2

Mitochondrial function in lymphocytes and platelets from calcium oxalate (CaOx) stone formers and healthy subjects (HS). OCR profiles of A) lymphocytes and (B) platelets isolated from healthy subjects (black circle) and CaOx stone formers (white circle) using the mitochondrial stress test. Results are presented as mean ± SE; n=5–6 replicates per group; n=14–17 healthy subjects and n=10–11 CaOx stone formers.

Figure 3

Plasma Interleukin-6 levels in calcium oxalate (CaOx) stone formers (SF) and healthy subjects (HS). Plasma from HS (black circle) and CaOx SF (white circle) were analyzed for the expression of IL-6 using ELISA. Results are represented in a box plot with lower 25th percentile, median, upper 75th percentile, and whiskers drawn at 1.5 × interquartile range; n = 10 healthy subjects and n=8 CaOx SF and presented as mean ± SE; *p<0.05 compared to HS monocytes.