Journal Article

Secreted Phosphoprotein 24 is a Biomarker of Mineral Metabolism

Anastassiades Tassos, Turner, Mandy E, Hopman, Wilma M, Holden, Rachel M, Adams, Michael A, White, Christine A, Rees-Milton, Karen, Neville, Kathryn, Taylor, Sarah M

2021

Overview

The 24 kD form of secreted phosphoprotein (SPP-24), a cytokine-binding bone matrix protein with various truncated C-terminal products, is primarily synthesized by the liver. SPP-24 shares homology with fetuin-A, a potent vascular and soft tissue calcification inhibitor and SPP-24 is one component of calciprotein particles (CPPs), a circulating fetuin–mineral complex. The limited molecular evidence to date suggests that SPP-24 may also function as an inhibitor of bone formation and ectopic vascular calcification, potentially through bone morphogenic protein 2 (BMP-2) and Wnt-signaling mediated actions. The C-terminal products of SPP-24 bind to BMP-2 and attenuate BMP-2-induced bone formation. The aim of this study was to assess circulating SPP-24 in relation to kidney function and in concert with markers of mineral metabolism in humans. SPP-24 was measured in the serum of total of 192 subjects using ELISA-based measurements. Subjects were participants of one of two cohorts: (1) mGFR Cohort (n = 80) was participants of a study of measured GFR (mGFR) using inulin urinary clearance, recruited mostly from a chronic kidney disease clinic with low-range kidney function (eGFR 38.7 ± 25.0 mL/min/1.73 m2) and (2) CaMOS Cohort (n = 112) was a subset of randomly selected, community-dwelling participants of year 10 of the Canadian Multicentre Osteoporosis Study with eGFR in the normal range of 75.0 ± 15.9 mL/min/1.73 m2. In the combined cohort, the mean SPP-24 was 167.7 ± 101.1 ng/mL (range 33.4–633.6 ng/mL). The mean age was 66.5 ± 11.3, 57.1% female and mean eGFR (CKD-EPI) was 59.9 ± 27.0 mL/min/1.73 m2 (range 8–122 mL/min/1.73 m2). There was a strong inverse correlation between SPP-24 and eGFR (R = − 0.58, p < 0.001) that remained after adjustment for age. Following adjustment for age, eGFR, and sex, SPP-24 was significantly associated with phosphate (R = − 0.199), PTH (R = 0.298), and the Wnt-signaling inhibitor Dickkopf-related protein 1 (R = − 0.156). The results of this study indicate that SPP-24 is significantly altered by kidney function and is the first human data linking levels of SPP-24 to other biomarkers involved in mineral metabolism. Whether there is a role for circulating SPP-24 in bone formation and ectopic mineralization requires further study.

• Publisher: Springer Nature B.V
• Subject: Age / Biomarkers / Bone growth / Bone matrix / Bone morphogenetic protein 2 / Bone morphogenetic proteins / Calcification / Calcification (ectopic) / Cytokines / Epidermal growth factor receptors / Homology / Inulin / Kidney diseases / Matrix protein / Metabolism / Mineralization / Osteogenesis / Osteoporosis / Parathyroid hormone / Proteins / Wnt protein