Carboxy terminus of secreted phosphoprotein‐24 kDa (spp24) is essential for full inhibition of BMP‐2 activity

Abstract

Secreted phosphoprotein-24 kDa (spp24) is a bone morphogenetic protein (BMP)-binding protein isolated from bone. It exists in a number of size forms and is hypothesized to function as a BMP latency protein and/or a ''slow release'' mechanism for BMPs involved in bone turnover and repair. We have examined the hypothesis that proteolytic modification of the C-terminus of spp24 affects its BMP-2-binding properties and bioactivity in the BMP-2-stimulated ectopic bone forming bioassay. Three different size forms of recombinant spp24 that correspond to predicted 18.1 kDa, 16.0 kDa, and 14.5 kDa proteolytic products were compared to full-length (fl) spp24. One of these forms (spp18.1) we hypothesize to be the protein which Urist initially, but apparently inaccurately, called ''BMP.'' Only full-length spp24 completely inhibited BMP-2-induced bone formation. The 18.1 kDa truncated isoform of spp24 which we hypothesize to be Urist's protein did not. The inhibitory capacity of the proteins was correlated with their kinetic constants, assessed by surface plasmon resonance. At the highest, inhibitory, dose of spp24 and its derivatives, k d (''stability'') best predicted the extent of ectopic bone formation whereas at the lowest dose, which was not inhibitory, k a (''recognition'') best predicted the extent of ectopic bone formation. We conclude that proteolytic processing of spp24 affects the interaction of this protein with BMP-2 and this affects the function of the protein. ß

Secreted phosphoprotein 24 kD (Spp24) inhibits growth of human pancreatic cancer cells caused by BMP-2

“…We have engineered a number of protein and peptide products that, while they are thought to function by a single mechanism (BMP binding), operationally they have seemingly paradoxically opposite outcomes along a spectrum from enhancement of BMP activity ("slow release") to inhibition of BMP activity ("sequestration") [9]. Among these peptides, full length Spp24, when applied to a collagen sponge to which BMP-2 was subsequently applied, dramatically inhibited the activity of BMP-2 in models of ectopic bone formation [29] and spinal fusion [9].…”

High-resolution mapping of a novel rat blood pressure locus on chromosome 9 to a region containing the Spp2 gene and colocalization of a QTL for bone mass

“…Spp2 is a bone matrix protein that is produced primarily in the liver and delivered to other tissues for function, especially the bone (1,11,24). The function of this protein in the normal bone environment is not clearly delineated.…”

“…The candidate gene, Secreted phosphoprotein-2 (Spp2), also known as secreted phosphoprotein-24 kDa (Spp24), has not been studied in the context of cardiovascular physiology but has been studied in the context of skeletal physiology (9) and in normal bone growth and turnover (11,50,51). Spp2 is a bone matrix protein that is produced primarily in the liver and delivered to other tissues for function, especially the bone (1,11,24).…”

Secreted Phosphoprotein 24 kD Inhibits Growth of Human Prostate Cancer Cells Stimulated by BMP-2

“…Spp24 can bind with BMP2 and inhibit its osteogenic activity (28,29). Several studies have indicated that spp24 could inhibit cancer cell proliferation and tumor growth (22,23).…”