Short and sweet: evolution of a small glycopeptide from a bladder disorder to an anticancer lead

Antiproliferative factor (APF) binds specifically to sites within the cytoskeleton-associated protein 4 (CKAP4) extracellular domain

Background

Antiproliferative factor (APF) is a sialoglycopeptide elevated in the urine of patients with interstitial cystitis—a chronic, painful bladder disease. APF inhibits the proliferation of normal bladder epithelial cells and cancer cells in vitro, presumably by binding to its cellular receptor, cytoskeleton associated-protein 4 (CKAP4); however, the biophysical interaction of APF with CKAP4 has not been characterized previously. In this study, we used surface plasmon resonance (SPR) to explore the binding kinetics of the interaction of APF and as-APF (a desialylated APF analogue with full activity) to CKAP4.

Results

We immobilized non-glycosylated APF (TVPAAVVVA) to the Fc1 channel as the control and as-APF to Fc2 channel as the ligand in order to measure the binding of CKAP4 recombinant proteins encompassing only the extracellular domain (Aa 127–602) or the extracellular domain plus the transmembrane domain (Aa 106–602). Positive binding was detected to both CKAP4_126–602 and CKAP4_106–602, suggesting that as-APF can bind specifically to CKAP4 and that the potential binding site(s) are located within the extracellular domain. To identify the primary APF binding site(s) within the CKAP4 extracellular domain, deletion mutants were designed according to structural predictions, and the purified recombinant proteins were immobilized on a CM5 chip through amine-coupling to measure as-APF binding activity. Importantly, both CKAP4_127–360 and CKAP4_361–524 exhibited a fast association rate (k_on) and a slow dissociation rate (k_off), thus generating high binding affinity and suggesting that both regions contribute relatively equally to overall as-APF binding. Therefore, two or more as-APF binding sites may exist within the Aa 127–524 region of the CKAP4 extracellular domain.

Conclusions

We determined that the CKAP4_127–360 and CKAP4_361–524 mutants exhibit improved binding activity to as-APF as compared to the full-length extracellular domain, making it possible to detect low concentrations of as-APF in urine, thereby establishing a foundation for a non-invasive diagnostic assay for IC. Further, these data have revealed novel APF binding site(s) suggesting that targeting this region of CKAP4 to inhibit APF binding may be a useful strategy for treating IC-related bladder pathology.

Antiproliferative factor-induced changes in phosphorylation and palmitoylation of cytoskeleton-associated protein-4 regulate its nuclear translocation and DNA binding

Cytoskeleton-associated protein 4 (CKAP4) is a reversibly palmitoylated and phosphorylated transmembrane protein that functions as a high-affinity receptor for antiproliferative factor (APF)—a sialoglycopeptide secreted from bladder epithelial cells of patients with interstitial cystitis (IC). Palmitoylation of CKAP4 by the palmitoyl acyltransferase, DHHC2, is required for its cell surface localization and subsequent APF signal transduction; however, the mechanism for APF signal transduction by CKAP4 is unknown. In this paper, we demonstrate that APF treatment induces serine phosphorylation of residues S3, S17, and S19 of CKAP4 and nuclear translocation of CKAP4. Additionally, we demonstrate that CKAP4 binds gDNA in a phosphorylation-dependent manner in response to APF treatment, and that a phosphomimicking, constitutively nonpalmitoylated form of CKAP4 localizes to the nucleus, binds DNA, and mimics the inhibitory effects of APF on cellular proliferation. These results reveal a novel role for CKAP4 as a downstream effecter for APF signal transduction.