The Dickkopf1-cytoskeleton-associated protein 4 axis creates a novel signalling pathway and may represent a molecular target for cancer therapy

Abstract

Dickkopf 1 (DKK1) is a secreted protein and antagonizes oncogenic Wnt signalling by binding to the Wnt co-receptor, low-density lipoprotein receptor-related protein 6. DKK1 has also been suggested to regulate its own signalling, associated with tumour aggressiveness. However, the underlying mechanism by which DKK1 promotes cancer cell proliferation has remained to be clarified for a long time. The cytoskeleton-associated protein 4 (CKAP4), originally identified as an endoplasmic reticulum membrane protein, was recently found to act as a novel DKK1 receptor. DKK1 stimulates cancer cell proliferation when CKAP4 is expressed on the cell surface membrane. Although there are no tyrosine residues in the intracellular region of CKAP4, CKAP4 forms a complex with PI3K upon the binding of DKK1, leading to the activation of Akt. Both DKK1 and CKAP4 are frequently expressed in pancreatic and lung tumours, and their simultaneous expression is negatively correlated with prognosis. Knockdown of CKAP4 in cancer cells and treatment of mice with the anti-CKAP4 antibody inhibit Akt activity in cancer cells and suppress xenograft tumour formation, suggesting that CKAP4 may represent a therapeutic target for cancers expressing both DKK1 and CKAP4. This review will provide details of the novel DKK1-CKAP4 signalling axis that promotes cancer proliferation and discuss the possibility of targeting this pathway in future cancer drug development.

Linked articles: This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc.

Figure 1

‘Two endocytic routes and two outputs model’ in the Wnt signalling pathways. See details in the text. AP‐2, adaptor protein complex 2; Cav‐1, caveolin‐1; Fz, frizzled; Ror2, receptor tyrosine kinase‐like orphan receptor 2; Dvl, dishevelled; GSK‐3, glycogen synthase kinase 3.

Figure 2

Polarized trafficking of Wnt proteins and receptors. Wnt3a, Wnt5a and Wnt5b are secreted basolaterally, and Wnt11 is secreted apically. Wnt1 is secreted in both ways. Wnt receptor frizzled2 is localized to the basolateral membrane, and Frizzled7 is localized to both apical and basolateral membranes. Wnt co‐receptors are localized to specific membrane domains (e.g. basolateral membrane, LRP6 and Ror2; apical membrane, Ryk). DKK1 is secreted apically. Fz, Frizzled; Ryk, receptor‐like tyrosine kinase.

Figure 3

Structure and functions of CKAP4 in the cell surface membrane. (A) Schematic diagram of CKAP4. Amino acid residues in the CKAP4 intracellular region are enlarged, showing no tyrosine residues. The proline‐rich motif that interacts with the p85 subunit of PI3K is underlined. Cys, cysteine; TM, transmembrane. (B) Possible functions of CKAP4 in the cell surface membrane. Left panel, tPA binds to CKAP4 and regulates plasmin production and progression of fibrinolysis in vascular smooth muscle cells. Middle panel, SP‐A binds to CKAP4 and induces CKAP4 internalization and regulates secretion of lamellar body. Right panel, APF binds to CKAP4 and inhibits cell proliferation through the suppression of Akt in bladder epithelial cells.

Figure 4

The DKK1 and CKAP4 signalling axis. DKK1 binds to the extracellular region of CKAP4. CRD‐1 of DKK1 and LZD of CKAP4 are required for their interaction. PRD of CKAP4 binds to the SH3 domain of the p85 subunit upon DKK1 binding to CKAP4, resulting in the activation of the PI3K and Akt signalling pathway and the promotion of cell proliferation. CRD, cysteine‐rich domain; LZD, leucine‐rich domain; PRD, proline‐rich domain.

Figure 5

Simultaneous expression of DKK1 and CKAP4 in cancers and inhibition of cancer cell proliferation by anti‐CKAP4 antibody. (A) In lung adenocarcinoma and pancreatic cancer tissues, DKK1 and CKAP4 are expressed in tumour lesions and minimally detected in non‐tumour regions. Scale bars, 100 μm in lung adenocarcinoma and 50 μm in pancreatic cancer. (B) In the xenograft tumour model using nude mice subcutaneously implanted with pancreatic cancer cells (S2‐CP8 cells, left panels) and lung cancer cells (A549 cells, right panels), anti‐CKAP4 antibody suppressed tumour formation. Anti‐Glutathione S‐transferase (GST) antibody was used as a control. Representative appearance of one mouse (left pictures) and extirpated xenograft tumours (right pictures) are shown. Scale bars, 10 mm (Kimura et al., 2016).

Figure 6

The DKK1–CKAP4 axis as a novel molecular target for cancer therapy and future perspectives. Upon binding to CKAP4, DKK1 promotes cancer cell proliferation through the activation of PI3K and Akt. Anti‐CKAP4 antibody inhibits the binding of DKK1 to CKAP4 and xenograft tumour formation. There are questions to be addressed. (1) ‘Trafficking routes’: it is unclear how subcellular localization of CKAP4 is regulated. Although CKAP4 functions as a receptor, CKAP4 is mainly localized to the ER. Post‐translational modification, such as palmitoylation at Cys100 by DHHC2, could be involved in the subcellular localization of CKAP4. (2) ‘Other Dkk family members’: it is unknown whether other DKK family members also act as a ligand for CKAP4. Since CRD‐1 is conserved among the four DKK genes, DKK2, 3 and 4 would interact with CKAP4. (3) ‘Ligand specificity’: Although several ligands for CKAP4 other than DKK1 have been identified, the specificity of the binding between CKAP4 and ligands has not been addressed. See details in the text.