The WNT/ROR Pathway in Cancer: From Signaling to Therapeutic Intervention

Abstract

The WNT pathway is one of the major signaling cascades frequently deregulated in human cancer. While research had initially focused on signal transduction centered on β-catenin as a key effector activating a pro-tumorigenic transcriptional response, nowadays it is known that WNT ligands can also induce a multitude of β-catenin-independent cellular pathways. Traditionally, these comprise WNT/planar cell polarity (PCP) and WNT/Ca²⁺ signaling. In addition, signaling via the receptor tyrosine kinase-like orphan receptors (RORs) has gained increasing attention in cancer research due to their overexpression in a multitude of tumor entities. Active WNT/ROR signaling has been linked to processes driving tumor development and progression, such as cell proliferation, survival, invasion, or therapy resistance. In adult tissue, the RORs are largely absent, which has spiked the interest in them for targeted cancer therapy. Promising results in preclinical and initial clinical studies are beginning to unravel the great potential of such treatment approaches. In this review, we summarize seminal findings on the structure and expression of the RORs in cancer, their downstream signaling, and its output in regard to tumor cell function. Furthermore, we present the current clinical anti-ROR treatment strategies and discuss the state-of-the-art, as well as the challenges of the different approaches.

Keywords: CAR T cells; ROR1; ROR2; WNT signaling; cancer; immunotherapy; non-canonical; targeted therapy.

Figure 1

The structure of non-canonical WNT co-receptors. Comparison of the schematic structures of potential non-canonical WNT co-receptors. ROR = receptor tyrosine kinase-like orphan receptor, RYK = receptor tyrosine kinase, PTK7 = protein tyrosine kinase 7, SDC = syndecan, GPC = glypican, MUSK = muscle-specific kinase, TKD = tyrosine kinase domain, KRD = kringle domain, CRD = cysteine-rich domain.

Figure 2

ROR1 signaling. WNT/ROR1 signaling is induced by binding of a non-canonical WNT ligand, which triggers the formation of a complex between ROR1 and ROR2, or ROR1 and a FZD receptor, respectively. Signal transduction is mediated by the phosphorylation of ROR1 by several kinases (orange) which on the one hand results in the inhibition of anti-apoptotic pathways (grey), while on the other hand activates downstream pathways such as WNT/PCP (magenta), MAPK/ERK (dark brown), PI3K/AKT (green) or NF-κB (light brown). These either trigger cytoskeletal rearrangements associated with enhanced tumor cell migration, or induce a transcriptional response (blue) leading to the expression of genes which promote cell proliferation, survival, EMT, or therapy resistance.

Figure 3

ROR2 signaling. Binding of a non-canonical WNT ligand induces the association of ROR2 with a FZD receptor (e.g., FZD2, 5, 7), or other WNT co-receptors (e.g., PTK7, ROR1). ROR2 activation is triggered by its interplay with several kinases (orange). The signal is then relayed to downstream pathways including WNT/PCP (magenta), PI3K/AKT (green), or MAPK/ERK (brown). Active WNT/ROR2 signaling results in the nuclear translocation and activation of transcription factors (blue) that mediate the expression of target genes associated with cell proliferation, survival, or EMT.

Figure 4

Targeting ROR1 with small molecules and immunotherapy—a selection of agents. While KAN0441571C functions as a tyrosine kinase inhibitor and already meets clinical evaluation, the inhibitor ARI-1, binding to the CRD of ROR1, is still in preclinical development. The most advanced monoclonal antibody (mAb) targeting ROR1 is cirmtuzumab. Other antibody-based targeting options comprise VLS-101, an antibody drug conjugate linking cirmtuzumab to MMAE, or bispecific antibodies such as ROR1 × CD3 or NBE-002. ROR1 is also an attractive target for chimeric antigen receptor-engineered (CAR) T cells since it does not lead to B cell depletion. Strictinin is a newly discovered, plant-derived tannin, which seemingly interacts with ROR1 and inhibits PI3K/AKT/GSK-3 activity.