Reciprocal Regulation of Hippo and WBP2 Signalling-Implications in Cancer Therapy

Abstract

Cancer is a global health problem. The delineation of molecular mechanisms pertinent to cancer initiation and development has spurred cancer therapy in the form of precision medicine. The Hippo signalling pathway is a tumour suppressor pathway implicated in a multitude of cancers. Elucidation of the Hippo pathway has revealed an increasing number of regulators that are implicated, some being potential therapeutic targets for cancer interventions. WW domain-binding protein 2 (WBP2) is an oncogenic transcriptional co-factor that interacts, amongst others, with two other transcriptional co-activators, YAP and TAZ, in the Hippo pathway. WBP2 was recently discovered to modulate the upstream Hippo signalling components by associating with LATS2 and WWC3. Exacerbating the complexity of the WBP2/Hippo network, WBP2 itself is reciprocally regulated by Hippo-mediated microRNA biogenesis, contributing to a positive feedback loop that further drives carcinogenesis. Here, we summarise the biological mechanisms of WBP2/Hippo reciprocal regulation and propose therapeutic strategies to overcome Hippo defects in cancers through targeting WBP2.

Keywords: WW-domain binding protein 2; cancer; hippo pathway; oncogene; reciprocal regulation; therapeutics.

Figure 1

Schematic diagram of the Hippo signalling pathway. The canonical core kinase cassette is composed of the kinases MST1/2 and LATS1/2, the adaptor proteins MOB1 and SAV and the transcriptional co-factors YAP and TAZ. MST1/2 phosphorylates LATS1/2, that in turn phosphorylates and inactivates YAP. Phosphorylated YAP is sequestered by the 14-3-3 protein in the cytoplasm and degraded via proteasome facilitated by the SCF^βTRCP E3 ligase. Other non-canonical regulators have been discovered to modulate the Hippo pathway core. The KIBRA–NF2–FERM complex and TOAK has been identified to play an activating role on MST1/2. WWC is an adaptor protein that binds and activates LATS while MAP4K may phosphorylate LATS directly. ITCH is a E3 ligase that promotes ubiquitination and proteasomal degradation of the LATS. AMOT directly binds to YAP/TAZ to promote YAP/TAZ cytoplasmic sequestration and phosphorylation. Integrin signalling is another non-canonical pathway that activates YAP through FAK/RhoA and VASP. Additionally, VASP can stabilise YAP/TAZ by inducing its dephosphorylation. YAP/TAZ associates with transcription factors such as TEAD and SMAD in the nucleus where they initiate gene transcription. Activity of the YAP/TAZ–TEAD complex in the nucleus may be inhibited by its binding with RUNX3. Created with BioRender.com.

Figure 2

Aberration of the core Hippo pathway components in cancer. (A) Aberrant expression of Hippo components in cancer. Tumour suppressor such as MST/LATS were found to be downregulated at the mRNA and protein level in various cancers, while oncogenic YAP/TAZ were found to be upregulated with tumour progression. (B) Genetic aberrations of the Hippo pathway in cancer. Examples of genetic aberrations: gene amplification, gene fusion, gene deletion and promoter methylation. Created with BioRender.com.

Figure 3

Structure and implications of WBP2 in various cancer types. (A) Schematic diagram of the WBP2 domain structure. WBP2 is comprised of the GRAM domain at the N terminus (1–84 amino acid (a.a)) and the proline-rich region at the C terminus (146–261 a.a). There are three PPxY motifs namely PY1 (167–170 a.a), PY2 (197–200 a.a) and PY3 (249–252 a.a) that have been identified as being involved in protein–protein interactions. Two phospho-tyrosine sites at Y192 and Y231 are found in the proline-rich region of WBP2. Phosphorylation of WBP2 is important for its nuclear translocation. (B) Implications of WBP2 in various cancer types based on the current literature. Created with BioRender.com.

Figure 4

WBP2 copy number alteration frequency in 32 cancer types from the TCGA Pan-cancer Atlas. The x-axis shows the frequency of WBP2 alteration and the y-axis shows the tumour types found in the TCGA Pan-Cancer Atlas [75]. Each dot represents the frequency of the indicated types of WBP2 alteration; a red circled dot represents amplification, a blue squared dot represents deletion and a green triangle dot represents mutation. Data were downloaded from cBioportal [76,77] and created using GraphPad Prism.

Figure 5

Schematic timeline of WBP2 implication in the Hippo pathway. WBP2 was first discovered to interact with YAP in 1995. Later in 2011, it was shown to promote growth in Drosophila by associating with Yorkie. In the same year, its oncogenic role affecting the Hippo pathway was first implicated in breast cancer through its association with TAZ. The oncogenic role of WBP2 in directing Hippo dysregulation was later extended to other cancer types including cutaneous squamous cell carcinoma (cSCC), non-small cell lung carcinoma (NSCLC) and gastric cancer. The Hippo pathway regulating WBP2 levels was identified to be through the Dicer–miRNA axis by MST. The text coloured in blue describes the transcriptional co-activator role of WBP2 in the Hippo pathway through binding to YAP and TAZ. The text coloured in red describes a non-transcriptional co-activator mechanism of WBP2 in the Hippo pathway. The arrow indicates the direction of regulation (i.e., WBP2 on Hippo/Hippo on WBP2). Created with BioRender.com.

Figure 6

Regulation of the Hippo signalling pathway by WBP2. WBP2 interacts with LATS2 to inhibit the LATS2-mediated phosphorylation of YAP. WBP2 binds competitively with WWC3 to displace LATS1, hence limiting LATS1 activation. WBP2 translocates into the nucleus and interacts with YAP/TAZ to activate target gene transcription. Created with BioRender.com.

Figure 7

Regulation of WBP2 by the Hippo signalling pathway. (A) Overview of the modes of action of how WBP2 can be regulated by Hippo pathway in cancer. In the nucleus, WBP2 negatively regulates miRNA biogenesis through its interaction with MPC components, hence disrupting the MPC complex and its miRNA processing capacity. On the other hand, the nuclear accumulation of YAP and the activation of MST maintains the high expression of Dicer. Pre-miRNAs are processed by Dicer in the cytosol to mature miRNAs in turns target WBP2. Some miRNAs known to target WBP2 include miR-19a/b, miR-23a, miR-27a, miR-206, miR-485 and miR-613. WBP2 level is also regulated by proteasomal degradation where the interaction of WBP2 with ITCH via the WW-PPxY domain facilitates the ubiquitin-mediated proteasomal degradation of WBP2. ITCH E3 ubiquitin ligase promotes the ubiquitin-mediated proteasomal degradation of LATS. (B) A positive feedback loop in the regulation of WBP2 and the Hippo signalling pathway drives cancer progression. Created with BioRender.com.

Figure 8

Summary of WBP2 translational implications in Hippo-dysregulated cancers. (A) Targeting WBP2 expression could be carried out at the transcriptional, post-transcriptional and post-translation levels. (B) Targeting WBP2 interacting partners could be facilitated by the design of small molecules that target the PPxY motifs in the C terminus of WBP2. This is expected to block the interaction of WBP2 with its interacting partners in the Hippo pathway to subvert WBP2-driven cancer progression. (C) Combinatorial therapy involving potential WBP2 inhibitors and other therapeutics to improve treatment efficacy and reduce possible side effects. Created with BioRender.com.