Research Overview
Understanding Cancer
— Why Genetic Alterations and Signaling Pathways Matter —
Inside cells, numerous signaling pathways operate in concert. In healthy cells, receptors on the cell surface receive survival signals—such as ligands—from the extracellular environment. This activates intracellular molecules through phosphorylation, which in turn phosphorylate and activate downstream targets. These sequential events ultimately promote the transcription of genes required for cell proliferation and survival, thereby enhancing cell growth.
In cancer, however, various signaling pathways are either abnormally activated or inactivated, leading to uncontrolled cell proliferation. At the root of these disruptions are genetic alterations. Mutations, gene amplifications, deletions, and gene fusions can activate oncogenes—genes that promote cell growth—or inactivate tumor suppressor genes, which normally function to prevent excessive proliferation. As a result, signaling cascades downstream of these genes are dysregulated.
Among these alterations, some have little biological consequence (known as passenger mutations), while others play a critical role in tumor growth (known as driver mutations). The concept behind molecular targeted therapy is to inhibit these driver mutations or their downstream signaling pathways, ideally damaging cancer cells while sparing normal ones. Today, molecular targeted drugs have become an essential component of cancer treatment.
For example, immune checkpoint inhibitors, a type of targeted therapy, have emerged in recent years. These agents block mechanisms by which cancer cells suppress immune responses. One such mechanism involves PD-L1 on cancer cells binding to PD-1 on T cells, thereby inactivating them. Antibodies like Nivolumab and Pembrolizumab block this interaction, reactivating T cells to attack tumors. In oral and head and neck squamous cell carcinoma, Cetuximab, an anti-EGFR antibody, is approved for use, targeting the often amplified and overexpressed EGFR gene.
In 2019, Japan launched a nationwide cancer genome medicine initiative. Using next-generation sequencing (NGS), clinicians can identify genetic alterations in a patient’s tumor and choose targeted therapies that inhibit driver mutations or downstream signaling pathways. Traditionally, drug approval was organ-specific—limited to the tissue where the cancer originated—but genome medicine is changing this paradigm. By focusing on the genetic profile rather than the tumor’s anatomical site, we are moving closer to precision medicine.
However, a significant challenge remains: even when a genetic alteration is identified, if the downstream signaling pathways are poorly understood or no targeted therapies exist, we cannot translate these findings into effective treatments.
This is why it is critically important to investigate which genetic alterations regulate which signaling pathways. Clarifying this relationship will not only enhance our understanding of cancer biology but also accelerate the development of future targeted therapies—bringing us closer to improved treatments for patients.
A Critical Signaling Pathway in Cancer Cells
— The Hippo Pathway and YAP/TAZ —
The Hippo pathway plays a central role in regulating cell proliferation and the development of tissues and organs. It consists primarily of the serine/threonine kinases MST1/2 (mammalian STE20-like kinase 1 and 2) and LATS1/2 (large tumor suppressor 1 and 2), along with their respective adaptor proteins, SAV1 (Salvador homolog 1) and MOB1A/B (MOB kinase activators 1A and 1B).
Downstream of this pathway lie the key effectors YAP (Yes-associated protein) and TAZ (transcriptional co-activator with PDZ binding motif). When the Hippo pathway is activated, LATS1/2 phosphorylate YAP/TAZ at five serine residues, resulting in their sequestration in the cytoplasm or degradation via the ubiquitin-proteasome system. Conversely, when the Hippo pathway is inactivated, dephosphorylated YAP/TAZ translocate into the nucleus, where they bind to TEAD (TEA domain family) transcription factors and act as co-activators to promote the expression of proliferation-related genes such as CTGF (connective tissue growth factor) and CYR61 (cysteine-rich angiogenic inducer 61).
YAP/TAZ activity is regulated by physiological factors such as cell density, mechanical stress, and serum availability. In addition, upstream regulators include GPCRs (G protein-coupled receptors) and receptor tyrosine kinases, which modulate the Hippo pathway under various conditions.
Significance of the Hippo Pathway in Oral Squamous Cell Carcinoma
We have demonstrated that various genetic alterations in oral squamous cell carcinoma (OSCC) lead to inactivation of the Hippo pathway, resulting in aberrant activation of YAP/TAZ, key transcriptional co-activators involved in cell proliferation.
We previously showed that Tissue Inhibitor of Metalloproteinases-1 (TIMP-1) is overexpressed in OSCC and promotes YAP/TAZ activation through interactions with CD63 and integrins, thereby enhancing tumor cell proliferation (Ando T et al., Oncogene, 2017).
Epidermal Growth Factor Receptor (EGFR) plays a central role in cell proliferation through ligand-induced dimerization and autophosphorylation, triggering downstream signaling cascades. In OSCC, EGFR is frequently amplified and overexpressed. We demonstrated that EGFR phosphorylates three tyrosine residues on MOB1, a key component of the Hippo pathway, leading to inactivation of LATS1/2 and subsequent activation of YAP/TAZ, promoting tumor growth (Ando T et al., Commun Biol, 2021).
Although EGFR inhibitors have been approved as molecular targeted therapies for OSCC, their clinical efficacy is limited, and resistance remains a major issue. Previous studies have suggested that reactivation of YAP and upregulation of other receptor tyrosine kinases (RTKs) may contribute to this resistance. We hypothesized that a specific RTK may drive YAP reactivation after transient inhibition by EGFR inhibitors. Through our investigation, we identified AXL as a novel RTK that activates YAP via the EGFR–LATS1/2 axis. Notably, the combined use of EGFR and AXL inhibitors led to sustained inactivation of YAP and marked suppression of tumor growth, underscoring the therapeutic potential of dual inhibition or direct YAP targeting in resistant OSCC (Okamoto K et al., Oncogene, 2023).
In recent years, nuclear interactions of YAP/TAZ have garnered increasing attention, but their OSCC-specific binding partners remained unclear. We identified RBM39, an RNA-binding protein, as a YAP interactor that enhances YAP/TEAD transcriptional activity. Interestingly, indisulam, a compound known to degrade RBM39, induces both transcriptional repression and aberrant splicing, leading to cell death. However, when YAP binds to RBM39, it delays RBM39 degradation and mitigates both transcriptional repression and splicing disruption, conferring resistance to indisulam. We are currently investigating the structural basis of the YAP–RBM39 interaction with the goal of developing novel therapeutics that disrupt this oncogenic axis (Ando T et al., Oncogenesis, 2024).
To sustain growth, cancer cells must evade immune surveillance. Tumors with a high tumor mutational burden (TMB) tend to produce neoantigens, attracting cytotoxic T cells. However, cancer cells can evade immune attack by expressing PD-L1, which binds to PD-1 on T cells and suppresses their activity. The link between the Hippo pathway and immune evasion, however, has been unclear. Our recent findings suggest that patients with genetic alterations in Hippo pathway components may respond better to immune checkpoint inhibitors such as anti–PD-1 antibodies. Activation of YAP/TAZ promotes genomic instability, leading to increased TMB and T-cell recruitment. At the same time, YAP/TAZ also upregulate PD-L2 transcription, contributing to immune evasion. These opposing roles may explain the enhanced sensitivity to PD-1 blockade therapy. We are now expanding our investigation into how Hippo-YAP/TAZ signaling shapes the tumor immune microenvironment (Ando T et al., under review, 2025).
In summary, although many genetic alterations that inactivate the Hippo pathway have been identified in OSCC, further research is needed to fully elucidate the mechanisms by which this pathway drives tumor progression and therapeutic resistance.
Targeting the Hippo–YAP/TAZ Pathway for Diagnosis and Therapy in Oral Cancer
The detailed mechanisms by which specific genetic alterations lead to inactivation of the Hippo pathway remain largely unclear.
As a result, cancer genome profiling may reveal genetic alterations that potentially affect the Hippo–YAP/TAZ signaling axis, yet their functional consequences are still unknown—making it difficult to translate such findings into therapeutic applications. Even when known alterations that inactivate the Hippo pathway are identified, there are currently no approved molecular targeted therapies that directly inhibit the Hippo–YAP/TAZ pathway.
One promising therapeutic candidate is TEAD inhibitors, which disrupt the interaction between YAP and TEAD by binding to a defined pocket within TEAD, thereby blocking transcriptional activation of YAP target genes.
Our ongoing research aims to uncover the molecular links between genetic alterations and Hippo–YAP/TAZ pathway dysregulation, while also working toward the development of next-generation diagnostics and targeted therapies that specifically address this critical oncogenic pathway.