Abstract 5880: A comprehensive approach to targeting PI3K and Hippo pathways in sarcomas

Samuel Y. Yu; Keith Garcia; Ali Khan; Souradip Sinha; Colleen Fullenkamp; Munir R. Tanas

doi:10.1158/1538-7445.AM2026-5880

Sarcomas are heterogeneous cancers of mesenchymal origin with limited effective targeted therapies. One frequently dysregulated pathway in sarcomas is PI3K signaling, due to PTEN loss in 30–60% of patient samples. PI3K activation promotes tumor growth via the canonical Akt-mTORC1 axis and a parallel PI3K-TAZ/YAP-TEAD axis. TAZ/YAP are transcriptional co-activators that drive oncogenic gene expression and are regulated by the Hippo pathway consisting of the core kinases MST1/2 and LATS1/2. TAZ/YAP lack DNA binding domains and complex with TEAD transcription factors in the nucleus. The TAZ/YAP-TEAD axis is regulated by PI3K in a LATS1/2 dependent manner. In sarcomas, loss of Hippo kinases is seen in 30-50% of samples, leading to aberrant TAZ/YAP activation. Epigenetic silencing of MST1/2 and MAP4Ks via histone deacetylation may account for a significant subset of these cases. Preliminary data shows HDAC inhibition with Romidepsin restores MST1 expression and reduces TAZ/YAP transcriptional activity, suggesting a therapeutic opportunity. We propose dual inhibition of PI3K-Akt-mTORC1 signaling and TAZ/YAP transcriptional activity to reduce sarcoma proliferation and survival. We hypothesize that combination therapy will be more effective due to the convergence of these pathways on tumor growth.We used three cell lines: RD (embryonal rhabdomyosarcoma), A204 (malignant extrarenal rhabdoid tumor) and RH30 (alveolar rhabdomyosarcoma). Cells were treated with MK2206 (Akt inhibitor), Everolimus (mTORC1 inhibitor), Romidepsin (HDAC inhibitor) and VT-107 (TEAD inhibitor) alone or in combination. Proliferation was assessed via cell viability assays and clonogenic outgrowth was measured to evaluate cell survival. Western blotting was used to assess signal transduction.MK2206 reduced proliferation in A204/RH30 cells at concentrations greater than 1 μM, with corresponding decreases in phosphorylation of Akt-mTORC1 substrates. Clonogenic assays showed reduced colony formation in both lines. Combination treatment with MK2206, VT-107 and Everolimus further suppressed proliferation/clonogenic outgrowth, suggesting additive or synergistic effects. In RD cells, Romidepsin at 5nM significantly reduced proliferation/clonogenic outgrowth, consistent with increased MST1 protein levels and reduced TAZ/YAP activity.PI3K and Hippo pathways are frequently dysregulated in sarcomas and combination therapy targeting both can more effectively inhibit tumor growth. TEAD inhibition disrupts TAZ/YAP transcriptional activity downstream, while HDAC inhibition restores Hippo signaling and suppresses TAZ/YAP transcription, and PI3K pathway inhibitors reduce survival signaling. These findings provide rationale for further investigation into dual-pathway inhibition as a therapeutic strategy, including mechanistic studies and in vivo validation to define therapeutic windows and potential clinical applications.

Abstract 5880: A comprehensive approach to targeting PI3K and Hippo pathways in sarcomas

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