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Targeted protein degradation reveals BET bromodomains as the cellular target of Hedgehog Pathway Inhibitor-1

ABSTRACT: Target deconvolution of small molecule hits from phenotypic screens presents a major challenge. Illustrative of these are the many screens that have been conducted to find inhibitors for the Hedgehog (Hh) signaling pathway – a major developmental pathway with many implications in health and disease - with many hits but very few identified cellular targets. We here present a strategy for target identification based on Proteolysis-Targeting Chimeras (PROTACs), combined with label-free quantitative proteomics. We developed a PROTAC based on the downstream Hedgehog Pathway Inhibitor-1 (HPI-1), a phenotypic screen hit with unknown cellular target. Using our Hedgehog Pathway PROTAC (HPP) we identified and validated BET bromodomains to be the cellular targets of HPI-1. Furthermore, we found that HPP-9 has a unique mechanism of action as a long-acting Hh pathway inhibitor through prolonged BET bromodomain degradation. Collectively, we provide a powerful PROTAC-based approach for target deconvolution, that has answered the longstanding question of the cellular target of HPI-1 and yielded the first PROTAC that acts on the Hh pathway.

ORGANISM(S): Mus musculus

PROVIDER: GSE228015 | GEO | 2023/06/05

REPOSITORIES: GEO

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Project description:Misactivation of the Hedgehog (Hh) pathway can cause cancers such as medulloblastomas, the most common malignant brain tumor in children, and basal cell carcinomas, the most common cancer in the United States. Hedgehog signals are transmitted through primary cilia, where Hedgehog ligands bind to Patched1 and activate Smoothened through interactions with cilia-associated sterol lipids. The gene expression programs driving cellular responses to ciliary Hh signals are incompletely understood. Thus, to define Hh target genes, we performed RNA sequencing of cells after treatment with Hh ligands (Shh, Dhh, Ihh), cilia-associated lipids (7b,27-dihydroxycholesterol, 24(S),25-epoxycholesterol), or synthetic lipids or small molecules that activate Smoothened (20(S)-hydroxycholesterol, SAG). Treatment with Hh pathway agonists identified a core gene expression program comprised of 155 genes driving lipid synthesis, metabolism, signaling, adhesion, or angiogenesis. These datasets were integrated with RNA sequencing of Hh-human medulloblastomas (n=?), a Math1-Cre SmoM2 mouse genetic model of Hh-associated medulloblastoma (n=?), and human basal cell carcinomas (n=10) to ascertain how malignant Hh signaling differs from canonical Hh signaling. We discover a conserved response to ciliary Hh signals in human or mouse medulloblastomas, including known target genes such as Gli1 or Ptch1, and novel target genes such as Hsd11b1 or Retnla. Importantly, mechanistic studies reveal Hsd11b1 to be a putative negative regulator of Hh signaling that is dysregulated in malignancies. We further demonstrate Retnla to be a positive regulator of Hh signaling that drives expression of Hsd11b2, a druggable dependency underlying Hedgehog-associated medulloblastoma. Orthotopic implantation of neuroepithelial stem cells that overexpress either Hsd11b1 and Retnla demonstrate that tumors derived Hsd11ß1 overexpression are more primitive and less aggressive whereas Retnla overexpression forms tumors that are more differentiated and behave more aggressively. In sum, we illuminate the first comprehensive transcriptome of Hh signaling and highlight the intricate interplay between Hh signaling and lipid metabolism that Hh-dependent malignancies dysregulate to drive tumor progression.

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Targeted protein degradation reveals BET bromodomains as the cellular target of Hedgehog Pathway Inhibitor-1

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