Abstract 7253: Regulation of TbRIII expression and ectodomain shedding via the sheddase, PRSS8, and ERG transcription factor

Abstract TGF-β signaling is frequently dysregulated in cancer and can contribute to cancer-associated phenotypes including immune evasion, migration, and metastasis. Unfortunately, attempts to inhibit TGF-β have had limited success, likely due to the critical roles of TGF-β in normal tissue. For this reason, this work aims to more fully understand the regulation of TGF-β signaling. The co-receptor, TβRIII, can bind ligand and stimulate the TβRI/TβRII complex. Conversely, TβRIII can be shed from the cell surface where it can sequester TGF-β ligand and prevent TβRI/TβRII activation. However, the identity of the enzyme that sheds TβRIII and the regulatory mechanisms that govern TβRIII shedding are unknown. This work aims to address two aspects of TβRIII shedding regulation: firstly, what enzyme is responsible for producing shed TβRIII, and secondly, what novel transcriptional networks are responsible for regulating the expression of TβRIII and its sheddase. PRSS8 increased TβRIII shedding, decreased expression of TGF-β target genes and EMT markers, and decreased TGF-β associated phenotypes of EMT, migration, and invasion. PRSS8 overexpression also sensitized cells to chemotherapy. Furthermore, PRSS8 expression could be induced pharmacologically with lovastatin to elicit similar reductions in TGF-β associated phenotypes. Treatment with cholesterol was able to increase migration of these cells, but PRSS8 overexpression prevented this effect of cholesterol. These data suggest cholesterol homeostasis pathways may regulate PRSS8 expression and therefore alter TGF-β signaling activity. Taken together, this establishes a novel cholesterol-PRSS8-TGF-β axis of regulation. Current work aims to identify novel transcriptional networks that are regulating TβRIII and its shedding. RNA-seq data between prostate cell lines with and without ERG overexpression has revealed ERG increased expression of pro-TGF-β signaling genes and suppressed genes associated with down-regulating TGF-β. In cervical cancer cell lines, knockdown of ERG has reduced phenotypes such as migration, invasion, and chemoresistance. ERG knockdown has also increased TβRIII shedding. ERG ChIP-seq in cervical cancer is being performed in the presence and absence of TGF-β activation to determine the effect of TGF-β on ERG binding. SMAD2/3 ChIP-seq will be performed with and without ERG knockdown to reveal if ERG affects SMAD2/3 binding to the genome. By discovering new transcriptional regulators of TβRIII, such as ERG, new possibilities for rescuing TβRIII expression and reducing dysregulated TGF-β signaling are revealed. Therapies directed at cholesterol homeostasis or ERG may be able to be repurposed and applied to cancers experiencing TGF-β dysregulation. Citation Format: Benjamin Michael Greulich, Kaitlyn Eidson, Logan Baker, Hannah Fitzgibbons, Shreya Sudakar, Emma Teng, Cion Kim, Huy Lam. Regulation of TbRIII expression and ectodomain shedding via the sheddase, PRSS8, and ERG transcription factor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7253.