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

TGF-beta signaling is frequently dysregulated in cancer and can contribute to cancer-associated phenotypes including immune evasion, migration, and metastasis. Unfortunately, attempts to inhibit TGF-beta have had limited success, likely due to the critical roles of TGF-beta in normal tissue. For this reason, this work aims to more fully understand the regulation of TGF-beta signaling. The co-receptor, TbetaRIII, can bind ligand and stimulate the TbetaRI/TbetaRII complex. Conversely, TbetaRIII can be shed from the cell surface where it can sequester TGF-beta ligand and prevent TbetaRI/TbetaRII activation. However, the identity of the enzyme that sheds TbetaRIII and the regulatory mechanisms that govern TbetaRIII shedding are unknown. This work aims to address two aspects of TbetaRIII shedding regulation: firstly, what enzyme is responsible for producing shed TbetaRIII, and secondly, what novel transcriptional networks are responsible for regulating the expression of TbetaRIII and its sheddase. PRSS8 increased TbetaRIII shedding, decreased expression of TGF-beta target genes and EMT markers, and decreased TGF-beta 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-beta 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-beta signaling activity. Taken together, this establishes a novel cholesterol-PRSS8-TGF-beta axis of regulation. Current work aims to identify novel transcriptional networks that are regulating TbetaRIII and its shedding. RNA-seq data between prostate cell lines with and without ERG overexpression has revealed ERG increased expression of pro-TGF-beta signaling genes and suppressed genes associated with down-regulating TGF-beta. In cervical cancer cell lines, knockdown of ERG has reduced phenotypes such as migration, invasion, and chemoresistance. ERG knockdown has also increased TbetaRIII shedding. ERG ChIP-seq in cervical cancer is being performed in the presence and absence of TGF-beta activation to determine the effect of TGF-beta 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 TbetaRIII, such as ERG, new possibilities for rescuing TbetaRIII expression and reducing dysregulated TGF-beta signaling are revealed. Therapies directed at cholesterol homeostasis or ERG may be able to be repurposed and applied to cancers experiencing TGF-beta dysregulation.