DISTINGUISHING SMAD2 and SMAD3-DRIVEN EFFECTS OF THE TGF-? SIGNALLING IN A METASTATIC BREAST CANCER MODEL

Abstract

The Transforming Growth Factor beta (TGF-β) signalling is simple in its basic architecture yet able to elicit a wide array of cellular responses in different cell types. Intracellular proteins, known as SMADs, play a critical role in mediating these intracellular responses to TGF-β stimulation. The ability to regulate diverse cellular activity enables TGF-β to maintain tissue homeostasis by regulating delicate and dynamic balance between the cellular process of proliferation and apoptosis. However in breast cancer metastasis, the TGF-β induced growth inhibition is lost and a pro-oncogenic effect is observed. Although this TGF- β paradox is widely accepted, the mechanism of how a single TGF-β stimulus, could lead to multiple contradictory outcomes remain elusive. SMAD2 and SMAD3 knockdown studies suggest their differential roles in metastatic breast carcinoma with SMAD3 playing an important role in migration. This study aims to elucidate the SMAD-dependent TGF-β signalling pathway and identify SMAD2 and SMAD3-driven effects in a metastatic breast cancer model through a genome-wide assessment of their differential transcriptional regulation. This research used BACtransgenomic technology to tag SMAD2 and SMAD3 proteins under native regulatory control. By combining BACtransgenomics with ChIP-Seq, specific SMAD2 and SMAD3 binding sites and their direct regulatory targets in response to TGF-β were identified. Genome-wide integrative analysis with RNA-Seq data indicates that SMAD2 plays a role in balancing cell death and survival through cell cycle regulation while SMAD3 is involved in cellular movement. Distinguishing the specific roles of SMAD2 and SMAD3 in the dynamic regulatory TGF-β signalling in metastatic breast cancer will serve as a molecular basis to the design of mechanism based cancer drugs and combination therapies, translating into the development of effective cancer strategy and potentially provide clinically relevant biomarkers for TGF-β directed therapy in breast cancer.