Breast cancer is an extremely heterogeneous disease comprising at least ten different subtypes, each exhibiting different characteristics in progression, prognosis and response to treatment. Transforming growth factor β (TGFβ) a member of the TGF superfamily of cytokines differentially regulates breast tumour-initiating cells (BTICs). In the claudin-low breast cancer subtype, TGFβ increases the tumour initiating capacity through the ability of the scaffolding protein NEDD9 to unite the TGFβ/SMAD and Rho-Actin-SRF pathways. Previously, oncogenicity has been ascribed to the level of NEDD9 protein expression. However, my analysis indicates this mediation by NEDD9 is irrespective of NEDD9 protein expression, which is ubiquitously overexpressed in the majority of cancer types including breast cancer. In this thesis, I demonstrate how in the Claudin-low breast cancer subtype NEDD9 protein expression and post-translational modification are influenced by TGFβ pathway activation. I identify significant NEDD9 interacting proteins and their downstream effectors which contribute towards oncogenic TGFβ signalling pathways. A key TGFβ specific NEDD9 interactor identified in this subtype is the metabolic isoenzyme PKM2. Through a variety of techniques, I explore the known roles of PKM2 in the regulation of oncogenic metabolic reprogramming and demonstrate how these processes are influenced by its association with NEDD9. Finally, I investigate potential translational applications and biomarkers of TGFβ mediated, NEDD9/PKM2 dependent downstream signalling pathways, using large clinical breast cancer datasets and patient-derived tumour xenograft (PDTX) models. These data suggest a novel mechanism by which oncogenic TGFβ signalling regulates cellular proliferation and self-renewal via β-Catenin/c-Myc regulation of altered metabolism in the Claudin-low breast cancer subtype, a process which is dependent upon the scaffolding protein NEDD9 and the metabolic enzyme PKM2. Together, these data suggest that a combined biomarker of TGFβ signalling and c-Myc expression may be useful in identifying a subset of Claudin-low breast cancer patients who would be sensitive to inhibition of Wnt/β-Catenin signalling. Additionally, due to the dependence of these tumours on c-Myc driven glutamine dependent metabolic processes, metabolic magnetic resonance imaging may be useful for monitoring response in these patients.