Date of Award


Degree Type


Degree Name

Doctor of Philosophy




Dr. Fred Dick


The ability to respond to anti-growth signals is critical to maintain tissue homeostasis and loss of this proliferative control mechanism is considered a hallmark of cancer. Negative growth regulation generally occurs during the G0/G1 phase of the cell cycle, yet the redundancy and complexity among components of this regulatory network have made it difficult to discern how negative growth cues protect cells from aberrant proliferation. .

Transforming growth factor (3(TGF-P) is a crucial mediator of mammary epithelial morphogenesis and can negatively regulate cell cycle progression. TGF-P has been shown to inhibit cyclin dependent kinase activity, which leads to activation of the retinoblastoma protein (pRB) and growth arrest. However, unlike other components of TGF-P cytostatic signalling, pRB is thought to be dispensable for mammary development. Using gene-targeted mice where the LXCXE binding cleft on pRB has been disrupted (R b l^ and RblNF), we have discovered that pRB plays a crucial role in mammary gland development. In particular, Rbl and RblNFmutant female mice have hyperplastic mammary epithelium due to insensitivity to TGF-P growth inhibition. In contrast with previous studies that highlight the inhibition of cyclin/CDK activity by TGF-P signalling, these experiments reveal that active transcriptional repression of E2F target genes by pRB is also a key component of TGF-p cytostatic signalling. However, loss of pRB-LXCXE interactions does not cause overt defects in other TGF-P signalling pathways such as apoptosis and differentiation. Taken together, this work demonstrates a unique functional connection between pRB and TGF-p in growth control and mammary development.


These findings were extended to explore the importance of the pRB anti­ proliferative response during tumour formation and progression. Cytostatic control is considered a key tumour suppressive mechanism in the mammary gland. Here I show that LXCXE-dependent growth control by pRB blocks formation of mammary tumours in Wap-p53R172Htransgenic mice. In contrast, the same growth control mechanism is unnecessary to protect against Neu or 7,72-dimethylbenz[a]anthracene-induced mammary tumorigenesis. Taken together, this work demonstrates that anti-proliferative control by pRB can act as a barrier against oncogenic transformation. Strikingly, these data also reveals that this tumour suppressive effect is context-dependent.



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