Post-translational modifications in mammary gland development and mammary tumor progression

Abstract

Breast cancer is one of the most common cancers in women. Estrogen receptor α (ERα) signaling and p53 signaling have important roles in breast cancer progression. Therefore, post-translational modifications of ERα and p53 play critical roles in breast cancer. The overall aim of this thesis is to characterize the role of RING-finger protein 31 (RNF31) on ERα and p53 signaling and the function of P21-activated kinase 4 (PAK4) on ERα signaling. Moreover, the role of PAK4 in mouse mammary development and mammary tumor progression was also analyzed.

In the first study, RNF31 was shown to active and stabilize ERα, and subsequently to increase estrogen-stimulated cell proliferation in breast cancer cells. In breast cancer clinical databases, the gene expression of RNF31 and ERα target genes were correlated. The suggested mechanism is that RNF31 interacts ERα via the RBR domain and facilitate ERα mono-ubiquitination.

In the second study, RNF31 depletion was shown to increase the gene expression of p53 target genes. RNF31 depletion caused cycle arrest and cisplatin-induced apoptosis in a p53- dependent manner in breast cancer cells. Depletion of RNF31 increased p53 protein levels and the mRNA levels of its downstream target genes. The suggested mechanism is that RNF31 interacts with the p53/MDM2 complex and stabilizes MDM2 and consequently facilitates p53 poly-ubiquitination and degradation.

In the third study, high PAK4 expression level was correlated with poor tamoxifen response in breast cancer patients in clinical databases, based on analysis of available mRNA expression. In MCF-7 cells, PAK4 overexpression promoted tamoxifen resistance, while PAK4 inhibition sensitized tamoxifen-resistant breast cancer cells to tamoxifen. Mechanistically, we identified a regulatory positive feedback loop, where PAK4 acts as a downstream target gene of ERα; while PAK4 can phosphorylate ERα at Ser305, thereby increasing ERα protein stability and activating ERα signaling. In conclusion, PAK4 may be a suitable target for tamoxifen resistance in breast cancer.

In the fourth study, we elucidated the function of PAK4 in mammary development and mammary tumor progression in vivo. We observed no difference in mammary gland development between control mice and PAK4 conditional knockout mice. To test the role of PAK4 in mammary tumor development, conditional depletion of PAK4 was introduced in the MMTV-PyMT breast cancer mouse model. Importantly, conditional PAK4 depletion caused an increased tumor latency in MMTV-PyMT mice, indicating a role for PAK4 in early mammary tumor development.