PBF phosphorylation in endocrine cancer

Kocbiyik, Merve (2024). PBF phosphorylation in endocrine cancer. University of Birmingham. Ph.D.

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Abstract

Pituitary tumor-transforming gene-binding factor (PTTG1IP/PBF) is a proto-oncogene that is upregulated and associated with tumour progression in various cancers including thyroid, breast, colorectal, liver, and lung. The impact of PBF varies across different cancers; predominantly heightened PBF expression is linked to cancer metastasis and recurrence in cancers such as breast and thyroid, while the downregulation of PBF has been linked with aggressive breast cancers. PBF is engaged in diverse cellular pathways in both normal and disease mechanisms, influencing processes such as cell proliferation, cell migration, cell adhesion, and cell invasion. Notably, PBF phosphorylation at tyrosine 174 is one of the most highly phosphorylated residues in the human proteome, and initial investigations have demonstrated that it may mediate PBF function in breast and thyroid cancer cell invasiveness. This study sought to elucidate the importance of PBF and its regulatory phosphorylation in the induction of cell motility.

Key elements of the C-terminal regulatory tail of PBF were evaluated in the migration and invasion of thyroid and breast cancer cells. Our findings indicate that not only phosphorylation but also endocytosis from the plasma membrane plays a crucial role in facilitating PBF-stimulated migration and invasion.

A potential relationship between PBF phosphorylation, endocytosis and protein stability was investigated. A role for Y174 phosphorylation in mediating PBF plasma membrane localisation and stability could not be determined. However, mechanisms of PBF protein turnover were elucidated with degradation via proteasomal and lysosomal pathways demonstrated.

Increasing evidence of PBF stimulation of cancer cell motility and interaction with related molecules and pathways led us to hypothesise that PBF has a physiological role in the regulation of cell motility.

To investigate this, mouse embryonic fibroblast (MEF) cells were isolated from a novel Pbf knockout (KO) mouse model and characterised. Pbf-KO MEF cells demonstrated significantly impaired migration and invasion capacity. KO cells heterozygous for Pbf deletion demonstrated an intermediate reduction in cell motility, suggesting a potential gene dosage effect. To establish an intermediate model bridging primary and cancer cells, primary MEF cells were immortalised. Functional assays once again showed a notable downregulation of MEF cell migration with Pbf deletion. Collectively, these findings strongly indicate a physiological role for Pbf in normal cell motility. Additionally, through RNA sequencing, molecular alterations downstream of Pbf-depletion in primary MEFs were identified, and cellular processes and pathways in which Pbf is involved in normal physiology were delineated. In addition to the involvement in cell motility as expected, a strong link with immune regulation was apparent.

In conclusion, this thesis highlights the significance of PBF as a crucial regulator of cell motility, impacting both normal physiology and dysregulation in disease, such as cancer progression. These findings contribute to our understanding of the function and regulation of this proto-oncogene in the critical cellular process of motility. A comprehensive understanding of the nature of PBF is essential for considering its potential utilisation as a biomarker or as a target in therapeutic approaches.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):