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FLJ22318: a novel binding partner of the NKX3-1 homeodomain protein in prostate cancer cells

Dawson, Linda (2006) FLJ22318: a novel binding partner of the NKX3-1 homeodomain protein in prostate cancer cells. PhD thesis, Murdoch University.

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Prostate cancer is a frequently diagnosed malignancy which ranges from an indolent asymptomatic tumour to an aggressive, rapidly lethal systemic disease. Determination of chromosomal, genetic and epigenetic alterations associated with prostate carcinogenesis have led to the characterisation of functional consequences of these alterations, thereby elucidating pathways contributing to malignant growth that can be utilised clinically and therapeutically.

FLJ22318 is a novel hypothetical protein that was identified by yeast two-hybrid analysis to interact with the prostatic homeodomain protein, NKX3-1. Expression of NKX3-1 is largely restricted to epithelial cells of the adult prostate where it is involved in maintaining the prostatic phenotype, while NKX3-1 expression is reduced or absent in prostate tumours. In contrast, FLJ22318 expression is documented in cDNA libraries derived from a variety of human adult and foetal tissues including the prostate, suggesting that it may be ubiquitously expressed and that it potentially interacts with a number of proteins in addition to NKX3-1. FLJ22318 expression is undocumented in human prostate tumours.

Bioinformatic analyses have postulated multiple FLJ22318 mRNA transcripts however the proposed open reading frames encoded by these transcripts predict the FLJ22318 protein to contain three strong protein-protein interaction domains, a Lissencephaly type-1-like homology (LisH), a C-terminal to LisH (CTLH) and a CT11-RanBPM (CRA) domain. Of the 44 single nucleotide polymorphisms identified within the FLJ22318 gene, none are located within the protein coding region suggesting that FLJ22318 may be critical for cell survival and/or function. Comparison of the amino acid sequence between human FLJ22318 and its orthologues in a diverse range of mammalian species identified >97% sequence homology, providing further strong evidence of the critical cellular function of FLJ22318.

To characterise the biological activity of FLJ22318 in prostate cancer cells, the FLJ22318 coding region was amplified by polymerase chain reaction (PCR) and ligated into mammalian and bacterial expression vectors to encode V5-, myc-, GFP-, HA-, and GST-FLJ22318 fusion proteins. Interaction between FLJ22318 and NKX3-1 was confirmed using (reverse) yeast two-hybrid, GST pull-down and co-immunoprecipitation assays. These data were supported by confocal microscopy which demonstrated the perinuclear and nuclear co-localisation of FLJ22318 and NKX3-1 in prostate cancer cells. Northern blotting identified expression of ~2Kb and ~4Kb FLJ22318 mRNA's in prostate cancer cell lines, which was consistent with bioinformatic analyses of mRNA species. Transfection of prostate cancer cells to overexpress FLJ22318 did not alter endogenous NKX3-1 levels, however FLJ22318 exhibited transcriptional repressor function on an NKX3-1 responsive element and increased NKX3-1 transcriptional repressor activity on this element.

To further investigate FLJ22318 function, additional yeast two-hybrid analyses were performed in prostate cancer cells to identify potential FLJ22318 binding proteins. These studies isolated cDNA's encoding 33 different proteins involved in cell metabolism and apoptosis as well as transcriptional regulators associated with control of cellular proliferation. One of the candidate FLJ22318 interactors, protein kinase, interferon-inducible double stranded RNA dependent activator (PRKRA/PACT) was shown using confocal microscopy to extensively co-localise with FLJ22318 in the cytoplasm and perinuclear regions of prostate cancer cells. Preliminary co-immunoprecipitation and GST pull-down assays have provided additional evidence of the interaction of PRKRA and FLJ22318.

Results of this thesis have generated important information characterising the structure of the FLJ22318 gene and protein, the interaction between FLJ22318 and NKX3-1 and the potential functions of FLJ22318 in prostate cancer cells. As the FLJ22318 gene is located on 5q35, a chromosomal region frequently disrupted in a variety of tumours, future studies of the biological activity of FLJ22318 will clarify its normal cellular functions and its contribution to tumorigenesis or malignant progression in the prostate and in other tissues.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Supervisor(s): Bentel, Jacky
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