Role of γ-catenin in acute myeloid leukaemia

• Main Author: Morgan, Rhys
• Published: Cardiff University 2011
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.585259

Acute myeloid leukaemia (AML) is a heterogeneous clonal disorder of haematopoietic cells that primarily affects the elderly. Previously, our laboratory identified y-catenin as significantly overexpressed in AML. y-Catenin shares close structural and functional homology with the more intensively studied /3-catenin. Both catenins have dual roles in cell adhesion complexes and in transcription. Their transcriptional role is regulated by Wnt signalling which is critical for normal development and is one of the most frequently dysregulated processes in AML. In spite of this, little is known regarding the specific role of y-catenin in normal haematopoiesis or AML pathology. This study devised an intracellular flow cytometric staining assay to characterise the expression of y-catenin in normal haematopoietic subsets. y-Catenin exhibited a similar expression profile to /3-catenin. Expression was relatively high in haematopoietic stem/progenitor cells (HSC/HPC) and showed increased expression in myeloid differentiated cells (granulocytes and monocytes) while expression was lower in lymphoid cells and undetectable in red blood cells. Studies of subcellular distribution by confocal imaging showed reciprocal localisation of catenins in CD34+ cells, with /3-catenin predominantly nuclear translocated and y-catenin nuclear excluded. Conversely, in granulocytic and monocytic cells nuclear y-catenin levels were relatively high whilst nuclear /3-catenin levels were reduced. A small subset of the CD14+ monocyte population exhibited heavily nuclear translocated y-catenin. Subsequent knock-down studies of y-catenin showed this protein to be required for normal haematopoietic development in vitro, evidenced by the inhibition of macrophage differentiation and apparent reprogramming of committed monocyte progenitors for granulocytic development. In AML patients, y- catenin mRNA expression conferred a reduced complete remission rate arising from resistant disease, however discordance was found between mRNA and protein level, implying post-translational control of y-catenin expression in AML. In primary AML blasts (undifferentiated) y-catenin was aberrantly localised to the nucleus suggesting a transcriptional role in AML pathology. A correlation was identified between y- and (3-catenin protein expression in primary AML blasts and an association between nuclear levels of these proteins. To determine whether this association was causal, y-catenin was ectopically expressed in normal human CD34+ haematopoietic progenitor cells but had no significant influence on /3-catenin expression or localisation even following subsequent differentiation. In contrast, overexpression of y-catenin in leukaemic cell lines stabilised /3-catenin protein and promoted its translocation to the nucleus suggesting that the influence of y-catenin on /5-catenin is a feature of leukaemic cells but not normal cells. Phenotypically, overexpression of y-catenin had little effect on normal progenitor cells but was able to block agonist-induced differentiation of AML cell lines, probably via stabilisation of jS-catenin. In summary, this study indicates a role for y-catenin in the regulation of normal haematopoietic development and that its nuclear translocation is strictly regulated independently of /3-catenin in normal haematopoiesis. In leukaemic cells, however, this control is dysfunctional allowing y-catenin to promote the stabilisation and translocation of /3-catenin. This relationship may represent a pathological mechanism active in AML blasts to block myeloid differentiation and promote a leukaemic phenotype.