Nuclear upregulation of PI3K p110β correlates with increased rRNA transcription in endometrial cancer cells

Abstract

Genes encoding for components of the phosphoinositide 3-kinase (PI3K) pathway are frequently mutated in cancer, including inactivating mutations of PTEN and activating mutations of PIK3CA, encoding the PI3K catalytic subunit p110α. PIK3CB, encoding p110β, is rarely mutated, but can contribute to tumourigenesis in some PTEN-deficient tumours. The underlying molecular mechanisms are however poorly understood. By analysing cell lines and annotated clinical samples, we have previously found that p110β is highly expressed in endometrial cancer (EC) cell lines and that PIK3CB mRNA levels increase early in primary tumours correlating with lower survival. Selective inhibition of p110α and p110β led to different effects on cell signalling and cell function, p110α activity being correlated to cell survival in PIK3CA mutant cells and p110β with cell proliferation in PTEN-deficient cells. To understand the mechanisms governing the differential roles of these isoforms, we assessed their sub-cellular localisation. p110α was cytoplasmic whereas p110β was both cytoplasmic and nuclear with increased levels in both compartments in cancer cells. Immunohistochemistry of p110β in clinically annotated patient tumour sections revealed high nuclear/cytoplasmic staining ratio, which correlated significantly with higher grades. Consistently, the presence of high levels of p110β in the nuclei of EC cells, correlated with high levels of its product phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) in the nucleus. Using immunofluorescence labelling, we observed both p110β and PtdIns(3,4,5)P3 in the nucleoli of EC cell lines. The production of nucleolar PtdIns(3,4,5)P3 was dependent upon p110β activity.EC cells with high levels of nuclear PtdIns(3,4,5)P3 and p110β showed elevated nucleolar activity as assessed by the increase in 47S pre-rRNA transcriptional levels in a p110βdependent manner. Altogether, these results present a nucleolar role for the PI3K pathway that may contribute to tumour progression in endometrial cancer.