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 p110. PIK3CB, encoding p110{beta}, 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 p110{beta} 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 p110 and p110{beta} led to different effects on cell signalling and cell function, p110 activity being correlated to cell survival in PIK3CA mutant cells and p110{beta} with cell proliferation in PTEN-deficient cells. To understand the mechanisms governing the differential roles of these isoforms, we assessed their sub-cellular localisation. p110 was cytoplasmic whereas p110{beta} was both cytoplasmic and nuclear with increased levels in both compartments in cancer cells. Immunohistochemistry of p110{beta} 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 p110{beta} 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 p110{beta} 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 p110{beta} activity. EC cells with high levels of nuclear PtdIns(3,4,5)P3 and p110{beta} showed elevated nucleolar activity as assessed by the increase in 47S pre-rRNA transcriptional levels in a p110{beta}-dependent manner. Altogether, these results present a nucleolar role for the PI3K pathway that may contribute to tumour progression in endometrial cancer.