Discovery and characterization of the mTOR-p73 signaling axis in human cancer

Abstract

p73, a transcription factor that plays essential roles during development and tumor suppression, shares significant sequence identity with p53. However, p53 and p73 are regulated by different pathways, and p73 is not mutated in human tumors. How tumors tolerate over-expression of p73, a protein with tumor suppressive properties, is unclear. We hypothesized that signaling pathways upstream of p73 inhibit its activity in tumors. We devised an approach, based on recognition of patterns within a p73 gene signature, that identified mTOR as an upstream inhibitor of p73. In addition, p73 is a substrate for mTOR and associates with mTOR in cells. mTOR inhibition leads to an elevation in p73 levels and an increase in its activity at target genes involved in apoptosis, metabolism, and autophagy. There is a critical need to understand not only genes and non-coding RNAs regulated by p73, but also how this regulation changes during treatment regimens. We used ChIP-on-Chip to obtain a comprehensive profile of p73 binding sites across the genome: the p73 cistrome. We measured this profile again in the presence of the mTOR inhibitor rapamycin, which did not alter the overall distribution of p73-bound loci but instead selectively increased p73 occupancy at ~9% of all binding sites.

Further, we propose an anti-cancer strategy that targets the mTOR-p73 signaling axis as demonstrated in two mesenchymal tumor types: basal-like breast cancers and rhabdomyosarcomas. Using a gene signature of the mTOR-p73 signaling axis, we demonstrated that p73 is functional in a subset of rhabdomyosarcomas and is a determinant of clinical outcome. These data are consistent with the observation that genes and miRNAs directly regulated by p73 exhibit patterns of expression conserved during both mesenchymal differentiation and tumorigenesis, functions that can be explored in mice and in human tumors using new tools that we have developed. Signatures similar to those presented herein might inform the use of cancer therapies such as mTOR inhibitors that engage p73 and are affected by differential p73 activities in tumor subtypes.