Investigating the role of p73 proteins in controlling the tumor microenvironment

Abstract

Cancer is a multifactorial disease governed by oncogenes and tumor suppressors that not only impact on the behavior of the cancer cells but also influence many processes in the surrounding tumor microenvironment. The most studied tumor suppressor family is the p53 family, which consists of the transcription factors p53, p63 and p73. While p53 is the most frequently mutated gene in the cancer genome, for both p63 and p73, a shift in the balance between different isoforms has been discovered and this deregulation of protein levels has been linked to tumor progression and survival. P73 can be transcribed from two separate promoters resulting in a tumor suppressing, full-length isoform (TAp73), and a N-terminally truncated version (ΔNp73), which lacks the transactivation domain and thus possesses oncogenic properties. Furthermore, alternative splicing in the C-terminus results in a number of additional isoforms. P73 has been shown to be able to support and overtake many processes p53 is regulating. However, p73 isoforms have also been shown to have p53-independent functions. Understanding how different p53 family members regulate tumor development and progression is essential for identifying possible treatment strategies.

In this thesis we identified several previously unknown roles for p73 isoforms in controlling the tumor microenvironment. Firstly, we discovered that loss of TAp73 results in a NF-κB-dependent upregulation of pro-inflammatory factors in breast cancer. Furthermore, this led to a concurrent increase in tumor-promoting macrophage infiltration. Secondly, we identified a role for ΔNp73 in the regulation of activating NK cell ligand expression on cancer cells. However, we observed a concomitant upregulation of inhibitory NK cell ligands upon loss of ΔNp73, leaving NK cell-mediated killing of tumor cells unaffected. Thirdly, a correlation between high levels of ΔNp73 and HIF-1α protein was observed. We demonstrate that ΔNp73 increases HIF-1α protein stability by interfering with the expression of genes of the ECV complex, normally involved in proteasomal degradation of HIF-1α. Finally, we further strengthen the involvement of ΔNp73 in the process of multidrug resistance. ΔNp73 was found to promote elevated expression of ABC transporters, ABCB1 and ABCB5, in breast cancer and melanoma, thereby supporting the efflux of drugs from the cancer cells and increasing their resistance to drug treatments. Taken together, these findings highlight the significant contributions of p73 isoforms on tumor progression and aid in unravelling the complex interactions of this network.