Reactivation of TAp73 tumor suppressor by protoporphyrin IX, a metabolite of aminolevulinic acid, induces apoptosis in TP53-deficient cancer cells

Abstract

BackgroundThe p73 protein is a tumor suppressor that shares structural and functional similarity with p53. p73 is expressed in two major isoforms; the TA isoform that interacts with p53 pathway, thus acting as tumor suppressor and the N-terminal truncated ΔN isoform that inhibits TAp73 and p53 and thus, acts as an oncogene.ResultsBy employing a drug repurposing approach, we found that protoporphyrin IX (PpIX), a metabolite of aminolevulinic acid applied in photodynamic therapy of cancer, stabilizes TAp73 and activates TAp73-dependent apoptosis in cancer cells lacking p53. The mechanism of TAp73 activation is via disruption of TAp73/MDM2 and TAp73/MDMX interactions and inhibition of TAp73 degradation by ubiquitin ligase Itch. Finally, PpIX showed potent antitumor effect and inhibited the growth of xenograft human tumors in mice.ConclusionOur findings may in future contribute to the successful repurposing of PpIX into clinical practice.Electronic supplementary materialThe online version of this article (10.1186/s13008-018-0043-3) contains supplementary material, which is available to authorized users.

Protoporphyrin IX is a dual inhibitor of p53/MDM2 and p53/MDM4 interactions and induces apoptosis in B-cell chronic lymphocytic leukaemia cells

“…Quantitative PCR was performed as described previously 25,29 . Briefly, cells were treated with 2.5 μg/ml PpIX for 8h.…”

“…Next, to determine that the inhibition of p53/MDM2 and p53/MDM4 complexes by PpIX is due to the direct interaction between the compound and p53 and not due to posttranslational modifications in the p53 protein, we employed a yeast-based reporter assay which allows measuring the transcription activity of human p53 using p53-dependent luciferase reporter 11,31 To determine that PpIX inhibits p53/MDM4 interactions also in cancer cells, we treated HCT 116 human colon cancer cells with PpIX for 16h. To stabilize MDM4 that is otherwise driven to degradation by MDM2 released from the complex with p53 by PpIX, cells were treated with MG132.…”

“…TAp73 and induces p53-downstream apoptotic genes 11,36,37 . Cancer cells harbouring wtTP53 gene, undergo cell death via a p53-dependent mechanism as PpIX inhibits p53/MDM2 complexes which leads to p53 stabilisation and a subsequent induction of apoptosis 36 55 .…”

“…In drug repurposing approach PpIX was identified as an activator of p53 and TAp73α tumor suppressors. Recent work demonstrates that PpIX inhibits TAp73/MDM2 and TAp73/MDM4 interactions, which leads to stabilisation of TAp73 protein and induction of TAp73-dependent apoptosis in cancers with TP53 gene mutations 11,12 .…”

“…, p53-, p73-dependent cell death in several cancer cell lines and to shrink tumors in vivo11,36 . To estimate the cytoxicity of PpIX in CLL cells, we conducted a 72-hour cell viability assay using the wt-p53 EHEB B-CLL cell line and the p53-null HL60 acute promyelocytic leukaemia cells.…”