The inhibitors of apoptosis (IAPs) constitute a family of proteins involved in the regulation of various cellular processes, including cell death, immune and inflammatory responses, cell proliferation, cell differentiation, and cell motility. There is accumulating evidence supporting IAP-targeting in tumors: IAPs regulate various cellular processes that contribute to tumor development, such as cell death, cell proliferation, and cell migration; their expression is increased in a number of human tumor samples, and IAP overexpression has been correlated with tumor growth, and poor prognosis or low response to treatment; and IAP expression can be rapidly induced in response to chemotherapy or radiotherapy because of the presence of an internal ribosome entry site (IRES)-dependent mechanism of translation initiation, which could contribute to resistance to antitumor therapy. The development of IAP antagonists is an important challenge and was subject to intense research over the past decade. Six molecules are currently in clinical trials. This review focuses on the role of IAPs in tumors and the development of IAP-targeting molecules for anticancer therapy.

Of critical importance in the stress response is the post-transcriptional control of the expression of important genes involved in the control of cell survival and apoptosis. Here we report that miR-19, an oncogenic component of the miR-17-92/Oncomir-1 microRNA polycistron, regulates the expression of Ras homolog B (RhoB) in keratinocytes upon exposure to ultraviolet (UV) radiation. Strikingly, we could not find any evidence for deregulated expression of miR-19 during UV treatment. However, we show that miR-19-mediated regulation of antiapoptotic RhoB expression requires the binding of human antigen R (HuR), an AU-rich element binding protein, to the 3 0 -untranslated region of the rhoB mRNA. We propose that the loss of the interdependent binding between HuR and miR-19 to the rhoB mRNA upon UV exposure relieves this mRNA from miR-19-dependent inhibition of translation and contributes to the apoptotic response.

The E2F transcription factor 1 is subtly regulated along the cell cycle progression and in response to DNA damage by post-translational modifications. Here, we demonstrated that the E3-ubiquitin ligase cellular inhibitor of apoptosis 1 (cIAP1) increases E2F1 K63-poly-ubiquitination on the lysine residue 161/164 cluster, which is associated with the transcriptional factor stability and activity. Mutation of these lysine residues completely abrogates the binding of E2F1 to CCNE, TP73 and APAF1 promoters, thus inhibiting transcriptional activation of these genes and E2F1-mediated cell proliferation control. Importantly, E2F1 stabilization in response to etoposide-induced DNA damage or during the S phase of cell cycle, as revealed by cyclin A silencing, is associated with K63-poly-ubiquitinylation of E2F1 on lysine 161/164 residues and involves cIAP1. Our results reveal an additional level of regulation of the stability and the activity of E2F1 by a non-degradative K63-poly-ubiquitination and uncover a novel function for the E3-ubiquitin ligase cIAP1.

The cellular inhibitor of apoptosis 1 (cIAP1) is an E3-ubiquitin ligase that regulates cell signaling pathways involved in fundamental cellular processes including cell death, cell proliferation, cell differentiation and inflammation. It recruits ubiquitination substrates thanks to the presence of three baculoviral IAP repeat (BIR) domains at its N-terminal extremity. We previously demonstrated that cIAP1 promoted the ubiquitination of the E2 factor 1 (E2F1) transcription factor. Moreover, we showed that cIAP1 was required for E2F1 stabilization during the S phase of cell cycle and in response to DNA damage. Here, we report that E2F1 binds within the cIAP1 BIR3 domain. The BIR3 contains a surface hydrophobic groove that specifically anchors a conserved IAP binding motif (IBM) found in a number of intracellular proteins including Smac. The Smac N-7 peptide that includes the IBM, as well as a Smac mimetic, competed with E2F1 for interaction with cIAP1 demonstrating the importance of the BIR surface hydrophobic groove. We demonstrated that the first alpha-helix of BIR3 was required for E2F1 binding, as well as for the binding of Smac and Smac mimetics. Overexpression of cIAP1 modified the ubiquitination profile of E2F1, increasing the ratio of E2F1 conjugated with K11- and K63-linked ubiquitin chains, and decreasing the proportion of E2F1 modified by K48-linked ubiquitin chains. ChIP-seq analysis demonstrated that cIAP1 was required for the recruitment of E2F1 onto chromatin. Lastly, we identified an E2F-binding site on the cIAP1-encoding birc2 gene promoter, suggesting a retro-control regulation loop.

Following publication of this article, it was noticed that an error during typesetting had led to the author Olivier Micheau being listed incorrectly as Olivier Michaud. The publisher would like to apologise for any inconvenience caused.

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Rho GTPases contribute to most steps of cancer initiation and progression, including the acquisition of unlimited proliferation potential, survival, tissue invasion and the establishment of metastases. By contrast to RhoA and RhoC, RhoB has been proposed to act as a tumor suppressor as it is activated in response to various stresses including UV exposure, and since it has been reported to inhibit tumor growth, cell migration and invasion. Unlike RhoA and RhoC, RhoB is often down regulated in human tumour tissues including lung and head and neck cancer and its expression is inversely correlated with tumor aggressiveness. However, the epigenetic regulation of RhoB expression in cancer cells remains to be elucidated. Since several recent works demonstrate that post-transcriptional deregulation of gene expression is involved in carcinogenesis, we have investigated the post-transcriptional regulation of RhoB expression. The RhoB 3’ untranslated region is well conserved between species and has numerous putative binding sites for post-transcriptional regulators such as mRNA binding proteins and microRNAs. By using luciferase assays and polysome analysis, we have found that the translation of the RhoB mRNA is regulated upon UV exposure. One specific miRNA, miR-19b whose expression is regulated by UV, directly targets the RhoB mRNA to control RhoB expression and UV-induced apoptosis. Interestingly, other post-transcriptionnal regulators involved in RhoB expression, such as the mRNA binding proteins HuR and TiaR were also indentified by functional assays using RNA interference strategy. Immunohistochemical analysis on a cohort of human lung tissues ranging from normal to invasive carcinoma are underway to detect a possible correlation between the expression of these proteins and RhoB downregulation in tumor. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2049.