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      Novel Apoptosis-Inducing Agents for the Treatment of Cancer, a New Arsenal in the Toolbox

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          Abstract

          Evasion from apoptosis is an important hallmark of cancer cells. Alterations of apoptosis pathways are especially critical as they confer resistance to conventional anti-cancer therapeutics, e.g., chemotherapy, radiotherapy, and targeted therapeutics. Thus, successful induction of apoptosis using novel therapeutics may be a key strategy for preventing recurrence and metastasis. Inhibitors of anti-apoptotic molecules and enhancers of pro-apoptotic molecules are being actively developed for hematologic malignancies and solid tumors in particular over the last decade. However, due to the complicated apoptosis process caused by a multifaceted connection with cross-talk pathways, protein–protein interaction, and diverse resistance mechanisms, drug development within the category has been extremely challenging. Careful design and development of clinical trials incorporating predictive biomarkers along with novel apoptosis-inducing agents based on rational combination strategies are needed to ensure the successful development of these molecules. Here, we review the landscape of currently available direct apoptosis-targeting agents in clinical development for cancer treatment and update the related biomarker advancement to detect and validate the efficacy of apoptosis-targeted therapies, along with strategies to combine them with other agents.

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          Most cited references241

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          Cell death: critical control points.

          Programmed cell death is a distinct genetic and biochemical pathway essential to metazoans. An intact death pathway is required for successful embryonic development and the maintenance of normal tissue homeostasis. Apoptosis has proven to be tightly interwoven with other essential cell pathways. The identification of critical control points in the cell death pathway has yielded fundamental insights for basic biology, as well as provided rational targets for new therapeutics.
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            IAP antagonists induce autoubiquitination of c-IAPs, NF-kappaB activation, and TNFalpha-dependent apoptosis.

            Inhibitor of apoptosis (IAP) proteins are antiapoptotic regulators that block cell death in response to diverse stimuli. They are expressed at elevated levels in human malignancies and are attractive targets for the development of novel cancer therapeutics. Herein, we demonstrate that small-molecule IAP antagonists bind to select baculovirus IAP repeat (BIR) domains resulting in dramatic induction of auto-ubiquitination activity and rapid proteasomal degradation of c-IAPs. The IAP antagonists also induce cell death that is dependent on TNF signaling and de novo protein biosynthesis. Additionally, the c-IAP proteins were found to function as regulators of NF-kappaB signaling. Through their ubiquitin E3 ligase activities c-IAP1 and c-IAP2 promote proteasomal degradation of NIK, the central ser/thr kinase in the noncanonical NF-kappaB pathway.
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              Proapoptotic Bak is sequestered by Mcl-1 and Bcl-xL, but not Bcl-2, until displaced by BH3-only proteins.

              Commitment of cells to apoptosis is governed largely by the interaction between members of the Bcl-2 protein family. Its three subfamilies have distinct roles: The BH3-only proteins trigger apoptosis by binding via their BH3 domain to prosurvival relatives, while the proapoptotic Bax and Bak have an essential downstream role involving permeabilization of organellar membranes and induction of caspase activation. We have investigated the regulation of Bak and find that, in healthy cells, Bak associates with Mcl-1 and Bcl-x(L) but surprisingly not Bcl-2, Bcl-w, or A1. These interactions require the Bak BH3 domain, which is also necessary for Bak dimerization and killing activity. When cytotoxic signals activate BH3-only proteins that can engage both Mcl-1 and Bcl-x(L) (such as Noxa plus Bad), Bak is displaced and induces cell death. Accordingly, the BH3-only protein Noxa could bind to Mcl-1, displace Bak, and promote Mcl-1 degradation, but Bak-mediated cell death also required neutralization of Bcl-x(L) by other BH3-only proteins. The results indicate that Bak is held in check solely by Mcl-1 and Bcl-x(L) and induces apoptosis only if freed from both. The finding that different prosurvival proteins have selective roles has notable implications for the design of anti-cancer drugs that target the Bcl-2 family.
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                Author and article information

                Journal
                Cancers (Basel)
                Cancers (Basel)
                cancers
                Cancers
                MDPI
                2072-6694
                31 July 2019
                August 2019
                : 11
                : 8
                : 1087
                Affiliations
                [1 ]Department of Breast Medical Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA
                [2 ]Women’s Malignancies Branch, Center for Cancer Research, National Cancer Institute, Bethesda, MD 20892, USA
                [3 ]Early Clinical Trials Development, Division of Cancer Treatment and Diagnosis, National Cancer Institute, Bethesda, MD 20892, USA
                Author notes
                Author information
                https://orcid.org/0000-0002-4182-6058
                https://orcid.org/0000-0002-5790-4954
                Article
                cancers-11-01087
                10.3390/cancers11081087
                6721450
                31370269
                c528d84f-f600-4076-b892-256996a0a189
                © 2019 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 13 June 2019
                : 17 July 2019
                Categories
                Review

                apoptosis,cancer,intrinsic apoptosis,extrinsic apoptosis,biomarker,targeted therapy

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