Insights into the Mechanisms of Action of MDA-7/IL-24: A Ubiquitous Cancer-Suppressing Protein

Abstract

Melanoma differentiation associated gene-7/interleukin-24 (MDA-7/IL-24), a secreted protein of the IL-10 family, was first identified more than two decades ago as a novel gene differentially expressed in terminally differentiating human metastatic melanoma cells. MDA-7/IL-24 functions as a potent tumor suppressor exerting a diverse array of functions including the inhibition of tumor growth, invasion, angiogenesis, and metastasis, and induction of potent "bystander" antitumor activity and synergy with conventional cancer therapeutics. MDA-7/IL-24 induces cancer-specific cell death through apoptosis or toxic autophagy, which was initially established in vitro and in preclinical animal models in vivo and later in a Phase I clinical trial in patients with advanced cancers. This review summarizes the history and our current understanding of the molecular/biological mechanisms of MDA-7/IL-24 action rendering it a potent cancer suppressor.

Keywords: MDA-7/IL-24; apoptosis; bystander antitumor activity; combinatorial therapy; cytokine.

Figure 1

Subtraction hybridization identifies MDA-7/IL-24. Schematic of DISH (differentiation induction subtraction hybridization), an approach for identifying and cloning genes associated with the induction of terminal differentiation in human melanoma cells. Treatment of HO-1 human melanoma cells with a combination of IFN-β + mezerein results in a rapid and irreversible loss of proliferation, extinction of tumorigenic potential, and terminal differentiation [12,13,14,15,16]. The DISH approach was developed to identify and clone genes associated with and causative of the physiologic changes associated with terminal differentiation. mRNAs were isolated from actively proliferating and IFN-β + mezerein (2000 units/mL + 10 ng/mL)-treated HO-1 cells that span the first 24 h of treatment, and were converted into cDNAs. Subtraction hybridization was then done between differentiation inducer-treated and control-proliferating cancer cells resulting in the production of a subtracted cDNA library enriched for melanoma differentiation associated (mda) genes. Probing of clones isolated from this cDNA library permitted cloning of mda genes involved in critical cellular processes, some are listed here [12,13,14,15,16]. Reprinted by permission from Fisher, 2005 [17].

Figure 2

mda-7/IL-24-mediated DICER regulation is controlled by the transcription factor MITF. mda-7/IL-24 downregulates MITF in different cancer cell lines (A,B), but not in normal RWPE-1 cells (A). DU-145 (C) and MDA-MB-231 cells (D) were transfected with vector or MITF, and then treated with Ad.null or Ad.mda-7. RNA was isolated 72 h post infection, and real-time quantitative PCR was done to check the level of DICER. DU-145 (E) and MDA-MB-231 cells (F) were treated as described in (C,D), total protein was isolated, and Western blotting was done with DICER and MITF antibodies. Actin was used as a loading control. (G) Schematic representation of regulation of the miRNA processing enzyme DICER by mda-7/IL-24. MDA-7/IL-24 downregulates the transcription factor MITF in a ROS-dependent manner, which in turn downregulates DICER. RWPE-1 (immortalized human prostate epithelial cell line); DU-145 (human prostate cancer cell line); MDA-MB-231 (human breast cancer cell line); and A549 (human lung cancer cell line). Reprinted by permission from Pradhan et al., 2019 [39]. *: p < 0.05.

Figure 3

The relationship between apoptosis and autophagy. The schematic model of autophagy-apoptosis crosstalk during ER stress. The autophagy inducer, the apoptosis inducer, and the ER stress sensor (ERSS) are denoted by isolated green, red, and blue boxes, respectively. Dashed line shows how the molecules can influence each other, while blocked end lines denote inhibition. Reprinted by permission from Holczer et al., 2015 [46].

Figure 4

Model of mda-7/IL-24-induced apoptosis in cancer cells. Outline of proposed selective cytotoxic effects of mda-7/IL-24 on cancer versus normal cells. When ectopically overexpressed, MDA-7/IL-24 localizes in the ER/Golgi compartments, regardless of the presence or absence of a secretory signal in the protein. Accumulation of MDA-7/IL-24 protein in transformed/tumor cells in this compartment triggers apoptosis, toxic autophagy and induction of ceramide that could involve induction of ER stress and/or reactive oxygen species in mitochondria. MDA-7/IL-24 activates signal transduction pathways and/or potentially enters cancer cells and activates pro-apoptotic pathways by localization and accumulation in the ER/Golgi compartment and/or by inducing mitochondrial dysfunction. A combination of pathways triggered by mda-7/IL-24 results in transformed cell-specific apoptosis (or toxic autophagy). Adapted by permission from Dash et al., 2010 [55].

Figure 5

A schematic representation of how MDA-7/IL-24 regulates toxic autophagy in cancer cells leading to cell death. MDA-7/IL-24 regulates autophagy mediated through ER stress and ceramide production. MDA-7/IL-24 first interacts with its receptors which leads to a downstream signaling pathway mediated by reactive oxygen species (ROS). ROS regulates DICER and through this molecule MDA-7/IL-24 downregulates miR-221, which in turn upregulates Beclin-1 to induce toxic autophagy leading to cell death. The transition of protective to toxic autophagy is explained by the cleavage of ATG5 by Calpain, which is also mediated by ROS induced from MDA-7/IL-24 treatment. Reprinted by permission from Emdad et al., 2020 [7].

Figure 6

Invasion and Metastasis. Clinically detectable metastases represent the end products of a complex series of cell-biological events, which are collectively termed the invasion-metastasis cascade. During metastatic progression, tumor cells exit their primary sites of growth (local invasion, intravasation), translocate systemically (survival in the circulation, arrest at a distant organ site, extravasation), and adapt to survive and thrive in the foreign microenvironments of distant tissues (micro metastasis formation, metastatic colonization). Reprinted by permission from De Ieso and Yool, 2018 [74].

Figure 7

Objective clinical response to INGN 241 (Ad.5-mda-7) in cohort 8 patient with metastatic melanoma (patient 83). Injected lesion was on right clavicle (dashed circle in (A)). (B) by day 4, the region is inflamed. (C) At the end of cycle 1 (day 30), the lesion has completely regressed. This patient was alive > 600 days post INGN 241 treatment. Reprinted by permission from Lebedeva et al. 2007 [65].

Figure 8

Strategies to improve MDA-7/IL-24 as a therapeutic for cancer. This figure provides various strategies that offer promise of enhancing the ability to use MDA-7/IL-24 as an effective therapy for multiple cancer subtypes. These approaches that include MDA-7/IL-24 include gene-based, protein-based, use of genetic or Pharmacological inducers/stabilizers, cell-based, cell-penetrating peptides or tumor homing peptides, antibody-based and combination therapies (chemotherapy, immunotherapy, repurposed drugs, Ab-therapy, small molecule therapy, radiation therapy, etc.). Abbreviations used in this figure: Next Gen MDA-7: Next Generation MDA-7; CTV: cancer terminator virus; Next Gen CTV: Next Generation cancer terminator virus; UTMD: ultrasound targeted microbubble destruction; NP: nanoparticle; ESC: embryonic stem cells; MSC: mesenchymal stem cells; CPP: cell penetrating peptides; THP: tumor homing peptides; Ab-therapy: antibody-based therapy.