Inhibition of coding region determinant binding protein sensitizes melanoma cells to chemotherapeutic agents

Development of a specific and potent IGF2BP1 inhibitor: A promising therapeutic agent for IGF2BP1-expressing cancers

IGF2BP1 is a protein that controls the stability, localization, and translation of various mRNA targets. Poor clinical outcomes in numerous cancer types have been associated with its overexpression. As it has been demonstrated to impede tumor growth and metastasis in animal models, inhibiting IGF2BP1 function is a promising strategy for combating cancer. A lead chemical, 7773, which specifically decreased IGF2BP1 RNA binding and cellular activities, was previously identified in a high-throughput screen for effective IGF2BP1 inhibitors. Additional optimization of 7773 described in this manuscript led to the discovery of six compounds that performed equally well or better than 7773. In cell lines with high levels of endogenous IGF2BP1, one of 7773 derivatives, AVJ16, was found to be most efficient at preventing cell migration. Further, AVJ16 was found to be IGF2BP1-specific because it had no effect on cell lines that expressed little or no IGF2BP1 protein. The direct binding of AVJ16 to IGF2BP1 was validated by binding tests, with a 12-fold increase in binding efficiency over the lead compound. AVJ16 was shown to bind to a hydrophobic region at the protein's KH34 di-domain interface between the KH3 and KH4 domains. Overall, the findings imply that AVJ16 is a potent and specific inhibitor of IGF2BP1 activity.

Small molecule inhibitor of Igf2bp1 represses Kras and a pro-oncogenic phenotype in cancer cells

Igf2bp1 is an oncofetal RNA binding protein whose expression in numerous types of cancers is associated with upregulation of key pro-oncogenic RNAs, poor prognosis, and reduced survival. Importantly, Igf2bp1 synergizes with mutations in Kras to enhance signalling and oncogenic activity, suggesting that molecules inhibiting Igf2bp1 could have therapeutic potential. Here, we isolate a small molecule that interacts with a hydrophobic surface at the boundary of Igf2bp1 KH3 and KH4 domains, and inhibits binding to Kras RNA. In cells, the compound reduces the level of Kras and other Igf2bp1 mRNA targets, lowers Kras protein, and inhibits downstream signalling, wound healing, and growth in soft agar, all in the absence of any toxicity. This work presents an avenue for improving the prognosis of Igf2bp1-expressing tumours in lung, and potentially other, cancer(s).

Inhibition of the mRNA-Binding Protein IGF2BP1 Suppresses Proliferation and Sensitizes Neuroblastoma Cells to Chemotherapeutic Agents

Gain at chromosome 17q21 in neuroblastoma is associated with a poor prognosis, independent of MYCN amplification status. Several potential proto-oncogenes have been identified in this region, one of them-insulin-like growth-factor-2 mRNA binding protein (IGF2BP1)-is expressed at high levels in stage 4 tumors, and associated with overall lower patient survival. Here, we demonstrate that down-regulation of IGF2BP1 activity, either by transcript silencing or chemical inhibition, suppresses neuroblastoma cell growth. Furthermore, the combination of IGF2BP1 inhibition along with commonly used chemotherapeutics that broadly affect DNA synthesis, or cyclin-dependent kinase (CDK) inhibitors that disrupt signal transduction, have a synergistic effect on the suppression of neuroblastoma cell proliferation.

The underestimated role of the microphthalmia-associated transcription factor (MiTF) in normal and pathological haematopoiesis

Haematopoiesis, the process by which a restrained population of stem cells terminally differentiates into specific types of blood cells, depends on the tightly regulated temporospatial activity of several transcription factors (TFs). The deregulation of their activity or expression is a main cause of pathological haematopoiesis, leading to bone marrow failure (BMF), anaemia and leukaemia. TFs can be induced and/or activated by different stimuli, to which they respond by regulating the expression of genes and gene networks. Most TFs are highly pleiotropic; i.e., they are capable of influencing two or more apparently unrelated phenotypic traits, and the action of a single TF in a specific setting often depends on its interaction with other TFs and signalling pathway components. The microphthalmia-associated TF (MiTF) is a prototype TF in multiple situations. MiTF has been described extensively as a key regulator of melanocyte and melanoma development because it acts mainly as an oncogene. Mitf-mutated mice show a plethora of pleiotropic phenotypes, such as microphthalmia, deafness, abnormal pigmentation, retinal degeneration, reduced mast cell numbers and osteopetrosis, revealing a greater requirement for MiTF activity in cells and tissue. A growing amount of evidence has led to the delineation of key roles for MiTF in haematopoiesis and/or in cells of haematopoietic origin, including haematopoietic stem cells, mast cells, NK cells, basophiles, B cells and osteoclasts. This review summarizes several roles of MiTF in cells of the haematopoietic system and how MiTFs can impact BM development.

Extracellular vesicle-dependent effect of RNA-binding protein IGF2BP1 on melanoma metastasis

Insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1) is a multifunctional RNA-binding protein with an oncofetal pattern of expression shown to be implicated in the development of a variety of malignancies. In this study, we explored the role and mechanisms of IGF2BP1 in melanoma development and progression. In two different in vivo models, we showed that while genetic deletion or shRNA-mediated suppression of IGF2BP1 did not affect primary tumor formation, it drastically suppressed lung metastasis. Here we demonstrated that extracellular vesicles (EVs) secreted by melanoma cells mediate the effects of IGF2BP1 on metastasis: EVs from the IGF2BP1 knockdown melanoma cells failed to promote metastasis whereas EVs isolated from IGF2BP1-overexpressed melanoma cells further accelerated EV-induced metastasis. Moreover, the EVs from IGF2BP1 knockdown melanoma cells inhibited fibronectin deposition and accumulation of CD45⁺ cells in the lungs compared to control EVs, thus blocking the pre-metastatic niche formation potential of EVs. IGF2BP1 knockdown did not affect size, number, or protein/RNA concentration of secreted EVs or their uptake by recipient cells in vitro or in vivo. However, RNA-sequencing and proteomics analysis of the EVs revealed differential expression in a number of mRNA, proteins and miRNAs. This suggested that IGF2BP1 is intimately involved in the regulation of the cargo of EVs, thereby affecting the pro-metastatic function of melanoma-derived EVs. To the best of our knowledge, this is the first study that demonstrates the role of RNA-binding protein IGF2BP1 in EV-mediated promotion of melanoma metastasis and may provide novel avenues for the development of metastatic inhibitors.

Reversal of Resistance in Targeted Therapy of Metastatic Melanoma: Lessons Learned from Vemurafenib (BRAFV600E-Specific Inhibitor)

Malignant melanoma is the most aggressive form of skin cancer and has a very low survival rate. Over 50% of melanomas harbor various BRAF mutations with the most common being the V600E. BRAF^V600E mutation that causes constitutive activation of the MAPK pathway leading to drug-, immune-resistance, apoptosis evasion, proliferation, survival, and metastasis of melanomas. The ATP competitive BRAF^V600E selective inhibitor, vemurafenib, has shown dramatic success in clinical trials; promoting tumor regression and an increase in overall survival of patients with metastatic melanoma. Regrettably, vemurafenib-resistance develops over an average of six months, which renders melanomas resistant to other therapeutic strategies. Elucidation of the underlying mechanism(s) of acquisition of vemurafenib-resistance and design of novel approaches to override resistance is the subject of intense clinical and basic research. In this review, we summarize recent developments in therapeutic approaches and clinical investigations on melanomas with BRAF^V600E mutation to establish a new platform for the treatment of melanoma.

Targeting insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1) in metastatic melanoma to increase efficacy of BRAFV600E inhibitors

Melanoma is one of the deadliest forms of skin cancer. Although BRAF inhibitors significantly enhance survival of metastatic melanoma patients, most patients relapse after less than a year of treatment. We previously reported that mRNA binding protein Insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1) is overexpressed in metastatic melanoma and that expression of IGF2BP1 confers resistance to chemotherapeutic agents. Here we demonstrate that IGF2BP1 plays an important role in the sensitivity of melanoma to targeted therapy. Inhibition of IGF2BP1 enhances the effects of BRAF-inhibitor and BRAF-MEK inhibitors in BRAF^V600E melanoma. Also, knockdown of IGF2BP1 alone is sufficient to reduce tumorigenic characteristics in vemurafenib-resistant melanoma. These findings suggest that IGF2BP1 can be a novel therapeutic target for melanoma.

Microphthalmia-associated transcription factor expression levels in melanoma cells contribute to cell invasion and proliferation

Microphthalmia-associated transcription factor (MITF) is a nodal point in melanoma transcriptional network that regulates dozens of genes with critical functions in cell differentiation, proliferation and survival. Highly variable MITF expression levels exist in tumor cell subpopulations conferring marked heterogeneity and plasticity in the tumor tissue. A model has been postulated whereby lower MITF levels favour cell invasion and suppress proliferation, whereas high levels stimulate differentiation and proliferation. Additionally, MITF is considered to be a prosurvival gene and a lineage addiction oncogene in melanoma. Herein, we review how MITF expression may affect the melanoma phenotype with consequences on the survival, invasion and metastasis of melanoma cells, and we discuss the research challenges.

Association of MITF and other melanosome-related proteins with chemoresistance in melanoma tumors and cell lines

Previous studies in cell lines have suggested a role for melanosomes and related protein trafficking pathways in melanoma drug response. We have investigated the expression of six proteins related to melanosomes and melanogenesis (MITF, GPR143, gp100/PMEL, MLANA, TYRP1, and RAB27A) in pretreatment metastases from melanoma patients (n = 52) with different response to dacarbazine/temozolomide. Microphthalmia-associated transcription factor (MITF) and G-protein coupled receptor 143 (GPR143) showed significantly higher expression in nonresponders compared with responders. The premelanosome protein (gp100/PMEL) has been indicated previously in resistance to cisplatin in melanoma cells, but the expression levels of gp100/PMEL showed no association with response to dacarbazine/temozolomide in our clinical material. We also investigated the effects on chemosensitivity of siRNA inhibition of gp100/PMEL in the MNT-1 melanoma cell line. As expected from the study of the tumor material, no effect was detected with respect to response to temozolomide. However, knockdown of gp100/PMEL sensitized the cells to both paclitaxel and cisplatin. Overall, our results suggest that MITF, and several MITF-regulated factors, are associated with resistance to chemotherapy in melanoma and that different MITF targets can be of importance for different drugs.

Two Isoforms of the RNA Binding Protein, Coding Region Determinant-binding Protein (CRD-BP/IGF2BP1), Are Expressed in Breast Epithelium and Support Clonogenic Growth of Breast Tumor Cells

CRD-BP/IGF2BP1 has been characterized as an "oncofetal" RNA binding protein typically highly expressed in embryonic tissues, suppressed in normal adult tissues, but induced in many tumor types. In this study, we show that adult breast tissues express ubiquitous but low levels of CRD-BP protein and mRNA. Although CRD-BP mRNA expression is induced in breast tumor cells, levels remain ∼1000-fold lower than in embryonic tissues. Despite low expression levels, CRD-BP is required for clonogenic growth of breast cancer cells. We reveal that because the most common protein isoform in normal adult breast and breast tumors has an N-terminal deletion (lacking two RNA recognition motif (RRM) domains) and is therefore missing antibody epitopes, CRD-BP expression has been under-reported by previous studies. We show that a CRD-BP mutant mouse strain retains expression of the shorter transcript (ΔN-CRD-BP), which originates in intron 2, suggesting that the impact of complete ablation of this gene in mice is not yet known. Either the full-length CRD-BP or the N-terminally truncated version can rescue the clonogenicity of CRD-BP knockdown breast cancer cells, suggesting that clonogenic function is served by either CRD-BP isoform. In summary, although CRD-BP expression levels are low in breast cancer cells, this protein is necessary for clonogenic activity.

MITF in melanoma: mechanisms behind its expression and activity

MITF (microphthalmia-associated transcription factor) represents a melanocytic lineage-specific transcription factor whose role is profoundly extended in malignant melanoma. Over the last few years, the function of MITF has been tightly connected to plasticity of melanoma cells. MITF participates in executing diverse melanoma phenotypes defined by distinct gene expression profiles. Mutation-dependent alterations in MITF expression and activity have been found in a relatively small subset of melanomas. MITF activity is rather modulated by its upstream activators and suppressors operating on transcriptional, post-transcriptional and post-translational levels. These regulatory mechanisms also include epigenetic and microenvironmental signals. Several transcription factors and signaling pathways involved in the regulation of MITF expression and/or activity such as the Wnt/β-catenin pathway are broadly utilized by various types of tumors, whereas others, e.g., BRAF^V600E/ERK1/2 are more specific for melanoma. Furthermore, the MITF activity can be affected by the availability of transcriptional co-partners that are often redirected by MITF from their own canonical signaling pathways. In this review, we discuss the complexity of a multilevel regulation of MITF expression and activity that underlies distinct context-related phenotypes of melanoma and might explain diverse responses of melanoma patients to currently used therapeutics.

MicroRNA-340-mediated degradation of microphthalmia-associated transcription factor (MITF) mRNA is inhibited by coding region determinant-binding protein (CRD-BP)

Alternative cleavage and polyadenylation generates multiple transcript variants producing mRNA isoforms with different length 3'-UTRs. Alternative cleavage and polyadenylation enables differential post-transcriptional regulation via the availability of different cis-acting elements in 3'-UTRs. Microphthalmia-associated transcription factor (MITF) is a master regulator of melanocyte development and melanogenesis. This central transcription factor is also implicated in melanoma development. Here, we show that melanoma cells favor the expression of MITF mRNA with a shorter 3'-UTR. We also establish that this isoform is regulated by a micro RNA (miRNA/miR), miR-340. miR-340 interacts with two of its target sites on the MITF 3'-UTR, causing mRNA degradation as well as decreased expression and activity of MITF. Conversely, the RNA-binding protein, coding region determinant-binding protein, was shown to be highly expressed in melanoma, directly binds to the 3'-UTR of MITF mRNA, and prevents the binding of miR-340 to its target sites, resulting in the stabilization of MITF transcripts, elevated expression, and transcriptional activity of MITF. This regulatory interplay between RNA-binding protein and miRNA highlights an important mechanism for the regulation of MITF in melanocytes and malignant melanomas.

The role of IRES trans-acting factors in carcinogenesis

Regulation of protein expression through RNA metabolism is a key aspect of cellular homeostasis. Upon specific cellular stresses, distinct transcripts are selectively controlled to modify protein output in order to quickly and appropriately respond to stress. Reprogramming of the translation machinery is one node of this strict control that typically consists of an attenuation of the global, cap-dependent translation and accompanying switch to alternative mechanisms of translation initiation, such as internal ribosome entry site (IRES)-mediated initiation. In cancer, many aspects of the RNA metabolism are frequently misregulated to provide cancer cells with a growth and survival advantage. This includes changes in the expression and function of RNA binding proteins termed IRES trans-acting factors (ITAFs) that are central to IRES translation. In this review, we will examine select emerging, as well as established, ITAFs with important roles in cancer initiation and progression, and in particular their role in IRES-mediated translation. This article is part of a Special Issue entitled: Translation and Cancer.

High-throughput fluorescence anisotropy screen for inhibitors of the oncogenic mRNA binding protein, IMP-1

Cancer cell proliferation is regulated by oncogenes, such as c-Myc. An alternative approach to directly targeting individual oncogenes is to target IMP-1, an oncofetal protein that binds to and stabilizes messenger RNAs (mRNAs), leading to elevated expression of c-Myc and other oncogenes. Expression of IMP-1 is tightly correlated with a poor prognosis and reduced survival in ovarian, lung, and colon cancer. Small-molecule inhibitors of IMP-1 have not been reported. We established a fluorescence anisotropy/polarization microplate assay (FAMA) for analyzing binding of IMP-1 to a fluorescein-labeled 93 nucleotide c-Myc mRNA target (flMyc), developed the assay as a highly robust (Z' factor = 0.60) FAMA-based high-throughput screen for inhibitors of binding of IMP-1 to flMyc, and carried out a successful pilot screen of 17,600 small molecules. Our studies support rapidly filtering out toxic nonspecific inhibitors using an early cell-based assay in control cells lacking the target protein. The physiologic importance of verified hits from the in vitro high-throughput screen was demonstrated by identification of the first small-molecule IMP-1 inhibitor, a lead compound that selectively inhibits proliferation of IMP-1-positive cancer cells with very little or no effect on proliferation of IMP-1-negative cells.

Insulin-like growth factor 2 mRNA-binding proteins (IGF2BPs): post-transcriptional drivers of cancer progression?

The insulin-like growth factor-2 mRNA-binding proteins 1, 2, and 3 (IGF2BP1, IGF2BP2, IGF2BP3) belong to a conserved family of RNA-binding, oncofetal proteins. Several studies have shown that these proteins act in various important aspects of cell function, such as cell polarization, migration, morphology, metabolism, proliferation and differentiation. In this review, we discuss the IGF2BP family’s role in cancer biology and how this correlates with their proposed functions during embryogenesis. IGF2BPs are mainly expressed in the embryo, in contrast with comparatively lower or negotiable levels in adult tissues. IGF2BP1 and IGF2BP3 have been found to be re-expressed in several aggressive cancer types. Control of IGF2BPs’ expression is not well understood; however, let-7 microRNAs, β-catenin (CTNNB1) and MYC have been proposed to be involved in their regulation. In contrast to many other RNA-binding proteins, IGF2BPs are almost exclusively observed in the cytoplasm where they associate with target mRNAs in cytoplasmic ribonucleoprotein complexes (mRNPs). During development, IGF2BPs are required for proper nerve cell migration and morphological development, presumably involving the control of cytoskeletal remodeling and dynamics, respectively. Likewise, IGF2BPs modulate cell polarization, adhesion and migration in tumor-derived cells. Moreover, they are highly associated with cancer metastasis and the expression of oncogenic factors (KRAS, MYC and MDR1). However, a pro-metastatic role of IGF2BPs remains controversial due to the lack of ‘classical’ in vivo studies. Nonetheless, IGF2BPs could provide valuable targets in cancer treatment with many of their in vivo roles to be fully elucidated.

Involvement of the mRNA binding protein CRD-BP in the regulation of metastatic melanoma cell proliferation and invasion by hypoxia

We have previously shown that the mRNA binding protein CRD-BP is overexpressed in human melanomas, where it promotes cell survival and resistance to chemotherapy. The present study investigates the role of hypoxia, a common characteristic of the tumor microenvironment, in the regulation of CRD-BP expression and melanoma cell responses. We found that hypoxia increases CRD-BP levels in metastatic melanoma cell lines but not in melanocytes or primary melanoma cells. Hypoxic stimulation transcriptionally regulates CRD-BP by facilitating the acetylation of histones within the CRD-BP gene and by modulating the extent of HIF1α binding to the CRD-BP promoter. Hypoxia significantly enhances the proliferative and invasive potential of metastatic melanoma cells but not that of normal or primary melanoma cells. Furthermore, inhibition of CRD-BP impairs the ability of metastatic cells to proliferate and invade in response to hypoxia. These findings identify CRD-BP as a novel effector of hypoxic responses that is relevant for the selection of metastatic cells. This work also describes a previously unknown role for CRD-BP in the regulation of melanoma cell invasion and highlights the importance of the hypoxic microenvironment in determining cell fate.