Retinoblastoma tumor suppressor functions shared by stem cell and cancer cell strategies

Targeting RB1 Loss in Cancers

Simple Summary

Irreversible defects in RB1 tumor suppressor functions often predict poor outcomes in cancer patients. However, the RB1-defecient status can be a benefit as well for them, as it generates a variety of vulnerabilities induced through the upregulation of RB1 targets, relief from functional restrictions due to RB1 binding, presence of genes whose inactivation cause synthetic lethality with RB1 loss, or collateral synthetic lethality owing to simultaneous loss of neighboring genes.

Abstract

Retinoblastoma protein 1 (RB1) is encoded by a tumor suppressor gene that was discovered more than 30 years ago. Almost all mitogenic signals promote cell cycle progression by braking on the function of RB1 protein through mono- and subsequent hyper-phosphorylation mediated by cyclin-CDK complexes. The loss of RB1 function drives tumorigenesis in limited types of malignancies including retinoblastoma and small cell lung cancer. In a majority of human cancers, RB1 function is suppressed during tumor progression through various mechanisms. The latter gives rise to the acquisition of various phenotypes that confer malignant progression. The RB1-targeted molecules involved in such phenotypic changes are good quarries for cancer therapy. Indeed, a variety of novel therapies have been proposed to target RB1 loss. In particular, the inhibition of a number of mitotic kinases appeared to be synthetic lethal with RB1 deficiency. A recent study focusing on a neighboring gene that is often collaterally deleted together with RB1 revealed a pharmacologically targetable vulnerability in RB1-deficient cancers. Here we summarize current understanding on possible therapeutic approaches targeting functional or genomic aberration of RB1 in cancers.

Mutation of SPINOPHILIN (PPP1R9B) found in human tumors promotes the tumorigenic and stemness properties of cells

Rationale: SPINOPHILIN (SPN, PPP1R9B) is an important tumor suppressor involved in the progression and malignancy of different tumors depending on its association with protein phosphatase 1 (PP1) and the ability of the PP1-SPN holoenzyme to dephosphorylate retinoblastoma (pRB).

Methods: We performed a mutational analysis of SPN in human tumors, focusing on the region of interaction with PP1 and pRB. We explored the effect of the SPN-A566V mutation in an immortalized non-tumorigenic cell line of epithelial breast tissue, MCF10A, and in two different p53-mutated breast cancer cells lines, T47D and MDA-MB-468.

Results: We characterized an oncogenic mutation of SPN found in human tumor samples, SPN-A566V, that affects both the SPN-PP1 interaction and its phosphatase activity. The SPN-A566V mutation does not affect the interaction of the PP1-SPN holoenzyme with pocket proteins pRB, p107 and p130, but it affects its ability to dephosphorylate them during G0/G1 and G1, indicating that the PP1-SPN holoenzyme regulates cell cycle progression. SPN-A566V also promoted stemness, establishing a connection between the cell cycle and stem cell biology via pocket proteins and PP1-SPN regulation. However, only cells with both SPN-A566V and mutant p53 have increased tumorigenic and stemness properties.

Conclusions: SPN-A566V, or other equivalent mutations, could be late events that promote tumor progression by increasing the CSC pool and, eventually, the malignant behavior of the tumor.

Human Pluripotent Stem Cells-Based Therapies for Neurodegenerative Diseases: Current Status and Challenges

Neurodegenerative diseases are characterized by irreversible cell damage, loss of neuronal cells and limited regeneration potential of the adult nervous system. Pluripotent stem cells are capable of differentiating into the multitude of cell types that compose the central and peripheral nervous systems and so have become the major focus of cell replacement therapies for the treatment of neurological disorders. Human embryonic stem cell (hESC) and human induced pluripotent stem cell (hiPSC)-derived cells have both been extensively studied as cell therapies in a wide range of neurodegenerative disease models in rodents and non-human primates, including Parkinson’s disease, stroke, epilepsy, spinal cord injury, Alzheimer’s disease, multiple sclerosis and pain. In this review, we discuss the latest progress made with stem cell therapies targeting these pathologies. We also evaluate the challenges in clinical application of human pluripotent stem cell (hPSC)-based therapies including risk of oncogenesis and tumor formation, immune rejection and difficulty in regeneration of the heterogeneous cell types composing the central nervous system.

Beyond What Your Retina Can See: Similarities of Retinoblastoma Function between Plants and Animals, from Developmental Processes to Epigenetic Regulation

The Retinoblastoma protein (pRb) is a key cell cycle regulator conserved in a wide variety of organisms. Experimental analysis of pRb's functions in animals and plants has revealed that this protein participates in cell proliferation and differentiation processes. In addition, pRb in animals and its orthologs in plants (RBR), are part of highly conserved protein complexes which suggest the possibility that analogies exist not only between functions carried out by pRb orthologs themselves, but also in the structure and roles of the protein networks where these proteins are involved. Here, we present examples of pRb/RBR participation in cell cycle control, cell differentiation, and in the regulation of epigenetic changes and chromatin remodeling machinery, highlighting the similarities that exist between the composition of such networks in plants and animals.

A tumor suppressor Retinoblastoma1 is essential for embryonic development in the sea urchin

BACKGROUND

Embryonic cells and cancer cells share various cellular characteristics important for their functions. It has been thus proposed that similar mechanisms of regulation may be present in these otherwise disparate cell types.

RESULTS

To explore how regulative embryonic cells are fundamentally different from cancerous cells, we report here that a fine balance of a tumor suppressor protein Retinoblastoma1 (Rb1) and a germline factor Vasa are important for proper cell proliferation and differentiation of the somatic cells during embryogenesis of the sea urchin. Rb1 knockdown blocked embryonic development and induced Vasa accumulation in the entire embryo, while its overexpression resulted in a smaller-sized embryo with differentiated body structures. These results suggest that a titrated level of Rb1 protein may be essential for a proper balance of cell proliferation and differentiation during development. Vasa knockdown or overexpression, on the other hand, reduced or increased Rb1 protein expression, respectively.

CONCLUSIONS

Taken together, it appears that Vasa protein positively regulates Rb1 protein while Rb1 protein negatively regulates Vasa protein, balancing the act of these two antagonistic molecules in somatic cells. This mechanism may provide a fine control of cell proliferation and differentiation, which is essential for regulative embryonic development.

In vitro evaluation of barium titanate nanoparticle/alginate 3D scaffold for osteogenic human stem cell differentiation

Nanomaterials can mimic properties of extracellular matrix molecules, promising great potential for scaffold composition in tissue engineering. In the present study, we investigated whether barium titanate nanoparticles (BT NP) combined with alginate polymer would provide a new cytocompatible three-dimensional (3D) scaffold to induce osteogenic stem cell differentiation. In vitro cytocompatibility and osteogenic differentiation potential were investigated using human mesenchymal stem cells (MSC). Firstly, we studied the cell viability and oxidative stress by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) thiazolyl blue tetrazolium bromide (MTT) and superoxide dismutase (SOD) assays. Overall, neither pure BT NP or BT NP/alginate 3D scaffold induced cytotoxicity. The scanning electron and atomic force microscopy revealed that BT NP/alginate 3D scaffold produced exhibited highly interconnected pores and surface nanotopography that were favorable for MSC differentiation. Von Kossa staining showed mineralization nodules and MSCs morphology changed from spindle to cuboid shape after 21 d. Finally, BMP-2 and ALP mRNA were significantly upregulated on cells grown into the BT NP/alginate 3D scaffold. Thus, the BT NP/alginate 3D scaffold showed an osteogenic differentiation induction potential, without the addition of osteogenic supplements. These results indicate that the BT NP/alginate 3D scaffold provides a cytocompatible and bioactive microenvironment for osteogenic human MSC differentiation.

RNA-Sequencing of Primary Retinoblastoma Tumors Provides New Insights and Challenges Into Tumor Development

Retinoblastoma is rare tumor of the retina caused by the homozygous loss of the Retinoblastoma 1 tumor suppressor gene (RB1). Loss of the RB1 protein, pRB, results in de-regulated activity of the E2F transcription factors, chromatin changes and developmental defects leading to tumor development. Extensive microarray profiles of these tumors have enabled the identification of genes sensitive to pRB disruption, however, this technology has a number of limitations in the RNA profiles that they generate. The advent of RNA-sequencing has enabled the global profiling of all of the RNA within the cell including both coding and non-coding features and the detection of aberrant RNA processing events. In this perspective, we focus on discussing how RNA-sequencing of rare Retinoblastoma tumors will build on existing data and open up new area's to improve our understanding of the biology of these tumors. In particular, we discuss how the RB-research field may be to use this data to determine how RB1 loss results in the expression of; non-coding RNAs, causes aberrant RNA processing events and how a deeper analysis of metabolic RNA changes can be utilized to model tumor specific shifts in metabolism. Each section discusses new opportunities and challenges associated with these types of analyses and aims to provide an honest assessment of how understanding these different processes may contribute to the treatment of Retinoblastoma.

MicroRNA-140 mediates RB tumor suppressor function to control stem cell-like activity through interleukin-6

We established an in vitro cell culture system to determine novel activities of the retinoblastoma (Rb) protein during tumor progression. Rb depletion in p53-null mouse-derived soft tissue sarcoma cells induced a spherogenic phenotype. Cells retrieved from Rb-depleted spheres exhibited slower proliferation and less efficient BrdU incorporation, however, much higher spherogenic activity and aggressive behavior. We discovered six miRNAs, including mmu-miR-18a, -25, -29b, -140, -337, and -1839, whose expression levels correlated tightly with the Rb status and spherogenic activity. Among these, mmu-miR-140 appeared to be positively controlled by Rb and to antagonize the effect of Rb depletion on spherogenesis and tumorigenesis. Furthermore, among genes potentially targeted by mmu-miR-140, Il-6 was upregulated by Rb depletion and downregulated by mmu-mir-140 overexpression. Altogether, we demonstrate the possibility that mmu-mir-140 mediates the Rb function to downregulate Il-6 by targeting its 3′-untranslated region. Finally, we detected the same relationship among RB, hsa-miR-140 and IL-6 in a human breast cancer cell line MCF-7. Because IL-6 is a critical modulator of malignant features of cancer cells and the RB pathway is impaired in the majority of cancers, hsa-miR-140 might be a promising therapeutic tool that disrupts linkage between tumor suppressor inactivation and pro-inflammatory cytokine response.

Intersection of retinoblastoma tumor suppressor function, stem cells, metabolism, and inflammation

The Retinoblastoma (RB) tumor suppressor regulates G₁/S transition during cell cycle progression by modulating the activity of E2F transcription factors. The RB pathway plays a central role in the suppression of most cancers, and RB mutation was initially discovered by virtue of its role in tumor initiation. However, as cancer genome sequencing has evolved to profile more advanced and treatment‐resistant cancers, it has become increasingly clear that, in the majority of cancers, somatic RB inactivation occurs during tumor progression. Furthermore, despite the presence of deregulation of cell cycle control due to an INK4A deletion, additional CCND amplification and/or other mutations in the RB pathway, mutation or deletion of the RB gene is often observed during cancer progression. Of note, RB inactivation during cancer progression not only facilitates G₁/S transition but also enhances some characteristics of malignancy, including altered drug sensitivity and a return to the undifferentiated state. Recently, we reported that RB inactivation enhances pro‐inflammatory signaling through stimulation of the interleukin‐6/STAT3 pathway, which directly promotes various malignant features of cancer cells. In this review, we highlight the consequences of RB inactivation during cancer progression, and discuss the biological and pathological significance of the interaction between RB and pro‐inflammatory signaling.

A distinct function of the retinoblastoma protein in the control of lipid composition identified by lipidomic profiling

Here, by combining lipidomics with transcriptome analysis, we demonstrate that Rb depletion in mouse embryonic fibroblastss induces significant alterations in their lipid composition. We discovered that Rb depletion induced increase in lysophosphatidylserine, diacylglycerol (DAG), fatty acid (FA), acylcarnitine, phosphatidylcholine (PC), arachidonoyl ethanolamine, and decrease in phosphatidylglycerol, monoacylglycerol, without change in total lipid per protein levels. Analysis of the acyl chain composition of DAG, PC and phosphatidylserine revealed increase of saturated and mono-unsaturated acyl chains with specific carbon chain length. Consistently, we observed that Rb depletion increased the levels of fatty acids with the corresponding carbon chain length and number of carbon-carbon double bondssuch as myristic acid (14:0), palmitic acid (16:0), stearic acid (18:0) and all forms of FA 18:1. Microarray analysis revealed that Rb depletion induced significant upregulation of enzymes involved in elongation and desaturation of fatty acids. Among these, we found that elongation of long chain fatty acid family member 6 (Elovl6) and stearoyl-CoA desaturase 1 (Scd1) are the most robustly controlled by Rb possibly through E2F and sterol regulatory element-binding protein transcription factors. Depletion of Elovl6 or Scd1 significantly suppressed colony formation, sphere formation and xenograft tumor growth of Rb-deficient tumor cells. Suppression of self-renewal by the SCD1 inhibitor was rescued upon supplementation of the mono-unsaturated fatty acids generated by this enzyme. This study suggests a novel role for Rb in suppressing the malignant progression of tumors by controlling the lipid composition.