Small ubiquitin-related modifier-1 modification regulates all-trans-retinoic acid-induced differentiation via stabilization of retinoic acid receptor α

Retinoic acid signaling in development and differentiation commitment and its regulatory topology

Retinoic acid (RA), the derivative of vitamin A/retinol, is a signaling molecule with important implications in health and disease. It is a well-known developmental morphogen that functions mainly through the transcriptional activity of nuclear RA receptors (RARs) and, uncommonly, through other nuclear receptors, including peroxisome proliferator-activated receptors. Intracellular RA is under spatiotemporally fine-tuned regulation by synthesis and degradation processes catalyzed by retinaldehyde dehydrogenases and P450 family enzymes, respectively. In addition to dictating the transcription architecture, RA also impinges on cell functioning through non-genomic mechanisms independent of RAR transcriptional activity. Although RA-based differentiation therapy has achieved impressive success in the treatment of hematologic malignancies, RA also has pro-tumor activity. Here, we highlight the relevance of RA signaling in cell-fate determination, neurogenesis, visual function, inflammatory responses and gametogenesis commitment. Genetic and post-translational modifications of RAR are also discussed. A better understanding of RA signaling will foster the development of precision medicine to improve the defects caused by deregulated RA signaling.

SUMOylation in Skeletal Development, Homeostasis, and Disease

The modification of proteins by small ubiquitin-related modifier (SUMO) molecules, SUMOylation, is a key post-translational modification involved in a variety of biological processes, such as chromosome organization, DNA replication and repair, transcription, nuclear transport, and cell signaling transduction. In recent years, emerging evidence has shown that SUMOylation regulates the development and homeostasis of the skeletal system, with its dysregulation causing skeletal diseases, suggesting that SUMOylation pathways may serve as a promising therapeutic target. In this review, we summarize the current understanding of the molecular mechanisms by which SUMOylation pathways regulate skeletal cells in physiological and disease contexts.

Retinoid receptor turnover mediated by sumoylation, ubiquitination and the valosin-containing protein is disrupted in glioblastoma

Resistance to therapeutic use of retinoids in glioma has been observed for over 20 years; however, the exact mechanism of resistance remains unknown. To understand retinoic acid resistance in glioma, we studied the turnover mechanism of retinoid receptor proteins in neural stem cells and glioma stem-like cells. Here, we show that in normal neural stem cells, proteasomal degradation of retinoid receptors involves sumoylation, ubiquitination and recognition by the valosin-containing protein (VCP/p97/Cdc48). We find that Sumo1 modification has a dual role to stabilize the retinoid receptor from unwanted degradation and signal additional modification via ubiquitination. Subsequently, the modified receptor binds to the VCP chaperone and both proteins are degraded by the proteasome. Additionally, we reveal that all trans retinoic acid (ATRA) induces VCP expression, creating a positive feedback loop that enhances degradation. In contrast, the pathway is impaired in the glioma stem-like cells resulting in the accumulation of sumoylated and high molecular weight forms of retinoid receptors that lack transcriptional activity and fail to be recognized by the proteasome. Moreover, modified receptor accumulation occurs before ATRA treatment; therefore, the transcritptional defect in glioma is due to a block in the proteasomal degradation pathway that occurs after the sumo modification step.

Advances in differentiation therapy for osteosarcoma

Differentiation therapy involves the use of agents that can induce differentiation in cancer cells, with the irreversible loss of tumour phenotype. The application of differentiation therapy in osteosarcoma has made progress because of a better understanding of the potential links between differentiation defects and tumorigenesis. Here, we review recent studies on differentiation therapy for osteosarcoma, describing a variety of differentiation inducers. By highlighting these examples of drug-induced osteosarcoma cell differentiation, we can acquire unique insights into not only osteosarcoma treatment, but also novel approaches to transform differentiating drugs into more effective therapies for other solid tumours.

The SUMO Pathway in Hematomalignancies and Their Response to Therapies

SUMO (Small Ubiquitin-related MOdifier) is a post-translational modifier of the ubiquitin family controlling the function and fate of thousands of proteins. SUMOylation is deregulated in various hematological malignancies, where it participates in both tumorigenesis and cancer cell response to therapies. This is the case for Acute Promyelocytic Leukemias (APL) where SUMOylation, and subsequent destruction, of the PML-RARα fusion oncoprotein are triggered by arsenic trioxide, which is used as front-line therapy in combination with retinoic acid to cure APL patients. A similar arsenic-induced SUMO-dependent degradation was also documented for Tax, a human T-cell lymphotropic virus type I (HTLV1) viral protein implicated in Adult T-cell Leukemogenesis. SUMOylation also participates in Acute Myeloid Leukemia (AML) response to both chemo- and differentiation therapies, in particular through its ability to regulate gene expression. In Multiple Myeloma, many enzymes of the SUMO pathway are overexpressed and their high expression correlates with lower response to melphalan-based chemotherapies. B-cell lymphomas overexpressing the c-Myc oncogene also overexpress most components of the SUMO pathway and are highly sensitive to SUMOylation inhibition. Targeting the SUMO pathway with recently discovered pharmacological inhibitors, alone or in combination with current therapies, might therefore constitute a powerful strategy to improve the treatment of these cancers.

Targeting the SUMO Pathway Primes All-trans Retinoic Acid-Induced Differentiation of Nonpromyelocytic Acute Myeloid Leukemias

Differentiation therapies using all-trans retinoic acid (ATRA) are highly efficient at treating acute promyelocytic leukemia (APL), a subtype of acute myeloid leukemia (AML). However, their efficacy, if any, is limited in the case of non-APL AML. We report here that inhibition of SUMOylation, a posttranslational modification related to ubiquitination, restores the prodifferentiation and antiproliferative activities of retinoids in non-APL AML. Controlled inhibition of SUMOylation with the pharmacologic inhibitors 2-D08 or anacardic acid, or via overexpression of SENP deSUMOylases, enhanced the ATRA-induced expression of key genes involved in differentiation, proliferation, and apoptosis in non-APL AML cells. This activated ATRA-induced terminal myeloid differentiation and reduced cell proliferation and viability, including in AML cells resistant to chemotherapeutic drugs. Conversely, enhancement of SUMOylation via overexpression of the SUMO-conjugating enzyme Ubc9 dampened expression of ATRA-responsive genes and prevented differentiation. Thus, inhibition of the SUMO pathway is a promising strategy to sensitize patients with non-APL AML to retinoids and improve the treatment of this poor-prognosis cancer.Significance: SUMOylation silences key ATRA-responsive genes in nonpromyelocytic acute myeloid leukemias. Cancer Res; 78(10); 2601-13. ©2018 AACR.