CDK2 destabilizes tumor suppressor C/EBPα expression through ubiquitin‐mediated proteasome degradation in acute myeloid leukemia

Ring finger protein 138 inhibits transcription factor C/EBPα protein turnover leading to differentiation arrest in acute myeloid leukemia

E3 ubiquitin ligase, ring finger protein 138 (RNF138) is involved in several biological processes; however, its role in myeloid differentiation or tumorigenesis remains unclear. RNAseq data from TNMplot showed that RNF138 mRNA levels are highly elevated in acute myeloid leukemia (AML) bone marrow samples as compared with bone marrow of normal volunteers. Here, we show that RNF138 serves as an E3 ligase for the tumor suppressor CCAAT/enhancer binding protein (C/EBPα) and promotes its degradation leading to myeloid differentiation arrest in AML. Wild-type RNF138 physically interacts with C/EBPα and promotes its ubiquitin-dependent proteasome degradation while a mutant RNF-138 deficient in ligase activity though interacts with C/EBPα, fails to down-regulate it. We show that RNF138 depletion enhances endogenous C/EBPα levels in peripheral blood mononuclear cells (PBMCs) isolated from healthy volunteers. Our data further shows that RNF138-mediated degradation of C/EBPα negatively affects its transactivation potential on its target genes. Furthermore, RNF138 overexpression inhibits all-trans-retinoic acid-induced differentiation of HL-60 cells whereas RNF138 RNAi enhances. In line with RNF138 inhibiting C/EBPα protein turnover, we also observed that RNF138 overexpression inhibited β-estradiol (E2)-induced C/EBPα driven granulocytic differentiation in C/EBPα inducible K562-p42C/EBPα-estrogen receptor cells. Furthermore, we also recapitulated these findings in PBMCs isolated from AML patients where depletion of RNF138 increased the expression of myeloid differentiation marker CD11b. These results suggest that RNF138 inhibits myeloid differentiation by targeting C/EBPα for proteasomal degradation and may provide a plausible mechanism for loss of C/EBPα expression often observed in myeloid leukemia. Also, targeting RNF138 may resolve differentiation arrest by restoring C/EBPα expression in AML.

The roles of ubiquitination in AML

Acute myeloid leukemia (AML) is a heterogeneously malignant disorder resulting in poor prognosis. Ubiquitination, a major post-translational modification (PTM), plays an essential role in regulating various cellular processes and determining cell fate. Despite these initial insights, the precise role of ubiquitination in AML pathogenesis and treatment remains largely unknown. In order to address this knowledge gap, we explore the relationship between ubiquitination and AML from the perspectives of signal transduction, cell differentiation, and cell cycle control; and try to find out how this relationship can be utilized to inform new therapeutic strategies for AML patients.

A review on the role of cyclin dependent kinases in cancers

The Cyclin-dependent kinase (CDK) class of serine/threonine kinases has crucial roles in the regulation of cell cycle transition and is mainly involved in the pathogenesis of cancers. The expression of CDKs is controlled by a complex regulatory network comprised of genetic and epigenetic mechanisms, which are dysregulated during the progression of cancer. The abnormal activation of CDKs results in uncontrolled cancer cell proliferation and the induction of cancer stem cell characteristics. The levels of CDKs can be utilized to predict the prognosis and treatment response of cancer patients, and further understanding of the function and underlying mechanisms of CDKs in human tumors would pave the way for future cancer therapies that effectively target CDKs. Defects in the regulation of cell cycle and mutations in the genes coding cell-cycle regulatory proteins lead to unrestrained proliferation of cells leading to formation of tumors. A number of treatment modalities have been designed to combat dysregulation of cell cycle through affecting expression or activity of CDKs. However, effective application of these methods in the clinical settings requires recognition of the role of CDKs in the progression of each type of cancer, their partners, their interactions with signaling pathways and the effects of suppression of these kinases on malignant features. Thus, we designed this literature search to summarize these findings at cellular level, as well as in vivo and clinical levels.

SOX4-mediated FBW7 transcriptional upregulation confers Tamoxifen resistance in ER+ breast cancers via GATA3 downregulation

AIM

Tamoxifen-mediated endocrine therapy has been standard treatment for ER+ breast cancers; however, majority of them acquire resistance leading to disease relapse. Although numerous substrates of E3 ligase FBW7 are known, only a handful of factors that regulate FBW7 expression and function are reported. In particular, there remains a lack of in-depth understanding of FBW7 transcriptional regulation.

MATERIALS AND METHODS

Luciferase reporter assay was performed after cloning full length and truncated FBW7 promoters followed by Chromatin immunoprecipitation assay to validate binding of SOX4 on FBW7 promoter. Transcriptional regulation of FBW7 by SOX4 and their biological consequences with respect to ER+ breast cancer was then evaluated using immunoblotting and other cell based assays.

KEY FINDINGS

SOX4 positively regulates FBW7 at transcriptional level by binding to three putative SOX4 biding sites within 3.1 kb long FBW7 promoter. Analysis of publicly available RNAseq datasets also showed a positive correlation between SOX4 and FBW7 mRNA in cancer cell lines and patient samples. qPCR and Immunoblotting confirmed that transiently or stably expressed SOX4 induced both endogenous FBW7 mRNA and protein levels. Our findings further demonstrated that increased levels of SOX4 and FBW7 in MCF7 mammospheres promoted cancer stemness and tumor cell dormancy. We further showed that both MCF7 mammospheres and MCF^TAMR cells had elevated SOX4 levels which apparently enhanced FBW7 to potentiate GATA3 degradation leading to enhanced stemness, tumor dormancy and Tamoxifen resistance in MCF7^TAMR as well as patients with ER+ breast cancers.

SIGNIFICANCE

Targeting SOX4-FBW7-GATA3 axis may overcome tamoxifen resistance in ER+ breast cancers.

Physiological Overview of the Potential Link between the UPS and Ca2+ Signaling

The ubiquitin–proteasome system (UPS) is the main proteolytic pathway by which damaged target proteins are degraded after ubiquitination and the recruit of ubiquitinated proteins, thus regulating diverse physiological functions and the maintenance in various tissues and cells. Ca²⁺ signaling is raised by oxidative or ER stress. Although the basic function of the UPS has been extensively elucidated and has been continued to define its mechanism, the precise relationship between the UPS and Ca²⁺ signaling remains unclear. In the present review, we describe the relationship between the UPS and Ca²⁺ signaling, including Ca²⁺-associated proteins, to understand the end point of oxidative stress. The UPS modulates Ca²⁺ signaling via the degradation of Ca²⁺-related proteins, including Ca²⁺ channels and transporters. Conversely, the modulation of UPS is driven by increases in the intracellular Ca²⁺ concentration. The multifaceted relationship between the UPS and Ca²⁺ plays critical roles in different tissue systems. Thus, we highlight the potential crosstalk between the UPS and Ca²⁺ signaling by providing an overview of the UPS in different organ systems and illuminating the relationship between the UPS and autophagy.

Acute Myeloid Leukemia-Related Proteins Modified by Ubiquitin and Ubiquitin-like Proteins

Acute myeloid leukemia (AML), the most common form of an acute leukemia, is a malignant disorder of stem cell precursors of the myeloid lineage. Ubiquitination is one of the post-translational modifications (PTMs), and the ubiquitin-like proteins (Ubls; SUMO, NEDD8, and ISG15) play a critical role in various cellular processes, including autophagy, cell-cycle control, DNA repair, signal transduction, and transcription. Also, the importance of Ubls in AML is increasing, with the growing research defining the effect of Ubls in AML. Numerous studies have actively reported that AML-related mutated proteins are linked to Ub and Ubls. The current review discusses the roles of proteins associated with protein ubiquitination, modifications by Ubls in AML, and substrates that can be applied for therapeutic targets in AML.

CDK2-instigates C/EBPα degradation through SKP2 in Acute myeloid leukemia

Transcription factor CCAAT/enhancer-binding protein-alpha (C/EBPα) regulates myelopoiesis by coupling growth arrest with differentiation of myeloid progenitors. Mutations in one or both alleles are observed in 10-14% AML cases that render C/EBPα functionally inactive. Besides, antagonistic protein-protein interactions also impair C/EBPα expression and function. In recent independent studies, we showed that CDK2 and SKP2 downregulated C/EBPα expression in an ubiquitin-dependent proteasome degradation manner leading to differentiation block in AML. Here, we demonstrate that CDK2-instigated C/EBPα downregulation is actually mediated by SKP2. Mechanistically, we show that CDK2 stabilizes SKP2 by phosphorylating it at Ser64 and thereby potentiates C/EBPα ubiquitination and subsequent degradation in AML cells. Immunoblot experiments showed that CDK2 inhibition downregulated SKP2 levels and concomitantly enhanced C/EBPα levels in myeloid cells. We further show that while CDK2 promoted C/EBPα ubiquitination and inhibited its protein levels, negatively affected its transactivation potential and DNA binding ability, simultaneous SKP2 depletion abrogated CDK2-promoted ubiquitination and restored C/EBPα expression and function. Taken together, these findings consolidate that CDK2 potentiates SKP2-mediated C/EBPα degradation in AML and targeting CDK2-SKP2 axis can be harnessed for therapeutic benefit in AML. Hypothetical model depicts that SKP2-mediated C/EBPα proteasomal degradation is reinforced by CDK2. CDK2 phopshorylates SKP2 leading to its enhanced stabilization which in turn exaggerates C/EBPα degradation leading to differentiation arrest in AML.

Cyclin G2 upregulation impairs migration, invasion, and network formation through RNF123/Dvl2/JNK signaling in the trophoblast cell line HTR8/SVneo, a possible role in preeclampsia

Disruption of extravillous trophoblast (EVT) migration and invasion is considered to be responsible for pathological placentation in preeclampsia (PE). Cyclin G2 (CCNG2) is an atypical cyclin that inhibits cell cycle progression. However, its biological function and underlying molecular mechanism in PE are poorly understood. In this study, clinical data demonstrated that CCNG2 was significantly upregulated in PE placenta and associated with invasive EVT dysfunction. Additionally, Ccng2 knockout led to an attenuation of PE-like symptoms in the PE mouse model produced via treatment with NG-nitro-L-arginine methyl ester (L-NAME). In vitro, CCNG2 inhibited the migration, invasion, and endothelial-like network formation of human trophoblast cell line HTR8/SVneo. Mechanically, CCNG2 suppressed JNK-dependent Wnt/PCP signaling and its downstream indicators including epithelial-to-mesenchymal transition (EMT) markers and matrix metalloproteinases (MMPs) via promoting the polyubiquitination degradation of dishevelled 2 (Dvl2) protein in HTR8/SVneo cells. We also discovered that the E3 ligase Ring finger protein 123 (RNF123), as a novel CCNG2 target among HTR8/SVneo cells, interacted with Dvl2 and participated in CCNG2-induced polyubiquitination degradation of Dvl2. Moreover, we verified that the treatment of HTR8/SVneo cells with RNF123-specific siRNA improved polyubiquitination-induced degradation of Dvl2 and the activity of Wnt/PCP-JNK signaling mediated by CCNG2. Taken together, our results reveal that the CCNG2/RNF123/Dvl2/JNK axis may be involved in the pathogenesis and progression of PE through trophoblastic cell function modulation, thus probably providing us with new therapeutic strategies for PE treatment.

FBW7 Inhibits Myeloid Differentiation in Acute Myeloid Leukemia via GSK3-Dependent Ubiquitination of PU.1

Glycogen synthase kinase 3β (GSK3β), an ubiquitously expressed serine/threonine kinase is reported to be overexpressed and hyperactivated in cancers including acute myeloid leukemia (AML) where it promotes self-renewal, growth, and survival of AML cells. Therefore, GSK3β inhibition results in AML cell growth inhibition and myeloid differentiation. Here we identified master transcription factor PU.1 of monocyte-macrophage differentiation pathway as potential GSK3β target. We demonstrate that GSK3β phosphorylates PU.1 at Ser41 and Ser140 leading to its recognition and subsequent ubiquitin-mediated degradation by E3 ubiquitin ligase FBW7. This GSK3-dependent degradation of PU.1 by FBW7 inhibited monocyte-macrophage differentiation. We further showed that a phospho-deficient PU.1 mutant (PU.1-S41, S140A) neither bound to FBW7 nor was degraded by it. Consequently, PU.1-S41, S140A retained its transactivation, DNA-binding ability and promoted monocyte-macrophage differentiation of U937 cells even without phorbol 12-myristate 13-acetate (PMA) treatment. We further showed that FBW7 overexpression inhibited both PMA as well as M-CSF-induced macrophage differentiation of myeloid cell lines and peripheral blood mononuclear cells (PBMC) from healthy volunteers, respectively. Contrarily, FBW7 depletion promoted differentiation of these cells even without any inducer suggesting for a robust role of GSK3β-FBW7 axis in negatively regulating myeloid differentiation. Furthermore, we also recapitulated these findings in PBMCs isolated from patients with leukemia where FBW7 overexpression markedly inhibited endogenous PU.1 protein levels. In addition, PBMCs also showed enhanced differentiation when treated with M-CSF and GSK3 inhibitor (SB216763) together compared with M-CSF treatment alone. IMPLICATIONS: Our data demonstrate a plausible mechanism behind PU.1 restoration and induction of myeloid differentiation upon GSK3β inhibition and further substantiates potential of GSK3β as a therapeutic target in AML.