SUMOylation of MCL1 protein enhances its stability by regulating the ubiquitin-proteasome pathway

KLF11 promotes the proliferation of breast cancer cells by inhibiting p53-MDM2 signaling

Abnormal Krüppel-like factor 11 (KLF11) expression is frequently found in tumor tissues and is associated with cancer prognosis, but its biological functions and corresponding mechanisms remain elusive. Here, we demonstrated that KLF11 functions as an oncoprotein to promote tumor proliferation in breast cancer cells. Mechanistically, at the transcription level, KLF11 decreased TP53 mRNA expression. Notably, KLF11 also interacted with and stabilized MDM2 through inhibiting MDM2 ubiquitination and subsequent degradation. This increase in MDM2 in turn accelerated the ubiquitin-mediated proteolysis of p53, leading to the reduced expression of p53 and its target genes, including CDKN1A, BAX, and NOXA1. Accordingly, data from animals further confirmed that KLF11 significantly upregulated the growth of breast cancer cells and was inversely correlated with p53 expression. Taken together, our findings reveal a novel mechanism for breast cancer progression in which the function of the tumor suppressor p53 is dramatically weakened.

Low-dose radiation-induced SUMOylation of NICD1 negatively regulates osteogenic differentiation in BMSCs

Various biological effects of ionizing radiation, especially continuous exposure to low-dose radiation (LDR), have attracted considerable attention. Impaired bone structure caused by LDR has been reported, but little is known about the mechanism involved in the disruption of bone metabolism. In this study, given that LDR was found to (at a cumulative dose of 0.10 Gy) disturb the serum Mg2+ level and Notch1 signal in the mouse femur tissues, the effects of LDR on osteogenesis and the underlying molecular mechanisms were investigated based on an in vitro culture system for bone marrow stromal cells (BMSCs). Our data showed that cumulative LDR suppressed the osteogenic potential in BMSCs as a result of upregulation of Notch1 signaling. Further analyses indicated that the upregulation of NICD1 (Notch1 intracellular domain), the key intracellular domain for Notch1 signaling, under LDR was a consequence of enhanced protein stabilization caused by SUMOylation (small ubiquitin-like modification). Specifically, the downregulation of SENP1 (sentrin/SUMO-specific protease 1) expression induced by LDR enhanced the SUMOylation of NICD1, causing the accumulation of Notch1 signaling, which eventually inhibited the osteogenic potential of BMSCs. In conclusion, this work expounded on the mechanisms underlying the impacts of LDR on bone metabolism and shed light on the research on bone regeneration under radiation.

SENP1 knockdown-mediated CTCF SUMOylation enhanced its stability and alleviated lipopolysaccharide-evoked inflammatory injury in human lung fibroblasts via regulation of FOXA2 transcription

BACKGROUND

Excessive inflammation is the main cause of treatment failure in neonatal pneumonia (NP). CCCTC-binding factor (CTCF) represents an important node in various inflammatory diseases. In the present study, we tried to clarify the function and underlying molecular mechanism of CTCF on an in vitro cellular model of NP, which was generated by simulating the human lung fibroblast cell line WI-38 with lipopolysaccharide (LPS).

METHODS

The SUMOylation level and protein interaction were verified by Co-immunoprecipitation assay. Cell viability was measured by Cell Counting Kit-8 assay. Inflammatory factors were examined by Enzyme-linked immunosorbent assay. Cell apoptosis was evaluated by TUNEL assay. The binding activity of CTCF to target promoter was tested by chromatin immunoprecipitation and luciferase reporter assay.

RESULTS

LPS treatment restrained cell viability, promoted the production of inflammatory factors, and enhanced cell apoptosis. CTCF overexpression played anti-inflammatory and anti-apoptotic roles. Furthermore, CTCF was modified by SUMOylation with small ubiquitin-like modifier protein 1 (SUMO1). Interfering with sumo-specific protease 1 (SENP1) facilitated CTCF SUMOylation and protein stability, thus suppressing LPS-evoked inflammatory and apoptotic injuries. Moreover, CTCF could bind to the forkhead box protein A2 (FOXA2) promoter region to promote FOXA2 expression. The anti-inflammatory and anti-apoptotic roles of CTCF are associated with FOXA2 activation. In addition, SENP1 knockdown increased FOXA2 expression by enhancing the abundance and binding ability of CTCF.

CONCLUSIONS

SUMOylation of CTCF by SENP1 knockdown enhanced its protein stability and binding ability and it further alleviated LPS-evoked inflammatory injury in human lung fibroblasts by positively regulating FOXA2 transcription.

SENP2 Promotes VSMC Phenotypic Switching via Myocardin De-SUMOylation

Myocardin is a master regulator of smooth muscle cell (SMC) differentiation, which induces the expression of smooth-muscle-specific genes through its direct association with serum response factor (SRF). During the past two decades, significant insights have been obtained regarding the regulatory control of myocardin expression and transcriptional activity at the transcriptional, post-transcriptional, and post-translational levels. However, whether and how SUMOylation plays important roles in modulating myocardin function remain elusive. In this study, we found that myocardin is modified by SUMO-1 at lysine 573, which can be reversibly de-conjugated by SENP2. SUMO-1 modification promotes myocardin protein stability, whereas SENP2 facilitates its proteasome-dependent degradation. Moreover, we found that PIAS4 is the SUMO E3 ligase that enhances the SUMOylation and protein stability of myocardin. Most importantly, we found that SENP2 promotes phenotypic switching of VSMC. We therefore concluded that SENP2 promotes VSMC phenotypic switching via de-SUMOylation of myocardin and regulation of its protein stability.

Mcl-1 Differentially Regulates Autophagy in Response to Changes in Energy Status and Mitochondrial Damage

Myeloid cell leukemia-1 (Mcl-1) is a unique antiapoptotic Bcl-2 member that is critical for mitochondrial homeostasis. Recent studies have demonstrated that Mcl-1′s functions extend beyond its traditional role in preventing apoptotic cell death. Specifically, data suggest that Mcl-1 plays a regulatory role in autophagy, an essential degradation pathway involved in recycling and eliminating dysfunctional organelles. Here, we investigated whether Mcl-1 regulates autophagy in the heart. We found that cardiac-specific overexpression of Mcl-1 had little effect on baseline autophagic activity but strongly suppressed starvation-induced autophagy. In contrast, Mcl-1 did not inhibit activation of autophagy during myocardial infarction or mitochondrial depolarization. Instead, overexpression of Mcl-1 increased the clearance of depolarized mitochondria by mitophagy independent of Parkin. The increase in mitophagy was partially mediated via Mcl-1′s LC3-interacting regions and mutation of these sites significantly reduced Mcl-1-mediated mitochondrial clearance. We also found that Mcl-1 interacted with the mitophagy receptor Bnip3 and that the interaction was increased in response to mitochondrial stress. Overall, these findings suggest that Mcl-1 suppresses nonselective autophagy during nutrient limiting conditions, whereas it enhances selective autophagy of dysfunctional mitochondria by functioning as a mitophagy receptor.

The Role of SUMO E3 Ligases in Signaling Pathway of Cancer Cells

Small ubiquitin-like modifier (SUMO)ylation is a reversible post-translational modification that plays a crucial role in numerous aspects of cell physiology, including cell cycle regulation, DNA damage repair, and protein trafficking and turnover, which are of importance for cell homeostasis. Mechanistically, SUMOylation is a sequential multi-enzymatic process where SUMO E3 ligases recruit substrates and accelerate the transfer of SUMO onto targets, modulating their interactions, localization, activity, or stability. Accumulating evidence highlights the critical role of dysregulated SUMO E3 ligases in processes associated with the occurrence and development of cancers. In the present review, we summarize the SUMO E3 ligases, in particular, the novel ones recently identified, and discuss their regulatory roles in cancer pathogenesis.

The regulation of neuronal autophagy and cell survival by MCL1 in Alzheimer's disease

Maintaining neuronal integrity and functions requires precise mechanisms controlling organelle and protein quality. Alzheimer's disease (AD) is characterized by functional defects in the clearance and recycling of intracellular components. As such, neuronal homeostasis involves autophagy, mitophagy, and apoptosis. Compromised activity in these cellular processes may cause pathological phenotypes of AD. Dysfunction of mitochondria is one of the hallmarks of AD. Mitophagy is a critical mitochondria quality control system, and the impaired mitophagy is observed in AD. Myeloid cell leukemia 1 (MCL1), a member of the pro-survival B-cell lymphoma protein 2 (BCL2) family, is a mitochondria-targeted protein that contributes to maintaining mitochondrial integrity. Mcl1 knockout mice display peri-implantation lethality. The studies on conditional Mcl1 knockout mice demonstrate that MCL1 plays a central role in neurogenesis and neuronal survival during brain development. Accumulating evidence reveals the critical role of MCL1 as a regulator of neuronal autophagy, mitophagy, and survival. In this review, we discuss the emerging neuroprotective function of MCL1 and how dysregulation of MCL1 signaling is involved in the pathogenesis of AD. As the pro-survival BCL2 family of proteins are promising targets of pharmacological intervention with BH3 mimetic drugs, we also discuss the promise of MCL1-targeting therapy in AD.

LncRNA GAL promotes colorectal cancer liver metastasis through stabilizing GLUT1

Colorectal cancer liver metastasis (CRLM) is the leading cause of colorectal cancer-related deaths and remains a clinical challenge. Enhancement of glucose uptake is involved in CRLM; however, whether long noncoding RNAs (lncRNAs) participate in these molecular events remains largely unclear. Here, we report an lncRNA, GAL (glucose transporter 1 (GLUT1) associated lncRNA), that was upregulated in CRLM tissues compared with primary colorectal cancer (CRC) tissues or matched normal tissues and was associated with the overall survival rates of CRLM patients. Functionally, GAL served as an oncogene because it promoted CRC cell migration and invasion in vitro and enhanced the ability of CRC cells to metastasize from the intestine to the liver in vivo. Mechanistically, GAL interacted with the GLUT1 protein to increase GLUT1 SUMOylation, inhibiting the effect of the ubiquitin-proteasome system on the GLUT1 protein. GLUT1-knockout (-/+) repressed the GAL-mediated increase in CRC cell uptake of glucose, migrate, and invade in vitro, as well as metastasis from the intestine to the liver in vivo, and enforced expression of GLUT1 rescued GAL knockout-induced biological functions in CRC cells. Taken together, our findings demonstrated that GAL promotes CRLM by stabilizing GLUT1, suggesting that the GAL-GLUT1 complex may act as a potential therapeutic target for CRLM.

Novel insights into the impact of the SUMOylation pathway in hematological malignancies (Review)

The small ubiquitin-like modifier (SUMO) system serves an important role in the regulation of protein stability and function. SUMOylation sustains the homeostatic equilibrium of protein function in normal tissues and numerous types of tumor. Accumulating evidence has revealed that SUMO enzymes participate in carcinogenesis via a series of complex cellular or extracellular processes. The present review outlines the physiological characteristics of the SUMOylation pathway and provides examples of SUMOylation participation in different cancer types, including in hematological malignancies (leukemia, lymphoma and myeloma). It has been indicated that the SUMO pathway may influence chromosomal instability, cell cycle progression, apoptosis and chemical drug resistance. The present review also discussed the possible relationship between SUMOylation and carcinogenic mechanisms, and evaluated their potential as biomarkers and therapeutic targets in the diagnosis and treatment of hematological malignancies. Developing and investigating inhibitors of SUMO conjugation in the future may offer promising potential as novel therapeutic strategies.

Myeloid cell leukemia 1 (MCL-1): Structural characteristics and application in cancer therapy

Apoptosis, a major hallmark of cancer cells, regulates cellular fate and homeostasis. BCL-2 (B-cell CLL/Lymphoma 2) protein family is popularly known to mediate the intrinsic mode of apoptosis, of which MCL-1 is a crucial member. Myeloid cell leukemia 1 (MCL-1) is an anti-apoptotic oncoprotein and one of the most investigated members of the BCL-2 family. It is commonly known to be genetically altered, aberrantly overexpressed, and primarily associated with drug resistance in various human cancers. Recent advancements in the development of selective MCL-1 inhibitors and evaluating their effectiveness in cancer treatment establish its popularity as a molecular target. The overall aim is the selective induction of apoptosis in cancer cells by using a single or combination of BCL-2 family inhibitors. Delineating the precise molecular mechanisms associated with MCL-1-mediated cancer progression will certainly improve the efficacy of clinical interventions aimed at MCL-1 and hence patient survival. This review is structured to highlight the structural characteristics of MCL-1, its specific interactions with NOXA, MCL-1-regulatory microRNAs, and at the same time focus on the emerging therapeutic strategies targeting our protein of interest (MCL-1), alone or in combination with other treatments.

The role of post-translational modifications in the regulation of MCL1

Apoptosis is an evolutionarily conserved form of programed cell death (PCD) that has a vital effect on early embryonic development, tissue homeostasis and clearance of damaged cells. Dysregulation of apoptosis can lead to many diseases, such as Alzheimer's disease, cancer, AIDS and heart disease. The anti-apoptotic protein MCL1, a member of the BCL2 family, plays important roles in these physiological and pathological processes. Its high expression is closely related to drug resistances in the treatment of tumor. This review summarizes the structure and function of MCL1, the types of post-translational modifications of MCL1 and their effects on the functions of MCL1, as well as the treatment strategies targeting MCL1 in cancer therapy. The research on the fine regulation of MCL1 will be favorable to the provision of a promising future for the design and screening of MCL1 inhibitors.

SUMO modification in apoptosis

Apoptosis and clearance of dead cells is highly evolutionarily conserved from nematode to humans, which is crucial to the growth and development of multicellular organism. Fail to remove apoptotic cells often lead to homeostasis imbalance, fatal autoimmune diseases, and neurodegenerative diseases. Small ubiquitin-related modifiers (SUMOs) modification is a post-translational modification of ubiquitin proteins mediated by the sentrin-specific proteases (SENPs) family. SUMO modification is widely involved in many cellular biological process, and abnormal SUMO modification is also closely related to many major human diseases. Recent researches have revealed that SUMO modification event occurs during apoptosis and clearance of apoptotic cells, and plays an important role in the regulation of apoptotic signaling pathways. This review summarizes some recent progress in the revelation of regulatory mechanisms of these pathways and provides some potential researching hotpots of the SUMO modification regulation to apoptosis.