SUMO-Specific Proteases and the Cell Cycle

Post-translational modification by SUMO is a dynamic and reversible process and several SUMO-specific proteases that remove SUMO from substrates have been identified. We have recently described the activities of a new SUMO-specific protease, SENP5. We found that SENP5 discriminates between SUMO-1 and SUMO-2/3 and cells depleted of SENP5 by RNAi failed to proliferate. Our findings support the idea that differential substrate selection by the mammalian SUMO-specific proteases underlies their regulation of distinct biological processes. Furthermore, our finding of a non-redundant function for SENP5 in cell proliferation provides further support for the model that, analogous to phosphorylation, cycles of SUMOylation and deSUMOylation regulate orderly progression through cell division.     

SUMO-specific protease 1 is critical for early lymphoid development through regulation of STAT5 activation

Small ubiquitin-like modifier (SUMO) modification has emerged as an important regulatory mechanism during embryonic development. However, it is not known whether SUMOylation plays a role in the development of the immune system. Here, we show that SUMO-specific protease 1 (SENP1) is essential for the development of early T and B cells. STAT5, a key regulator of lymphoid development, is modified by SUMO-2 and is specifically regulated by SENP1. In the absence of SENP1, SUMO-2 modified STAT5 accumulates in early lymphoid precursors, resulting in a block in its acetylation and subsequent signaling. These results demonstrate a crucial role of SENP1 in?the regulation of STAT5 activation during early lymphoid development.     

SENP1 modulates microglia-mediated neuroinflammation toward intermittent hypoxia-induced cognitive decline through the de-SUMOylation of NEMO

Intermittent hypoxia (IH)-induced cognition decline is related to the neuroinflammation in microglia. SUMOylation is associated with multiple human diseases, which can be reversed by sentrin/SUMO-specific proteases 1 (SENP1). Herein, we investigated the role of SENP1 in IH-induced inflammation and cognition decline. BV-2 microglial cells and mice were used for inflammatory response and cognition function evaluation following IH treatment. Biochemical analysis and Morris water maze methods were used to elaborate the mechanism of SENP1 in IH impairment. Molecular results revealed that IH induced the inflammatory response, as evidenced by the up-regulation of NF-κB activation, IL-1β and TNF-α in vitro and in vivo. Moreover, IH decreased the expression of SENP1, and increased the SUMOylation of NEMO, not NF-κB P65. Moreover, SENP1 overexpression inhibited IH-induced inflammatory response and SUMOylation of NEMO. However, the inhibitions were abolished by siRNA-NEMO. In contrast, SENP1 depletion enhanced IH-induced inflammatory response and SUMOylation of NEMO, accompanying with increased latency and reduced dwell time in mice. Overall, the results demonstrated that SENP1 regulated IH-induced neuroinflammation by modulating the SUMOylation of NEMO, thus activating the NF-κB pathway, revealing that targeting SENP1 in microglia may represent a novel therapeutic strategy for IH-induced cognitive decline.     

Antileukemic effects of topoisomerase I inhibitors mediated by de-SUMOylase SENP1

SUMO-specific proteases (SENPs) play pivotal roles in maintaining the balance of SUMOylation/de-SUMOylation and in SUMO recycling. Deregulation of SENPs leads to cellular dysfunction and corresponding diseases. As a key member of the SENP family, SENP1 is highly correlated with various cancers. However, the potential role of SENP1 in leukemia, especially in acute lymphoblastic leukemia (ALL), is not clear. This study shows that ALL cells knocking down SENP1 display compromised growth rather than significant alterations in chemosensitivity, although ALL relapse samples have a relatively higher expression of SENP1 than the paired diagnosis samples. Camptothecin derivatives 7-ethylcamptothecin (7E-CPT, a monomer compound) and topotecan (TPT, an approved clinical drug) induce specific SENP1 reduction and severe apoptosis of ALL cells, showing strong anticancer effects against ALL. Conversely, SENP1 could attenuate this inhibitory effect by targeting DNA topoisomerase I (TOP1) for de-SUMOylation, indicating that specific reduction in SENP1 induced by 7E-CPT and/or topotecan inhibits the proliferation of ALL cells.     

Characterization of a family of nucleolar SUMO-specific proteases with preference for SUMO-2 or SUMO-3

SUMOylation is a reversible process regulated by a family of sentrin/SUMO-specific proteases (SENPs). Of the six SENP family members, except for SENP1 and SENP2, the substrate specificities of the rest of SENPs are not well defined. Here, we have described SENP5, which has restricted substrate specificity. SENP5 showed SUMO-3 C-terminal hydrolase activity but could not process pro-SUMO-1 in vitro. Furthermore, SENP5 showed more limited isopeptidase activity in vitro. In vivo, SENP5 showed isopeptidase activity against SUMO-2 and SUMO-3 conjugates but not against SUMO-1 conjugates. Native SENP5 localized mainly to the nucleolus but was also found in the nucleus. The N terminus of SENP5 contains a stretch of amino acids responsible for the nucleolar localization of SENP5. N-terminal-truncated SENP5 co-localized with PML, a known SUMO substrate. Using PML SUMOylation mutants as model substrates, we showed that SENP5 can remove poly-SUMO-2 or poly-SUMO-3 from the Lys160 or Lys490 positions of PML. However, SENP5 could not remove SUMO-1 from the Lys160 or Lys490 positions of PML. Nonetheless, SENP5 could remove SUMO-1, -2, and -3 from the Lys65 position of PML. Thus, SENP5 also possesses limited SUMO-1 isopeptidase activity. We were also able to show that SENP3 has substrate specificity similar to that of SENP5. Thus, SENP3 and SENP5 constitute a subfamily of SENPs that regulate the formation of SUMO-2 or SUMO-3 conjugates and, to a less extent, SUMO-1 modification.     

SENP3‐mediated deSUMOylation of dynamin‐related protein 1 promotes cell death following ischaemia

Global increases in small ubiquitin‐like modifier (SUMO)‐2/3 conjugation are a neuroprotective response to severe stress but the mechanisms and specific target proteins that determine cell survival have not been identified. Here, we demonstrate that the SUMO‐2/3‐specific protease SENP3 is degraded during oxygen/glucose deprivation (OGD), an in vitro model of ischaemia, via a pathway involving the unfolded protein response (UPR) kinase PERK and the lysosomal enzyme cathepsin B. A key target for SENP3‐mediated deSUMOylation is the GTPase Drp1, which plays a major role in regulating mitochondrial fission. We show that depletion of SENP3 prolongs Drp1 SUMOylation, which suppresses Drp1‐mediated cytochrome c release and caspase‐mediated cell death. SENP3 levels recover following reoxygenation after OGD allowing deSUMOylation of Drp1, which facilitates Drp1 localization at mitochondria and promotes fragmentation and cytochrome c release. RNAi knockdown of SENP3 protects cells from reoxygenation‐induced cell death via a mechanism that requires Drp1 SUMOylation. Thus, we identify a novel adaptive pathway to extreme cell stress in which dynamic changes in SENP3 stability and regulation of Drp1 SUMOylation are crucial determinants of cell fate.     

pVHL-independent ubiquitination of HIF1alpha and its stabilization by cobalt ion

Recent studies have shown that hypoxia-inducible factor1alpha (HIF1alpha) is ubiquitinated by an E3-ligase complex containing von Hippel-Lindau gene product (pVHL) after which it is targeted for proteasomal degradation. In this study, we showed that HIF1alpha was stabilized in the pVHL-deficient cell line 786-0 treated with a proteasome inhibitor or Co(2+). This suggests that HIF1alpha is also ubiquitinated by a pVHL-independent pathway and that its stability is regulated by Co(2+). Indeed, using the COS cell expression system, we confirmed that HIF1alpha is ubiquitinated at the N-terminal region by a pVHL-independent pathway and that its degradation is inhibited by Co(2+). We also demonstrated that Co(2+) binds to both PAS domains in the N-terminal region of HIF1alpha. These observations imply that the stability of HIF1alpha is regulated by an additional pathway through the cobalt binding of PAS domains.