A subset of RAB proteins modulates PP2A phosphatase activity

Protein phosphatase 2A holoenzymes regulate leucine-rich repeat kinase 2 phosphorylation and accumulation

LRRK2 is a highly phosphorylated multidomain protein and mutations in the gene encoding LRRK2 are a major genetic determinant of Parkinson's disease (PD). Dephosphorylation at LRRK2's S910/S935/S955/S973 phosphosite cluster is observed in several conditions including in sporadic PD brain, in several disease mutant forms of LRRK2 and after pharmacological LRRK2 kinase inhibition. However, the mechanism of LRRK2 dephosphorylation is poorly understood. We performed a phosphatome-wide reverse genetics screen to identify phosphatases involved in the dephosphorylation of the LRRK2 phosphosite S935. Candidate phosphatases selected from the primary screen were tested in mammalian cells, Xenopus oocytes and in vitro. Effects of PP2A on endogenous LRRK2 phosphorylation were examined via expression modulation with CRISPR/dCas9. Our screening revealed LRRK2 phosphorylation regulators linked to the PP1 and PP2A holoenzyme complexes as well as CDC25 phosphatases. We showed that dephosphorylation induced by different kinase inhibitor triggered relocalisation of phosphatases PP1 and PP2A in LRRK2 subcellular compartments in HEK-293 T cells. We also demonstrated that LRRK2 is an authentic substrate of PP2A both in vitro and in Xenopus oocytes. We singled out the PP2A holoenzyme PPP2CA:PPP2R2 as a powerful phosphoregulator of pS935-LRRK2. Furthermore, we demonstrated that this specific PP2A holoenzyme induces LRRK2 relocalization and triggers LRRK2 ubiquitination, suggesting its involvement in LRRK2 clearance. The identification of the PPP2CA:PPP2R2 complex regulating LRRK2 S910/S935/S955/S973 phosphorylation paves the way for studies refining PD therapeutic strategies that impact LRRK2 phosphorylation.

Long noncoding RNA ZFAS1 promotes tumorigenesis through regulation of miR-150-5p/RAB9A in melanoma

Melanoma is the deadliest form of skin cancer and one of the most aggressive cancers. ZFAS1 is a newly identified lncRNA, playing an oncogenic role in several types of cancer. The present study aimed to investigate the function and mechanism of ZFAS1-induced regulation of melanoma. ZFAS1 expression was increased in melanoma tissues and cells compared with normal controls. ZFAS1 expression in metastatic tissues was higher than that in nonmetastatic subjects. Higher expression of ZFAS1 predicted lower survival rates. Knockdown of ZFAS1 decreased proliferation, increased apoptosis, decreased migration and invasion, and reduced epithelial-mesenchymal transition potential in melanoma cells. Moreover, ZFAS1 knockdown inhibited tumor growth in nude mice. There was a direct binding between ZFAS1 and miR-150-5p. ZFAS1 negatively regulated miR-150-5p expression and upregulation of miR-150-5p was involved in ZFAS1 knockdown-induced effect on proliferation, apoptosis, migration, and invasion. Using bioinformatics, we predicted the binding between RAB9A and miR-150-5p, and the direct interaction between RAB9A and miR-150-5p was confirmed by luciferase reporter and RNA immunoprecipitation assays. We also showed that RAB9A expression was regulated negatively by miR-150-5p, but was regulated positively by ZFAS1. Downregulation of RAB9A significantly inhibited the increase in proliferation, decrease in apoptosis, and increase in migration and invasion induced by miR-150-5p inhibitors. Moreover, RAB9A knockdown decreased proliferation, increased apoptosis, and decreased migration and invasion in melanoma cells. In summary, we confirmed the tumor-promoting role of ZFAS1 in melanoma and provide evidence for the role and mechanism of the ZFAS1/miR-150-5p/RAB9A axis. These findings may lead to novel therapeutic strategies for melanoma.

Protein phosphatase 2A promotes stomatal development by stabilizing SPEECHLESS in Arabidopsis

Stomatal guard cells control gas exchange that allows plant photosynthesis but limits water loss from plants to the environment. In Arabidopsis, stomatal development is mainly controlled by a signaling pathway comprising peptide ligands, membrane receptors, a mitogen-activated protein kinase (MAPK) cascade, and a set of transcription factors. The initiation of the stomatal lineage requires the activity of the bHLH transcription factor SPEECHLESS (SPCH) with its partners. Multiple kinases were found to regulate SPCH protein stability and function through phosphorylation, yet no antagonistic protein phosphatase activities have been identified. Here, we identify the conserved PP2A phosphatases as positive regulators of Arabidopsis stomatal development. We show that mutations in genes encoding PP2A subunits result in lowered stomatal production in Arabidopsis Genetic analyses place the PP2A function upstream of SPCH. Pharmacological treatments support a role for PP2A in promoting SPCH protein stability. We further find that SPCH directly binds to the PP2A-A subunits in vitro. In plants, nonphosphorylatable SPCH proteins are less affected by PP2A activity levels. Thus, our research suggests that PP2A may function to regulate the phosphorylation status of the master transcription factor SPCH in stomatal development.

RAB9A Plays an Oncogenic Role in Human Liver Cancer Cells

Background

RAB9, as a member of the Rab GTPase family, is required for the transport of the mannose-6-phosphate receptor (MPR) from late endosomes to trans-Golgi network (TGN). However, the role of RAB9A in tumors, including liver cancer, is still unknown.

Methods

We used pcDNA3.1 plasmid to upregulate the expression of RAB9A in Hep3b cells and used specific shRNA to downregulate the expression of RAB9A in HepG2 cells. Biological functions of RAB9A were performed by CCK-8 assay, colony formation assay, apoptosis analysis, transwell assays, and wound healing assays. Finally, an in-depth mechanism study was performed by western blot.

Results

RAB9A promoted the proliferation and clonality of Hep3b and HepG2 cells. RAB9A also inhibited apoptosis and the activation of mitochondrial apoptotic pathway. In addition, RAB9A promoted the invasion and migration of Hep3b and HepG2 cells. Importantly, RAB9A activated the AKT/mTOR signaling pathway in human liver cancer cells. A double-effect inhibitor (BEZ235) significantly hindered the effect of RAB9A overexpression on the proliferation and invasion of Hep3b cells.

Conclusion

Our data suggest that RAB9A plays a carcinogenic role in human liver cancer progression partially through AKT signaling pathways, suggesting that RAB9A may serve as a potential therapeutic target for liver cancer therapy.

RABL6A inhibits tumor-suppressive PP2A/AKT signaling to drive pancreatic neuroendocrine tumor growth

Hyperactivated AKT/mTOR signaling is a hallmark of pancreatic neuroendocrine tumors (PNETs). Drugs targeting this pathway are used clinically, but tumor resistance invariably develops. A better understanding of factors regulating AKT/mTOR signaling and PNET pathogenesis is needed to improve current therapies. We discovered that RABL6A, a new oncogenic driver of PNET proliferation, is required for AKT activity. Silencing RABL6A caused PNET cell-cycle arrest that coincided with selective loss of AKT-S473 (not T308) phosphorylation and AKT/mTOR inactivation. Restoration of AKT phosphorylation rescued the G1 phase block triggered by RABL6A silencing. Mechanistically, loss of AKT-S473 phosphorylation in RABL6A-depleted cells was the result of increased protein phosphatase 2A (PP2A) activity. Inhibition of PP2A restored phosphorylation of AKT-S473 in RABL6A-depleted cells, whereas PP2A reactivation using a specific small-molecule activator of PP2A (SMAP) abolished that phosphorylation. Moreover, SMAP treatment effectively killed PNET cells in a RABL6A-dependent manner and suppressed PNET growth in vivo. The present work identifies RABL6A as a new inhibitor of the PP2A tumor suppressor and an essential activator of AKT in PNET cells. Our findings offer what we believe is a novel strategy of PP2A reactivation for treatment of PNETs as well as other human cancers driven by RABL6A overexpression and PP2A inactivation.