The role of vitamin D through SphK1/S1P in the regulation of MS progression.

Sphingosine-1-phosphate (S1P) is biologically active lipid, leading to neuroinflammation and macrophage invasion in central nervous system, plays an important role in the development of multiple sclerosis (MS) model in experimental allergic encephalomyelitis (EAE) rats. Vitamin D is observed to be a key factor in regulating cell S1P levels. We detected vitamin D can alleviate the symptoms of EAE rats, but the exact mechanism is unclear. In PC12 cells, vitamin D can reverse S1P-induced cell death, but the signaling pathway unclear. This study was aimed to investigate S1P regulation mechanism or signaling pathway mediated by vitamin D in EAE and PC12 model. In our experiments, S1P and Sphingosine kinase type 1 (SphK1) mRNA and protein expression in EAE rats group, control group, vitamin D feeding group were detected by HPLC, ELISA, RT-PCR and western blot. PC12 cell death was detected by Propidium (PI) staining. VDR plasmid overexpression and RNA interference, immunofluorescence, real-time cell analysis, protein immunoblotting was used to detect SphK1 transcriptional regulation, cell-substrate attachment quality, the signaling pathway of cell apoptosis and inflammation related gene expression (Bax/Bcl-2, Casepase-3, Il-6, TGF-ß, TNF-α). Our study showed vitamin D can reverse the elevation of S1P level in EAE rats, reduce the severity and shorten the course of EAE. 1,25-(OH) 2D3 coupled with vitamin D receptor (VDR) inhibited SphK1 transcription. 1,25-(OH)2D3 significantly reduced PC12 cell death rate induced by S1P, in addition improved the cell substrate attachment quality. 1,25-(OH) 2D3 can block S1P-induced p-ERK activation and PI3K /Akt signaling pathway reduced Il-6, TGF-ß, TNF-α cytokine release and Bax/Bcl-2, Casepase-3 apoptosis protein expression. On the other hand, immunofluorescence staining showed 1,25-(OH) 2D3 can increase the expression of neuronal perinuclear protein MAP2 in PC12 cells probably protect nerve cells further. In summary, the ameliorative effect of vitamin D was derived from its ability to reduce S1P levels, provides an idea for vitamin D as a combination therapy for disease.

Computational Strategy for Bound State Structure Prediction in Structure-Based Virtual Screening: A Case Study of Protein Tyrosine Phosphatase Receptor Type O Inhibitors.

Accurate protein structure in the ligand-bound state is a prerequisite for successful structure-based virtual screening (SBVS). Therefore, applications of SBVS against targets for which only an apo structure is available may be severely limited. To address this constraint, we developed a computational strategy to explore the ligand-bound state of a target protein, by combined use of molecular dynamics simulation, MM/GBSA binding energy calculation, and fragment-centric topographical mapping. Our computational strategy is validated against low-molecular weight protein tyrosine phosphatase (LMW-PTP) and then successfully employed in the SBVS against protein tyrosine phosphatase receptor type O (PTPRO), a potential therapeutic target for various diseases. The most potent hit compound GP03 showed an IC50 value of 2.89 µM for PTPRO and possessed a certain degree of selectivity toward other protein phosphatases. Importantly, we also found that neglecting the ligand energy penalty upon binding partially accounts for the false positive SBVS hits. The preliminary structure-activity relationships of GP03 analogs are also reported.

Allosteric modulation of the catalytic VYD loop in Slingshot by its N-terminal domain underlies both Slingshot auto-inhibition and activation.

Slingshots are phosphatases that modulate cytoskeleton dynamics, and their activities are tightly regulated in different physiological contexts. Recently, abnormally elevated Slingshot activity has been implicated in many human diseases, such as cancer, Alzheimer's disease, and vascular diseases. Therefore, Slingshot-specific inhibitors have therapeutic potential. However, an enzymological understanding of the catalytic mechanism of Slingshots and of their activation by actin is lacking. Here, we report that the N-terminal region of human Slingshot2 auto-inhibits its phosphatase activity in a noncompetitive manner. pH-dependent phosphatase assays and leaving-group dependence studies suggested that the N-terminal domain of Slingshot2 regulates the stability of the leaving group of the product during catalysis by modulating the general acid Asp361 in the catalytic VYD loop. F-actin binding relieved this auto-inhibition and restored the function of the general acid. Limited tryptic digestion and biophysical studies identified large conformational changes in Slingshot2 after the F-actin binding. The dissociation of N-terminal structural elements, including Leu63, and the exposure of the loop between α-helix-2 and ß-sheet-3 of the phosphatase domain served as the structural basis for Slingshot activation via F-actin binding in vitro and via neuregulin stimulation in cells. Moreover, we designed a FlAsH-BRET-based Slingshot2 biosensor whose readout was highly correlated with the in vivo phosphatase activities of Slingshot2. Our results reveal the auto-inhibitory mechanism and allosteric activation mechanisms of a human Slingshot phosphatase. They also contribute to the design of new strategies to study Slingshot regulation in various cellular contexts and to screen for new activators/inhibitors of Slingshot activity.

Switching of the substrate specificity of protein tyrosine phosphatase N12 by cyclin-dependent kinase 2 phosphorylation orchestrating 2 oncogenic pathways.

The protein tyrosine phosphatase nonreceptor type 12 (PTPN12) is a multifunctional protein and has elicited much research attention because its decreased protein level has been associated with poor prognosis of several types of cancers. Recently, we have solved the crystal structure of the phosphatase domain of PTPN12, which disclosed a specific PTPN12-insert-loop harboring a cyclin-dependent kinase 2 (CDK2) phosphorylation site. However, the functional significance of this phosphorylation is undefined. In the present study, we found that S19 site phosphorylation of PTPN12 by CDK2 discharged its antitumor activity by down-regulation of its inhibitory role in cell migration, but not affecting its other regulatory functions. Phosphorylation of PTPN12 at the S19 site changed its substrate interface, and by doing so, selectively decreased its activity toward the human epidermal growth factor receptor 2 (HER2)- pY1196 site, but not other HER2 phosphorylation sites or other known PTPN12 substrates. A further in-depth mechanism study revealed that the phosphorylation of PTPN12 by CDK2 impaired recruitment of the serine/threonine-protein kinase 1 (PAK1) to HER2, resulted in the blockade of the HER2-pY1196-PAK1-T423 signaling pathway, thus increased tumor cell motility. Taken together, our results identified a new phosphorylation-based substrate recognition mechanism of PTPN12 by CDK2, which orchestrated signaling crosstalk between the oncogenic CDK2 and HER2 pathways. The newly identified governing mechanism of the substrate selectivity of a particular phosphatase was previously unappreciated and exemplifies how a phospho-network is precisely controlled in different cellular contexts.-Li, H., Yang, D., Ning, S., Xu, Y., Yang, F., Yin, R., Feng, T., Han, S., Guo, L., Zhang, P., Qu, W., Guo, R., Song, C., Xiao, P., Zhou, C., Xu, Z., Sun, J.-P., Yu, X. Switching of the substrate specificity of protein tyrosine phosphatase N12 by cyclin-dependent kinase 2 phosphorylation orchestrating 2 oncogenic pathways.

Identification of para-Substituted Benzoic Acid Derivatives as Potent Inhibitors of the Protein Phosphatase Slingshot.

Slingshot proteins form a small group of dual-specific phosphatases that modulate cytoskeleton dynamics through dephosphorylation of cofilin and Lim kinases (LIMK). Small chemical compounds with Slingshot-inhibiting activities have therapeutic potential against cancers or infectious diseases. However, only a few Slingshot inhibitors have been investigated and reported, and their cellular activities have not been examined. In this study, we identified two rhodanine-scaffold-based para-substituted benzoic acid derivatives as competitive Slingshot inhibitors. The top compound, (Z)-4-((4-((4-oxo-2-thioxo-3-(o-tolyl)thiazolidin-5-ylidene)methyl)phenoxy)methyl)benzoic acid (D3) had an inhibition constant (Ki) of around 4 µm and displayed selectivity over a panel of other phosphatases. Moreover, compound D3 inhibited cell migration and cofilin dephosphorylation after nerve growth factor (NGF) or angiotensin II stimulation. Therefore, our newly identified Slingshot inhibitors provide a starting point for developing Slingshot-targeted therapies.

The catalytic region and PEST domain of PTPN18 distinctly regulate the HER2 phosphorylation and ubiquitination barcodes.

The tyrosine phosphorylation barcode encoded in C-terminus of HER2 and its ubiquitination regulate diverse HER2 functions. PTPN18 was reported as a HER2 phosphatase; however, the exact mechanism by which it defines HER2 signaling is not fully understood. Here, we demonstrate that PTPN18 regulates HER2-mediated cellular functions through defining both its phosphorylation and ubiquitination barcodes. Enzymologic characterization and three crystal structures of PTPN18 in complex with HER2 phospho-peptides revealed the molecular basis for the recognition between PTPN18 and specific HER2 phosphorylation sites, which assumes two distinct conformations. Unique structural properties of PTPN18 contribute to the regulation of sub-cellular phosphorylation networks downstream of HER2, which are required for inhibition of HER2-mediated cell growth and migration. Whereas the catalytic domain of PTPN18 blocks lysosomal routing and delays the degradation of HER2 by dephosphorylation of HER2 on pY(1112), the PEST domain of PTPN18 promotes K48-linked HER2 ubiquitination and its rapid destruction via the proteasome pathway and an HER2 negative feedback loop. In agreement with the negative regulatory role of PTPN18 in HER2 signaling, the HER2/PTPN18 ratio was correlated with breast cancer stage. Taken together, our study presents a structural basis for selective HER2 dephosphorylation, a previously uncharacterized mechanism for HER2 degradation and a novel function for the PTPN18 PEST domain. The new regulatory role of the PEST domain in the ubiquitination pathway will broaden our understanding of the functions of other important PEST domain-containing phosphatases, such as LYP and PTPN12.