PREX2 contributes to radiation resistance by inhibiting radiotherapy-induced tumor immunogenicity via cGAS/STING/IFNs pathway in colorectal cancer.

BACKGROUND: Colorectal cancer (CRC) lacks established biomarkers or molecular targets for predicting or enhancing radiation response. Phosphatidylinositol-3,4,5-triphosphate-dependent Rac exchange factor 2 (PREX2) exhibits intricate implications in tumorigenesis and progression. Nevertheless, the precise role and underlying mechanisms of PREX2 in CRC radioresistance remain unclear. METHODS: RNA-seq was employed to identify differentially expressed genes between radioresistant CRC cell lines and their parental counterparts. PREX2 expression was scrutinized using Western blotting, real-time PCR, and immunohistochemistry. The radioresistant role of PREX2 was assessed through in vitro colony formation assay, apoptosis assay, comet assay, and in vivo xenograft tumor models. The mechanism of PREX2 was elucidated using RNA-seq and Western blotting. Finally, a PREX2 small-molecule inhibitor, designated PREX-in1, was utilized to enhance the efficacy of ionizing radiation (IR) therapy in CRC mouse models. RESULTS: PREX2 emerged as the most significantly upregulated gene in radioresistant CRC cells. It augmented the radioresistant capacity of CRC cells and demonstrated potential as a marker for predicting radioresistance efficacy. Mechanistically, PREX2 facilitated DNA repair by upregulating DNA-PKcs, suppressing radiation-induced immunogenic cell death, and impeding CD8+ T cell infiltration through the cGAS/STING/IFNs pathway. In vivo, the blockade of PREX2 heightened the efficacy of IR therapy. CONCLUSIONS: PREX2 assumes a pivotal role in CRC radiation resistance by inhibiting the cGAS/STING/IFNs pathway, presenting itself as a potential radioresistant biomarker and therapeutic target for effectively overcoming radioresistance in CRC.

Small-molecule inhibitors of P-Rex guanine-nucleotide exchange factors.

P-Rex1 and P-Rex2 are guanine-nucleotide exchange factors (GEFs) that activate Rac small GTPases in response to the stimulation of G protein-coupled receptors and phosphoinositide 3-kinase. P-Rex Rac-GEFs regulate the morphology, adhesion and migration of various cell types, as well as reactive oxygen species production and cell cycle progression. P-Rex Rac-GEFs also have pathogenic roles in the initiation, progression or metastasis of several types of cancer. With one exception, all P-Rex functions are known or assumed to be mediated through their catalytic Rac-GEF activity. Thus, inhibitors of P-Rex Rac-GEF activity would be valuable research tools. We have generated a panel of small-molecule P-Rex inhibitors that target the interface between the catalytic DH domain of P-Rex Rac-GEFs and Rac. Our best-characterized compound, P-Rex inhibitor 1 (PREX-in1), blocks the Rac-GEF activity of full-length P-Rex1 and P-Rex2, and of their isolated catalytic domains, in vitro at low-micromolar concentration, without affecting the activities of several other Rho-GEFs. PREX-in1 blocks the P-Rex1 dependent spreading of PDGF-stimulated endothelial cells and the production of reactive oxygen species in fMLP-stimulated mouse neutrophils. Structure-function analysis revealed critical structural elements of PREX-in1, allowing us to develop derivatives with increased efficacy, the best with an IC50 of 2 µM. In summary, we have developed PREX-in1 and derivative small-molecule compounds that will be useful laboratory research tools for the study of P-Rex function. These compounds may also be a good starting point for the future development of more sophisticated drug-like inhibitors aimed at targeting P-Rex Rac-GEFs in cancer.

Genetic Predisposition to Hepatocellular Carcinoma.

Liver preneoplastic and neoplastic lesions of the genetically susceptible F344 and resistant BN rats cluster, respectively, with human HCC with better (HCCB) and poorer prognosis (HCCP); therefore, they represent a valid model to study the molecular alterations determining the genetic predisposition to HCC and the response to therapy. The ubiquitin-mediated proteolysis of ERK-inhibitor DUSP1, which characterizes HCC progression, favors the unrestrained ERK activity. DUSP1 represents a valuable prognostic marker, and ERK, CKS1, or SKP2 are potential therapeutic targets for human HCC. In DN (dysplastic nodule) and HCC of F344 rats and human HCCP, DUSP1 downregulation and ERK1/2 overexpression sustain SKP2-CKS1 activity through FOXM1, the expression of which is associated with a susceptible phenotype. SAM-methyl-transferase reactions and SAM/SAH ratio are regulated by GNMT. In addition, GNMT binds to CYP1A, PARP1, and NFKB and PREX2 gene promoters. MYBL2 upregulation deregulates cell cycle and induces the progression of premalignant and malignant liver. During HCC progression, the MYBL2 transcription factor positively correlates with cells proliferation and microvessel density, while it is negatively correlated to apoptosis. Hierarchical supervised analysis, regarding 6132 genes common to human and rat liver, showed a gene expression pattern common to normal liver of both strains and BN nodules, and a second pattern is observed in F344 nodules and HCC of both strains. Comparative genetics studies showed that DNs of BN rats cluster with human HCCB, while F344 DNs and HCCs cluster with HCCP.

PREX2 gene's expression in gastric antral epithelial cells of patients with H. pylori infection / Expressão do gene PREX2 em células epiteliais antrais gástricas em pacientes com infecção por H. pylori

ABSTRACT BACKGROUND: The Prex2 protein is a member of the Rac family proteins that belongs to small G proteins with a critical role in cell migration, cell proliferation, and apoptosis through its effects on PI3K cell signaling pathway and phosphatase activity of PTEN protein. The effect of PREX2 gene expression has been shown in some cancer cells. A survey of PREX2 gene expression in gastric antral epithelial cells of gastric cancer patients with Helicobacter pylori various genotypes infection can conduct to better understanding H. pylori infection's carcinogenesis. METHODS: In a case-control study, PREX2 gene expression was evaluated in gastric antral biopsy samples on four groups of patients referred to Sanandaj hospitals, including gastritis with (n=23) and without (n=27) H. pylori infection and gastric cancer with (n=21) and without (n=32) H. pylori infection. Each gastric biopsy sample's total RNA was extracted and cDNA synthesized by using Kits (Takara Company). The PREX2 gene expression was measured using the relative quantitative real-time RT-PCR method and ΔΔCt formula. RESULTS: The PREX2 gene expression increased in gastric antral biopsy samples of gastritis and gastric cancer patients with H. pylori infection (case groups) than patients without H. pylori infection (control groups) 2.38 and 2.27 times, respectively. The patients with H. pylori vacA s1m1 and sabB genotypes infection showed a significant increase of PREX2 gene expression in gastric cancer antral epithelial cells. CONCLUSION: H. pylori vacA s1m1 and sabB genotypes have the positive correlations with PREX2 gene expression in gastric antral epithelial cells of gastritis and gastric cancer patients.

RESUMO CONTEXTO: A proteína Prex2 é membro das proteínas da família Rac que pertencem a pequenas proteínas G com um papel crítico na migração celular, na proliferação celular e na apoptose através de seus efeitos na via de sinalização celular PI3K e atividade fosfatase da proteína PTEN. O efeito da expressão genética PREX2 tem sido mostrada em algumas células cancerosas. Um levantamento da expressão genética PREX2 em células epiteliais antrais gástricas de pacientes infectados com vários genótipos de Helicobacter pylori pode conduzir a um melhor entendimento da carcinogênese da infecção por H. pylori. MÉTODOS: Em estudo de caso-controle, a expressão genética PREX2 foi avaliada em amostras de biópsia antral gástrica em quatro grupos de pacientes encaminhados aos hospitais de Sanandaj, incluindo gastrite com (n=23) e sem (n=27) infecção por H. pylori e de câncer gástrico com (n=21) e sem (n=32) infecção por H. pylori. O RNA total de cada amostra de biópsia gástrica foi extraído e cDNA sintetizado por meio de kits (Takara Company). A expressão genética PREX2 foi medida utilizando-se o método RT-PCR em tempo real quantitativo relativo e a fórmula ΔΔCt. RESULTADOS: A expressão genética PREX2 aumentou em amostras de biópsia antral gástrica de pacientes com gastrite e câncer gástrico com infecção por H. pylori (grupos de casos) em relação aos sem infecção por H. pylori (grupos de controle) 2,38 e 2,27 vezes, respectivamente. Os pacientes com infecção por genótipos H. pylori vacA s1m1 e sabB apresentaram um aumento significativo da expressão genética PREX2 em células epiteliais antrais de câncer gástrico. CONCLUSÃO: Os genótipos H. pylori vacA s1m1 e sabB têm correlações positivas com a expressão genética PREX2 em células epiteliais antrais gástricas de pacientes com câncer gástrico e gastrites.

Phosphatidylinositol 3,4,5-Trisphosphate-Dependent Rac Exchanger 2 Protein Facilitates Glioma Progression via Akt and Stat3 Signaling.

Glioblastoma multiforme (GBM) is the recognized as the most aggressive brain tumor with poor prognosis and low 1-year and 5-year survival rate. The treatment methods for GBM are limited and inefficient, and novel strategies for GBM treatment are urgently warranted. MiR-338-3p is described as a tumor suppressor in a variety of malignancies, including GBM. However, its role in GBM is not fully understood. The mRNA or protein levels of targets in cells or tissues were determined by quantitative reverse transcription PCR (RT-qPCR) or Western blot, respectively. The GBM cell growth rate in vitro or in vivo was measured by Cell Counting Kit-8 or bioluminescence imaging, respectively. Upregulation of hsa-miR-338-3p and downregulation of phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 2 protein (Prex2) were observed in GBM tissues compared to normal brain tissues. We further confirmed that murine Prex2 was a target of mmu-miR-338-3p in GBM. Mmu-miR-338-3p exerted profound inhibition effects on GBM cell growth in vitro or in vivo through targeting Prex2, leading to attenuation of (Protein kinase B) AKT/Signal transducer and activator of transcription 3 (STAT3) signaling activation. Restoration of mmu-miR-338-3p or inhibition of Prex2 may facilitate the development of innovative therapies for GBM treatment.

The Molecular Landscape of Primary Acral Melanoma: A Multicenter Study of the Italian Melanoma Intergroup (IMI).

Acral melanoma (AM) is a rare and aggressive subtype of melanoma affecting the palms, soles, and nail apparatus with similar incidence among different ethnicities. AM is unrelated to ultraviolet radiation and has a low mutation burden but frequent chromosomal rearrangements and gene amplifications. Next generation sequencing of 33 genes and somatic copy number variation (CNV) analysis with genome-wide single nucleotide polymorphism arrays were performed in order to molecularly characterize 48 primary AMs of Italian patients in association with clinicopathological and prognostic features. BRAF was the most commonly mutated gene, followed by NRAS and TP53, whereas TERT promoter, KIT, and ARID1A were less frequently mutated. Gains and losses were recurrently found in the 1q, 6p, 7, 8q, 20 and 22 chromosomes involving PREX2, RAC1, KMT2C, BRAF, CCND1, TERT, and AKT3 genes, and in the 6q, 9, 10, 11q and 16q chromosomes including CDKN2A, PTEN, and ADAMTS18 genes, respectively. This study confirmed the variety of gene mutations and the high load of CNV in primary AM. Some genomic alterations were associated with histologic prognostic features. BRAF mutations, found with a higher rate than previously reported, correlated with a low Breslow thickness, low mitotic count, low CNV of the AMs, and with early-stage of disease.

Genome-wide association studies for iris pigmentation and heterochromia patterns in Large White pigs.

Eye colour genetics have been extensively studied in humans since the rediscovery of Mendel's laws. This trait was first interpreted using simplistic genetic models but soon it was realised that it is more complex. In this study, we analysed eye colour variability in a Large White pig population (n = 897) and report the results of GWASs based on several comparisons including pigs having four main eye colour categories (three with both pigmented eyes of different brown grades: pale, 17.9%; medium, 14.8%; and dark, 54.3%; another one with both eyes completely depigmented, 3.8%) and heterochromia patterns (heterochromia iridis - depigmented iris sectors in pigmented irises, 3.2%; heterochromia iridum - one whole eye iris of depigmented phenotype and the other eye with the iris completely pigmented, 5.9%). Pigs were genotyped with the Illumina PorcineSNP60 BeadChip and GEMMA was used for the association analyses. The results indicated that SLC45A2 (on chromosome 16, SSC16), EDNRB (SSC11) and KITLG (SSC5) affect the different grades of brown pigmentation of the eyes, the bilateral eye depigmentation defect and the heterochromia iridis defect recorded in this white pig population respectively. These genes are involved in several mechanisms affecting pigmentation. Significant associations for the eye depigmented patterns were also identified for SNPs on two SSC4 regions (including two candidate genes: NOTCH2 and PREX2) and on SSC6, SSC8 and SSC14 (including COL17A1 as candidate gene). This study provided useful information to understand eye pigmentation mechanisms, further valuing the pig as animal model to study complex phenotypes in humans.

LncRNA MYCNOS facilitates proliferation and invasion in hepatocellular carcinoma by regulating miR-340.

Hepatocellular carcinoma (HCC) remains one of the most common and aggressive human cancers worldwide. Accumulating evidences indicate that non-coding RNAs are critical regulators implicated in various physiological processes including HCC development. Long non-coding RNA (lncRNA) MYCN opposite-strand (MYCNOS) was reported to be up-regulated in several human cancers, yet its role in HCC progression is still elusive. In the present study, MYCNOS was up-regulated in both HCC tissues and cell lines, and elevated MYCNOS expression was correlated to shorter survival time of HCC patients. We knocked down MYCNOS expression using short hairpin RNAs specifically targeting MYCNOS. MYCNOS knockdown significantly inhibited proliferation in HCC cells in vitro accompanied by exacerbated cell apoptosis; it also suppressed tumor growth in mouse model in vivo. Besides, the migration and invasion of HCC cells were remarkably inhibited after MYCNOS knockdown. In addition, MYCNOS acted as a negative regulator of miR-340 in HCC cells, and all effects of MYCNOS knockdown were abrogated by further miR-340 inhibition. We also discovered that oncogene phosphatidylinositol-3, 4, 5-trisphosphate-dependent Rac exchange factor 2 (PREX2) was a downstream target of miR-340, and PREX2 expression was positively correlated to that of MYCNOS in HCC tissues. In conclusion, our findings demonstrated that MYCNOS knockdown inhibited HCC progression through regulating miR-340.

P-Rex1 and P-Rex2 RacGEFs and cancer.

Phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger (P-Rex) proteins are RacGEFs that are synergistically activated by phosphatidylinositol 3,4,5-trisphosphate and Gßγ subunits of G-protein-coupled receptors. P-Rex1 and P-Rex2 share similar amino acid sequence homology, domain structure, and catalytic function. Recent evidence suggests that both P-Rex proteins may play oncogenic roles in human cancers. P-Rex1 and P-Rex2 are altered predominantly via overexpression and mutation, respectively, in various cancer types, including breast cancer, prostate cancer, and melanoma. This review compares the similarities and differences between P-Rex1 and P-Rex2 functions in human cancers in terms of cellular effects and signalling mechanisms. Emerging clinical data predict that changes in expression or mutation of P-Rex1 and P-Rex2 may lead to changes in tumour outcome, particularly in breast cancer and melanoma.

Characterization of the GNMT-HectH9-PREX2 tripartite relationship in the pathogenesis of hepatocellular carcinoma.

The pathogenesis of hepatocellular carcinoma (HCC) involves many molecular pathways. Glycine N-methyltransferase (GNMT) is downregulated in almost all HCC and its gene knockout mice developed HCC with high penetrance. We identified PREX2, a novel PTEN inhibitor, as a GNMT-interacting protein. Such interaction enhanced degradation of PREX2 through an E3 ligase HectH9-mediated proteasomal ubiquitination pathway. Depletion of GNMT or HectH9 resulted in AKT activation in a PREX2 dependent manner and enhanced cell proliferation. An elevated PREX2 protein expression accompanied by activation of AKT was observed in the liver of Gnmt knockout mice. PREX2 protein expression was upregulated in 54.9% of human HCC samples, while its mRNA level was comparable in tumor and tumor-adjacent tissue, suggesting a post-translational alteration of PREX2 expression. Higher level of PREX2 in the tumor tissues was associated with poorer survival. These results reveal a novel mechanism in which GNMT participates in AKT signaling and HCC tumorigenesis by promoting HectH9-mediated PREX2 degradation.

Mechanistic insights into the role of truncating PREX2 mutations in melanoma.

PREX2 is a PTEN binding protein that is significantly mutated in melanoma and pancreatic ductal adenocarcinoma. We recently reported the molecular mechanism of tumorigenesis associated with PREX2 mutations: truncating PREX2 mutations activate its RAC1 guanine nucleotide exchanger activity leading to increased PI3K/AKT signaling and enhanced cell proliferation.

Interplay between PREX2 mutations and the PI3K pathway and its effect on epigenetic regulation of gene expression in NRAS-mutant melanoma.

PREX2 is a PTEN interacting protein that is significantly mutated in melanoma and pancreatic ductal adenocarcinoma. Recently, we reported the mechanistic basis of melanomagenesis by PREX2 mutations. Truncating PREX2 mutations activate its guanine nucleotide exchange factor activity for its substrate RAC1. This leads to increased PI3K/AKT signaling associated with reduced DNA methylation and increased cell proliferation in NRAS-mutant melanoma. Here, we provide additional data that indicates a reciprocal regulation of PREX2 by PTEN whereby loss of PTEN results in a dramatic increase in expression of PREX2 at the protein level. Pharmacologic studies revealed destabilization of PREX2 by inhibition of PI3K/AKT signaling. Additionally, we provide data to show a selective decrease in a particular histone mark, H4 Lys20 trimethylation, in cells expressing PREX2 (E824*) truncating mutation globally and at the imprint control region of CDKN1C (also known as p57) and IGF2. The decrease in H4K20 trimethylation coupled with DNA hypomethylation at this particular locus is associated with genomic imprinting and regulation of expression of p57 and IGF2. Taken together, these results demonstrate the complex signaling mechanisms that involve PREX2, PI3K/AKT/PTEN and downstream epigenetic machinery to deregulate expression of key cell cycle regulators.

Truncating PREX2 mutations activate its GEF activity and alter gene expression regulation in NRAS-mutant melanoma.

PREX2 (phosphatidylinositol-3,4,5-triphosphate-dependent Rac-exchange factor 2) is a PTEN (phosphatase and tensin homolog deleted on chromosome 10) binding protein that is significantly mutated in cutaneous melanoma and pancreatic ductal adenocarcinoma. Here, genetic and biochemical analyses were conducted to elucidate the nature and mechanistic basis of PREX2 mutation in melanoma development. By generating an inducible transgenic mouse model we showed an oncogenic role for a truncating PREX2 mutation (PREX2(E824)*) in vivo in the context of mutant NRAS. Using integrative cross-species gene expression analysis, we identified deregulated cell cycle and cytoskeleton organization as significantly perturbed biological pathways in PREX2 mutant tumors. Mechanistically, truncation of PREX2 activated its Rac1 guanine nucleotide exchange factor activity, abolished binding to PTEN and activated the PI3K (phosphatidyl inositol 3 kinase)/Akt signaling pathway. We further showed that PREX2 truncating mutations or PTEN deletion induces down-regulation of the tumor suppressor and cell cycle regulator CDKN1C (also known as p57(KIP2)). This down-regulation occurs, at least partially, through DNA hypomethylation of a differentially methylated region in chromosome 11 that is a known regulatory region for expression of the CDKN1C gene. Together, these findings identify PREX2 as a mediator of NRAS-mutant melanoma development that acts through the PI3K/PTEN/Akt pathway to regulate gene expression of a cell cycle regulator.

Norbin Stimulates the Catalytic Activity and Plasma Membrane Localization of the Guanine-Nucleotide Exchange Factor P-Rex1.

P-Rex1 is a guanine-nucleotide exchange factor (GEF) that activates the small G protein (GTPase) Rac1 to control Rac1-dependent cytoskeletal dynamics, and thus cell morphology. Three mechanisms of P-Rex1 regulation are currently known: (i) binding of the phosphoinositide second messenger PIP3, (ii) binding of the Gßγ subunits of heterotrimeric G proteins, and (iii) phosphorylation of various serine residues. Using recombinant P-Rex1 protein to search for new binding partners, we isolated the G-protein-coupled receptor (GPCR)-adaptor protein Norbin (Neurochondrin, NCDN) from mouse brain fractions. Coimmunoprecipitation confirmed the interaction between overexpressed P-Rex1 and Norbin in COS-7 cells, as well as between endogenous P-Rex1 and Norbin in HEK-293 cells. Binding assays with purified recombinant proteins showed that their interaction is direct, and mutational analysis revealed that the pleckstrin homology domain of P-Rex1 is required. Rac-GEF activity assays with purified recombinant proteins showed that direct interaction with Norbin increases the basal, PIP3- and Gßγ-stimulated Rac-GEF activity of P-Rex1. Pak-CRIB pulldown assays demonstrated that Norbin promotes the P-Rex1-mediated activation of endogenous Rac1 upon stimulation of HEK-293 cells with lysophosphatidic acid. Finally, immunofluorescence microscopy and subcellular fractionation showed that coexpression of P-Rex1 and Norbin induces a robust translocation of both proteins from the cytosol to the plasma membrane, as well as promoting cell spreading, lamellipodia formation, and membrane ruffling, cell morphologies generated by active Rac1. In summary, we have identified a novel mechanism of P-Rex1 regulation through the GPCR-adaptor protein Norbin, a direct P-Rex1 interacting protein that promotes the Rac-GEF activity and membrane localization of P-Rex1.

Gene polymorphisms and oral cancer risk in tobacco habitués.

Oral cancer incidence of 77,003 poses a major health concern in India, with 5-10 % tobacco habitués developing oral cancer. The current study examined the role of specific genomic variants in oral cancer. We examined five genomic variants represented as single nucleotide polymorphisms (SNPs) in genes associated with cell proliferation and cellular invasion. The SNPs rs2124437 (RASGRP3), rs1335022 (GRIK2), rs4512367 (PREX2), rs4748011 (CCDC3), and rs1435218 (LNX1) were analyzed in 500 histopathologically confirmed oral cancers and 500 healthy controls with a minimum of 10 years of tobacco usage. Allelic discrimination real-time PCR SYBR Green assay was used. The genotypic and allelic frequencies between cases and controls were analyzed using SPSS software (version 19) and odds ratio (OR) using Hutchon.net, indicating increased risk to oral cancers. A significant association of the SNPs in oral cancer was observed in RASGRP3 AA (rs2124437) (p < 0.000, OR 1.34, 95 % confidence interval (CI) 1.01-1.76), GRIK2 TT (rs1335022) (p = 0.008, OR 1.58, 95 % CI 1.23-2.03), PREX2 CC (p = 0.008, OR 1.56, 95 % CI 1.15-2.1), and TT (p < 0.000, OR 2.77, 1.68-4.57) genotypes, whereas the heterozygous genotypes showed higher frequencies in controls, i.e., GRIK2 CT (rs1335022) (p = 0.029, OR 0.68, 95 % CI 0.53-0.87) and PREX2 CT (p = 0.004, OR 0.49, 95 % CI 0.37-0.64), indicating protection. Coinheritance of the SNPs was associated with further increase in the risk. Thus, the SNP genotypes in the three genes, present singly or as a coinherited panel constituted "Predictive Biomarkers" indicating increased risk of oral cancer in tobacco habitués.

p21-activated Kinases (PAKs) Mediate the Phosphorylation of PREX2 Protein to Initiate Feedback Inhibition of Rac1 GTPase.

Phosphatidylinositol 3,4,5-trisphosphate (PIP3)-dependent Rac exchanger 2 (PREX2) is a guanine nucleotide exchange factor (GEF) for the Ras-related C3 botulinum toxin substrate 1 (Rac1) GTPase, facilitating the exchange of GDP for GTP on Rac1. GTP-bound Rac1 then activates its downstream effectors, including p21-activated kinases (PAKs). PREX2 and Rac1 are frequently mutated in cancer and have key roles within the insulin-signaling pathway. Rac1 can be inactivated by multiple mechanisms; however, negative regulation by insulin is not well understood. Here, we show that in response to being activated after insulin stimulation, Rac1 initiates its own inactivation by decreasing PREX2 GEF activity. Following PREX2-mediated activation of Rac1 by the second messengers PIP3 or Gßγ, we found that PREX2 was phosphorylated through a PAK-dependent mechanism. PAK-mediated phosphorylation of PREX2 reduced GEF activity toward Rac1 by inhibiting PREX2 binding to PIP3 and Gßγ. Cell fractionation experiments also revealed that phosphorylation prevented PREX2 from localizing to the cellular membrane. Furthermore, the onset of insulin-induced phosphorylation of PREX2 was delayed compared with AKT. Altogether, we propose that second messengers activate the Rac1 signal, which sets in motion a cascade whereby PAKs phosphorylate and negatively regulate PREX2 to decrease Rac1 activation. This type of regulation would allow for transient activation of the PREX2-Rac1 signal and may be relevant in multiple physiological processes, including diseases such as diabetes and cancer when insulin signaling is chronically activated.

The Phosphatidylinositol (3,4,5)-Trisphosphate-dependent Rac Exchanger 1·Ras-related C3 Botulinum Toxin Substrate 1 (P-Rex1·Rac1) Complex Reveals the Basis of Rac1 Activation in Breast Cancer Cells.

The P-Rex (phosphatidylinositol (3,4,5)-trisphosphate (PIP3)-dependent Rac exchanger) family (P-Rex1 and P-Rex2) of the Rho guanine nucleotide exchange factors (Rho GEFs) activate Rac GTPases to regulate cell migration, invasion, and metastasis in several human cancers. The family is unique among Rho GEFs, as their activity is regulated by the synergistic binding of PIP3 and Gßγ at the plasma membrane. However, the molecular mechanism of this family of multi-domain proteins remains unclear. We report the 1.95 Å crystal structure of the catalytic P-Rex1 DH-PH tandem domain in complex with its cognate GTPase, Rac1 (Ras-related C3 botulinum toxin substrate-1). Mutations in the P-Rex1·Rac1 interface revealed a critical role for this complex in signaling downstream of receptor tyrosine kinases and G protein-coupled receptors. The structural data indicated that the PIP3/Gßγ binding sites are on the opposite surface and markedly removed from the Rac1 interface, supporting a model whereby P-Rex1 binding to PIP3 and/or Gßγ releases inhibitory C-terminal domains to expose the Rac1 binding site.

Regulation and function of P-Rex family Rac-GEFs.

The P-Rex family are Dbl-type guanine-nucleotide exchange factors for Rac family small G proteins. They are distinguished from other Rac-GEFs through their synergistic mode of activation by the lipid second messenger phosphatidyl inositol (3,4,5) trisphosphate and the Gßγ subunits of heterotrimeric G proteins, thus acting as coincidence detectors for phosphoinositide 3-kinase and G protein coupled receptor signaling. Work in genetically-modified mice has shown that P-Rex1 has physiological importance in the inflammatory response and the migration of melanoblasts during development, whereas P-Rex2 controls the dendrite morphology of cerebellar Purkinje neurons as well as glucose homeostasis in liver and adipose tissue. Deregulation of P-Rex1 and P-Rex2 expression occurs in many types of cancer, and P-Rex2 is frequently mutated in melanoma. Both GEFs promote tumor growth or metastasis. This review critically evaluates the P-Rex literature and tools available and highlights exciting recent developments and open questions.

miR-338-3p suppresses neuroblastoma proliferation, invasion and migration through targeting PREX2a.

MicroRNAs (miRNA) can regulate cancer cell proliferation and metastasis. Here, we show that miR-338-3p is down-regulated in metastatic tumor tissues compared to primary tumors, and that that miR-338-3p can inhibit cell proliferation by inducing cell cycle arrest, as well as restrain cell migration and invasion. PREX2a is confirmed as a direct target of miR-338-3p. Knockdown of PREX2a inhibits cell proliferation, migration and invasion through the PTEN/Akt pathway. miR-338-3p-dependent inhibition of proliferation and invasion can be rescued by PREXa. Overall, this study demonstrates that miR-338-3p affects the PTEN/Akt pathway by down-regulating PREX2a. This newly identified function of miR-338-3p provides novel insights into neuroblastoma and may foster therapeutic applications.

Genetics of melanoma.

Genomic variation is a trend observed in various human diseases including cancer. Genetic studies have set out to understand how and why these variations result in cancer, why some populations are pre-disposed to the disease, and also how genetics affect drug responses. The melanoma incidence has been increasing at an alarming rate worldwide. The burden posed by melanoma has made it a necessity to understand the fundamental signaling pathways involved in this deadly disease. Signaling cascades such as mitogen-activated protein kinase and PI3K/AKT have been shown to be crucial in the regulation of processes that are commonly dysregulated during cancer development such as aberrant proliferation, loss of cell cycle control, impaired apoptosis, and altered drug metabolism. Understanding how these and other oncogenic pathways are regulated has been integral in our challenge to develop potent anti-melanoma drugs. With advances in technology and especially in next generation sequencing, we have been able to explore melanoma genomes and exomes leading to the identification of previously unknown genes with functions in melanomagenesis such as GRIN2A and PREX2. The therapeutic potential of these novel candidate genes is actively being pursued with some presenting as druggable targets while others serve as indicators of therapeutic responses. In addition, the analysis of the mutational signatures of melanoma tumors continues to cement the causative role of UV exposure in melanoma pathogenesis. It has become distinctly clear that melanomas from sun-exposed skin areas have distinct mutational signatures including C to T transitions indicative of UV-induced damage. It is thus necessary to continue spreading awareness on how to decrease the risk factors of developing the disease while at the same time working for a cure. Given the large amount of information gained from these sequencing studies, it is likely that in the future, treatment of melanoma will follow a highly personalized route that takes into account the differential mutational signatures of each individual's cancer.