Dilazep analogues for the study of equilibrative nucleoside transporters 1 and 2 (ENT1 and ENT2).

As ENT inhibitors including dilazep have shown efficacy improving oHSV1 targeted oncolytic cancer therapy, a series of dilazep analogues was synthesized and biologically evaluated to examine both ENT1 and ENT2 inhibition. The central diamine core, alkyl chains, ester linkage and substituents on the phenyl ring were all varied. Compounds were screened against ENT1 and ENT2 using a radio-ligand cell-based assay. Dilazep and analogues with minor structural changes are potent and selective ENT1 inhibitors. No selective ENT2 inhibitors were found, although some analogues were more potent against ENT2 than the parent dilazep.

Identification of the ENT1 antagonists dipyridamole and dilazep as amplifiers of oncolytic herpes simplex virus-1 replication.

Oncolytic herpes simplex virus-1 (oHSV) vectors selectively replicate in tumor cells, where they kill through oncolysis while sparing normal cells. One of the drawbacks of oHSV vectors is their limited replication and spread to neighboring cancer cells. Here, we report the outcome of a high-throughput chemical library screen to identify small-molecule compounds that augment the replication of oHSV G47Delta. Of the 2,640-screened bioactives, 6 compounds were identified and subsequently validated for enhanced G47Delta replication. Two of these compounds, dipyridamole and dilazep, interfered with nucleotide metabolism by potently and directly inhibiting the equilibrative nucleoside transporter-1 (ENT1). Replicative amplification promoted by dipyridamole and dilazep were dependent on HSV mutations in ICP6, the large subunit of ribonucleotide reductase. Our results indicate that ENT1 antagonists augment oHSV replication in tumor cells by increasing cellular ribonucleoside activity.

TSAP6 facilitates the secretion of translationally controlled tumor protein/histamine-releasing factor via a nonclassical pathway.

Translationally controlled tumor protein (TCTP) is cytoplasmic and structurally related to guanine-nucleotide free chaperones. TCTP (also called histamine-releasing factor) has been described previously as a secreted protein that participates in inflammatory responses by promoting the release of histamine. How TCTP is eventually exported out of the cell to promote such activities is unknown. Here we show that TCTP secretion was insensitive to either brefeldin A or monensin, suggesting that it proceeds via an endoplasmic reticulum/Golgi-independent or nonclassical pathway. Moreover, our analyses also suggest that secreted TCTP originates from pre-existing pools. TSAP6, a p53-inducible 5-6 transmembrane protein, was found to interact with TCTP in a yeast two-hybrid hunt. GST pull down assays confirmed their direct interaction, and immunofluorescence analysis revealed their partial co-distribution to vesicular-like structures at the plasma membrane and around the nucleus. Functionally, the overexpression of TSAP6 consistently leads to enhanced secretion of both endogenously and exogenously expressed TCTP. Finally, we found TCTP in preparations of small secreted vesicles called exosomes, which have been suggested as a possible pathway for nonclassical secretion. Overexpression of TSAP6 also increased TCTP levels in exosome preparations. Altogether, these data identify a novel role for TSAP6 in the export of TCTP and indicate that this multipass membrane protein could have a general role in the regulation of vesicular trafficking and secretion.

Self-regulated cleavage of the mitochondrial intramembrane-cleaving protease PARL yields Pbeta, a nuclear-targeted peptide.

Regulated intramembrane proteolysis (RIP) is an emerging paradigm in signal transduction. RIP is mediated by intramembrane-cleaving proteases (I-CliPs), which liberate biologically active nuclear or secreted domains from their membrane-tethered precursor proteins. The yeast Pcp1p/Rbd1p protein is a Rhomboid-like I-CliP that regulates mitochondrial membrane remodeling and fusion through cleavage of Mgm1p, a regulator of these essential activities. Although this ancient function is conserved in PARL (Presenilins-associated Rhomboid-like protein), the mammalian ortholog of Pcp1p/Rbd1p, the two proteins show a strong divergence at their N termini. However, the N terminus of PARL is significantly conserved among vertebrates, particularly among mammals, suggesting that this domain evolved a distinct but still unknown function. Here, we show that the cytosolic N-terminal domain of PARL is cleaved at positions 52-53 (alpha-site) and 77-78 (beta-site). Whereas alpha-cleavage is constitutive and removes the mitochondrial targeting sequence, beta-cleavage appears to be developmentally controlled and dependent on PARL I-CliP activity supplied in trans. The beta-cleavage of PARL liberates Pbeta, a nuclear targeted peptide whose sequence is conserved only in mammals. Thus, in addition to its evolutionarily conserved function in regulating mitochondrial dynamics, PARL might mediate a mammalian-specific, developmentally regulated mitochondria-to-nuclei signaling through regulated proteolysis of its N terminus and release of the Pbeta peptide.

Translationally controlled tumor protein acts as a guanine nucleotide dissociation inhibitor on the translation elongation factor eEF1A.

Recently, we demonstrated that the expression levels of the translationally controlled tumor protein (TCTP) were strongly down-regulated at the mRNA and protein levels during tumor reversion/suppression and by the activation of p53 and Siah-1. To better characterize the function of TCTP, a yeast two-hybrid hunt was performed. Subsequent analysis identified the translation elongation factor, eEF1A, and its guanine nucleotide exchange factor, eEF1Bbeta, as TCTP-interacting partners. In vitro and in vivo studies confirmed that TCTP bound specifically eEF1Bbeta and eEF1A. Additionally, MS analysis also identified eEF1A as a TCTP interactor. Because eEF1A is a GTPase, we investigated the role of TCTP on the nucleotide exchange reaction of eEF1A. Our results show that TCTP preferentially stabilized the GDP form of eEF1A, and, furthermore, impaired the GDP exchange reaction promoted by eEF1Bbeta. These data suggest that TCTP has guanine nucleotide dissociation inhibitor activity, and, moreover, implicate TCTP in the elongation step of protein synthesis.

The p53-inducible TSAP6 gene product regulates apoptosis and the cell cycle and interacts with Nix and the Myt1 kinase.

The p53 tumor suppressor protein plays a crucial role in tumorigenesis by controlling cell-cycle progression and apoptosis. We have previously described a transcript designated tumor suppressor activated pathway-6 (TSAP6) that is up-regulated in the p53-inducible cell line, LTR6. Cloning of the murine and human full-length TSAP6 cDNA revealed that it encodes a 488-aa protein with five to six transmembrane domains. This gene is the murine and human homologue of the recently published rat pHyde. Antibodies raised against murine and human TSAP6 recognize a 50- to 55-kDa band induced by p53. Analysis of the TSAP6 promoter identified a functional p53-responsive element. Functional studies demonstrated that TSAP6 antisense cDNA diminished levels of the 50- to 55-kDa protein and decreased significantly the levels of p53-induced apoptosis. Furthermore, TSAP6 small interfering RNA inhibited apoptosis in TSAP6-overexpressing cells. Yeast two-hybrid analysis followed by GST/in vitro-transcribed/translated pull-down assays and in vivo coimmunoprecipitations revealed that TSAP6 associated with Nix, a proapoptotic Bcl-2-related protein and the Myt1 kinase, a negative regulator of the G(2)/M transition. Moreover, TSAP6 enhanced the susceptibility of cells to apoptosis and cooperated with Nix to exacerbate this effect. Cell-cycle studies indicated that TSAP6 could augment Myt1 activity. Overall, these data suggest that TSAP6 may act downstream to p53 to interface apoptosis and cell-cycle progression.

PAMP and PARL, two novel putative metalloproteases interacting with the COOH-terminus of Presenilin-1 and -2.

The familial Alzheimer's disease gene products, presenilin-1 and presenilin-2 (PS1 and PS2), are involved in amyloid beta-protein precursor processing (AbetaPP), Notch receptor signaling, and programmed cell death. However, the molecular mechanisms by which presenilins regulate these processes remain unknown. Clues about the function of a protein can be obtained by seeing whether it interacts with another protein of known function. Using the yeast two-hybrid system, we identified two proteins that interact and colocalize with the presenilins. One of these newly detected presenilin-interacting proteins belongs to the FtsH family of ATP-dependent proteases, and the other one belongs to Rhomboid superfamily of membrane proteins that are highly conserved in eukaryotes, archaea and bacteria. Based on the pattern of amino acid residues conservation in the Rhomboid superfamily, we hypothesize that these proteins possess a metal-dependent enzymatic, possibly protease activity. The two putative proteases interacting with presenilins could mediate specific proteolysis of membrane proteins and contribute to the network of interactions in which presenilins are involved.