The signal peptide of human preproendothelin-1

Interplay of the Circadian Clock and Endothelin System

The peptide hormone endothelin-1 and its receptors are linked to several disease states. Pharmacological inhibition of this pathway has proven beneficial in pulmonary hypertension, yet its potential in other disease states remains to be realized. This review considers an often understudied aspect of endothelin biology, circadian rhythm regulation and how understanding the intersection between endothelin signaling and the circadian clock may be leveraged to realize the potential of endothelin-based therapeutics.

Endothelin-1 gene regulation

Over two decades of research have demonstrated that the peptide hormone endothelin-1 (ET-1) plays multiple, complex roles in cardiovascular, neural, pulmonary, reproductive, and renal physiology. Differential and tissue-specific production of ET-1 must be tightly regulated in order to preserve these biologically diverse actions. The primary mechanism thought to control ET-1 bioavailability is the rate of transcription from the ET-1 gene (edn1). Studies conducted on a variety of cell types have identified key transcription factors that govern edn1 expression. With few exceptions, the cis-acting elements bound by these factors have been mapped in the edn1 regulatory region. Recent evidence has revealed new roles for some factors originally believed to regulate edn1 in a tissue or hormone-specific manner. In addition, other mechanisms involved in epigenetic regulation and mRNA stability have emerged as important processes for regulated edn1 expression. The goal of this review is to provide a comprehensive overview of the specific factors and signaling systems that govern edn1 activity at the molecular level.

Fluctuating gene expression and localized cellular distribution of vasoactive intestinal contractor (VIC) in mouse uterus

To understand the physiological roles of vasoactive intestinal contractor (VIC) and endothelin-2 (ET-2) in the uterus, we examined the expression levels of VIC mRNA by real-time quantitative reverse transcription-linked polymerase chain reaction (RT-PCR) and characterized the cellular distribution of VIC peptide and mRNA by immunostaining and in situ hybridization in mouse uterus. In pregnant mouse uterus, VIC mRNA expression changed considerably between Days 10.5 and 12.5 of pregnancy. The expression levels were significantly (p<0.05) higher (approximately fivefold) in the later stage of pregnancy (Days 12.5-17.5) than in the earlier stage (Days 7.5-10.5). In nonpregnant uterus, VIC mRNA expression was significantly (p <0.05) higher (approximately threefold) in proestrus and estrus than in diestrus. Immunohistochemical studies demonstrated the presence of VIC peptide in endometrial epithelial cells, myometrial cells, and vascular smooth muscle cells during the estrous cycle and pregnancy and after parturition. Notably, myometrial cells showed dominant immunostaining in proestrus and estrus, in the later pregnancy stage, and in the early postpartum period, analogous to the expression pattern of VIC mRNA. In situ hybridization confirmed localization of VIC mRNA in myometrial cells. These findings suggest that VIC may play an important role in the function of myometrial cells.

Two di-leucine-based motifs account for the different subcellular localizations of the human endothelin-converting enzyme (ECE-1) isoforms

Endothelin-converting enzyme (ECE-1) is a type II integral membrane protein which plays a key role in the biosynthetic pathway of the vasoconstricting endothelins. Three ECE-1 isoforms, differing by their N-terminal cytoplasmic tails, are generated from a single gene. When expressed in CHO cells, they display comparable enzymatic activity but whereas ECE-1a is strongly expressed at the cell surface, ECE-1b is exclusively intracellular and ECE-1c presents an intermediate distribution. In the present study these different localizations were further described at the ultrastructural level, by electron microscope immunocytochemistry. To characterize the motifs responsible for the intracellular localization of ECE-1b we constructed chimeric proteins and point mutants. Two di-leucine-based motifs, contained in the N-terminal part of ECE-1b, were thus identified. One of these motifs (LV), displayed by both ECE-1b and ECE-1c, accounts for the reduced surface expression of ECE-1c as compared to ECE-1a. Mutation of both motifs (LL and LV) induces a very strong appearance of ECE-1b at the cell surface indicating that their presence in the N-terminal extremity of ECE-1b is critical for its exclusively intracellular localization.

Cloning and recombinant expression of rat and human kynureninase

Kynureninase [E.C.3.7.1.3.] is one of the enzymes involved in the biosynthesis of NAD cofactors from tryptophan through the kynurenine pathway. By tryptic and CNBr digestion of purified rat liver kynureninase, we obtained about 28% of the amino acid sequence of the enzyme. The rat kynureninase cDNA, isolated by means of reverse-transcribed polymerase chain reaction and hybridization screening, codes for a polypeptide of 464 amino acids. Northern blot analysis revealed the synthesis of a 2.0 kb rat kynureninase mRNA. A cDNA encoding human liver kynureninase was also isolated. The deduced amino acid sequence is 85% identical to that of the rat protein. COS-1 cells were transfected with both cDNAs. The Km values of the rat enzyme, for L-kynurenine and DL-3-hydroxykynurenine, were 440 +/- 20 microM and 32 +/- 5 microM and of the human enzyme 440 /- 20 microM and 49 +/- 6 microM, respectively. Interestingly, COS-1 cells transfected with the cDNA coding for rat kynureninase also display cysteine-conjugate beta-lyase activity.

Characterization of a saporin isoform with lower ribosome-inhibiting activity

We have expressed in Escherichia coli five isoforms of saporin, a single-chain ribosome-inactivating protein (RIP). Translation inhibition activities of the purified recombinant polypeptides in vitro were compared with those of recombinant dianthin 30, a less potent and closely related RIP, and of ricin A chain. Dianthin 30, and a saporin isoform encoded by a cDNA from leaf tissue (SAP-C), both had about one order of magnitude lower activity in translation inhibition assays than all other isoforms of saporin tested. We recently demonstrated that saporin extracted from seeds of Saponaria officinalis binds to alpha2-macroglobulin receptor (alpha2MR; also termed low density lipoprotein-receptor-related-protein), indicating a general mechanism of interaction of plant RIPs with the alpha2MR system [Cavallaro, Nykjaer, Nielsen and Soria (1995) Eur. J. Biochem. 232, 165-171]. Here we report that SAP-C bound to alpha2MR equally well as native saporin. However, the same isoform had about ten times lower cytotoxicity than the other saporin isoforms towards different cell lines. This indicates that the lower cell-killing ability of the SAP-C isoform is presumably due to its altered interaction with the protein synthesis machinery of target cells. Since saporin binding to the alpha2MR is competed by heparin, we also tested in cell-killing experiments Chinese hamster ovary cell lines defective for expression of either heparan sulphates or proteoglycans. No differences were observed in cytotoxicity using native saporin or the recombinant isoforms. Therefore saporin binding to the cell surface should not be mediated by interaction with proteoglycans, as is the case for other alpha2MR ligands.

Inhibitors of endothelin

With the advent of the first generation of both selective and nonselective endothelin antagonists being a relatively recent event, the manifold therapeutic potentials of these compounds are only now being explored clinically. Undoubtedly, numerous clinical utilities for these compounds will soon be realized.

Molecular cloning of rat kynurenine aminotransferase: identity with glutamine transaminase K

The enzyme kynurenine aminotransferase (KAT) catalyses the conversion of L-kynurenine to kynurenic acid. A combination of polymerase chain reaction techniques and hybridization screening was used to isolate a cDNA clone encompassing the entire coding region of KAT from rat kidney. Identification of the cDNA as coding for KAT was based both on the comparison of amino acid sequences obtained from purified rat KAT and on the expression of KAT activity in COS-1 cells transfected with the cDNA. RNA blot analysis indicated that KAT mRNA is widely expressed in rat tissues. Cultured cells transfected with the cDNA for KAT also showed glutamine transaminase K activity. Based mainly on sequence data, these results demonstrate that rat kidney KAT is identical with glutamine transaminase K.

In vivo expression of mutant preproendothelins: hierarchy of processing events but no strict requirement of Trp-Val at the processing site

Endothelin-1 (ET-1), a 21-residue vasoconstrictor peptide, originates in human cells from a 212-amino acid precursor (preproET-1). Big ET-1, an intermediate form of 38 amino acids, is generated by cleavage at basic-pair residues of proET-1, while a specific "ET-converting enzyme" was proposed to process the unusual Trp-Val site at positions 21 and 22 of big ET-1. We have previously shown that expression of synthetic RNA encoding human preproET-1 in Xenopus oocytes results in secretion of putative ET-1 and big ET-1. Here, to further dissect the processing pathway of preproET-1, we designed and expressed in oocytes a set of preproET-1 mutants. Four mutants affecting the Trp-Val site always originated putative ET-1(s) at levels comparable to the wild type, suggesting that there is only a conformational requirement for cleavage at this site. An Arg-->Ile mutation at the basic-pair site after the C terminus of big ET-1 fully inhibited the formation of both big ET-1 and ET-1, indicating that processing at this site is an early event and that big ET-1 is an obligate intermediate for the synthesis of ET-1 in vivo. Also, a truncated mutant bearing a stop codon after the C terminus of the big ET-1 sequence was totally stable and further processed into mature big ET-1 and ET-1, indicating that the second part of the precursor is not necessary for maturation.

Two preproendothelin 1 mRNAs transcribed by alternative promoters

Endothelin-1, initially identified as potent vasoconstrictor secreted by vascular endothelial cells, was subsequently found to have many effects on both vascular and nonvascular tissues. We have identified from a human placenta cDNA library a clone (cDNA-2) which corresponds to a novel 5'-extended preproendothelin 1 (preproET-1) mRNA. Comparison with the known preproET-1 mRNA (cDNA-1), showed that the two molecules share the same coding sequence but differ in the 5'-untranslated region. Interestingly, cDNA-2 extends upstream of promoter regions previously shown to be essential for full preproET-1 expression. Primer extension and PCR analysis of human placenta RNA demonstrated the presence of additional transcription initiation sites located upstream of the previously identified preproET-1 CAP site. Moreover, the two mRNAs show different pattern of expression. To elucidate the mechanisms controlling the production of alternative transcripts we transfected COS-1 cells with a series of preproET-1 promoter deletion mutants. This analysis revealed that the human preproET-1 gene can be transcribed from a proximal and a distal promoter element which has hitherto been undetected. In addition, we demonstrate the presence of a region in the down-epithelial specific expression.

Human preproendothelin-1 is converted into active endothelin-1 by baculovirus-infected insect cells

To investigate biochemical and biological parameters involved in preproendothelin-1 (preproET-1) maturation we infected Spodoptera frugiperda (Sf21) cells with a suitable engineered baculovirus vector carrying the cDNA encoding the entire human 212 amino acids precursor. Culture supernatants were tested by RIA using an anti-ET-1 serum, ET-1-like immunoreactive material (IRM) was detected in the infected Sf21 cells but not in control, wild-type or mock-infected cells. Fractionation of the culture supernatant by RP-HPLC coupled to an ET-1 specific RIA yielded two main peaks corresponding to the retention times of human bigET-1 and ET-1. Furthermore, culture supernatant of preproET-1 expressing Sf21 cells elicited a characteristic dose-response vasoconstrictive activity on rabbit vena cava, consistent with the amount of ET-1 as estimated by RP-HPLC coupled to RIA. These results suggest that insect cells possess the enzymatic activities necessary for human preproET-1 full maturation even though no such peptide has ever been found in insect cells.

Heterologous in vivo processing of human preproendothelin 1 into bioactive peptides

Endothelin (ET) is an extremely potent vasoconstrictor peptide of 21 amino acids, originally found in the supernatant of cultured vascular endothelial cells. To gain insights into its biosynthetic pathway, we expressed a synthetic RNA coding for the 212-amino acid precursor of human ET-1 (preproET-1) in Xenopus oocytes. Cell homogenates and oocyte incubation medium were tested by RIA using an anti-ET-1 serum. ET-1-like immunoreactivity was detected in oocytes injected with preproET-1 synthetic RNA but not in control oocytes and was much higher in medium than in cell homogenates. When preproET-1 was expressed in oocytes treated with monensin, a dramatic decrease in secretion of immunoreactive material was observed, indicating that secretion is mediated by the Golgi complex. ET-1-like immunoreactive material present in oocyte incubation medium was fractionated by reverse-phase HPLC into two main peaks, corresponding to the retention times of human big ET-1 and ET-1. Incubation medium of oocytes expressing the synthetic preproET-1 RNA elicited a characteristic vasoconstrictor response on rabbit vena cava, consistent with the biological activity that would be predicted from the amount of ET-1-like immunoreactivity measured. These results suggest that common pathways of ET maturation exist in widely different cells and that Xenopus oocytes may represent a useful tool in studying the cell biology of ET-1 synthesis.