Interaction of human GTP cyclohydrolase I with its splice variants

GTP Cyclohydrolase Drives Breast Cancer Development and Promotes EMT in an Enzyme-Independent Manner

GTP cyclohydrolase (GCH1) is the rate-limiting enzyme for tetrahydrobiopterin (BH4) biosynthesis. The catalysis of BH4 biosynthesis is tightly regulated for physiological neurotransmission, inflammation, and vascular tone. Paradoxically, BH4 has emerged as an oncometabolite regulating tumor growth, but the effects on tumor development remain controversial. Here, we found that GCH1 potentiated the growth of triple-negative breast cancer (TNBC) and HER2+ breast cancer and transformed nontumor breast epithelial cells. Independent of BH4 production, GCH1 protein induced epithelial-to-mesenchymal transition by binding to vimentin (Vim), which was mediated by HSP90. Conversely, GCH1 ablation impaired tumor growth, suppressed Vim in TNBC, and inhibited EGFR/ERK signaling while activating the p53 pathway in estrogen receptor-positive tumor cells. GCH1 deficiency increases tumor cell sensitivity to HSP90 inhibition and endocrine treatments. In addition, high GCH1 correlated with poor breast cancer survival. Together, this study reveals an enzyme-independent oncogenic role of GCH1, presenting it as a potential target for therapeutic development.

SIGNIFICANCE

GTP cyclohydrolase functions as an oncogene in breast cancer and binds vimentin to induce epithelial-to-mesenchymal transition independently of its enzyme activity, which confers targetable vulnerabilities for developing breast cancer treatment strategies.

A novel FADS2 isoform identified in human milk fat globule suppresses FADS2 mediated Δ6-desaturation of omega-3 fatty acids

INTRODUCTION

The only known non-pharmacological means to alter long chain polyunsaturated fatty acid (LCPUFA) abundance in mammalian tissue is by altering substrate fatty acid ratios. Alternative mRNA splicing is increasingly recognized as a modulator of protein structure and function. Here we report identification of a novel alternative transcript (AT) of fatty acid desaturase 2 (FADS2) that inhibits production of omega-3 but not omega-6 LCPUFA, discovered during study of ATs in human milk fat globules (MFG).

METHODS

Human breastmilk collected from a single donor was used to isolate MFG. An mRNA-sequencing library was constructed from the total RNA isolated from the MFG. The constructed library was sequenced using an Illumina HiSeq instrument operating in high output mode. Expression levels of evolutionary conserved FADSAT were measured using cDNA from MFG by semi-quantitative RT-PCR assay.

RESULTS

RNA sequencing revealed >15,000 transcripts, including moderate expression of the FADS2 classical transcript (CS). A novel FADS2 alternative transcript (FADS2AT2) with 386 amino acids was discovered. When FADS2AT2 was transiently transfected into MCF7 cells stably expressing FADS2, delta-6 desaturation (D6D) of alpha-linolenic acid 18:3n-3 → 18:4n-3 was suppressed as were downstream products 20:4n-3 and 20:5n-3. In contrast, no significant effect on D6D of linoleic acid 18:2n-6 → 18:3n-6 or downstream products was observed. FADS2, FADS2AT1 and 5 out of 8 known FADS3AT were expressed in MFG. FADS1, FADS3AT3, and FADS3AT5 are undetectable.

CONCLUSION

The novel, noncatalytic FADS2AT2 regulates FADS2CS-mediated Δ6-desaturation of omega-3 but not omega-6 PUFA biosynthesis. This spliced isoform mediated interaction is the first molecular mechanism by which desaturation of one PUFA family but not the other is modulated.

Preparation and characterization of monoclonal antibodies against VSTM1

VSTM1 (V-set and transmembrane domain containing 1) is a novel membrane molecule identified from immunogenomics, which has two main isoforms, VSTM1-v1 and VSTM1-v2. VSTM1-v1 is a type I transmembrane protein, and VSTM1-v2 is a classical secretory protein, lacking only the transmembrane domain compared with VSTM1-v1. This study was designed to generate VSTM1-specific monoclonal antibodies (MAbs) for further exploration of its expression and function. Mice were immunized with two recombinant prokaryotic proteins of GST-VSTM1-v2 and VSTM1-v2 without any tag. Hybridomas were generated by the fusion of the splenocytes to Sp2/0 myeloma cells. Three hybridoma cell lines (2C11, 6E11, and 7A8) stable in secreting anti-VSTM1 MAb were obtained and further characterized. All three MAbs were IgG2b isotype and effective in detecting the overexpressed VSTM1 in both Western blot and flow cytometry assays, while recognizing the endogenous VSTM1 in Western blot analysis only. These MAbs could be helpful in the basic study of VSTM1 and in revealing the interesting conformation difference between the overexpressed and endogenous proteins.

Constitutive nuclear localization of an alternatively spliced sirtuin-2 isoform

Sirtuin-2 (SIRT2), the cytoplasmic member of the sirtuin family, has been implicated in the deacetylation of nuclear proteins. Although the enzyme has been reported to be located to the nucleus during G2/M phase, its spectrum of targets suggests functions in the nucleus throughout the cell cycle. While a nucleocytoplasmic shuttling mechanism has been proposed for SIRT2, recent studies have indicated the presence of a constitutively nuclear isoform. Here we report the identification of a novel splice variant (isoform 5) of SIRT2 that lacks a nuclear export signal and encodes a predominantly nuclear isoform. This novel isoform 5 fails to show deacetylase activity using several assays, both in vitro and in vivo, and we are led to conclude that this isoform is catalytically inactive. Nevertheless, it retains the ability to interact with p300, a known interaction partner. Moreover, changes in intrinsic tryptophan fluorescence upon denaturation indicate that the protein is properly folded. These data, together with computational analyses, confirm the structural integrity of the catalytic domain. Our results suggest an activity-independent nuclear function of the novel isoform.

The Epithelial Sodium Channel alpha subunit (alpha ENaC) alternatively spliced form "b" in Dahl rats: What's next?

Background

The amiloride-sensitive Epithelial Sodium Channel (ENaC) is critical in maintaining Na⁺ balance, extracellular fluid volume and long term blood pressure control. ENaC is composed of three main subunits α, β, & γ. While α ENaC is critical for channel functionality, β & γ ENaC maximize channel function. To date, there are four alternatively spliced forms of the α subunit of ENaC (α ENaC-a, -b, -c, & -d) that have been published in rats, in addition to the major α ENaC transcript. While α ENaC-a, -c & -d transcripts are low abundance transcripts compared to full-length α ENaC, α ENaC-b is a higher abundance and salt-sensitive transcript compared to full-length α ENaC.

Presentation of the hypothesis

α ENaC-b protein, which is preferentially produced in Dahl R rats, to a greater extent on high salt diet, exerts a dominant negative effect on full-length α ENaC subunit by physically binding to and trapping full-length α ENaC subunit in the endoplasmic reticulum, and finally accelerating full-length α ENaC proteolytic degradation in a dose-dependent manner.

Testing the hypothesis

1) To examine the mRNA and protein abundance of α ENaC-b relative to α ENaC full-length in kidney, lung, and taste tissues of Dahl rats. 2) To compare the expression (mRNA and protein) of α ENaC-b in kidneys of Dahl S and R rats on regular and high salt diet. 3) To examine the putative binding of α ENaC-b proteins to full-length α ENaC in vitro and to determine the impact of such binding on full-length α ENaC expression in vitro.

Implications of the hypothesis

Our studies will be the first to demonstrate the over-expression of salt-sensitive α ENaC-b spliced form in kidney tissues of Dahl R rats at the expense of full-length α ENaC. The current proposal will provide highly novel insights into the putative mechanisms leading to ENaC hypoactivity in high-salt-fed Dahl R rats. Finally, findings from the present proposal will uncover a new mechanism by which alternative splicing may control the regulation of ENaC expression/function.

Regulation of the epithelial sodium channel [ENaC] in kidneys of salt-sensitive Dahl rats: insights on alternative splicing

The epithelial sodium channel [ENaC] is critical for the maintenance of sodium balance, extracellular fluid volume and long term blood pressure control. Monogenic disorders causing ENaC hyperactivity have led to a severe form of hereditary hypertension in humans, known as Liddle's syndrome. Similarly, in animal models, ENaC hyperactivity has been well documented in kidneys of salt-sensitive [S] Dahl rats [a genetic model of salt-sensitive hypertension] versus their normotensive control [Dahl salt-resistant [R] rats]. The purpose of the present review is to highlight the differential regulation of ENaC in kidneys of Dahl S versus R rats. A systematic overview of the putative role of alternative splicing of the main alpha subunit of ENaC [alpha ENaC] in modulating ENaC expression in kidneys of Dahl rats will be discussed. Finally, a better understanding of the meaningful contribution of ENaC in the pathogenesis of salt-sensitive hypertension will be achieved upon completion of this review.

A Novel Mechanism in Regulating the Alpha-Subunit of the Epithelial Sodium Channel (α ENaC) by the Alternatively Spliced Form α ENaC-b

Introduction

In Dahl rats’ kidney cortex, the alternatively spliced form of the epithelial sodium channel α subunit (α ENaC-b) is the most abundant mRNA transcript (32+/-3 fold > α ENaC-wt) as was investigated by quantitative RT-PCR analysis. α ENaC-b mRNA levels were significantly higher in Dahl R versus S rats, and were further augmented by high salt diet.

Objectives

In the present study, we described the molecular cloning and searched for a possible role of α ENaC-b by testing its potential expression in COS7 cells as well as its impact on α ENaC-wt expression levels when co-expressed in COS7 cells in a dose-dependent manner.

Methods

Using RT-PCR strategy, the full-length wildtype α ENaC transcript and the alternatively spliced form α ENaC-b were amplified, sequenced, cloned, subcloned into PCMV-sport6 expression vector, expressed and co-expressed into COS7 cells in a dose-dependent manner. A combination of denaturing and native western blotting techniques was employed to examine the expression of α ENaC-b in vitro, and to determine if an interaction between α ENaC-b and α ENaC-wt occurs in vitro, and finally to demonstrate if degradation of α ENaC-wt protein does occur.

Results

α ENaC-b is translated in COS7 cells. Co-expression of α ENaC-b together with α ENaC-wt reduced α ENaC-wt levels in a dose-dependent manner. α ENaC-wt and α ENaC-b appear to form a complex that enhances the degradation of α ENaC-wt.

Conclusions

Western blots suggest a novel mechanism in α ENaC regulation whereby α ENaC-b exerts a dominant negative effect on α ENaC-wt expression. This is potentially by sequestering α ENaC-wt, enhancing its proteolytic degradation, and possibly explaining the mechanism of salt-resistance in Dahl R rats.

Characterization of the epithelial sodium channel alpha subunit coding and non-coding transcripts and their corresponding mRNA expression levels in Dahl R versus S rat kidney cortex on normal and high salt diet

Aims/hypothesis

The α subunit of the amiloride-sensitive epithelial sodium channel (α ENaC) is critical for the expression of functional channels. In humans and rats, non functional alternatively spliced forms of α ENaC have been proposed to act as negative regulatory components for ENaC. The purpose of this study was to examine the presence and consequently investigate the mRNA expression levels of alternatively spliced forms of α ENaC in kidney cortex of Dahl salt-resistant rats (R) versus Dahl salt-sensitive rats (S) on high salt and normal diets.

Methods

Using quantitative RT-PCR strategy, we examined the mRNA expression levels of previously reported α ENaC-a and -b alternatively spliced forms in kidney cortex of Dahl S and R rats on normal and four-week high salt diet and compared their corresponding abundance to wildtype α ENaC mRNA levels. We identified 2 novel non-coding C-terminus spliced forms and examined their mRNA expression in Dahl R versus S rat kidney cortex. We also tested the presence of five previously reported lung-specific α ENaC spliced forms in Dahl rat kidney cortex (CK479583, CK475461, CK364785, CK475819, and CB690980).

Results

Previously reported α ENaC-a and -b alternatively spliced forms are present in Dahl rat kidney cortex and are significantly higher in Dahl R versus S rats (P < 0.05). Four-week high salt diet significantly increases α ENaC-b (P < 0.05), but not α ENaC-a transcript abundance in Dahl R, but not S rats. Two non-coding α ENaC spliced forms -c and -d are newly identified in the present study, whose levels are comparable in Dahl R and S rats. Compared to α ENaC-wt, α ENaC-a, -c and -d are low abundance transcripts (4 +/- 2, 110 +/- 20, and 10 +/- 2 fold less respectively), in contrast to α ENaC-b abundance that exceeds α ENaC-wt by 32 +/- 3 fold. We could not identify any of the five previously reported lung-specific α ENaC spliced forms (CK479583, CK475461, CK364785, CK475819, and CB690980) in Dahl rat kidney cortex.

Conclusion/interpretation

α ENaC alternative splicing might regulate α ENaC by the formation of coding RNA species (α ENaC-a and -b) and non-coding RNA species (α ENaC-c and -d). α ENaC-a and -b mRNA levels are significantly higher in Dahl R versus S rats. Additionally, α ENaC-b is a salt-sensitive transcript whose levels are significantly higher 4-weeks post high salt diet compared to normal salt diet in Dahl R rats. Among the four α ENaC transcripts (-a, -b, -c and -d), α ENaC-b is a predominant transcript that exceeds α ENaC-wt abundance by ~32 fold. α ENaC-a and -b spliced forms, particularly, α ENaC-b, might potentially act as dominant negative proteins for ENaC activity, thereby rescuing Dahl R rats from developing salt-sensitive hypertension on high salt diet. On the other hand, non-coding α ENaC-c and -d might assist alternative splicing, facilitate RNA processing, or regulate α ENaC as well as each other.

Deficient BH4 production via de novo and salvage pathways regulates NO responses to cytokines in adult cardiac myocytes

Adult rat cardiac myocytes typically display a phenotypic response to cytokines manifested by low or no increases in nitric oxide (NO) production via inducible NO synthase (iNOS) that distinguishes them from other cell types. To better characterize this response, we examined the expression of tetrahydrobiopterin (BH4)-synthesizing and arginine-utilizing genes in cytokine-stimulated adult cardiac myocytes. Intracellular BH4 and 7,8-dihydrobiopterin (BH2) and NO production were quantified. Cytokines induced GTP cyclohydrolase and its feedback regulatory protein but with deficient levels of BH4 synthesis. Despite the induction of iNOS protein, cytokine-stimulated adult cardiac myocytes produced little or no increase in NO versus unstimulated cells. Western blot analysis under nonreducing conditions revealed the presence of iNOS monomers. Supplementation with sepiapterin (a precursor of BH4) increased BH4 as well as BH2, but this did not enhance NO levels or eliminate iNOS monomers. Similar findings were confirmed in vivo after treatment of rat cardiac allograft recipients with sepiapterin. It was found that expression of dihydrofolate reductase, required for full activity of the salvage pathway, was not detected in adult cardiac myocytes. Thus, adult cardiac myocytes have a limited capacity to synthesize BH4 after cytokine stimulation. The mechanisms involve posttranslational factors impairing de novo and salvage pathways. These conditions are unable to support active iNOS protein dimers necessary for NO production. These findings raise significant new questions about the prevailing understanding of how cytokines, via iNOS, cause cardiac dysfunction and injury in vivo during cardiac inflammatory disease states since cardiac myocytes are not a major source of high NO production.

Cytokines and lipopolysaccharides induce inducible nitric oxide synthase but not enzyme activity in adult rat cardiomyocytes

There is evidence that nitric oxide (NO) formation in adult cardiomyocytes stimulated with lipopolysaccharide (LPS) is not commensurate with iNOS levels. Tetrahydrobiopterin (BH(4)) is a key factor in the stabilization and NO production by iNOS homodimer. Thus we hypothesized that BH(4) is a limiting factor for NO production in adult cardiomyocytes in response to LPS and cytokines (TNF-alpha, IL-1, IFN-gamma alone, or mixed). It was verified that LPS and cytokines induced iNOS expression which did not translate into increased nitrite or [(14)C]citrulline production. This response coincided with defective BH(4) synthesis and low GTP cyclohydrolase activity. Furthermore, supplementation with BH(4) and ascorbate failed to increase iNOS activity. This effect was related to preferential accumulation of BH(2) rather than BH(4) in these cells. Uncoupled iNOS activity in stimulated cells was examined using mitochondrial aconitase activity as an endogenous marker of superoxide anion radical (O(2)(-)) formation, and found not to be significantly inhibited. 2-Hydroxyethidium also was not significantly increased. We conclude that adult cardiomyocytes are an unlikely source of NO and O(2)(-) in inflammatory conditions. This finding adds a new and unexpected layer of complexity to our understanding of the responses of the adult heart to inflammation.