STK11 loss leads to YAP1-mediated transcriptional activation in human KRAS-driven lung adenocarcinoma cell lines

Serine Threonine Kinase 11 (STK11) loss of function (LoF) correlates with anti-PD-1 therapy resistance in patients with KRAS-driven lung adenocarcinoma (LUAD). The molecular mechanisms governing this observation remain unclear and represent a critical outstanding question in the field of lung oncology. As an initial approach to understand this phenomenon, we knocked-out (KO) STK11 in multiple KRAS-driven, STK11-competent human LUAD cell lines and performed whole transcriptome analyses to identify STK11-loss-dependent differential gene expression. Subsequent pathway enrichment studies highlighted activation of the HIPPO/YAP1 signaling axis, along with the induction of numerous tumor-intrinsic cytokines. To validate that YAP1-mediated transcriptional activation occurs in response to STK11 loss, we pursued YAP1 perturbation as a strategy to restore an STK11-competent gene expression profile in STK11-KO LUAD cell lines. Together, our data link STK11 loss with YAP1-mediated transcriptional activation, including the upregulation of immune-evasion promoting cytokines IL-6, CXCL8 and CXCL2. Further, our results raise the intriguing possibility that YAP1 antagonism may represent a therapeutic approach to counter anti-PD-1 therapy resistance in STK11-null, KRAS-driven LUADs by modulating tumor-intrinsic gene expression to promote a "hot" tumor immune microenvironment.

Glutaredoxin attenuates glutathione levels via deglutathionylation of Otub1 and subsequent destabilization of system xC. SCIENCE ADVANCES 2023;9:eadi5192. [PMID: 37703360 PMCID: PMC10499329 DOI: 10.1126/sciadv.adi5192]

Glutathione (GSH) is a critical component of the cellular redox system that combats oxidative stress. The glutamate-cystine antiporter, system xC-, is a key player in GSH synthesis that allows for the uptake of cystine, the rate-limiting building block of GSH. It is unclear whether GSH or GSH-dependent protein oxidation [protein S-glutathionylation (PSSG)] regulates the activity of system xC-. We demonstrate that an environment of enhanced PSSG promotes GSH increases via a system xC--dependent mechanism. Absence of the deglutathionylase, glutaredoxin (GLRX), augmented SLC7A11 protein and led to significant increases of GSH content. S-glutathionylation of C23 or C204 of the deubiquitinase OTUB1 promoted interaction with the E2-conjugating enzyme UBCH5A, leading to diminished ubiquitination and proteasomal degradation of SLC7A11 and augmentation of GSH, effects that were reversed by GLRX. These findings demonstrate an intricate link between GLRX and GSH via S-glutathionylation of OTUB1 and system xC- and illuminate a previously unknown feed-forward regulatory mechanism whereby enhanced GSH protein oxidation augments cellular GSH.

Spatiotemporal higher-order chromatin landscape of human histone gene clusters at Histone Locus Bodies during the cell cycle in breast cancer progression

Human Histone Locus Bodies (HLBs) are nuclear subdomains comprised of clustered histone genes that are coordinately regulated throughout the cell cycle. We addressed temporal-spatial higher-order genome organization for time-dependent chromatin remodeling at HLBs that supports control of cell proliferation. Proximity distances of specific genomic contacts within histone gene clusters exhibit subtle changes during the G1 phase in MCF10 breast cancer progression model cell lines. This approach directly demonstrates that the two principal histone gene regulatory proteins, HINFP (H4 gene regulator) and NPAT, localize at chromatin loop anchor-points, denoted by CTCF binding, supporting the stringent requirement for histone biosynthesis to package newly replicated DNA as chromatin. We identified a novel enhancer region located ∼2MB distal to histone gene sub-clusters on chromosome 6 that consistently makes genomic contacts with HLB chromatin and is bound by NPAT. During G1 progression the first DNA loops form between one of three histone gene sub-clusters bound by HINFP and the distal enhancer region. Our findings are consistent with a model that the HINFP/NPAT complex controls the formation and dynamic remodeling of higher-order genomic organization of histone gene clusters at HLBs in early to late G1 phase to support transcription of histone mRNAs in S phase.

Abstract 270: Metabolic rewiring and metastatic potential in STK11-null lung adenocarcinoma

Non-small cell lung adenocarcinomas with concurrent oncogenic KRAS and SKT11 loss-of-function mutations represent an aggressive subtype that is characterized, in part, by increased metastasis and enhanced dependence on glutamine metabolism. The purpose of this study was to elucidate the metabolic rewiring of STK11-null KRAS-driven lung adenocarcinoma cells and determine how such altered metabolism may promote metastasis. Both parental and ΔSTK11 cells, generated via CRISPR-Cas9, were assessed for metabolic flux and measures of metastatic potential at baseline, and when deprived of exogenous glutamine. Seahorse analysis revealed a “metabolically exhausted” mitochondrial phenotype in ΔSTK11 cells at baseline. Upon exogenous glutamine deprivation, both ΔSTK11 and parental cells similarly decreased mitochondrial respiration, however, through the employment of RNA sequencing and heavy carbon labeling, we demonstrate an upregulation of the hexosamine biosynthetic pathway (HBP) in ΔSTK11 cells. Moreover, we observed an increase in global O-GlcNAcylation, the product of the HBP, in ΔSTK11 cells. The HBP is an offshoot of glycolysis/gluconeogenesis that serves as a central hub to regulate many cancer fitness pathways via O-GlcNAcylation, and it has been established that cancer cells upregulate the HBP resulting in activation of proteins involved in promoting metastasis. To assess transcriptional characteristics of metastatic spread, KEGG pathway analysis was performed on transcriptomic data obtained from parental vs ΔSTK11 cells subjected to glutamine deprivation and revealed significant upregulation of NFkB and MAPK signaling in ΔSTK11 cells. Additionally, EMT markers downstream of these pathways, such as SNAI2 and ZEB1, were upregulated in ΔSTK11 cells. Trypan Blue staining was used as a preliminary method to measure anoikis resistance, an additional characteristic of metastatic potential. Results revealed a significant increase in live detached cells upon glutamine deprivation-induced detachment in ΔSTK11 cells. Correspondingly, differential gene expression indicated significant upregulation of BCL2A1, an anti-apoptotic protein, in ΔSTK11 cells following glutamine deprivation. Future studies aim to further clarify the altered metabolic profile of ΔSTK11 lung adenocarcinoma cells to identify the mechanism(s) by which such metabolic rewiring drives metastatic progression. In conclusion, this work reveals novel findings regarding the molecular mechanisms altered downstream of STK11-loss that influence glutamine metabolism and enhanced metastasis in KRAS-driven lung adenocarcinomas.

Citation Format: Shannon M. Prior, Sean M. Lenahan, Hailey M. Sarausky, David J. Seward, Paula B. Deming. Metabolic rewiring and metastatic potential in STK11-null lung adenocarcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 270.

Investigation of discordant sibling pairs from hereditary breast cancer families and analysis of a rare PMS1 variant

BACKGROUND

It is likely that additional genes for hereditary breast cancer can be identified using a discordant sib pair design. Using this design we identified individuals harboring a rare PMS1 c.605G>A variant previously predicted to result in loss of function.

OBJECTIVES

A family-based design and predictive algorithms were used to prioritize candidate variants possibly associated with an increased risk of hereditary breast cancer. Functional analyses were performed for one of the candidate variants, PMS1 c.605G>A.

METHODS

1) 14 discordant sister-pairs from hereditary breast cancer families were identified. 2) Whole exome sequencing was performed and candidate risk variants identified. 3) A rare PMS variant was identified in 2 unrelated affected sisters but no unaffected siblings. 4) Functional analysis of this variant was carried out using targeted mRNA sequencing.

RESULTS

Genotype-phenotype correlation did not demonstrate tracking of the variant with cancer in the family. Functional analysis revealed no difference in exon 6 incorporation, which was validated by analyzing PMS1 allele specific expression.

CONCLUSIONS

The PMS1 c.605G>A variant did not segregate with disease, and there was no variant-dependent impact on PMS1 exon 6 splicing, supporting this variant is likely benign. Functional analyses are imperative to understanding the clinical significance of predictive algorithms.

Functional assessment of somatic STK11 variants identified in primary human non-small cell lung cancers

Serine/Threonine Kinase 11 (STK11) encodes an important tumor suppressor that is frequently mutated in lung adenocarcinoma. Clinical studies have shown that mutations in STK11 resulting in loss of function correlate with resistance to anti-PD-1 monoclonal antibody therapy in KRAS-driven non-small cell lung cancer (NSCLC), but the molecular mechanisms responsible remain unclear. Despite this uncertainty, STK11 functional status is emerging as a reliable biomarker for predicting non-response to anti-PD-1 therapy in NSCLC patients. The clinical utility of this biomarker ultimately depends upon accurate classification of STK11 variants. For nonsense variants occurring early in the STK11 coding region, this assessment is straightforward. However, rigorously demonstrating the functional impact of missense variants remains an unmet challenge. Here we present data characterizing four STK11 splice-site variants by analyzing tumor mRNA, and 28 STK11 missense variants using an in vitro kinase assay combined with a cell-based p53-dependent luciferase reporter assay. The variants we report were identified in primary human NSCLC biopsies in collaboration with the University of Vermont Genomic Medicine group. Additionally, we compare our experimental results with data from 22 in silico predictive algorithms. Our work highlights the power, utility and necessity of functional variant assessment and will aid STK11 variant curation, provide a platform to assess novel STK11 variants and help guide anti-PD-1 therapy utilization in KRAS-driven NSCLCs.

Glutathionylation chemistry promotes interleukin-1 beta-mediated glycolytic reprogramming and pro-inflammatory signaling in lung epithelial cells

Glycolysis is a well-known process by which metabolically active cells, such as tumor or immune cells meet their high metabolic demands. Previously, our laboratory has demonstrated that in airway epithelial cells, the pleiotropic cytokine, interleukin-1 beta (IL1B) induces glycolysis and that this contributes to allergic airway inflammation and remodeling. Activation of glycolysis is known to increase NADPH reducing equivalents generated from the pentose phosphate pathway, linking metabolic reprogramming with redox homeostasis. In addition, numerous glycolytic enzymes are known to be redox regulated. However, whether and how redox chemistry regulates metabolic reprogramming more generally remains unclear. In this study, we employed a multi-omics approach in primary mouse airway basal cells to evaluate the role of protein redox biochemistry, specifically protein glutathionylation, in mediating metabolic reprogramming. Our findings demonstrate that IL1B induces glutathionylation of multiple proteins involved in metabolic regulation, notably in the glycolysis pathway. Cells lacking Glutaredoxin-1 (Glrx), the enzyme responsible for reversing glutathionylation, show modulation of multiple metabolic pathways including an enhanced IL1B-induced glycolytic response. This was accompanied by increased secretion of thymic stromal lymphopoietin (TSLP), a cytokine important in asthma pathogenesis. Targeted inhibition of glycolysis prevented TSLP release, confirming the functional relevance of enhanced glycolysis in cells stimulated with IL1B. Collectively, data herein point to an intriguing link between glutathionylation chemistry and glycolytic reprogramming in epithelial cells and suggest that glutathionylation chemistry may represent a therapeutic target in pulmonary pathologies with perturbations in the glycolysis pathway.

Downregulation of DUOX1 function contributes to aging-related impairment of innate airway injury responses and accelerated senile emphysema

Aging is associated with a gradual loss of lung function due to increased cellular senescence, decreased regenerative capacity, and impaired innate host defense. One important aspect of innate airway epithelial host defense to nonmicrobial triggers is the secretion of alarmins such as IL-33 and activation of type 2 inflammation, which were previously found to depend on activation of the NADPH oxidase (NOX) homolog DUOX1, and redox-dependent signaling pathways that promote alarmin secretion. Here, we demonstrate that normal aging of C57BL/6J mice resulted in markedly decreased lung innate epithelial type 2 responses to exogenous triggers such as the airborne allergen Dermatophagoides pteronyssinus, which was associated with marked downregulation of DUOX1, as well as DUOX1-mediated redox-dependent signaling. DUOX1 deficiency was also found to accelerate age-related airspace enlargement and decline in lung function but did not consistently affect other features of lung aging such as senescence-associated inflammation. Intriguingly, observations of age-related DUOX1 downregulation and enhanced airspace enlargement due to DUOX1 deficiency in C57BL/6J mice, which lack a functional mitochondrial nicotinamide nucleotide transhydrogenase (NNT), were much less dramatic in C57BL/6NJ mice with normal NNT function, although the latter mice also displayed impaired innate epithelial injury responses with advancing age. Overall, our findings indicate a marked aging-dependent decline in (DUOX1-dependent) innate airway injury responses to external nonmicrobial triggers, but the impact of aging on DUOX1 downregulation and its significance for age-related senile emphysema development was variable between different C57BL6 substrains, possibly related to metabolic alterations due to differences in NNT function.

Peroxiredoxins and Beyond; Redox Systems Regulating Lung Physiology and Disease

Significance: The lung is a unique organ, as it is constantly exposed to air, and thus it requires a robust antioxidant defense system to prevent the potential damage from exposure to an array of environmental insults, including oxidants. The peroxiredoxin (PRDX) family plays an important role in scavenging peroxides and is critical to the cellular antioxidant defense system. Recent Advances: Exciting discoveries have been made to highlight the key features of PRDXs that regulate the redox tone. PRDXs do not act in isolation as they require the thioredoxin/thioredoxin reductase/NADPH, sulfiredoxin (SRXN1) redox system, and in some cases glutaredoxin/glutathione, for their reduction. Furthermore, the chaperone function of PRDXs, controlled by the oxidation state, demonstrates the versatility in redox regulation and control of cellular biology exerted by this class of proteins. Critical Issues: Despite the long-known observations that redox perturbations accompany a number of pulmonary diseases, surprisingly little is known about the role of PRDXs in the etiology of these diseases. In this perspective, we review the studies that have been conducted thus far to address the roles of PRDXs in lung disease, or experimental models used to study these diseases. Intriguing findings, such as the secretion of PRDXs and the formation of autoantibodies, raise a number of questions about the pathways that regulate secretion, redox status, and immune response to PRDXs. Future Directions: Further understanding of the mechanisms by which individual PRDXs control lung inflammation, injury, repair, chronic remodeling, and cancer, and the importance of PRDX oxidation state, configuration, and client proteins that govern these processes is needed.

Dysregulation of the glutaredoxin/S-glutathionylation redox axis in lung diseases

Glutathione is a major redox buffer, reaching millimolar concentrations within cells and high micromolar concentrations in airways. While glutathione has been traditionally known as an antioxidant defense mechanism that protects the lung tissue from oxidative stress, glutathione more recently has become recognized for its ability to become covalently conjugated to reactive cysteines within proteins, a modification known as S-glutathionylation (or S-glutathiolation or protein mixed disulfide). S-glutathionylation has the potential to change the structure and function of the target protein, owing to its size (the addition of three amino acids) and charge (glutamic acid). S-glutathionylation also protects proteins from irreversible oxidation, allowing them to be enzymatically regenerated. Numerous enzymes have been identified to catalyze the glutathionylation/deglutathionylation reactions, including glutathione S-transferases and glutaredoxins. Although protein S-glutathionylation has been implicated in numerous biological processes, S-glutathionylated proteomes have largely remained unknown. In this paper, we focus on the pathways that regulate GSH homeostasis, S-glutathionylated proteins, and glutaredoxins, and we review methods required toward identification of glutathionylated proteomes. Finally, we present the latest findings on the role of glutathionylation/glutaredoxins in various lung diseases: idiopathic pulmonary fibrosis, asthma, and chronic obstructive pulmonary disease.

Age-dependent dysregulation of redox genes may contribute to fibrotic pulmonary disease susceptibility

Aging is associated with enhanced oxidative stress and increased susceptibility to numerous diseases. This relationship is particularly striking with respect to the incidence of fibrotic lung disease. To identify potential mechanisms underlying the association between aging and susceptibility to fibrotic lung disease we analyzed transcriptome data from 342 disease-free human lung samples as a function of donor age. Our analysis reveals that aging in lung is accompanied by modest yet progressive changes in genes modulating redox homeostasis, the TGF-beta 1 signaling axis, and the extracellular matrix (ECM), pointing to an aging lung functional network (ALFN). Further, the transcriptional changes we document are tissue-specific, with age-dependent gene expression patterns differing across organ systems. Our findings suggest that the age-associated increased incidence of fibrotic pulmonary disease occurs in the context of tissue-specific, age-dependent transcriptional changes. Understanding the relationship between age-associated gene expression and susceptibility to fibrotic pulmonary disease may allow for more accurate risk stratification and effective therapeutic interventions within this challenging clinical space.

Active Surveillance for Medically Inoperable Stage IA Lung Cancer in the Elderly

Objectives

Treatment for stage IA lung cancer may be too aggressive an approach in elderly patients with competing co-morbidities. We report outcomes for those electing active surveillance (AS) and investigate factors that may predict indolent disease.

Materials and methods

Retrospective review was performed for 12 consecutive patients, ≥70 years old, with medically inoperable stage IA, T1N0M0 lung cancer and significant co-morbidities, who chose AS with radiation therapy (RT) reserved for clear disease progression. Collected data included Charlson-Deyo Comorbidity Index (CDCI) grades, histology, and tumor size changes. Volume doubling time (VDT) calculations used a modified Schwartz equation.

Results

Fifteen nodules underwent AS in 12 patients; three patients had more than one nodule. Median age of all patients was 78 (range, 71–85). All patients’ CDCI grades were ≥1, 7 were ≥2. Eleven of 12 patients were deemed to be at high-risk for falls. Twelve nodules in 12 patients were biopsied; adenocarcinoma the prevailing common (47%) histology. The median, one, two and three year patient freedom-from-RT values were 21.4 months (95% CI: 11.6-not reached), 81%, 43%, and 29%, respectively. Median VDT of treated vs. untreated nodules was 189 days (range, 62-infinite) vs. 1153 days (range, 504-infinite), respectively. No patient progressed regionally or distantly, and there have been no cancer-related deaths. Due to cardiovascular events, two patients died and one remains on hospice. Median duration of AS for those still continuing computed tomography (CT) surveillance is 35.1 months.

Conclusion

Selected elderly patients with stage IA lung cancer and significant co-morbidities may undergo AS without detriment in outcome. Prospective AS studies are warranted.

Reducing protein oxidation reverses lung fibrosis

Idiopathic pulmonary fibrosis is characterized by excessive deposition of collagen in the lung, leading to chronically impaired gas exchange and death^1-3. Oxidative stress is believed to be critical in this disease pathogenesis^4-6, although the exact mechanisms remain enigmatic. Protein S-glutathionylation (PSSG) is a post-translational modification of proteins that can be reversed by glutaredoxin-1 (GLRX)⁷. It remains unknown whether GLRX and PSSG play a role in lung fibrosis. Here, we explored the impact of GLRX and PSSG status on the pathogenesis of pulmonary fibrosis, using lung tissues from subjects with idiopathic pulmonary fibrosis, transgenic mouse models and direct administration of recombinant Glrx to airways of mice with existing fibrosis. We demonstrate that GLRX enzymatic activity was strongly decreased in fibrotic lungs, in accordance with increases in PSSG. Mice lacking Glrx were far more susceptible to bleomycin- or adenovirus encoding active transforming growth factor beta-1 (AdTGFB1)-induced pulmonary fibrosis, whereas transgenic overexpression of Glrx in the lung epithelium attenuated fibrosis. We furthermore show that endogenous GLRX was inactivated through an oxidative mechanism and that direct administration of the Glrx protein into airways augmented Glrx activity and reversed increases in collagen in mice with TGFB1- or bleomycin-induced fibrosis, even when administered to fibrotic, aged animals. Collectively, these findings suggest the therapeutic potential of exogenous GLRX in treating lung fibrosis.

Higher order genomic organization and regulatory compartmentalization for cell cycle control at the G1/S-phase transition

Fidelity of histone gene regulation, and ultimately of histone protein biosynthesis, is obligatory for packaging of newly replicated DNA into chromatin. Control of histone gene expression within the 3-dimensional context of nuclear organization is reflected by two well documented observations. DNA replication-dependent histone mRNAs are synthesized at specialized subnuclear domains designated histone locus bodies (HLBs), in response to activation of the growth factor dependent Cyclin E/CDK2/HINFP/NPAT pathway at the G1/S transition in mammalian cells. Complete loss of the histone gene regulatory factors HINFP or NPAT disrupts HLB integrity that is necessary for coordinate control of DNA replication and histone gene transcription. Here we review the molecular histone-related requirements for G1/S-phase progression during the cell cycle. Recently developed experimental strategies, now enable us to explore mechanisms involved in dynamic control of histone gene expression in the context of the temporal (cell cycle) and spatial (HLBs) remodeling of the histone gene loci.

Paradoxical roles of dual oxidases in cancer biology

Dysregulated oxidative metabolism is a well-recognized aspect of cancer biology, and many therapeutic strategies are based on targeting cancers by altering cellular redox pathways. The NADPH oxidases (NOXes) present an important enzymatic source of biological oxidants, and the expression and activation of several NOX isoforms are frequently dysregulated in many cancers. Cell-based studies have demonstrated a role for several NOX isozymes in controlling cell proliferation and/or cell migration, further supporting a potential contributing role for NOX in promoting cancer. While various NOX isoforms are often upregulated in cancers, paradoxical recent findings indicate that dual oxidases (DUOXes), normally prominently expressed in epithelial lineages, are frequently suppressed in epithelial-derived cancers by epigenetic mechanisms, although the functional relevance of such DUOX silencing has remained unclear. This review will briefly summarize our current understanding regarding the importance of reactive oxygen species (ROS) and NOXes in cancer biology, and focus on recent observations indicating the unique and seemingly opposing roles of DUOX enzymes in cancer biology. We will discuss current knowledge regarding the functional properties of DUOX, and recent studies highlighting mechanistic consequences of DUOX1 loss in lung cancer, and its consequences for tumor invasiveness and current anticancer therapy. Finally, we will also discuss potentially unique roles for the DUOX maturation factors. Overall, a better understanding of mechanisms that regulate DUOX and the functional consequences of DUOX silencing in cancer may offer valuable new diagnostic insights and novel therapeutic opportunities.

Demethylation of trimethylated histone H3 Lys4 in vivo by JARID1 JmjC proteins

Histone H3 Lys4 trimethylation (H3-K4me3) is a conserved mark of actively transcribed chromatin. Using a conditional mutant of the yeast H3-K4 methyltransferase, Set1p, we demonstrate rapid turnover of H3-K4me3 and H3-K4me2 in vivo and show this process requires Yjr119Cp, of the JARID1 family of JmjC proteins. Ectopic overexpression of mouse Jarid1B, a Yjr119Cp homolog, greatly diminished H3-K4me3 and H3-K4me2 in HeLa cells, suggesting these proteins function as K4 demethylases in vivo.

Retention of structural and functional polarity in cultured skate hepatocytes undergoing in vitro morphogenesis

The present study characterized a primary culture model of hepatocytes isolated from the little skate, Leucoraja erinacea, that maintain remarkable structural and functional polarity over 7 days in culture. Skate hepatocytes were isolated as clusters of 3-20 hepatocytes surrounding a bile canaliculus, rather than as single cells. Trypan blue and propidium iodide exclusion was found to be >98%, and the cells maintained high intracellular concentrations of K+, ATP, and reduced glutathione (GSH), and high ratios of ATP/ADP and GSH/GSSG. Glutathione S-transferase activity remained constant, whereas cytochrome P450 activity declined to 16% of initial levels after 7 days. Quantitative RT-PCR analysis revealed that the mRNA levels of several genes remained constant over the 7-day period, whereas Bsep, the canalicular bile salt export pump, levels declined slowly to 30% of initial values. In the presence of dexamethasone, the cells underwent a morphogenesis in which the clusters reannealed into a three-dimensional network of chords. During this morphogenesis, skate hepatocytes clusters maintained a polarized distribution of actin filaments and microtubules, as well as apical and basolateral membrane domains. Polarity of membrane transport systems was confirmed both morphologically, using antibodies raised against Bsep and Mrp2, the canalicular multispecific organic anion transporter, and functionally, by monitoring secretion of the fluorescent organic anions NBD-taurocholate, a Bsep substrate, and fluorescein-methotrexate, an Mrp2 substrate, into the bile canalicular spaces. Overall, the results indicate that in contrast with mammalian hepatocytes, isolated skate hepatocyte clusters retain polarity in culture, and provide an excellent system for investigating long-term effects of drugs and xenobiotics on hepatobiliary functions, and for studying in vitro morphogenesis.

Lack of biliary lipid excretion in the little skate, Raja erinacea, indicates the absence of functional Mdr2, Abcg5, and Abcg8 transporters

The ABC transporters bile salt export pump (BSEP; encoded by the ABCB11 gene), MDR3 P-glycoprotein (ABCB4), and sterolin 1 and 2 (ABCG5 and ABCG8) are crucial for the excretion of bile salt, phospholipid, and cholesterol, respectively, into the bile of mammals. The current paradigm is that phospholipid excretion mainly serves to protect membranes of the biliary tree against bile salt micelles. Bile salt composition and cytotoxicity, however, differ greatly between species. We investigated whether biliary phospholipid and cholesterol excretion occurs in a primitive species, the little skate, which almost exclusively excretes the sulphated bile alcohol scymnolsulphate. We observed no phospholipid and very little cholesterol excretion into bile of these animals. Conversely, when scymnolsulphate was added to the perfusate of isolated mouse liver perfusions, it was very well capable of driving biliary phospholipid and cholesterol excretion. Furthermore, in an erythrocyte cytolysis assay, scymnolsulphate was found to be at least as cytotoxic as taurocholate. These results demonstrate that the little skate does not have a system for the excretion of phospholipid and cholesterol and that both the MDR3 and the two half-transporter genes, ABCG5 and ABCG8, have evolved relatively late in evolution to mediate biliary lipid excretion. Little skate plasma membranes may be protected against bile salt micelles mainly by their high sphingomyelin content.

Functional complementation between a novel mammalian polygenic transport complex and an evolutionarily ancient organic solute transporter, OSTalpha-OSTbeta

These studies identify an organic solute transporter (OST) that is generated when two novel gene products are co-expressed, namely human OSTalpha and OSTbeta or mouse OSTalpha and OSTbeta. The results also demonstrate that the mammalian proteins are functionally complemented by evolutionarily divergent Ostalpha-Ostbeta proteins recently identified in the little skate, Raja erinacea, even though the latter exhibit only 25-41% predicted amino acid identity with the mammalian proteins. Human, mouse, and skate OSTalpha proteins are predicted to contain seven transmembrane helices, whereas the OSTbeta sequences are predicted to have a single transmembrane helix. Human OSTalpha-OSTbeta and mouse Ostalpha-Ostbeta cDNAs were cloned from liver mRNA, sequenced, expressed in Xenopus laevis oocytes, and tested for their ability to functionally complement the corresponding skate proteins by measuring transport of [3H]estrone 3-sulfate. None of the proteins elicited a transport signal when expressed individually in oocytes; however, all nine OSTalpha-OSTbeta combinations (i.e. OSTalpha-OSTbeta pairs from human, mouse, or skate) generated robust estrone 3-sulfate transport activity. Transport was sodium-independent, saturable, and inhibited by other steroids and anionic drugs. Human and mouse OSTalpha-OSTbeta also were able to mediate transport of taurocholate, digoxin, and prostaglandin E2 but not of estradiol 17beta-d-glucuronide or p-aminohippurate. OSTalpha and OSTbeta were able to reach the oocyte plasma membrane when expressed either individually or in pairs, indicating that co-expression is not required for proper membrane targeting. Interestingly, OSTalpha and OSTbeta mRNAs were highly expressed and widely distributed in human tissues, with the highest levels occurring in the testis, colon, liver, small intestine, kidney, ovary, and adrenal gland.

Expression cloning of two genes that together mediate organic solute and steroid transport in the liver of a marine vertebrate

Uptake of organic solutes and xenobiotics by mammalian cells is mediated by ATP-independent transporters, and four families of transporters have now been identified. To search for novel organic solute transporters, a liver cDNA library from an evolutionarily primitive marine vertebrate, the little skate Raja erinacea, was screened for taurocholate transport activity by using Xenopus laevis oocytes. In contrast to the organic anion transporters identified to date, a transport activity was identified in this library that required the coexpression of two distinct gene products, termed organic solute transporter alpha and beta (Ostalpha, Ostbeta). Ostalpha cDNA encodes for a protein of 352 aa and seven putative transmembrane (TM) domains. Ostbeta contains 182 aa and has at least one and perhaps two TM domains. There is no significant sequence identity between Ostalpha and Ostbeta, and only low identity with sequences in the databases; however, Ostalpha bears a resemblance to some G protein-coupled receptors, and Ostbeta exhibits 22% amino acid identity with the C-terminal TM and intracellular domains of protocadherin-gamma, a cell surface glycoprotein. Xenopus oocytes injected with the cRNA for both Ostalpha and Ostbeta, but not each separately, were able to take up taurocholate, estrone sulfate, digoxin, and prostaglandin E(2), but not p-aminohippurate or S-dinitrophenyl glutathione. Transport was sodium-independent, saturable, and inhibited by organic anions and steroids, including the major skate bile salt, scymnol sulfate. These results identify an organic anion transporter composed of a putative seven-helix TM protein and an ancillary membrane polypeptide.

bHLH transcription factor MyoD affects myosin heavy chain expression pattern in a muscle-specific fashion

A strong correlative pattern between MyoD gene expression and myosin heavy chain IIB (MHC IIB) gene expression exists. To test whether this correlative relationship is causative, MHC gene expression in muscles from MyoD(-/-) mice was analyzed. The MHC IIB gene was not detectable in the MyoD(-/-) diaphragm, whereas the MHC IIB protein made up 10.0 +/- 1.7% of the MHC protein pool in the wild-type (WT) mouse diaphragm. Furthermore, the MHC IIA protein was not detectable in the MyoD(-/-) biceps brachii, and the MHC IIB protein was overexpressed in the masseter. To examine whether MyoD is required for the upregulation of the MHC IIB gene within slow muscle after disuse, MyoD(-/-) and WT hindlimb musculature was unweighted. MyoD(-/-) exhibited a diminished response in the upregulation of the MHC IIB mRNA within the soleus muscle as a result of the hindlimb unweighting. Collectively, these data suggest that MyoD plays a role in the MHC profile in a muscle-specific fashion.

Bile salt excretion in skate liver is mediated by a functional analog of Bsep/Spgp, the bile salt export pump

Biliary secretion of bile salts in mammals is mediated in part by the liver-specific ATP-dependent canalicular membrane protein Bsep/Spgp, a member of the ATP-binding cassette superfamily. We examined whether a similar transport activity exists in the liver of the evolutionarily primitive marine fish Raja erinacea, the little skate, which synthesizes mainly sulfated bile alcohols rather than bile salts. Western blot analysis of skate liver plasma membranes using antiserum raised against rat liver Bsep/Spgp demonstrated a dominant protein band with an apparent molecular mass of 210 kDa, a size larger than that in rat liver canalicular membranes, approximately 160 kDa. Immunofluorescent localization with anti-Bsep/Spgp in isolated, polarized skate hepatocyte clusters revealed positive staining of the bile canaliculi, consistent with its selective apical localization in mammalian liver. Functional characterization of putative ATP-dependent canalicular bile salt transport activity was assessed in skate liver plasma membrane vesicles, with [(3)H]taurocholate as the substrate. [(3)H]taurocholate uptake into the vesicles was mediated by ATP-dependent and -independent mechanisms. The ATP-dependent component was saturable, with a Michaelis-Menten constant (K(m)) for taurocholate of 40+/-7 microM and a K(m) for ATP of 0.6+/-0.1 mM, and was competitively inhibited by scymnol sulfate (inhibition constant of 23 microM), the major bile salt in skate bile. ATP-dependent uptake of taurocholate into vesicles was inhibited by known substrates and inhibitors of Bsep/Spgp, including other bile salts and bile salt derivatives, but not by inhibitors of the multidrug resistance protein-1 or the canalicular multidrug resistance-associated protein, indicating a distinct transport mechanism. These findings provide functional and structural evidence for a Bsep/Spgp-like protein in the canalicular membrane of the skate liver. This transporter is expressed early in vertebrate evolution and transports both bile salts and bile alcohols.

Hepatic toxicity and persistence of ser/thr protein phosphatase inhibition by microcystin in the little skate Raja erinacea

Microcystin-induced ser/thr protein phosphatase (PP) inhibition and toxicity were examined in the little skate (Raja erinacea), an evolutionarily primitive marine vertebrate. As in mammals, PP inhibition and toxicity were exclusively hepatocellular, but were much more persistent in the skate. A dose of 63 microg/kg given iv to adult male skates resulted in the near complete inhibition of hepatic PP activity at 24 h. PP activity was still 95% inhibited 7 days after dosing in skates given 125 microg/kg microcystin. Mortality occurred at doses of 500 microg/kg or more. Hepatic lesions were only seen in animals with fully inhibited PP activity in liver. The histological changes seen at 125 microg/kg were mild periportal inflammatory changes increasing in severity together with hepatocyte necrosis at higher doses of microcystin. Microcystin persisted and could be detected in plasma up to 7 days after dosing. This finding shows that, in the skate, as in mammals, the liver is the only organ capable of uptake of microcystin, since there was no significant inhibition of PP activity in the rectal gland and small decreases in PP activity of the kidney that were not time or dose dependent. In vitro microcystin caused dose-dependent inhibition of PP activity in isolated skate hepatocytes, while it was without effect in cultured rectal glands. Uptake of microcystin and the accompanying inhibition of PP activity in skate hepatocytes was prevented by the addition of a series of organic dyes and bile acids. The spectrum of inhibitors of microcystin uptake in skate is similar to that seen in the rat, indicating common features of the carrier(s) in these diverse species.