High glucose upregulates pantothenate kinase 4 (PanK4) and thus affects M2-type pyruvate kinase (Pkm2)

Pantothenate Kinase 4 Governs Lens Epithelial Fibrosis by Negatively Regulating Pyruvate Kinase M2-Related Glycolysis

Lens fibrosis is one of the leading causes of cataract in the elderly population. The primary energy substrate of the lens is glucose from the aqueous humor, and the transparency of mature lens epithelial cells (LECs) is dependent on glycolysis for ATP. Therefore, the deconstruction of reprogramming of glycolytic metabolism can contribute to further understanding of LEC epithelial-mesenchymal transition (EMT). In the present study, we found a novel pantothenate kinase 4 (PANK4)-related glycolytic mechanism that regulates LEC EMT. The PANK4 level was correlated with aging in cataract patients and mice. Loss of function of PANK4 significantly contributed to alleviating LEC EMT by upregulating pyruvate kinase M2 isozyme (PKM2), which was phosphorylated at Y105, thus switching oxidative phosphorylation to glycolysis. However, PKM2 regulation did not affect PANK4, demonstrating the downstream role of PKM2. Inhibition of PKM2 in Pank4-/- mice caused lens fibrosis, which supports the finding that the PANK4-PKM2 axis is required for LEC EMT. Glycolytic metabolism-governed hypoxia inducible factor (HIF) signaling is involved in PANK4-PKM2-related downstream signaling. However, HIF-1α elevation was independent of PKM2 (S37) but PKM2 (Y105) when PANK4 was deleted, which demonstrated that PKM2 and HIF-1α were not involved in a classic positive feedback loop. Collectively, these results indicate a PANK4-related glycolysis switch that may contribute to HIF-1 stabilization and PKM2 phosphorylation at Y105 and inhibit LEC EMT. The mechanism elucidation in our study may also shed light on fibrosis treatments for other organs.

An extensive β1-adrenergic receptor gene signaling network regulates molecular remodeling in dilated cardiomyopathies

We investigated the extent, biologic characterization, phenotypic specificity, and possible regulation of a β1-adrenergic receptor-linked (β1-AR-linked) gene signaling network (β1-GSN) involved in left ventricular (LV) eccentric pathologic remodeling. A 430-member β1-GSN was identified by mRNA expression in transgenic mice overexpressing human β1-ARs or from literature curation, which exhibited opposite directional behavior in interventricular septum endomyocardial biopsies taken from patients with beta-blocker-treated, reverse remodeled dilated cardiomyopathies. With reverse remodeling, the major biologic categories and percentage of the dominant directional change were as follows: metabolic (19.3%, 81% upregulated); gene regulation (14.9%, 78% upregulated); extracellular matrix/fibrosis (9.1%, 92% downregulated); and cell homeostasis (13.3%, 60% upregulated). Regarding the comparison of β1-GSN categories with expression from 19,243 nonnetwork genes, phenotypic selection for major β1-GSN categories was exhibited for LV end systolic volume (contractility measure), ejection fraction (remodeling index), and pulmonary wedge pressure (wall tension surrogate), beginning at 3 months and persisting to study completion at 12 months. In addition, 121 lncRNAs were identified as possibly involved in cis-acting regulation of β1-GSN members. We conclude that an extensive 430-member gene network downstream from the β1-AR is involved in pathologic ventricular remodeling, with metabolic genes as the most prevalent category.

Influence of a Polyherbal Mixture in Dairy Calves: Growth Performance and Gene Expression

A polyherbal feed mixture containing (Achyrantes aspera, Trachyspermum ammi, Citrullus colocynthis, Andrographis paniculata, and Azadirachta indica) was evaluated in growing calves through blood chemistry, blood biometry, and gene expression during the pre-ruminant to weaning period. Forty Holstein calves (initial BW 45.6 ± 3.2 kg; 22.8 ± 0.9 days post birth) from a dairy farm were randomly assigned to the following treatments: 0, 3, 4, and 5 g/d of a polyherbal mixture, dosed in colloid gels with gelatin. Calves were housed in individual outdoor boxes with ad libitum access to a 21.5% CP calf starter and water and fed individually with a mixture of milk and a non-medicated milk replacer (22% CP). Blood samples were collected on day 59 for blood chemistry, blood biometry, and gene expression analysis in leukocyte through microarray assays. Immunoglobulins were quantified by enzyme-linked immunosorbent assay. The animals treated with the polyherbal mixture showed a quadratic effect on final body weight, daily weight gain, final hip height, and final thoracic girth. The best performance results were obtained with a treatment dose of 4 g/d. The serum IgG increased linearly with the treatment doses. Gene set enrichment analysis of upregulated genes revealed that the three biological processes with higher fold change were tight junction, mucin type O-Glycan biosynthesis, and intestinal immune network for IgA production. Also, these upregulated genes influenced arachidonic acid metabolism, and pantothenate and CoA biosynthesis. Gene ontology enrichment analysis indicated that the pathways enriched were PELP1 estrogen receptor interacting protein pathways, nuclear receptors in lipid metabolism and toxicity, tight junction, ECM-receptor interaction, thyroid hormone signaling pathways, vascular smooth muscle contraction, ribosome function, glutamatergic synapse pathway, focal adhesion, Hippo, calcium, and MAPK signaling pathways.

Regulation of coenzyme A levels by degradation: the 'Ins and Outs'

Coenzyme A (CoA) is the predominant acyl carrier in mammalian cells and a cofactor that plays a key role in energy and lipid metabolism. CoA and its thioesters (acyl-CoAs) regulate a multitude of metabolic processes at different levels: as substrates, allosteric modulators, and via post-translational modification of histones and other non-histone proteins. Evidence is emerging that synthesis and degradation of CoA are regulated in a manner that enables metabolic flexibility in different subcellular compartments. Degradation of CoA occurs through distinct intra- and extracellular pathways that rely on the activity of specific hydrolases. The pantetheinase enzymes specifically hydrolyze pantetheine to cysteamine and pantothenate, the last step in the extracellular degradation pathway for CoA. This reaction releases pantothenate in the bloodstream, making this CoA precursor available for cellular uptake and de novo CoA synthesis. Intracellular degradation of CoA depends on specific mitochondrial and peroxisomal Nudix hydrolases. These enzymes are also active against a subset of acyl-CoAs and play a key role in the regulation of subcellular (acyl-)CoA pools and CoA-dependent metabolic reactions. The evidence currently available indicates that the extracellular and intracellular (acyl-)CoA degradation pathways are regulated in a coordinated and opposite manner by the nutritional state and maximize the changes in the total intracellular CoA levels that support the metabolic switch between fed and fasted states in organs like the liver. The objective of this review is to update the contribution of these pathways to the regulation of metabolism, physiology and pathology and to highlight the many questions that remain open.

Human pantothenate kinase 4 is a pseudo-pantothenate kinase

Pantothenate kinase generates 4'-phosphopantothenate in the first and rate-determining step of coenzyme A (CoA) biosynthesis. The human genome encodes three well-characterized and nearly identical pantothenate kinases (PANK1-3) plus a putative bifunctional protein (PANK4) with a predicted amino-terminal pantothenate kinase domain fused to a carboxy-terminal phosphatase domain. Structural and phylogenetic analyses show that all active, characterized PANKs contain the key catalytic residues Glu138 and Arg207 (HsPANK3 numbering). However, all amniote PANK4s, including human PANK4, encode Glu138Val and Arg207Trp substitutions which are predicted to inactivate kinase activity. Biochemical analysis corroborates bioinformatic predictions-human PANK4 lacks pantothenate kinase activity. Introducing Glu138Val and Arg207Trp substitutions to the human PANK3 and plant PANK4 abolished their robust pantothenate kinase activity. Introducing both catalytic residues back into human PANK4 restored kinase activity, but only to a low level. This result suggests that epistatic changes to the rest of the protein already reduced the kinase activity prior to mutation of the catalytic residues in the course of evolution. The PANK4 from frog, an anamniote living relative encoding the catalytically active residues, had only a low level of kinase activity, supporting the view that HsPANK4 had reduced kinase activity prior to the catalytic residue substitutions in amniotes. Together, our data show that human PANK4 is a pseudo-pantothenate kinase-a catalytically deficient variant of the catalytically active PANK4 found in plants and fungi. The Glu138Val and Arg207Trp substitutions in amniotes (HsPANK3 numbering) completely deactivated the pantothenate kinase activity that had already been reduced by prior epistatic mutations.

A critical review of the role of M2PYK in the Warburg effect

It is becoming generally accepted in recent literature that the Warburg effect in cancer depends on inhibition of M₂PYK, the pyruvate kinase isozyme most commonly expressed in tumors. We remain skeptical. There continues to be a general lack of solid experimental evidence for the underlying idea that a bottle neck in aerobic glycolysis at the level of M₂PYK results in an expanded pool of glycolytic intermediates (which are thought to serve as building blocks necessary for proliferation and growth of cancer cells). If a bottle neck at M₂PYK exists, then the remarkable increase in lactate production by cancer cells is a paradox, particularly since a high percentage of the carbons of lactate originate from glucose. The finding that pyruvate kinase activity is invariantly increased rather than decreased in cancer undermines the logic of the M₂PYK bottle neck, but is consistent with high lactate production. The "inactive" state of M₂PYK in cancer is often described as a dimer (with reduced substrate affinity) that has dissociated from an active tetramer of M₂PYK. Although M₂PYK clearly dissociates easier than other isozymes of pyruvate kinase, it is not clear that dissociation of the tetramer occurs in vivo when ligands are present that promote tetramer formation. Furthermore, it is also not clear whether the dissociated dimer retains any activity at all. A number of non-canonical functions for M₂PYK have been proposed, all of which can be challenged by the finding that not all cancer cell types are dependent on M₂PYK expression. Additional in-depth studies of the Warburg effect and specifically of the possible regulatory role of M₂PYK in the Warburg effect are needed.

A novel mutation of PANK4 causes autosomal dominant congenital posterior cataract

Though many mutations have been identified to be associated with the occurrence of congenital cataract, pathogenic loci in some affected families are still unknown. Clinical data and genomic DNA were collected from a four-generation Chinese family. Candidate mutations were independently verified for cosegregation in the whole pedigree. Linkage analysis showed that the disease-causing mutation was located between 1p36.21 and 1p36.33. Analysis of the whole-exome sequencing data combined with linkage analysis identified a novel pathogenic variant (g.2451906C>T) at intron 4 of Pantothenate kinase 4 (PANK4 protein, PANK4 gene) in 1p36.32|606162. This variant showed complete cosegregation with the phenotype in the pedigree. The mutation was not detected in 106 normal controls nor in 40 sporadic congenital cataract patients. The mutation was demonstrated to significantly reduce the expression of the PANK4 protein level in the blood of cataract patients than that in normal individuals by ELISA. Pank4^-/- mice showed a cataract phenotype with increased numbers of apoptotic lens epithelial cells, fiber cell aggregation, and significant mRNA variation of crystallin family members. Thus, the association of a new entity of an autosomal dominant cataract with mutations in PANK4, which influences cell proliferation, apoptosis of lens epithelial cells, crystallin abnormalities, and fiber cell derangement, subsequently induces cataract.

Vnn1 pantetheinase limits the Warburg effect and sarcoma growth by rescuing mitochondrial activity

Expression of the Vnn1 pantetheinase by sarcomas is tumor suppressive by limiting the use of aerobic glycolysis for growth and rescuing mitochondrial activity through CoA regeneration.

Like other tumors, aggressive soft tissue sarcomas (STS) use glycolysis rather than mitochondrial oxidative phosphorylation (OXPHOS) for growth. Given the importance of the cofactor coenzyme A (CoA) in energy metabolism, we investigated the impact of Vnn1 pantetheinase—an enzyme that degrades pantetheine into pantothenate (vitamin B5, the CoA biosynthetic precursor) and cysyteamine—on tumor growth. Using two models, we show that Vnn1⁺ STS remain differentiated and grow slowly, and that in patients a detectable level of VNN1 expression in STS is associated with an improved prognosis. Increasing pantetheinase activity in aggressive tumors limits their growth. Using combined approaches, we demonstrate that Vnn1 permits restoration of CoA pools, thereby maintaining OXPHOS. The simultaneous production of cysteamine limits glycolysis and release of lactate, resulting in a partial inhibition of STS growth in vitro and in vivo. We propose that the Warburg effect observed in aggressive STS is reversed by induction of Vnn1 pantetheinase and the rewiring of cellular energy metabolism by its products.

Kinome Screen Identifies PFKFB3 and Glucose Metabolism as Important Regulators of the Insulin/Insulin-like Growth Factor (IGF)-1 Signaling Pathway

The insulin/insulin-like growth factor (IGF)-1 signaling pathway (ISP) plays a fundamental role in long term health in a range of organisms. Protein kinases including Akt and ERK are intimately involved in the ISP. To identify other kinases that may participate in this pathway or intersect with it in a regulatory manner, we performed a whole kinome (779 kinases) siRNA screen for positive or negative regulators of the ISP, using GLUT4 translocation to the cell surface as an output for pathway activity. We identified PFKFB3, a positive regulator of glycolysis that is highly expressed in cancer cells and adipocytes, as a positive ISP regulator. Pharmacological inhibition of PFKFB3 suppressed insulin-stimulated glucose uptake, GLUT4 translocation, and Akt signaling in 3T3-L1 adipocytes. In contrast, overexpression of PFKFB3 in HEK293 cells potentiated insulin-dependent phosphorylation of Akt and Akt substrates. Furthermore, pharmacological modulation of glycolysis in 3T3-L1 adipocytes affected Akt phosphorylation. These data add to an emerging body of evidence that metabolism plays a central role in regulating numerous biological processes including the ISP. Our findings have important implications for diseases such as type 2 diabetes and cancer that are characterized by marked disruption of both metabolism and growth factor signaling.

Identification of putative ortholog gene blocks involved in gestant and lactating mammary gland development: a rodent cross-species microarray transcriptomics approach

The mammary gland (MG) undergoes functional and metabolic changes during the transition from pregnancy to lactation, possibly by regulation of conserved genes. The objective was to elucidate orthologous genes, chromosome clusters and putative conserved transcriptional modules during MG development. We analyzed expression of 22,000 transcripts using murine microarrays and RNA samples of MG from virgin, pregnant, and lactating rats by cross-species hybridization. We identified 521 transcripts differentially expressed; upregulated in early (78%) and midpregnancy (89%) and early lactation (64%), but downregulated in mid-lactation (61%). Putative orthologous genes were identified. We mapped the altered genes to orthologous chromosomal locations in human and mouse. Eighteen sets of conserved genes associated with key cellular functions were revealed and conserved transcription factor binding site search entailed possible coregulation among all eight block sets of genes. This study demonstrates that the use of heterologous array hybridization for screening of orthologous gene expression from rat revealed sets of conserved genes arranged in chromosomal order implicated in signaling pathways and functional ontology. Results demonstrate the utilization power of comparative genomics and prove the feasibility of using rodent microarrays to identification of putative coexpressed orthologous genes involved in the control of human mammary gland development.

Pyruvate kinase M2: multiple faces for conferring benefits on cancer cells

The M2 splice isoform of pyruvate kinase (PKM2), an enzyme that catalyzes the later step of glycolysis, is a key regulator of aerobic glycolysis (known as the Warburg effect) in cancer cells. Expression and low enzymatic activity of PKM2 confer on cancer cells the glycolytic phenotype, which promotes rapid energy production and flow of glycolytic intermediates into collateral pathways to synthesize nucleic acids, amino acids, and lipids without the accumulation of reactive oxygen species. PKM2 enzymatic activity has also been shown to be negatively regulated by the interaction with CD44 adhesion molecule, which is a cell surface marker for cancer stem cells. In addition to the glycolytic functions, nonglycolytic functions of PKM2 in cancer cells are of particular interest. PKM2 is induced translocation into the nucleus, where it activates transcription of various genes by interacting with and phosphorylating specific nuclear proteins, endowing cancer cells with a survival and growth advantage. Therefore, inhibitors and activators of PKM2 are well underway to evaluate their anticancer effects and suitability for use as novel therapeutic strategies.

Insulin resistance and metabolic hepatocarcinogenesis with parent-of-origin effects in A×B mice

Insulin resistance is a defining feature of metabolic syndrome and type 2 diabetes mellitus but also may occur independently of these conditions. Nonalcoholic fatty liver disease (NAFLD), the hepatic manifestation of these disorders, increases the risk of hepatocellular carcinoma (HCC). However, mechanisms linking hyperinsulinemia to NAFLD and HCC require clarification. We describe a novel model of primary insulin resistance and HCC with strong parent-of-origin effects. Male AB6F1 (A/JCr dam × C57BL/6 sire) but not B6AF1 (B6 dam × A/J sire) mice developed spontaneous insulin resistance, NAFLD, and HCC without obesity or diabetes. A survey of mitochondrial, imprinted, and sex-linked traits revealed modest associations with X-linked genes. However, a diet-induced obesity study, including B6.A chromosome substitution-strain (consomic) mice, showed no segregation by sex chromosome. Thus, parent-of-origin effects were specified within the autosomal genome. Next, we interrogated mechanisms of insulin-associated hepatocarcinogenesis. Steatotic hepatocytes exhibited adipogenic transition characterized by vacuolar metaplasia and up-regulation of vimentin, adipsin, fatty acid translocase (CD36), peroxisome proliferator-activated receptor-γ, and related products. This profile was largely recapitulated in insulin-supplemented primary mouse hepatocyte cultures. Importantly, pyruvate kinase M2, a fetal anabolic enzyme implicated in the Warburg effect, was activated by insulin in vivo and in vitro. Thus, our study reveals parent-of-origin effects in heritable insulin resistance, implicating adipogenic transition with acquired anabolic metabolism in the progression from NAFLD to HCC.

Transient gastric irritation in the neonatal rats leads to changes in hypothalamic CRF expression, depression- and anxiety-like behavior as adults

AIMS

A disturbance of the brain-gut axis is a prominent feature in functional bowel disorders (such as irritable bowel syndrome and functional dyspepsia) and psychological abnormalities are often implicated in their pathogenesis. We hypothesized that psychological morbidity in these conditions may result from gastrointestinal problems, rather than causing them.

METHODS

Functional dyspepsia was induced by neonatal gastric irritation in male rats. 10-day old male Sprague-Dawley rats received 0.1% iodoacetamide (IA) or vehicle by oral gavage for 6 days. At 8-10 weeks of age, rats were tested with sucrose preference and forced-swimming tests to examine depression-like behavior. Elevated plus maze, open field and light-dark box tests were used to test anxiety-like behaviors. ACTH and corticosterone responses to a minor stressor, saline injection, and hypothalamic CRF expression were also measured.

RESULTS

Behavioral tests revealed changes of anxiety- and depression-like behaviors in IA-treated, but not control rats. As compared with controls, hypothalamic and amygdaloid CRF immunoreactivity, basal levels of plasma corticosterone and stress-induced ACTH were significantly higher in IA-treated rats. Gastric sensory ablation with resiniferatoxin had no effect on behaviors but treatment with CRF type 1 receptor antagonist, antalarmin, reversed the depression-like behavior in IA-treated rats

CONCLUSIONS

The present results suggest that transient gastric irritation in the neonatal period can induce a long lasting increase in depression- and anxiety-like behaviors, increased expression of CRF in the hypothalamus, and an increased sensitivity of HPA axis to stress. The depression-like behavior may be mediated by the CRF1 receptor. These findings have significant implications for the pathogenesis of psychological co-morbidity in patients with functional bowel disorders.

Proteomic analysis of mineralising osteoblasts identifies novel genes related to bone matrix mineralisation

Bone matrix mineralisation plays a critical role in the determination of the overall biomechanical competence of bone. However, the molecular mechanisms of bone matrix mineralisation have not been fully elucidated. We used a proteomic approach to identify proteins and genes that may play a role in osteoblast matrix mineralisation. Proteomic differential display revealed a protein band that appeared only in mineralising mouse 7F2 osteoblasts. In-gel protein digestion and mass spectrometry proteomic analysis of this protein band identified 16 proteins. Furthermore, their corresponding transcripts were upregulated. This identification of proteins that may be associated with bone matrix mineralisation presents important new information toward a better understanding of the precise mechanisms of this process.

Human pyruvate kinase M2: a multifunctional protein

Glycolysis, a central metabolic pathway, harbors evolutionary conserved enzymes that modulate and potentially shift the cellular metabolism on requirement. Pyruvate kinase, which catalyzes the last but rate-limiting step of glycolysis, is expressed in four isozymic forms, depending on the tissue requirement. M2 isoform (PKM2) is exclusively expressed in embryonic and adult dividing/tumor cells. This tetrameric allosterically regulated isoform is intrinsically designed to downregulate its activity by subunit dissociation (into dimer), which results in partial inhibition of glycolysis at the last step. This accumulates all upstream glycolytic intermediates as an anabolic feed for synthesis of lipids and nucleic acids, whereas reassociation of PKM2 into active tetramer replenishes the normal catabolism as a feedback after cell division. In addition, involvement of this enzyme in a variety of pathways, protein-protein interactions, and nuclear transport suggests its potential to perform multiple nonglycolytic functions with diverse implications, although multidimensional role of this protein is as yet not fully explored. This review aims to provide an overview of the involvement of PKM2 in various physiological pathways with possible functional implications.

Gene expression profile of human skeletal muscle and adipose tissue of Chinese Han patients with type 2 diabetes mellitus

OBJECTIVE

To study the differential patterns of gene expression in skeletal muscle and adipose tissue between type 2 diabetes mellitus (T2DM) patients and healthy subjects using DNA microarray analysis.

METHODS

T2DM patiens were divided into female group, young male group and old male group. DNA microarray analysis and quantitative real-time PCR were carried out to analyze the relation between gene expressions and T2DM.

RESULTS

The mRNA expression of 298, 578, and 350 genes was changed in the skeletal muscle of diabetes mellitus patients compared with control subjects. The 1320, 1143, and 2847 genes were modified in adipose tissue of the three groups. Among the genes surveyed, the change of 25 and 39 gene transcripts in skeletal muscle and adipose tissue was > or = 2 folds. These differentially expressed genes were classified into 15 categories according to their functions.

CONCLUSION

New genes are found and T2DM can be prevented or cured.

Coenzyme A biosynthesis: an antimicrobial drug target

Pantothenic acid, a precursor of coenzyme A (CoA), is essential for the growth of pathogenic microorganisms. Since the structure of pantothenic acid was determined, many analogues of this essential metabolite have been prepared. Several have been demonstrated to exert an antimicrobial effect against a range of microorganisms by inhibiting the utilization of pantothenic acid, validating pantothenic acid utilization as a potential novel antimicrobial drug target. This review commences with an overview of the mechanisms by which various microorganisms acquire the pantothenic acid they require for growth, and the universal CoA biosynthesis pathway by which pantothenic acid is converted into CoA. A detailed survey of studies that have investigated the inhibitory activity of analogues of pantothenic acid and other precursors of CoA follows. The potential of inhibitors of both pantothenic acid utilization and biosynthesis as novel antibacterial, antifungal and antimalarial agents is discussed, focusing on inhibitors and substrates of pantothenate kinase, the enzyme catalysing the rate-limiting step of CoA biosynthesis in many organisms. The best strategies are considered for identifying inhibitors of pantothenic acid utilization and biosynthesis that are potent and selective inhibitors of microbial growth and that may be suitable for use as chemotherapeutic agents in humans.

Anterior thalamic lesions produce chronic and profuse transcriptional de-regulation in retrosplenial cortex: A model of retrosplenial hypoactivity and covert pathology

Anterior thalamic lesions are thought to produce 'covert pathology' in retrosplenial cortex, but the causes are unknown. Microarray analyses tested the hypothesis that thalamic damage causes a chronic, hypo-function of metabolic and plasticity-related pathways (Experiment 1). Rats with unilateral, anterior thalamic lesions were exposed to a novel environment for 20 minutes, and granular retrosplenial tissue sampled from both hemispheres 30 minutes, 2h, or 8h later. Complementary statistical approaches (analyses of variance, predictive patterning and gene set enrichment analysis) revealed pervasive gene expression differences between retrosplenial cortex ipsilateral to the thalamic lesion and contralateral to the lesion. Selected gene differences were validated by QPCR, immunohistochemistry (Experiment 1), and in situ hybridisation (Experiment 2). Following thalamic lesions, the retrosplenial cortex undergoes profuse cellular transcriptome changes including lower relative levels of specific mRNAs involved in energy metabolism and neuronal plasticity. These changes in functional gene expression may be largely driven by decreases in the expression of multiple transcription factors, including brd8, c-fos, fra-2, klf5, nfix, nr4a1, smad3, smarcc2, and zfp9, with a much smaller number (nfat5, neuroD1, RXRγ) showing increases. These findings have implications for conditions such as diencephalic amnesia and Alzheimer's disease, where both anterior thalamic pathology and retrosplenial cortex hypometabolism are prominent.

Plant coenzyme A biosynthesis: characterization of two pantothenate kinases from Arabidopsis

In bacterial and animal coenzyme A (CoA) biosynthesis, pantothenate kinase (PANK) activity is critical in regulating intracellular CoA levels. Less is known about the role of PANK in plants, although a single plant isozyme from Arabidopsis, AtPANK1, was previously cloned and analyzed in vitro. We report here the characterization of a second pantothenate kinase of Arabidopsis, AtPANK2, as well as characterization of the physiological roles of both plant enzymes. The activity of the second pantothenate kinase, AtPANK2, was confirmed by its ability to complement the temperature-sensitive mutation of the bacterial pantothenate kinase in E. coli strain ts9. Knock-out mutation of either AtPANK1 or AtPANK2 did not inhibit plant growth, whereas pank1-1/pank2-1 double knockout mutations were embryo lethal. The phenotypes of the mutant plants demonstrated that only one of the AtPANK enzymes is necessary and sufficient for producing adequate CoA levels, and that no other enzyme can compensate for the loss of both isoforms. Real-time PCR measurements of AtPANK1 and AtPANK2 transcripts indicated that both enzymes are expressed with similar patterns in all tissues examined, further suggesting that AtPANK1 and AtPANK2 have complementary roles. The two enzymes have homologous pantothenate kinase domains, but AtPANK2 also carries a large C-terminal protein domain. Sequence comparisons indicate that this type of "bifunctional" pantothenate kinase is conserved in other higher eukaryotes as well. Although the function of the C-terminal domain is not known, homology structure modeling suggests it contains a highly conserved cluster of charged residues that likely constitute a metal-binding site.