Biomedical subjects

A R Kimmel

Publications and source records attributed to A R Kimmel.

At least 19 recordsLinked to original sources

PubMed2000

Perilipin A increases triacylglycerol storage by decreasing the rate of triacylglycerol hydrolysis.

The perilipins are the most abundant proteins at the surfaces of lipid droplets in adipocytes and are also found in steroidogenic cells. To investigate perilipin function, perilipin A, the predominant isoform, was ectopically expressed in fibroblastic 3T3-L1 pre-adipocytes that normally lack the perilipins. In control cells, fluorescent staining of neutral lipids with Bodipy 493/503 showed a few minute and widely dispersed lipid droplets, while in cells stably expressing perilipin A, the lipid droplets were more numerous and tightly clustered in one or two regions of the cytoplasm. Immunofluorescence microscopy revealed that the ectopic perilipin A localized to the surfaces of the tiny clustered lipid droplets; subcellular fractionation of the cells using sucrose gradients confirmed that the perilipin A localized exclusively to lipid droplets. Cells expressing perilipin A stored 6-30-fold more triacylglycerol than control cells due to reduced lipolysis of triacylglycerol stores. The lipolysis of stored triacylglycerol was 5 times slower in lipid-loaded cells expressing perilipin A than in lipid-loaded control cells, when triacylglycerol synthesis was blocked with 6 microm triacsin C. This stabilization of triacylglycerol was not due to the suppression of triacylglycerol lipase activity by the expression of perilipin A. We conclude that perilipin A increases the triacylglycerol content of cells by forming a barrier that reduces the access of soluble lipases to stored lipids, thus inhibiting triacylglycerol hydrolysis. These studies suggest that perilipin A plays a major role in the regulation of triacylglycerol storage and lipolysis in adipocytes.

3T3 Cells↗

PubMed2000

GSK3, a master switch regulating cell-fate specification and tumorigenesis.

Until recently, protein kinase GSK3 (glycogen synthase kinase 3), an essential component for cell-fate specification, had been considered a constitutively activated enzyme subject to developmentally regulated inhibition through hierarchical, linear signaling paths. Data from various systems now indicate more complex scenarios involving activating as well as inhibiting circuits, and the differential formation of multi-protein complexes that antagonistically affect GSK3 function.

Animals↗

PubMed1999

On the control of lipolysis in adipocytes.

The lipolytic reaction in adipocytes is one of the most important reactions in the management of bodily energy reserves, and dysregulation of this reaction may contribute to the symptoms of Type 2 diabetes mellitus. Yet, progress on resolving the molecular details of this reaction has been relatively slow. However, recent developments at the molecular level begin to paint a clearer picture of lipolysis and point to a number of unanswered questions. While HSL has long been known to be the rate-limiting enzyme of lipolysis, the mechanism by which HSL attacks the droplet lipids is not yet firmly established. Certainly, the immunocytochemical evidence showing the movement of HSL to the lipid droplet upon stimulation leaves little doubt that this translocation is a key aspect of the lipolytic reaction, but whether or not HSL phosphorylation contributes to the translocation, and at which site(s), is as yet unresolved. It will be important to establish whether there is an activation step in addition to the translocation reaction. The participation of perilipin A is indicated by the findings that this protein can protect neutral lipids within droplets from hydrolysis, but active participation in the lipolytic reaction is yet to be proved. Again, it will be important to determine whether mutations of serine residues of PKA phosphorylation sites of perilipins prevent lipolysis, and whether such modifications abolish the physical changes in the droplet surfaces that accompany lipolysis.

Adipocytes↗

PubMed1999

The novel tyrosine kinase ZAK1 activates GSK3 to direct cell fate specification.

Inhibition of GSK3 by 7-TM Wnt/wg receptor signaling is critical for specifying embryonic cell fate patterns. In Dictyostelium, the 7-TM cAMP receptors regulate GSK3 by parallel, antagonistic pathways to establish a developmental body plan. We describe here a novel tyrosine kinase, ZAK1, downstream of 7-TM cAMP receptor signaling that is required for GSK3 activation during development. zak1-nulls have reduced GSK3 activity and are defective in GSK3-regulated developmental pathways. Moreover, recombinant ZAK1 phosphorylates and activates GSK3 in vitro. We propose that ZAK1 is a positive regulator of GSK3 activity required for cell pattern formation in Dictyostelium and speculate that similar mechanisms exist to antagonize Wnt/wg signaling for metazoan cell fate specification.

Amino Acid Sequence↗

PubMed1999

Proteolytic processing of HIV-1 protease precursor, kinetics and mechanism.

Previously it was demonstrated using a model precursor that processing at the N terminus of the HIV-1 protease (PR) precedes processing at its C terminus. We now show the expression, purification, and kinetics of the autoprocessing reaction of a PR precursor linked to 53 amino acids of the native flanking transframe region (DeltaTFP-p6(pol)) of Gag-Pol and containing its two native cleavage sites. The PR contains the two cysteine residues exchanged to alanines, mutations that do not alter the kinetics or the structural stability of the mature PR. DeltaTFP-p6(pol)-PR, which encompasses the known PR inhibitor sequence Glu-Asp-Leu within DeltaTFP, undergoes cleavage at the DeltaTFP/p6(pol) and p6(pol)/PR sites in two consecutive steps to produce the mature PR. Both DeltaTFP-p6(pol)-PR and p6(pol)-PR exhibit low intrinsic enzymatic activity. The appearance of the mature PR is accompanied by a large increase in catalytic activity. It follows first-order kinetics in protein concentration with a rate constant of 0.13 +/- 0.01 min(-1) in 0.1 M acetate at pH 4.8. The pH-rate profile for the observed first-order rate constant is bell-shaped with two ionizable groups of pK(a) 4.9 and 5.1. The rate constant also exhibits approximately 7-fold higher sensitivity to urea denaturation as compared with that of the mature PR, suggesting that the cleavage at the N terminus of the PR domain from the precursor leads to the stabilization of the dimeric structure.

Amino Acid Sequence↗

PubMed1999

Perilipins, ADRP, and other proteins that associate with intracellular neutral lipid droplets in animal cells.

Although all animal cells package and store neutral lipids in discrete intracellular storage droplets, there is little information on the molecular processes that govern either the deposition or catabolism of the stored lipid components. Studies on adipocytes have uncovered the perilipins and ADRP, related proteins that appear to be intrinsic to the surfaces of intracellular lipid storage droplets. We discuss the properties, distribution, localization, and potential functions of these proteins, as well as those of vimentin and the recently-described 'capsular' proteins, in lipid storage and metabolism.

3T3 Cells↗

PubMed1998

Temperature-sensitive Gbeta mutants discriminate between G protein-dependent and -independent signaling mediated by serpentine receptors.

Deletion of the single gene for the Dictyostelium G protein beta-subunit blocks development at an early stage. We have now isolated temperature-sensitive alleles of Gbeta to investigate its role in later development. We show that Gbeta is directly required for adenylyl cyclase A activation and for morphogenetic signaling during the entire developmental program. Gbeta was also essential for induction of aggregative gene expression by cAMP pulses, a process that is mediated by serpentine cAMP receptors (cARs). However, Gbeta was not required for cAR-mediated induction of prespore genes and repression of stalk genes, and neither was Gbeta needed for induction of prestalk genes by the differentiation inducing factor (DIF). cAMP induction of prespore genes and repression of stalk genes is mediated by the protein kinase GSK-3. GSK-3 also determines cell-type specification in insects and vertebrates and is regulated by the wingless/wnt morphogens that are detected by serpentine fz receptors. The G protein-dependent and -independent modes of cAR-mediated signaling reported here may also exist for the wingless/wnt signaling pathways in higher organisms.

Adenylyl Cyclases↗

PubMed1998

Hydrophilic peptides derived from the transframe region of Gag-Pol inhibit the HIV-1 protease.

The HIV-1 transframe region (TFR) is between the structural and functional domains of the Gag-Pol polyprotein, flanked by the nucleocapsid and the protease domains at its N and C termini, respectively. Transframe octapeptide (TFP) Phe-Leu-Arg-Glu-Asp-Leu-Ala-Phe, the N terminus of TFR, and its analogues are competitive inhibitors of the action of the mature HIV-1 protease. The smallest, most potent analogues are tripeptides: Glu-Asp-Leu and Glu-Asp-Phe with Ki values of approximately 50 and approximately 20 microM, respectively. Substitution of the acidic amino acids in the TFP by neutral amino acids and d or retro-d configurations of Glu-Asp-Leu results in an >40-fold increase in Ki. Protease inhibition by Glu-Asp-Leu is dependent on a protonated form of a group with a pKa of 3.8; unlike other inhibitors of HIV-1 protease which are highly hydrophobic, Glu-Asp-Leu is extremely soluble in water, and its binding affinity decreases with increasing NaCl concentration. However, Glu-Asp-Leu is a poor inhibitor (Ki approximately 7.5 mM) of the mammalian aspartic acid protease pepsin. X-ray crystallographic studies at pH 4.2 show that the interactions of Glu at P2 and Leu at P1 of Glu-Asp-Leu with residues of the active site of HIV-1 protease are similar to those of other product-enzyme complexes. It was not feasible to understand the interaction of intact TFP with HIV-1 protease under conditions of crystal growth due to its hydrolysis giving rise to two products. The sequence-specific, selective inhibition of the HIV-1 protease by the viral TFP suggests a role for TFP in regulating protease function during HIV-1 replication.

Amino Acid Sequence↗

PubMed1998

Sequence-specific protein interaction with a transcriptional enhancer involved in the autoregulated expression of cAMP receptor 1 in Dictyostelium.

Major stages of Dictyostelium development are regulated by secreted, extracellular cAMP through activation of a serpentine receptor family. During early development, oscillations of extracellular cAMP mobilize cells for aggregation; later, continuous exposure to higher extracellular cAMP concentrations downregulates early gene expression and promotes cytodifferentiation and cell-specific gene expression. The cAMP receptor 1 gene CAR1 has two promoters that are differentially responsive to these extracellular cAMP stimuli. The early CAR1 promoter is induced by nM pulses of cAMP, which in turn are generated by CAR1-dependent activation of adenylyl cyclase (AC). Higher, non-fluctuating concentrations of cAMP will adapt this AC stimulus-response, repress the activated early promoter and induce the dormant late promoter. We now identify a critical element of the pulse-induced CAR1 promoter and a nuclear factor with sequence-specific interaction. Mutation of four nucleotides within the element prevents both in vitro protein binding and in vivo expression of an otherwise fully active early CAR1 promoter and multimerization of the wild-type, but not mutant, sequence will confer cAMP regulation to a quiescent heterologous promoter. These cis and trans elements, thus, constitute a part of the molecular response to the cAMP transmembrane signal cascade that regulates early development of Dictyostelium.

Animals↗

PubMed1998

Non-autonomous regulation of a graded, PKA-mediated transcriptional activation signal for cell patterning.

The pseudoplasmodium or migrating slug of Dictyostelium is composed of non-terminally differentiated cells, organized along an anteroposterior axis. Cells in the anterior region of the slug define the prestalk compartment, whereas most of the posterior zone consists of prespore cells. We now present evidence that the cAMP-dependent protein kinase (PKA) and the RING domain/leucine zipper protein rZIP interact genetically to mediate a transcriptional activation gradient that regulates the differentiation of prespore cells within the posterior compartment of the slug. PKA is absolutely required for prespore differentiation. In contrast, rZIP negatively regulates prespore patterning; rzpA- cells, which lack rZIP, have reduced prestalk differentiation and a corresponding increase in prespore-specific gene expression. Using cell-specific markers and chimaeras of wild-type and rzpA- cells, we show that rZIP functions non-autonomously to establish a graded, prespore gene activation signal but autonomously to localize prespore expression. Overexpression of either the catalytic subunit or a dominant-negative regulatory subunit of PKA further demonstrates that PKA lies within the intracellular pathway that mediates the extracellular signal and regulates prespore patterning. Finally, we show that a 5'-distal segment within a prespore promoter that is responsive to a graded signal is also sensitive to PKA and rZIP, indicating that it acts directly at the level of prespore-specific gene transcription for regulation.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1997

Crystallographic analysis of human immunodeficiency virus 1 protease with an analog of the conserved CA-p2 substrate -- interactions with frequently occurring glutamic acid residue at P2' position of substrates.

Human immunodeficiency virus type 1 (HIV-1) protease hydrolysis of the Gag CA-p2 cleavage site is crucial for virion maturation and is optimal at acidic pH. To understand the processing of the CA-p2 site, we have determined the structure of HIV-1 protease complexed with an analog of the CA-p2 site, the reduced peptide inhibitor Arg-Val-Leu-r-Phe-Glu-Ala-Ahx-NH2 [r denotes the reduced peptide bond and Ahx 2-aminohexanoic acid (norleucine), respectively]. The crystal structure was refined to an R-factor of 0.17 at 0.21-nm resolution. The crystals have nearly the same lattice as related complexes in P2(1)2(1)2(1) which have twofold disordered inhibitor, but are in space group P2(1). and the asymmetric unit contains two dimers of HIV-1 protease related by 180 degrees rotation. An approximate non-crystallographic symmetry has replaced the exact crystal symmetry resulting in well-ordered inhibitor structure. Each protease dimer binds one ordered inhibitor molecule, but in opposite orientations. The interactions of the inhibitor with the two dimers are very similar for the central P2 Val to P2' Glu residues, but show more variation for the distal P3 Arg and P4' Ahx residues. Importantly, the carboxylate oxygens of Glu at P2' in the inhibitor are within hydrogen-bonding distance of a carboxylate oxygen of Asp30 of the protease suggesting that the two side chains share a proton. This interaction suggests that the enzyme-substrate complex is additionally stabilized at lower pH. The importance of this interaction is emphasized by the absence of polymorphisms of Asp30 in the protease and variants of P2' Glu in the critical CA-p2 cleavage site.

Amino Acid Sequence↗

PubMed1997

Autonomous and nonautonomous regulation of axis formation by antagonistic signaling via 7-span cAMP receptors and GSK3 in Dictyostelium.

Early during Dictyostelium development a fundamental cell-fate decision establishes the anteroposterior (prestalk/prespore) axis. Signaling via the 7-transmembrane cAMP receptor CAR4 is essential for creating and maintaining a normal pattern; car4-null alleles have decreased levels of prestalk-specific mRNAs but enhanced expression of prespore genes. car4- cells produce all of the signals required for prestalk differentiation but lack an extracellular factor necessary for prespore differentiation of wild-type cells. This secreted factor decreases the sensitivity of prespore cells to inhibition by the prestalk morphogen DIF-1. At the cell autonomous level, CAR4 is linked to intracellular circuits that activate prestalk but inhibit prespore differentiation. The autonomous action of CAR4 is antagonistic to the positive intracellular signals mediated by another cAMP receptor, CAR1 and/or CAR3. Additional data indicate that these CAR-mediated pathways converge at the serine/threonine protein kinase GSK3, suggesting that the anterior (prestalk)/posterior (prespore) axis of Dictyostelium is regulated by an ancient mechanism that is shared by the Wnt/Fz circuits for dorsoventral patterning during early Xenopus development and establishing Drosophila segment polarity.

Animals↗

PubMed1997

Post-translational regulation of perilipin expression. Stabilization by stored intracellular neutral lipids.

The perilipins are a family of polyphosphorylated proteins found exclusively surrounding neutral lipid storage droplets in adipocytes and steroidogenic cells. In steroidogenic cells, the cholesterol ester-rich lipid storage droplets are encoated with perilipins A and C. This study describes the dependence of perilipin levels on neutral lipid storage in cultured Y-1 adrenal cortical cells. The addition of fatty acids and cholesterol to the culture medium of Y-1 adrenal cortical cells greatly increased the storage of cholesterol esters and triacylglycerols concomitant with the formation of many new lipid storage droplets. The addition of fatty acids to the culture medium also produced a transient 6-fold increase in levels of perilipin A, but not C, mRNA, while much larger and stable increases in both perilipin A and C proteins were observed. The increases in perilipin protein levels were dependent upon the metabolism of fatty acids to triacylglycerol or cholesterol esters, since the incubation of cells with bromopalmitate, a poorly metabolized fatty acid, failed to yield large increases in lipid content or perilipin levels. Constitutive expression of epitope-tagged perilipins in transfected Y-1 adrenal cortical cells was regulated by lipid similarly to expression of the endogenous perilipins despite an absence of untranslated perilipin mRNA sequences in the expression constructs. Epitope-tagged perilipin A mRNAs were efficiently loaded with polyribosomes whether or not fatty acids were added to the culture medium; therefore, the increase in perilipin levels in the presence of fatty acids is likely due to factors other than increased translational efficiency. We suggest that the large increase in cellular perilipin levels upon lipid loading of cells is the result of post-translational stabilization of newly synthesized perilipins by stored neutral lipids.

3T3 Cells↗

PubMed1997

Control of cell-type specific gene expression in Dictyostelium by the general transcription factor GBF.

To understand how positional information within an organism specifies patterning during development, we are analyzing spatially regulated gene expression in Dictyostelium. CAR3 is a member of the cAMP, 7-span receptor family which directs the transition from unicellular to multicellular organism and regulates cellular differentiation and pattern formation. CAR3 mRNA is expressed maximally at 8-10 hours of development, as individual cells aggregate and differentiate, and is accumulated to equivalent levels in all cells. CAR3 is also induced in shaking cultures by response to extracellular cAMP. We now show, by extensive mutagenesis, that the maximum length of contiguous sequences required for accurate spatiotemporal regulation of CAR3 is approx. 350 bp. These sequences include three significant elements located in upstream and transcribed regions. Arrays of G-boxes (GBF regulatory sites) are centered near positions -165 and +50 and, although either is sufficient for induction by cAMP and expression in prespore cells, both are required for expression in prestalk cells. Another GC-rich element near position -80 is required for maximal expression of prespore-specific constructs, although full-length promoters carrying clustered mutations through the -80 region are still expressed in all cells, but with slightly reduced expression. Spatiotemporal expression of CAR3 during development, thus, requires cell-specific combinatorial interactions of multiple but redundant regulatory components. These essential elements are located in upstream and transcribed regions. However, most surprisingly, a primary control for spatial patterning of CAR3 expression appears to be mediated by GBF, a general transcription factor expressed ubiquitously during Dictyostelium development following early aggregation.

Animals↗

PubMed1997

rZIP, a RING-leucine zipper protein that regulates cell fate determination during Dictyostelium development.

rZIP is an approx. 32 kDa, multi-domain protein of Dictyostelium discoideum whose structural motifs include a RING (zinc-binding) domain, a leucine zipper, a glutamine repeat, an SH3-binding region and a consensus phosphorylation site for MAP kinase. In vitro, rZIP forms homodimers and interacts specifically with the SH3 domain(s) of the Nck adaptor protein. rZIP is expressed maximally during cell differentiation at approximately equivalent levels in all cells. Disruption of the rZIP gene rzpA results in altered cellular aggregation, impaired slug migration, and aberrant patterning of prespore and prestalk cells, the major progenitor classes. In rzpA- strains, prespore-specific genes are overexpressed and prestalk expression zones are reduced. Conversely, constitutive overexpression of rzpA markedly decreases prespore-specific gene expression and significantly increases the expression of prestalk-specific genes. Further, induced transdifferentiation of prespore cells into prestalk cells is inhibited in rzpA-slugs. In light of these patterning defects, we suggest that the RING/zipper protein rZIP plays an important role in early cell fate decisions in Dictyostelium, acting as a positive regulator of prestalk differentiation and an inhibitor of prespore differentiation.

Amino Acid Sequence↗

PubMed1996

Perilipin: possible roles in structure and metabolism of intracellular neutral lipids in adipocytes and steroidogenic cells.

Perilipins are a family of unique proteins intimately associated with the limiting surface of neutral lipid storage droplets in adipocytes and in steroidogenic cells. Lipid hydrolysis in these cells is initiated by cAMP, which leads to phosphorylation of hormone-sensitive lipase in adipocytes and cholesteryl esterase in steroidogenic cells by protein kinase A. Although the concurrent phosphorylation of perilipin by this kinase suggests a role for these proteins in lipid breakdown, a role for these proteins in lipid packaging or in maintaining the lipid droplet structure cannot be excluded.

Adipocytes↗

PubMed1995

Perilipins are associated with cholesteryl ester droplets in steroidogenic adrenal cortical and Leydig cells.

Steroidogenic cells store cholesteryl esters, precursors for steroid hormone synthesis, in intracellular lipid droplets. Cholesteryl ester hydrolysis is activated by protein kinase A and catalyzed by cholesteryl esterase. The esterase is similar, if not identical, to hormone-sensitive lipase in adipocytes where an analogous lipolytic mechanism occurs. Perilipins, proteins located exclusively at lipid droplet surfaces in adipocytes, are polyphosphorylated by protein kinase A in response to lipolytic stimuli, suggesting a role for these proteins in mediating lipid metabolism. The present study reveals that perilipins are associated with cholesteryl ester droplets in two steroidogenic cell lines: Y-1 adrenal cortical cells and MA-10 Leydig cells. The relative abundance of perilipin mRNAs and protein is much less in steroidogenic cells than in adipocytes. Like adipocytes, steroidogenic cells express perilipin A; additionally, the latter cells contain relatively abundant amounts of perilipin C, a protein that is not detectable in adipocytes by Western analysis. The data suggest a strong link between perilipins and lipid hydrolysis that is mediated by the hormone-sensitive lipase/cholesteryl esterase class of enzymes.

3T3 Cells↗

PubMed1995

Perilipin: unique proteins associated with intracellular neutral lipid droplets in adipocytes and steroidogenic cells.

For several reasons it seems reasonable to suspect that perilipins participate in lipid hydrolysis. First, they are located at the lipid droplet surface, the presumed site of HSL and cholesteryl esterase action. Secondly, they are polyphosphorylated by PKA in concert with lipid hydrolysis. Finally, these proteins appear to be expressed primarily, if not solely, in adipocytes and steroidogenic cells, cells in which lipid hydrolysis is stimulated by cyclic AMP and mediated by HSL or cholesteryl esterase(s), whereas other cells that contain abundant neutral lipid depositions contain no perilipin [13]. Interestingly, these closely related hydrolases share no homology with other mammalian lipases [3]. Although such attributes provide a link between perilipin and lipid hydrolysis, we have no evidence that perilipins participate directly in, or are necessary for, lipid catabolism. The basis for the strong affinity between the perilipins and neutral lipids is unknown. Clearly, lipids and perilipins are tightly linked, as evidenced by selective targeting of epitope-tagged perilipin to lipid droplets and by the paradoxical appearance of lipid droplets in pre-adipocytes transfected with a sense perilipin A construct. Indeed, in differentiating adipocytes the earliest lipid depositions are associated with perilipins, and restriction of perilipin synthesis with anti-sense constructs may impede lipid formation and deposition. It remains to be determined if, in the normal course of events, perilipins are directed toward lipid depositions or if lipids are transported to perilipin foci. Whatever the temporal sequence, the result is that neutral lipids are encased in perilipin-bounded droplets.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗