Pig liver carnitine palmitoyltransferase I, with low Km for carnitine and high sensitivity to malonyl-CoA inhibition, is a natural chimera of rat liver and muscle enzymes

The outer mitochondrial membrane enzyme carnitine palmitoyltransferase I (CPTI) catalyzes the initial and regulatory step in the beta-oxidation of fatty acids. The genes for the two isoforms of CPTI-liver (L-CPTI) and muscle (M-CPTI) have been cloned and expressed, and the genes encode for enzymes with very different kinetic properties and sensitivity to malonyl-CoA inhibition. Pig L-CPTI encodes for a 772 amino acid protein that shares 86 and 62% identity, respectively, with rat L- and M-CPTI. When expressed in Pichia pastoris, the pig L-CPTI enzyme shows kinetic characteristics (carnitine, K(m) = 126 microM; palmitoyl-CoA, K(m) = 35 microM) similar to human or rat L-CPTI. However, the pig enzyme, unlike the rat liver enzyme, shows a much higher sensitivity to malonyl-CoA inhibition (IC(50) = 141 nM) that is characteristic of human or rat M-CPTI enzymes. Therefore, pig L-CPTI behaves like a natural chimera of the L- and M-CPTI isotypes, which makes it a useful model to study the structure--function relationships of the CPTI enzymes.     

Two-color,rolling-circle amplification on antibody microarrays for sensitive,multiplexed serum-protein measurements

The ability to conveniently and rapidly profile a diverse set of proteins has valuable applications. In a step toward further enabling such a capability, we developed the use of rolling-circle amplification (RCA) to measure the relative levels of proteins from two serum samples, labeled with biotin and digoxigenin, respectively, that have been captured on antibody microarrays. Two-color RCA produced fluorescence up to 30-fold higher than direct-labeling and indirect-detection methods using antibody microarrays prepared on both polyacrylamide-based hydrogels and nitrocellulose. Replicate RCA measurements of multiple proteins from sets of 24 serum samples were highly reproducible and accurate. In addition, RCA enabled reproducible measurements of distinct expression profiles from lower-abundance proteins that were not measurable using the other detection methods. Two-color RCA on antibody microarrays should allow the convenient acquisition of expression profiles from a great diversity of proteins for a variety of applications.     

Endogenous adenosine inhibits CNS terminal Ca(2+) currents and exocytosis

Bursts of action potentials (APs) are crucial for the release of neurotransmitters from dense core granules. This has been most definitively shown for neuropeptide release in the hypothalamic neurohypophysial system (HNS). Why such bursts are necessary, however, is not well understood. Thus far, biophysical characterization of channels involved in depolarization-secretion coupling cannot completely explain this phenomenon at HNS terminals, so purinergic feedback mechanisms have been proposed. We have previously shown that ATP, acting via P2X receptors, potentiates release from HNS terminals, but that its metabolite adenosine, via A(1) receptors acting on transient Ca(2+) currents, inhibit neuropeptide secretion. We now show that endogenous adenosine levels are sufficient to cause tonic inhibition of transient Ca(2+) currents and of stimulated exocytosis in HNS terminals. Initial non-detectable adenosine levels in the static bath increased to 2.9 microM after 40 min. These terminals exhibit an inhibition (39%) of their transient inward Ca(2+) current in a static bath when compared to a constant perfusion stream. CPT, an A(1) adenosine receptor antagonist, greatly reduced this tonic inhibition. An ecto-ATPase antagonist, ARL-67156, similarly reduced tonic inhibition, but CPT had no further effect, suggesting that endogenous adenosine is due to breakdown of released ATP. Finally, stimulated capacitance changes were greatly enhanced (600%) by adding CPT to the static bath. Thus, endogenous adenosine functions at terminals in a negative-feedback mechanism and, therefore, could help terminate peptide release by bursts of APs initiated in HNS cell bodies. This could be a general mechanism for controlling transmitter release in these and other CNS terminals.     

Substrate specificity of a metalloprotease of the pappalysin family revealed by an inhibitor and a product complex

Human pappalysin-1 is a multi-domain metalloprotease engaged in the homeostasis of insulin-like growth factors and the founding member of the pappalysin family within the metzincin clan of metalloproteases. We have recently identified an archaeal relative, ulilysin, encompassing only the protease domain. It is a 262-residue active protease with a novel 3D structure with two subdomains separated by an active-site cleft. Despite negligible overall sequence similarity, noticeable similarity is found with other metzincin prototypes, adamalysins/ADAMs and matrix metalloproteinases. Ulilysin has been crystallised in a product complex with an arginine-valine dipeptide occupying the active-site S(1') and S(2') positions and in a complex with the broad-spectrum hydroxamic acid-based metalloprotease inhibitor, batimastat. This molecule inhibits mature ulilysin with an IC(50) value of 61 microM under the conditions assayed. The binding of batimastat to ulilysin evokes binding to vertebrate matrix metalloproteases but is much weaker. These data give insight into substrate specificity and mechanism of action and inhibition of the novel pappalysin family.     

Molecular Mechanisms of Fenofibrate-Induced Metabolic Catastrophe and Glioblastoma Cell Death

Fenofibrate (FF) is a common lipid-lowering drug and a potent agonist of the peroxisome proliferator-activated receptor alpha (PPARα). FF and several other agonists of PPARα have interesting anticancer properties, and our recent studies demonstrate that FF is very effective against tumor cells of neuroectodermal origin. In spite of these promising anticancer effects, the molecular mechanism(s) of FF-induced tumor cell toxicity remains to be elucidated. Here we report a novel PPARα-independent mechanism explaining FF''s cytotoxicity in vitro and in an intracranial mouse model of glioblastoma. The mechanism involves accumulation of FF in the mitochondrial fraction, followed by immediate impairment of mitochondrial respiration at the level of complex I of the electron transport chain. This mitochondrial action sensitizes tested glioblastoma cells to the PPARα-dependent metabolic switch from glycolysis to fatty acid β-oxidation. As a consequence, prolonged exposure to FF depletes intracellular ATP, activates the AMP-activated protein kinase–mammalian target of rapamycin–autophagy pathway, and results in extensive tumor cell death. Interestingly, autophagy activators attenuate and autophagy inhibitors enhance FF-induced glioblastoma cytotoxicity. Our results explain the molecular basis of FF-induced glioblastoma cytotoxicity and reveal a new supplemental therapeutic approach in which intracranial infusion of FF could selectively trigger metabolic catastrophe in glioblastoma cells.     

Activity of ulilysin, an archaeal PAPP-A-related gelatinase and IGFBP protease

Human growth and development are conditioned by insulin-like growth factors (IGFs), which have also implications in pathology. Most IGF molecules are sequestered by IGF-binding proteins (IGFBPs) so that exertion of IGF activity requires disturbance of these complexes. This is achieved by proteolysis mediated by IGFBP proteases, among which the best characterised is human PAPP-A, the first member of the pappalysin family of metzincins. We have previously identified and studied the only archaeal homologue found to date, Methanosarcina acetivorans ulilysin. This is a proteolytically functional enzyme encompassing a pappalysin catalytic domain and a pro-domain involved in maintenance of latency of the zymogen, proulilysin. Once activated, the protein hydrolyses IGFBP-2 to -6 and insulin chain beta in vitro. We report here that ulilysin is also active against several other substrates, viz (azo)casein, azoalbumin, and extracellular matrix components. Ulilysin has gelatinolytic but not collagenolytic activity. Moreover, the proteolysis-resistant skeletal proteins actin and elastin are also cleaved, as is fibrinogen, but not plasmin and alpha1-antitrypsin from the blood coagulation cascade. Ulilysin develops optimal activity at pH 7.5 and strictly requires peptide bonds preceding an arginine residue, as determined by means of a novel fluorescence resonance energy transfer assay, thus pointing to biotechnological applications as an enzyme complementary to trypsin.