Intravenous administration of replication-incompetent adenovirus to rhesus monkeys induces thrombocytopenia by increasing in vivo platelet clearance.

A replication-incompetent adenovirus vector was administered to rhesus macaques at 1, 3 and 6 x 1012 particles/kg doses to investigate its toxicity. Platelet count decrements of 28%, 82% and 90%, respectively, were observed, with corresponding platelet half-lives of 69.0, 25.2 and 22.2 h (compared with 111 h in untreated animals). The platelet decline was equivalent for all three doses for 8 h, and platelet count recovery began as early as 8 h after infusion for low-dose recipients, or as late as 24 h for the medium and high dose recipients. These observations suggest that thrombocytopenia is a saturable, reversible consumptive process.

TIP47 associates with lipid droplets.

Most mammalian cells package neutral lipids into droplets that are surrounded by a monolayer of phospholipids and a specific set of proteins including the adipose differentiation-related protein (ADRP; also called adipophilin), which is found in a wide array of cell types, and the perilipins, which are restricted to adipocytes and steroidogenic cells. TIP47 was initially identified in a yeast two-hybrid screen for proteins that interact with the cytoplasmic tail of the mannose 6-phosphate receptor, yet its sequence is highly similar to the lipid droplet protein, ADRP, and more distantly related to perilipins. Hence, we hypothesized that TIP47 might be associated with lipid droplets. In HeLa cells grown in standard low lipid-containing culture media, immunofluorescence microscopy revealed that the cells had few lipid droplets; however, TIP47 and ADRP were found on the surfaces of the small lipid droplets present. When the cells were grown in media supplemented with physiological levels of fatty acids, the amount of neutral lipid stored in lipid droplets increased dramatically, as did the staining of TIP47 and ADRP surrounding these droplets. TIP47 was found primarily in the cytosolic fractions of HeLa cells and murine MA10 Leydig cells grown in low lipid-containing culture medium, while ADRP was undetectable in these fractionated cell homogenates. When HeLa and MA10 Leydig cells were lipid-loaded, significant levels of ADRP were found in the floating lipid droplet fractions and TIP47 levels remained constant, but the distribution of a significant portion of TIP47 shifted from the cytosolic fractions to the lipid droplet fractions. Thus, we conclude that TIP47 associates with nascent lipid droplets and can be classified as a lipid droplet-associated protein.

Molecular characterization of the protein encoded by the Hermansky-Pudlak syndrome type 1 gene.

Hermansky-Pudlak syndrome (HPS) comprises a group of genetic disorders characterized by defective lysosome-related organelles. The most common form of HPS (HPS type 1) is caused by mutations in a gene encoding a protein with no homology to any other known protein. Here we report the identification and biochemical characterization of this gene product, termed HPS1p. Endogenous HPS1p was detected in a wide variety of human cell lines and exhibited an electrophoretic mobility corresponding to a protein of approximately 80 kDa. In contrast to previous theoretical analysis predicting that HPS1p is an integral membrane protein, we found that this protein was predominantly cytosolic, with a small amount being peripherally associated with membranes. The sedimentation coefficient of the soluble form of HPS1p was approximately 6 S as inferred from ultracentrifugation on sucrose gradients. HPS1p-deficient cells derived from patients with HPS type 1 displayed normal distribution and trafficking of the lysosomal membrane proteins, CD63 and Lamp-1. This was in contrast to cells from HPS type 2 patients, having mutations in the beta3A subunit of the AP-3 adaptor complex, which exhibited increased routing of these lysosomal proteins through the plasma membrane. Similar analyses performed on fibroblasts from 10 different mouse models of HPS revealed that only the AP-3 mutants pearl and mocha display increased trafficking of Lamp-1 through the plasma membrane. Taken together, these observations suggest that the product of the HPS1 gene is a cytosolic protein capable of associating with membranes and involved in the biogenesis and/or function of lysosome-related organelles by a mechanism distinct from that dependent on the AP-3 complex.

Adipose differentiation-related protein is an ubiquitously expressed lipid storage droplet-associated protein.

The adipose differentiation-related protein (ADRP) was first characterized as a mRNA induced early during adipocyte differentiation (Jiang, H. P., and G. Serrero. 1992. Proc. Natl. Acad. Sci. USA. 89:7856-7860). The present study demonstrates that ADRP mRNA is expressed in a variety of tissues and cultured cell lines. Immunocytochemical examination revealed that ADRP localizes to neutral lipid storage droplets in cultured murine 3T3-L1 adipocytes, murine MA-10 Leydig cells, Chinese hamster ovary (CHO) fibroblasts, and human HepG2 hepatoma cells; the association of ADRP with lipid droplets was confirmed by subcellular fractionation of MA-10 Leydig cells. In addition to ADRP, steroidogenic cells and adipocytes express the perilipins, a family of lipid droplet-associated proteins that share a highly related sequence domain with ADRP. ADRP and perilipins co-localize on lipid droplets in MA-10 Leydig cells. While ADRP was found on small lipid droplets in 3T3-L1 preadipocytes and early differentiated adipocytes, it was absent in maturing adipocytes. In contrast, perilipins were absent early during differentiation, but were found on small and large lipid droplets at later stages. The transition in surface protein composition of adipocyte lipid droplets from ADRP to perilipins occurred 3 days after the initiation of differentiation when cells displayed co-localizatioin of both proteins on the same lipid droplets. The specific localization of adipose differentiation-related protein to lipid droplets in a wide variety of cells suggests that ADRP plays a role in management of neutral lipid stores.

Binding of the peroxisomal targeting sequence SKL is specified by a low-affinity site in castor bean glyoxysomal membranes. A domain next to the SKL binds to a high-affinity site.

The carboxyl-terminal amino acid sequence serine-lysine-leucine (SKL) is the consensus peroxisomal targeting sequence 1 (PTS1) and is sufficient to direct a polypeptide to peroxisomes in vivo in plants, animals, and yeasts. However, there are also two sites on alkali-stripped glyoxysomal membranes from castor bean (Ricinus communis) endosperm that bind the peptide YHKHLKPLQSKL (SKLp), the sequence of the last 12 amino acids of acyl-coenzyme A oxidase (N.E. Wollins, R.P. Donaldson [1994] J Biol Chem 289: 1149-1153). It was hypothesized that one of these sites interacts with information other than the PTS1. To explore the sequence requirements for each SKLp binding site, we tested the peptides YHKHLKPQSKG and YHKHLKPLQS and found that they bound to the high-affinity site, but not to the low-affinity site. When the high-affinity site was blocked with YHKHLKPQSKG, SKLp bound to the low-affinity site with a dissociation constant (Kd) of 8.5 microM. In an attempt to disrupt high-affinity binding, two the upstream, positively charged residues were replaced with negatively charged residues to make the peptide YHKETEPLQSKL. YHKETEPLQSKL did not bind to either site on the glyoxysomal membranes. These results indicate that the PTS1 binds to the low-affinity site and that the adjacent, positively charged domain binds to the high-affinity site.

Aggregation as a determinant of protein fate in post-Golgi compartments: role of the luminal domain of furin in lysosomal targeting.

The mammalian endopeptidase furin is a type 1 integral membrane protein that is predominantly localized to the TGN and is degraded in lysosomes with a t1/2 = 2-4 h. Whereas the localization of furin to the TGN is largely mediated by sorting signals in the cytosolic tail of the protein, we show here that targeting of furin to lysosomes is a function of the luminal domain of the protein. Inhibition of lysosomal degradation results in the accumulation of high molecular weight aggregates of furin; aggregation is also dependent on the luminal domain of furin. Temperature and pharmacologic manipulations suggest that furin aggregation occurs in the TGN and thus precedes delivery to lysosomes. These findings are consistent with a model in which furin becomes progressively aggregated in the TGN, an event that leads to its transport to lysosomes. Our observations indicate that changes in the aggregation state of luminal domains can be potent determinants of biosynthetic targeting to lysosomes and suggest the possible existence of quality control mechanisms for disposal of aggregated proteins in compartments of the secretory pathway other than the endoplasmic reticulum.

Specific binding of the peroxisomal protein targeting sequence to glyoxysomal membranes.

It has been demonstrated that the carboxyl-terminal amino acid sequence, serine-lysine-leucine (SKL), is sufficient to direct a polypeptide to peroxisomes in vivo, and that this sequence is functional in plants, animals, and yeasts. Furthermore, many peroxisomal proteins have SKL carboxyl termini, including rat acyl-CoA oxidase. We have synthesized a 125I-peptide with the sequence of the last 12 amino acids of acyl-CoA oxidase, D-Tyr-HKHLKPLQSKL (SKLp), and used it to detect a receptor that recognizes SKL containing proteins targeted to glyoxysomes. SKLp binding to alkali-stripped glyoxysomal membranes was saturable and 80% of the binding could be displaced by 1 microM unlabeled SKLp or 8 micrograms/ml glyoxysomal matrix proteins. Very little specific binding was associated with endoplasmic reticulum or mitochondrial membranes. Specific binding was affected by the ionic composition of the medium; the binding was optimal at pH 6.5 and was inhibited by mono- and divalent cations. Scatchard analysis of SKLp binding to glyoxysomal membranes indicated that there were two binding sites with Kd values of 160 and 1450 nM and abundances of 17 and 43 nmol/mg glyoxysomal membrane protein, respectively. Protease treatment of the alkali-stripped glyoxysomal membranes lowered the number of high affinity sites and destroyed all the low affinity sites. These results demonstrate, for the first time, that there is an integral membrane protein in glyoxysomes that has the characteristics of a receptor for protein import.

Elevated cholesterol and decreased sterol carrier protein-2 in peroxisomes from AS-30D hepatoma compared to normal rat liver.

Peroxisomes were isolated from AS-30D hepatoma and compared to normal rat liver cells for the purpose of investigating the cholesterol accumulation in the hepatoma cells. Cholesterol was found to be approximately 10-fold higher relative to protein in AS-30D peroxisomes as compared to peroxisomes from normal liver. The peroxisomes from the hepatoma cells were found to be more stable; catalase was not released from these peroxisomes during isolation or osmotic shock of the peroxisomal fraction. The elevated cholesterol level may stabilize the peroxisomal membrane. Sterol carrier protein-2 (SCP-2) levels were measured using a radioimmunoassay (RIA), which indicated the highest concentration of SCP-2 to be in peroxisomes. Hepatoma peroxisomes had a lower concentration of SCP-2 (2.5 micrograms/mg) than normal liver peroxisomes (8 micrograms/mg). Approximately half of all SCP-2 detected was found to be soluble in both hepatoma and normal rat liver cells. Immunoblots from both rat liver and AS-30D fractions demonstrated the presence of the 14-kDa form of SCP-2. The liver fractions also had a 57-kDa immunoreactive protein, which was barely detectable in the AS-30D fractions. The low abundance of the high molecular weight form of SCP-2 from hepatoma peroxisomes and the lower amounts of SCP-2 detected in the AS-30D peroxisomes may be related to the accumulation of cholesterol in the cells.