Individual and Combined Effects of Peroxisome Proliferator-Activated Receptor α and γ Agonists, Fenofibrate and Rosiglitazone, on Biomarkers of Lipid and Glucose Metabolism in Healthy Nondiabetic Volunteers

This open-label, randomized, placebo-controlled, incomplete-block, 3-period crossover pilot study investigated the effects of peroxisome proliferator-activated receptor alpha- and gamma-agonists on biomarkers of lipid and glucose metabolism in 12 nondiabetic subjects. Plasma samples were collected before and after each 14-day treatment with placebo, fenofibrate (201 mg/d), rosiglitazone (4 mg twice daily), and combined fenofibrate (201 mg/d) plus rosiglitazone (4 mg twice daily). Except for triglycerides (P < .042) and free fatty acids (P < .074), no significant interaction was demonstrated between fenofibrate and rosiglitazone; thus, the effect due to each drug alone was evaluated (presence/absence of drug). Fenofibrate significantly (P < .050) increased lipoprotein lipase activity (35%) and decreased apolipoproteins B (13%) and C-III (20%). Rosiglitazone significantly (P < .050) decreased fasting glucose (7.3%) and increased apolipoprotein C-III (19%) and adiponectin (137%). Fenofibrate and rosiglitazone also produced effects on triglycerides and free fatty acids, but it was not possible to determine if these effects were synergistic in nature.

Evidence for peroxisome proliferator-activated receptor (PPAR)alpha-independent peroxisome proliferation: effects of PPARgamma/delta-specific agonists in PPARalpha-null mice

Peroxisome proliferators are a diverse group of compounds that cause hepatic hypertrophy and hyperplasia, increase peroxisome number, and on chronic high-dose administration, lead to rodent liver tumorigenesis. Various lines of evidence have led to the conclusion that these agents induce their pleiotropic effects exclusively via agonism of peroxisome proliferator-activated receptor (PPAR)alpha, a member of the steroid receptor superfamily involved in the regulation of fatty acid metabolism. Recently, agonists of two other members of this receptor family have been identified. PPARgamma is predominantly expressed in adipocytes where it mediates differentiation; PPARdelta is a widely expressed orphan receptor with yet unresolved physiologic functions. In the course of characterizing newer PPAR ligands, we noted that highly selective PPARgamma agonists or dual PPARgamma/PPARdelta agonists, lacking apparent murine PPARalpha agonist activity, cause peroxisome proliferation in CD-1 mice. We therefore made use of PPARalpha knockout mice to investigate whether these effects resulted from agonism of PPARalpha by these agents at very high dose levels or whether PPARgamma (or PPARdelta) agonism alone can result in peroxisome proliferation. We report here that several parameters linked to the hepatic peroxisome proliferation response in mice that were seen with these agents resulted from PPARalpha-independent effects.

Novel peroxisome proliferator-activated receptor (PPAR) gamma and PPARdelta ligands produce distinct biological effects

The peroxisome proliferator-activated receptors (PPARs) include three receptor subtypes encoded by separate genes: PPARalpha, PPARdelta, and PPARgamma. PPARgamma has been implicated as a mediator of adipocyte differentiation and the mechanism by which thiazolidinedione drugs exert in vivo insulin sensitization. Here we characterized novel, non-thiazolidinedione agonists for PPARgamma and PPARdelta that were identified by radioligand binding assays. In transient transactivation assays these ligands were agonists of the receptors to which they bind. Protease protection studies showed that ligand binding produced specific alterations in receptor conformation. Both PPARgamma and PPARdelta directly interacted with a nuclear receptor co-activator (CREB-binding protein) in an agonist-dependent manner. Only the PPARgamma agonists were able to promote differentiation of 3T3-L1 preadipocytes. In diabetic db/db mice all PPARgamma agonists were orally active insulin-sensitizing agents producing reductions of elevated plasma glucose and triglyceride concentrations. In contrast, selective in vivo activation of PPARdelta did not significantly affect these parameters. In vivo PPARalpha activation with WY-14653 resulted in reductions in elevated triglyceride levels with minimal effect on hyperglycemia. We conclude that: 1) synthetic non-thiazolidinediones can serve as ligands of PPARgamma and PPARdelta; 2) ligand-dependent activation of PPARdelta involves an apparent conformational change and association of the receptor ligand binding domain with CREB-binding protein; 3) PPARgamma activation (but not PPARdelta or PPARalpha activation) is sufficient to potentiate preadipocyte differentiation; 4) non-thiazolidinedione PPARgamma agonists improve hyperglycemia and hypertriglyceridemia in vivo; 5) although PPARalpha activation is sufficient to affect triglyceride metabolism, PPARdelta activation does not appear to modulate glucose or triglyceride levels.

Peroxisome proliferator-activated receptors gamma and alpha mediate in vivo regulation of uncoupling protein (UCP-1, UCP-2, UCP-3) gene expression

A role for peroxisome proliferator-activated receptors, PPAR gamma and PPAR alpha, as regulators of energy homeostasis and lipid metabolism, has been suggested. Recently, three distinct uncoupling protein isoforms, UCP-1, UCP-2, and UCP-3, have also been identified and implicated as mediators of thermogenesis. Here, we examined whether in vivo PPAR gamma or PPAR alpha activation regulates the expression of all three UCP isoforms. Rats or lean and db/db mice were treated with PPAR gamma [thiazolidinedione (TZD)] or PPAR alpha (WY-14643) agonists, followed by measurement of messenger RNAs (mRNAs) for UCP-1, UCP-2, and UCP-3 in selected tissues where they are expressed. TZD treatment (AD 5075 at 5 mg/kg x day) of rats (14 days) increased brown adipose tissue (BAT) depot size and induced the expression of each UCP mRNA (3x control levels for UCP-1 and UCP-2, 2.5x control for UCP-3). In contrast, UCP-2 and UCP-3 mRNA levels were not affected in white adipose tissue or skeletal muscle. Chronic (30 days) low-dose (0.3 mg/kg x day) TZD treatment induced UCP-1 mRNA and protein in BAT (2.5x control). In contrast, chronic TZD treatment (30 mg/kg x day) suppressed UCP-1 mRNA (>80%) and protein (50%) expression in BAT. This was associated with further induction of UCP-2 expression (>10-fold) and an increase in the size of lipid vacuoles, a decrease in the number of lipid vacuoles in each adipocyte, and an increase in the size of the adipocytes. TZD treatment of db/db mice (BRL 49653 at 10 mg/kg x day for 10 days) also induced UCP-1 and UCP-3 (but not UCP-2) expression in BAT. PPAR alpha is present in BAT, as well as liver. Treatment of rats or db/db mice with WY-14643 did not affect expression of UCP-1, -2, or -3 in BAT. Hepatic UCP-2 mRNA was increased (4x control level) in db/db and lean mice, although this effect was not observed in rats. Thus, in vivo PPAR gamma activation can induce expression of UCP-1, -2, and -3 in BAT; whereas chronic-intense PPAR gamma activation may cause BAT to assume white adipose tissue-like phenotype with increased UCP-2 levels. PPAR alpha activation in mice is sufficient to induce liver UCP-2 expression.

Thiazolidinediones produce a conformational change in peroxisomal proliferator-activated receptor-gamma: binding and activation correlate with antidiabetic actions in db/db mice

The thiazolidinediones are novel insulin sensitizers that serve as orally active antidiabetic agents, in rodents, nonhuman primates, and man. We have examined the effects of 4-week oral administration of three thiazolidinediones (AD-5075, BRL 49653, and CS-045) on plasma glucose and triglyceride concentrations in obese hyperglycemic db/db mice. All three agents lower plasma glucose and triglyceride concentrations. Normal levels of glucose are achieved after treatment with AD-5075 (> 1.7 mg/kg) or BRL 49653 (> or = 30 mg/kg), whereas CS-045 (100 or 300 mg/kg) produces only modest reductions in either parameter. Although the thiazolidinediones have demonstrated insulin-sensitizing activities both in vivo and in vitro, their primary molecular target has been unclear. We have compared the in vivo antidiabetic actions described above with the in vitro activities on peroxisomal proliferator-activated receptor-gamma (PPAR gamma). Hamster PPAR gamma 1 was transiently expressed in COS-1 cells to study the binding of [3H]AD-5075. The concentrations of compounds needed to displace radiolabeled AD-5075 from PPAR gamma correlate with their in vivo potency; the Ki values for displacement by cold AD-5075, BRL 49653, and CS-045 are 22, 68, and 1600 nM, respectively. To examine activation of the receptor, it was transiently cotransfected into COS-1 cells with a reporter plasmid containing two copies of a peroxisome proliferator response element. The EC50 values for activation are 2, 6, and 140 nM for AD-5075, BRL 49653, and CS-045, respectively. We have also analyzed limited proteolytic digests of in vitro translated hamster PPAR gamma. The thiazolidinediones produce a conformational change in PPAR gamma analogous to those produced by agonists of other nuclear hormone receptors. In the presence of saturating concentrations of either AD-5075 or BRL 49653, a receptor fragment of 27 kDa is protected from proteolysis by trypsin. These data support the conclusion that the antidiabetic actions of the thiazolidinediones are directly mediated through binding to PPAR gamma and the resulting active conformation of the receptor. Therefore, binding and transactivation assays using PPAR gamma should serve to identify other novel therapeutic agents with potential antidiabetic activities.

An antidiabetic thiazolidinedione potentiates insulin stimulation of glycogen synthase in rat adipose tissues

Thiazolidinedione derivatives are a novel class of insulin-sensitizing agents that have demonstrated effective antidiabetic activity in vivo. Here, the effects of the potent thiazolidinedione derivative, AD-5075, on the rate-limiting enzyme of glycogen synthesis, glycogen synthase, were investigated in cultured rat adipose tissue. Short term preincubation of adipose tissue with AD-5075 potentiated acute insulin stimulation of I-form glycogen synthase activity in a concentration-dependent (EC50 approximately, 61nM) and time-dependent (t1/2 approximately, 2.3 h) manner. The thiazolidinedione derivative increased the responsiveness of I-form glycogen synthase activity to insulin stimulation at both maximal and submaximal insulin concentrations. In contrast, it had no effect on total glycogen synthase activity. Isoproterenol inhibited acute insulin activation of I-form glycogen synthase activity in a dose-dependent manner; maximal inhibition was attained at a concentration of 3 nM. AD-5075 antagonized isoproterenol inhibition of insulin's action. The concentration of glycogenolytic agent required to attain maximal inhibition was increased an order of magnitude in tissue treated with the antidiabetic agent. Short term preincubation of adipose tissue under hyperglycemic conditions (15 or 25 mM glucose) decreased insulin-stimulated I-form glycogen synthase activity. Concurrent treatment of the tissue with AD-5075 abrogated this glucose toxicity-induced inhibition of insulin action. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, blocked insulin activation of glycogen synthase in a dose-dependent manner. Half-maximal inhibition was observed at approximately 0.3 microM, and maximal inhibition occurred at 1.0 microM. AD-5075 did not antagonize wortmannin's inhibitory action. These results indicate that thiazolidinediones can act directly on adipose tissues to augment an important metabolic effect of insulin and counteract the inhibitory effects of catecholamines or hyperglycemia. As insulin stimulation of glycogen synthase remains wortmannin inhibitable in the presence of AD-5075, the effects of thiazolidinediones on insulin signal transduction may be phosphatidylinositol 3-kinase-dependent.

Down-regulation of the expression of the obese gene by an antidiabetic thiazolidinedione in Zucker diabetic fatty rats and db/db mice

Obese (ob) is a recently identified gene involved in the regulation of energy balance in the mouse. We report here that AD-5075, a potent thiazolidinedione which lowered plasma glucose and triglyceride in Zucker diabetic fatty (ZDF) rats and db/db mice, decreased the expression of the ob gene in these animal models of obesity and non-insulin-dependent diabetes mellitus. The level of adipose ob mRNA in ZDF rats was 3-fold greater than that detected in the Zucker lean littermates. Chronic treatment with AD-5075 elicited a 67 and 70% reduction of ob mRNA in ZDF and control lean rats, respectively. Furthermore, the amount of adipose ob mRNA in db/db mice was 7 times higher than that detected in lean littermates. Treatment of db/db mice with AD-5075 resulted in a 78% reduction of the level of ob mRNA with parallel changes in circulating level of the ob gene product, leptin. The reduction of the ob mRNA in the Zucker lean rats was accompanied by significantly greater food intake and weight gain. However, in ZDF rats and db/db mice, there was profound increase in body weight without hyperphagia. The results demonstrate that the expression of the ob gene is up-regulated in these two rodent models of diabetes compared to their lean counterparts and that such overexpression is attenuated by treatment with an agent that improves insulin sensitivity and glucose homeostasis in vivo.

Low density lipoprotein is protected from oxidation and the progression of atherosclerosis is slowed in cholesterol-fed rabbits by the antioxidant N,N'-diphenyl-phenylenediamine

The oxidative modification of low density lipoprotein (LDL) may play an important role in atherosclerosis. We found that the antioxidant N,N'-diphenyl-1,4-phenylenediamine (DPPD) inhibits in vitro LDL oxidation at concentrations much lower than other reported antioxidants. To test whether DPPD could prevent atherosclerosis, New Zealand White rabbits were fed either a diet containing 0.5% cholesterol and 10% corn oil (control group) or the same diet also containing 1% DPPD (DPPD-fed group) for 10 wk. Plasma total cholesterol levels were not different between the two groups, but DPPD feeding increased the levels of triglyceride (73%, P = 0.007) and HDL cholesterol (26%, P = 0.045). Lipoproteins from DPPD-fed rabbits contained DPPD and were much more resistant to oxidation than control lipoproteins. After 10 wk, the DPPD-fed animals had less severe atherosclerosis than did the control animals: thoracic aorta lesion area was decreased by 71% (P = 0.0007), and aortic cholesterol content was decreased by 51% (P = 0.007). Although DPPD cannot be given to humans because it is a mutagen, our results indicate that orally active antioxidants can have antiatherosclerotic activity. This strongly supports the theory that oxidized LDL plays an important role in the pathogenesis of atherosclerosis.

Platelet-activating factor (PAF) stimulates the lysoPAF acetyltransferase in leukocyte-rich plasma: use in PAF antagonist studies

Addition of platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to leukocyte-rich plasma from several species resulted in the rapid and pronounced activation of the PAF biosynthetic enzyme acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine acetyltransferase (EC 2.3.1.67). Activation of acetyltransferase by PAF occurred in leukocyte-rich plasma from human, chimpanzee, rhesus monkey, and dog. The neutrophil was indicated to be the major cellular source of the activatable acetyltransferase in leukocyte-rich plasma. The induction of acetyltransferase was substantial with 10 nM PAF, and maximal at 10-30 seconds. Measurable acetyltransferase activation was significantly greater when the PAF-activated cells were separated from the plasma by centrifugation before the acetyltransferase assay. This may be due in part to the removal of the PAF-specific acetylhydrolase present in plasma which can cleave the acetyl group from PAF. Measuring PAF activation of acetyltransferase in leukocyte-rich plasma can be useful to determine the potency of PAF antagonists with neutrophils in plasma compared to isolated neutrophils in aqueous buffer, and as an ex vivo assay to determine the efficacy and plasma concentration equivalents of antagonists administered to whole animals. The PAF antagonist L-659,989 was shown to be 3-5 times more potent in inhibiting PAF induction of acetyltransferase in isolated human neutrophils than in human leukocyte-rich plasma, with IC50 values of 10 nM and 40 nM, respectively. In the ex vivo assay, oral administration of the PAF antagonist L-667,131 to dogs resulted in very substantial inhibition of PAF induction of acetyltransferase in the leukocyte-rich plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

(+/-)-trans-2-(3-Methoxy-5-methylsulfonyl-4-propoxyphenyl)-5-(3,4,5- trimethoxyphenyl)tetrahydrofuran (L-659,989), a novel, potent PAF receptor antagonist

The title compound, L-659,989, is a highly potent, competitive, and selective antagonist of the binding of [3H]PAF to its receptors in platelet membranes from rabbits and humans. It exhibits equilibrium inhibition constants for PAF binding of 1.1 nM (rabbit) to 9.0 nM (human), values that are at least 1-2 orders of magnitude lower than those of other PAF antagonists tested. L-659,989 potently inhibits PAF-induced aggregation of rabbit platelets and degranulation of rat (ED50 4.5 nM) and human (ED50 10 nM) neutrophils. L-659,989 inhibits PAF-induced extravasation and lysosomal enzyme release in rats, and is active orally in female rats (ED50 0.2 mg/kg) with an extraordinary oral duration of action of 12 to 16 hours at 1.0 mg/kg p.o.

Platelet-activating factor (PAF) stimulates the PAF-synthesizing enzyme acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine O2-acetyltransferase and PAF synthesis in neutrophils

Platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine; PAF) induced in isolated rat peritoneal and human peripheral neutrophils a rapid and potent activation of the PAF biosynthetic enzyme acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine O2-acetyltransferase (EC 2.3.1.67). The PAF-induced activation of the neutrophil acetyltransferase (8-10 times basal neutrophil activity) was maximal within 30 sec after PAF addition, as was the PAF-stimulated degranulation. After 1 min of PAF stimulation, the elevated acetyltransferase activity steadily decreased. Within 2 min of stimulation of neutrophils with 10(-6) M PAF, the 7-fold increase in acetyltransferase activity was coincident with substantial PAF synthesis (as measured by [3H]acetate incorporation into PAF), which was 14% of the PAF synthesis induced by the Ca2+ ionophore A23187 at 10(-5) M. PAF activation of the acetyltransferase and PAF synthesis required intact neutrophils as they did not occur in cells broken by sonication. The neutrophil acetyltransferase was 10-30 times more sensitive to activation by PAF than was degranulation as the acetyltransferase activation was evident with 10(-9) M PAF and was about maximal with 3 x 10(-8) M PAF. The unstimulated and PAF-induced acetyltransferase exhibited the same Km for acetyl-CoA (67 microM), but the Vmax for the PAF-induced enzyme (1667 pmol/min per 10(7) cells) was 10 times that of the unstimulated enzyme (175 pmol/min per 10(7) cells). The PAF induction of the acetyltransferase was less sensitive to inhibition by the specific PAF receptor antagonist L-652,731 than was PAF-induced degranulation. This, along with the differing sensitivities to PAF, suggests that acetyltransferase activation and degranulation induced by PAF either involve two different PAF receptors or involve one receptor type with different receptor occupancy requirements. Escherichia coli alkaline phosphatase, which greatly decreased the activity of the acetyltransferase in spleen microsomes, had little or no effect on the basal or PAF-induced neutrophil acetyltransferase. Thus, by stimulating the activity of acetyltransferase, PAF induces in neutrophils the synthesis of more PAF, thereby probably augmenting the neutrophil response to the initial PAF.

Inhibition of the platelet activating factor (PAF)-induced in vivo responses in rats by trans-2,5-(3,4,5-trimethoxyphenyl) tetrahydrofuran (L-652,731), a PAF receptor antagonist

Trans-2,5-bis-(3,4,5-trimethoxyphenyl)tetrahydrofuran (L652, 731), a potent and specific receptor antagonist of platelet activating factor (PAF) (Hwang et al., J. Biol. Chem. 260: 15639-15645, 1985), potently inhibits several PAF-induced in vivo responses in rats. Intravenously and p.o. administered L-652,731 gave a dose-response inhibition of PAF-induced lysosomal hydrolase secretion and extravasation with ED50 values of 1 and 3 mg/kg, respectively. Inhibitions of 87% were achieved with 50 mg/kg p.o. After a single 5-mg/kg p.o. dose, L-652,731 achieved 50 to 60% inhibition of PAF-induced lysosomal hydrolase secretion and extravasation by 0.5 to 1.5 hr with near maximum inhibition lasting through 6 hr. A 20-mg/kg p.o. dose of L-652,731 inhibited PAF-induced leukopenia and neutropenia by 96 and 73%, respectively. The most substantial inhibitions of the extravasation and lysosomal hydrolase secretion induced by PAF or soluble immune complexes were achieved by p.o. L-652,731 (20 mg/kg) with moderate inhibition by dexamethasone and little or no inhibition by antagonists/inhibitors of histamine H1 or H2 or serotonin receptors or cyclooxygenase. Intravenous infusion of a 0.4 mg of L-652,731 bolus inhibited the hypotensive responses from subsequent PAF infusions by a maximum of 72% and with a half-life duration of action of 2.5 hr. Intravenous infusion of L-652,731 results in an immediate 87% reversal of the extreme hypotension induced by a previous endotoxin injection. Thus, with its good p.o. activity, long duration of action and specificity in inhibiting PAF-induced responses in vivo, L-652,731 is a very useful tool in determining the role of PAF in mediating different pathophysiological processes.

Platelet-activating factor (PAF) mediation of rat anaphylactic responses to soluble immune complexes. Studies with PAF receptor antagonist L-652,731

A new synthetic compound, L-652,731 (trans-2,5-(3,4,5-trimethoxyphenyl) tetrahydrofuran), which has been demonstrated by Hwang et al. to be a potent and specific platelet-activating factor (PAF) receptor antagonist causes 100% inhibition of 1 microM PAF-induced neutrophil degranulation at 50 microM, but has no effect on neutrophil degranulation induced by precipitating immune complexes (323 micrograms/ml), fMet-Leu-Phe (10(-7) M), or the calcium ionophore A23187 (10(-5) M). Intravenous infusion of 1 mumol L-652,731 results in almost 100% inhibition of hypotension induced by PAF but not that induced by isoproterenol, histamine, bradykinin, or acetylcholine. With the use of this novel PAF receptor antagonist, the in vivo mediator role of PAF in the soluble immune complex-induced hypotension, extravasation, vascular lysosomal hydrolase secretion, and neutropenia in rats was determined. The hypotension, extravasation, and lysosomal hydrolase release induced by immune complex infusion take 2 to 10 min longer to occur than the same responses elicited by PAF infusion. The neutropenia response is immediate with both stimuli. L-652,731 when orally administered to rats (20 mg/kg, 1.5 hr before PAF infusion) inhibited PAF-induced hypotension (69%), extravasation (76%), vascular lysosomal hydrolase release (79%), and neutropenia (73%). The same L-652,731-dosing regimen inhibited immune complex-stimulated hypotension (87%), extravasation (77%), and vascular lysosomal hydrolase release (31%). The initial and complete neutropenia induced by immune complex infusion was not inhibited in L-652,731-pretreated rats, but the rate of return of neutrophils to the blood was faster in the latter rats. Rats with blocked circulation to the liver still exhibited extensive extravasation and vascular lysosomal hydrolase release in response to PAF, but there was no extravasation and greatly reduced hydrolase release in response to immune complexes. Thus PAF is indicated to be a major mediator of soluble immune complex-induced hypotension and vascular permeability and a minor mediator of immune complex-induced lysosomal hydrolase release in rats. PAF probably does not mediate the initial and complete neutropenia stimulated by immune complexes. The liver is probably the major site for PAF production in response to circulating immune complexes.

Platelet-activating factor (PAF) mediation of rat anaphylactic responses to soluble immune complexes. Studies with PAF receptor antagonist L-652,731

A new synthetic compound, L-652,731 (trans-2,5-(3,4,5-trimethoxyphenyl) tetrahydrofuran), which has been demonstrated by Hwang et al. to be a potent and specific platelet-activating factor (PAF) receptor antagonist causes 100% inhibition of 1 microM PAF-induced neutrophil degranulation at 50 microM, but has no effect on neutrophil degranulation induced by precipitating immune complexes (323 micrograms/ml), fMet-Leu-Phe (10(-7) M), or the calcium ionophore A23187 (10(-5) M). Intravenous infusion of 1 mumol L-652,731 results in almost 100% inhibition of hypotension induced by PAF but not that induced by isoproterenol, histamine, bradykinin, or acetylcholine. With the use of this novel PAF receptor antagonist, the in vivo mediator role of PAF in the soluble immune complex-induced hypotension, extravasation, vascular lysosomal hydrolase secretion, and neutropenia in rats was determined. The hypotension, extravasation, and lysosomal hydrolase release induced by immune complex infusion take 2 to 10 min longer to occur than the same responses elicited by PAF infusion. The neutropenia response is immediate with both stimuli. L-652,731 when orally administered to rats (20 mg/kg, 1.5 hr before PAF infusion) inhibited PAF-induced hypotension (69%), extravasation (76%), vascular lysosomal hydrolase release (79%), and neutropenia (73%). The same L-652,731-dosing regimen inhibited immune complex-stimulated hypotension (87%), extravasation (77%), and vascular lysosomal hydrolase release (31%). The initial and complete neutropenia induced by immune complex infusion was not inhibited in L-652,731-pretreated rats, but the rate of return of neutrophils to the blood was faster in the latter rats. Rats with blocked circulation to the liver still exhibited extensive extravasation and vascular lysosomal hydrolase release in response to PAF, but there was no extravasation and greatly reduced hydrolase release in response to immune complexes. Thus PAF is indicated to be a major mediator of soluble immune complex-induced hypotension and vascular permeability and a minor mediator of immune complex-induced lysosomal hydrolase release in rats. PAF probably does not mediate the initial and complete neutropenia stimulated by immune complexes. The liver is probably the major site for PAF production in response to circulating immune complexes.

Targeting of synthetically glycosylated human placental glucocerebrosidase

Human placental beta-glucocerebrosidase modified by covalent attachment of N2-(N2, N6-bis [3-(alpha-D-mannopyranosylthio)propionyl]-L- lysyl)-N6-[3-(alpha-D-mannopyranosylthio)propionyl]-L-lysine was administered to rats by intravenous injection. Comparison of enzyme distribution in isolated liver cell populations indicates an increase in enzyme-specific activity of 18-fold in nonparenchymal cells and only 1.5-fold to hepatocytes compared to uninjected control animals. This macrophage-specific delivery of an active lysosomal enzyme has potential for application in enzyme replacement trials.

Platelet activating factor (PAF) involvement in endotoxin-induced hypotension in rats. Studies with PAF-receptor antagonist kadsurenone

Evidence from three types of experiments indicates that platelet activating factor (PAF)1 is an important mediator of endotoxin-induced hypotension in rats. a) Endotoxin infusion stimulates the time-dependent appearance of PAF in the blood. b) PAF infusion results immediately (less than 30 sec) in hypotension while endotoxin-induced hypotension takes 3-5 min to occur, allowing time for PAF production. c) Infusion of the specific PAF-receptor antagonist kadsurenone (2.2 mumole/kg bolus, 0.9 mumoles/min/kg continuous infusion), which inhibits PAF-induced hypotension by 67%, causes a 67% reversal of endotoxin-elicited hypotension. An additional finding of this study is that rats respond hypotensively to each of a series of low-dose PAF infusions but only to the first low-dose endotoxin infusion. These endotoxin-refractory rats do respond to subsequent PAF infusions.

Platelet activating factor intravenous infusion in rats stimulates vascular lysosomal hydrolase secretion independent of blood neutrophils

Intravenous infusion of platelet activating factor into rats at doses of 5-20 nmoles/kg results in a rapid and dose-dependent increase in the plasma level of several lysosomal hydrolases, notably glucosaminidase. This hydrolase secretion occurs concomitantly with the increased vascular permeability but subsequent to the neutropenia and hypotensive responses to platelet activating factor. The glucosaminidase release in vivo does not exhibit any lasting desensitization, does not require cytochalasin B, and is quantitatively the same in rats made neutropenic with anti-neutrophil serum, and thus is different from the published in vitro degranulating effect of this lipid with neutrophils. Therefore, lysosomal hydrolase secretion may be an important pathophysiologic response to very low blood levels of platelet activating factor.

Cell-specific ligands for selective drug delivery to tissues and organs

Various numbers of D-mannose residues have been attached via spacer arms to lysine, dilysine, and oligolysine backbones. These D-mannosyl peptide analogues were found to be potent competitive inhibitors of the uptake of 125I-labeled D-mannose-bovine serum albumin conjugate by rat alveolar macrophages. The inhibitory potency of these synthetic ligands increased with increasing number of carbohydrate moieties. The chirality of the peptide backbone did not appear to play a major role in binding, whereas variations of the length and linkage of the spacer arm notably affected the inhibitory activities. The saccharide specificity of the macrophage receptor was demonstrated by the inactivity of the corresponding D-galactosyl peptide analogues. The L-fucosyl peptide derivative was only weakly active. The trimannosyldilysine ligand (KI = 3.9 microM) and its analogues are potentially useful in selective delivery of therapeutic agents to macrophages.

The role of extra-hepatic tissues in the receptor-mediated plasma clearance of glycoproteins terminated by mannose or N-acetylglucosamine

The mannose- and N-acetylglucosamine-specific pathway for the clearance of mammalian glycoproteins has been characterized by using 125I-labelled neoglycoproteins, glycosidase-treated orosomucoid and lysosomal glycosidases (beta-glucuronidase and beta-N-acetylglucosaminidase) as probes. There are two components to this pathway in vivo; one liver-dependent and the other extrahepatic or liver-independent. Cells that mediate clearance by the latter component of the pathway are present in spleen, bone and in elements of the reticuloendothelial system, but not in the kidney. Glycoproteins that possess terminal mannose, glucose or N-acetylglucosamine residues, including various lysosomal enzymes, are rapidly cleared from plasma via this pathway. Glucose-terminated glycoproteins are recognized by two pathways in the intact animal; the hepatic galactose-specific pathway and the mannose/N-acetylglycosamine-specific pathway, which is present in liver and in peripheral tissues. Following removal of the liver by surgical evisceration, glucose-terminated glycoproteins are cleared whereas glycoproteins bearing galactose are not cleared. Uptake of 125I-labelled neoglycoproteins and agalacto-orosomucoid by isolated alveolar macrophages closely mimics clearance in vivo by the mannose/N-acetylglucosamine pathway. Neoglycoproteins terminated by mannose, glucose or N-acetylglucosamine all compete with 125I-labelled agalacto-orosomucoid for uptake by receptor-mediated pinocytosis. The extent of substitution of the neoglycoproteins is a critical determinant of their inhibitory potency. It is proposed that mononuclear phagocytes are in important component of the clearance pathway in vivo. The mannose/N-acetylglucosamine pathway may be important in the regulation of extracellular levels of various glycosylated macromolecules, including lysosomal hydrolases.

Identification of a tripeptidyl aminopeptidase in the anterior pituitary gland: effect on the chemical and biological properties of rat and bovine growth hormones

The NH2-terminal heterogeneity which is generated in bovine GH during its extraction from mildly acidified pituitary homogenates is attributable to a newly identified peptidase. The beta-naphthylamide of Phe-Pro-Ala, modeled after the NH2-terminal tripeptide sequence of the phenylalanyl monomer of bovine growth hormone, was cleaved by the peptidase into the tripeptide and B-naphthylamine and served as a substrate for assay of the eznyme. However, the B-naphthylamide of Ala-Phe-Pro, modeled after the NH2-terminal tripeptide sequence of the alanyl monomer, was not cleaved. In harmony with this specificity, the peptidase cleaved 11 tripeptides sequentially from the NH2-terminus of the phenylalanyl monomer of bovine GH but none from the alanyl monomer. Six of the tripeptides nearest the NH2-terminus were unequivocally identified and their sequences were consistent with the NH2-terminal octadecapeptide sequence of the phenylalanyl monomer of bovine GH. Five additional peptides were by composition consistent with their being tripeptides derived from residues 19--33. Because of the apparent specificity for the hydrolytic release of tripeptides and inability to cleave substituted tripeptidyl derivatives, the enzyme is considered to be a tripeptidyl aminopeptidase. In its hydrolysis of phenylalanyl monomers of rat growth hormone, a similar number of tripeptides was released, associated with which there was a 70% loss of biological activity but no reduction in immunological activity. The enzyme could be solubilized by extraction with 1% Triton X-100 at pH 3.0, precipitated between 2 and 3 M (NH4)2SO4, and further purified by gel filtration on G-75 in M/10 acetic acid. The enzyme has a mol wt of 57,000 and is optimally active at pH 4. It can be differentiated from cathepsin D by its insensitivity to inhibition by pepstatin.

Plasma clearance of glycoproteins with terminal mannose and N-acetylglucosamine by liver non-parenchymal cells. Studies with beta-glucuronidase, N-acetyl-beta-D-glucosaminidase, ribonuclease B and agalacto-orosomucoid

Glycoproteins having mannose and/or N-acetylglucosamine in the terminal non-reducing position [Stockert, Morell & Scheinberg (1976) Biochem. Biophys. Res. Commun. 68, 988--993], and various lysosomal enzymes [Stahl, Schlesinger, Rodman & Doebber (1976) Nature (London) 264, 86--8] are rapidly cleared from plasma by the liver after intravenous administration. A liver cell-separation technique was used to determine the cellular localization of 125I-labelled beta-glucuronidase, ribonuclease B, agalacto-orosomucoid and asialo-orosomucoid. On a specific readioactivity basis, all ligands except 125I-labelled asialo-orosomucoid were enriched in the non-parenchymal cell fraction. Isolated cells, fixed and stained for beta-glucuronidase or N-acetyl-beta-D-glucosaminidase activity after intravenous injection of the enzymes, showed enrichment in the non-parenchymal cell fraction (probably Kupffer cells). After uptake by the non-parenchymal cells, liver lysosomal beta-glucuronidase and N-acetyl-beta-D-glucosaminidase showed degradation half-times of 2.2 and 0.4 days respectively.