Risk factors for hepatocellular carcinoma among patients with chronic liver disease

BACKGROUND AND METHODS

To detect potentially curable cases of hepatocellular carcinoma, outpatients with chronic hepatitis or compensated liver cirrhosis who were seen at the Center for Adult Diseases (Osaka, Japan) were examined periodically by means of ultrasonography and measurement of serum alpha-fetoprotein. Risk factors for hepatocellular carcinoma were identified with a Cox proportional-hazards model.

RESULTS

A total of 917 patients, 40 to 69 years old, were registered from May 1987 to March 1991. By the end of September 1991, liver cancer had developed in 54. The three-year cumulative risk of liver cancer was 12.5 percent for 240 patients with liver cirrhosis at enrollment and 3.8 percent for 677 patients with chronic hepatitis. Cox regression analysis showed that the risk of liver cancer was increased almost sevenfold in patients with hepatitis B surface antigen (rate ratio, 6.92; 95 percent confidence interval, 2.92 to 16.39) and fourfold in patients with hepatitis C antibody (rate ratio, 4.09; 95 percent confidence interval, 1.30 to 12.85). A high alpha-fetoprotein value at enrollment was also a risk marker for liver cancer.

CONCLUSIONS

Patients with hepatitis C virus infection have a greatly increased risk of liver cancer. Further studies are required to clarify the roles of other risk factors, including drinking and smoking habits.

The neuron-specific kinesin superfamily protein KIF1A is a unique monomeric motor for anterograde axonal transport of synaptic vesicle precursors

Axonal transport has been intensively examined as a good model for studying the mechanism of organelle transport in cells, but it is still unclear how different types of membrane organelles are transported through the nerve axon. To elucidate the function of this mechanism, we have cloned KIF1A, a novel neuron-specific kinesin superfamily motor that was discovered to be a monomeric, globular molecule and that had the fastest reported anterograde motor activity (1.2 microns/s). To identify its cargo, membranous organelles were isolated from the axon. KIF1A was associated with organelles that contained synaptic vesicle proteins such as synaptotagmin, synaptophysin, and Rab3A. However, this organelle did not contain SV2, another synaptic vesicle protein, nor did it contain presynaptic membrane proteins, such as syntaxin 1A or SNAP-25, or other known anterograde motor proteins, such as kinesin and KIF3. Thus, we suggest that the membrane proteins are sorted into different classes of transport organelles in the cell body and are transported by their specific motor proteins through the axon.

KIF1B, a novel microtubule plus end-directed monomeric motor protein for transport of mitochondria

To further elucidate the mechanism of organelle transport, we cloned a novel member of the mouse kinesin superfamily, KIF1B. This N-terminal-type motor protein is expressed ubiquitously in various kinds of tissues. In situ hybridization revealed that KIF1B is expressed abundantly in differentiated nerve cells. Interestingly, K1F1B works as a monomer, having a microtubule plus end-directed motility. Our rotary shadowing electron microscopy revealed mostly single globular structures. Immunocytochemically, KIF1B was colocalized with mitochondria in vivo. Furthermore, a subcellular fractionation study showed that KIF1B was concentrated in the mitochondrial fraction, and purified K1F1B could transport mitochondria along microtubules in vitro. These data strongly suggested that KIF1B works as a monomeric motor for anterograde transport of mitochondria.

Cytochrome P450 2E1 and 2A6 enzymes as major catalysts for metabolic activation of N-nitrosodialkylamines and tobacco-related nitrosamines in human liver microsomes

An acetyltransferase-overexpressing strain of Salmonella typhimurium (NM2009) has been used to investigate roles of human liver microsomal cytochrome P450 (P450) enzymes in the activation of carcinogenic nitrosamine derivatives, including N-nitrosodialkylamines and tobacco-smoke-related nitrosamines, to genotoxic products. Studies employing correlation of activities with several P450-dependent monooxygenase reactions in different human liver samples, inhibition of microsomal activities by antibodies raised against human P450 enzymes and by specific P450 inhibitors, and reconstitution of activities with purified P450 enzymes suggest that the tobacco-smoke-related nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and N-nitrosonornicotine (NNN) as well as N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) are oxidized to genotoxic products by different P450 enzymes, particularly P450 2E1 and 2A6. The activation of NDMA and NNN by liver microsomes was suggested to be catalyzed more actively by P450 2E1 than by other P450 enzymes because the activities were well correlated with NDMA N-demethylation and aniline p-hydroxylation in different human samples, and purified P450 2E1 had the highest activities in reconstituted monooxygenase systems. The relatively high contribution of P450 2A6 to the activation of NDEA and NNK was supported by the correlation seen with coumarin 7-hydroxylation in human liver microsomes, and antibodies raised against P450 2A6 inhibited both activities by approximately 50%. P450 3A4, 2D6 and 2C enzymes appear not to be extensively involved in the activation of these nitrosamines as judged by several criteria examined. Thus, this work indicates that several P450 enzymes, particularly P450 2E1 and 2A6, catalyze metabolic activation of nitrosamine derivatives including N-nitrosodialkylamines and tobacco-smoke-related nitrosamines in human liver microsomes.

KIF3A/B: a heterodimeric kinesin superfamily protein that works as a microtubule plus end-directed motor for membrane organelle transport

We cloned a new member of the murine brain kinesin superfamily, KIF3B, and found that its amino acid sequence is highly homologous but not identical to KIF3A, which we previously cloned and named KIF3 (47% identical). KIF3B is localized in various organ tissues and developing neurons of mice and accumulates with anterogradely moving membranous organelles after ligation of nerve axons. Immunoprecipitation assay of the brain revealed that KIF3B forms a complex with KIF3A and three other high molecular weight (approximately 100 kD)-associated polypeptides, called the kinesin superfamily-associated protein 3 (KAP3). In vitro reconstruction using baculovirus expression systems showed that KIF3A and KIF3B directly bind with each other in the absence of KAP3. The recombinant KIF3A/B complex (approximately 50-nm rod with two globular heads and a single globular tail) demonstrated plus end-directed microtubule sliding activity in vitro. In addition, we showed that KIF3B itself has motor activity in vitro, by making a complex of wild-type KIF3B and a chimeric motor protein (KIF3B head and KIF3A rod tail). Subcellular fractionation of mouse brain homogenates showed a considerable amount of the native KIF3 complex to be associated with membrane fractions other than synaptic vesicles. Immunoprecipitation by anti-KIF3B antibody-conjugated beads and its electron microscopic study also revealed that KIF3 is associated with membranous organelles. Moreover, we found that the composition of KAP3 is different in the brain and testis. Our findings suggest that KIF3B forms a heterodimer with KIF3A and functions as a new microtubule-based anterograde translocator for membranous organelles, and that KAP3 may determine functional diversity of the KIF3 complex in various kinds of cells in vivo.

Progesterone and testosterone hydroxylation by cytochromes P450 2C19, 2C9, and 3A4 in human liver microsomes

Roles of human cytochrome P450 (P450 or CYP) 2C9, 2C19, and 3A4 in the oxidation of progesterone and testosterone were studied in recombinant P450 enzymes and in human liver microsomes. In vitro inhibition experiments showed that progesterone and its 17alpha- and 21-hydroxylated metabolites and 11-deoxycortisol suppressed the CYP2C19-dependent R-warfarin 7-hydroxylation activities, with progesterone being the most active. These steroid chemicals also inhibited CYP2C9-dependent S-warfarin 7-hydroxylation activities though lesser extents seen with those in CYP2C19 enzyme. Progesterone was found to be a competitive inhibitor of CYP2C19 and CYP2C9 in human liver microsomes. Recombinant CYP2C19 catalyzed progesterone to form 21-hydroxyprogesterone as a major product and 16alpha- and 17alpha-hydroxyprogesterone as minor products. CYP2C9 also had progesterone 21-hydroxylation activities, although the activities were lower than those catalyzed by CYP2C19. Vmax/Km ratios for the progesterone 21-hydroxylation activity of CYP2C19 were determined to be 13- and 32-fold higher than those of CYP2C9 and 3A4, respectively. CYP3A4 oxidized progesterone to form 16alpha-, 6beta-, and 2beta-hydroxyprogesterone as major products and 21-hydroxyprogesterone as a minor product, but did not produce detectable levels of 17alpha-hydroxyprogesterone. Immunoinhibition experiments suggested that anti-CYP2C9 (which inhibits both CYP2C9 and CYP2C19 catalytic activities) suppressed the progesterone 21-hydroxylation activities catalyzed by liver microsomes of humans and monkeys and that anti-CYP2C11 inhibited the progesterone 21-hydroxylation activities catalyzed by liver microsomes of male rats. CYP2C19 was also found to oxidize testosterone at 17-position to form androstenedione. Androstenedione formation catalyzed by liver microsomes of humans and monkeys and of male rats was suppressed by anti-CYP2C9 and anti-CYP2C11, respectively. These results suggest that CYP2C19 plays important roles in the oxidation of progesterone and testosterone in human liver microsomes, although the physiological significance of these metabolic pathways remains unclear. CYP2C9 may have some, but lesser extent than those by CYP2C19, of the catalytic roles for the metabolism of progesterone and testosterone by human liver microsomes.

Vascular endothelial growth factor (VEGF) mRNA isoform expression pattern is correlated with liver metastasis and poor prognosis in colon cancer

Vascular endothelial growth factor (VEGF) is a well known factor that induces angiogenesis. Four isoforms, i.e. VEGF206, 189, 165, and 121, have been identified. We examined the isoform patterns of VEGF mRNA using reverse transcription polymerase chain reaction (RT-PCR) analysis in 61 colon cancers. All the colon cancers examined expressed VEGF121. The isoform patterns were classified into three groups: type 1, VEGF121; type 2, VEGF121 + VEGF165; type 3, VEGF121 + VEGF165 + VEGF189. Three of the 61 colon cancers examined showed type 1 expression, 26 showed type 2 expression and 32 showed the type 3 pattern. The patients with liver metastases showed the type 3 isoform expression pattern at a significantly higher incidence (12 of 16, 75%) than those without liver metastasis (20 of 45, 44%) (P=0.036). The type 3 isoform pattern was significantly associated with M1 stage (P=0.019). The patients with colon cancer and the type 3 isoform pattern showed significantly poor prognosis (P < 0.01, Cox-Mantel). The colon cancers with the type 3 pattern showed a significantly higher involvement of veins (P=0.006). These observations suggest that the aberrant type 3 expression pattern of VEGF189 mRNA isoforms is correlated with liver metastasis, M stage, and poor prognosis in colon cancer.

Activation and detoxication of aflatoxin B1

Aflatoxin B1 (AFB1) is a potent hepatocarcinogen in experimental animals and a hazard to human health in several parts of the world. Implementation of rational intervention plans requires understanding of aspects of the roles of individual chemical steps involved in its disposition. AFB1 is activated to AFB1 exo-8,9-epoxide primarily by cytochrome P450 (P450) enzymes, particularly P450 3A4. However, P450 3A4 and other P450s also oxidize AFB1 to less dangerous products. The exo-epoxide is unstable in H2O (t1/2 1 s at 25 degreesC, k=0.6 s-1) and the diol product undergoes base-catalyzed rearrangement to a dialdehyde that reacts with protein lysine residues. AFB1 exo-8, 9-epoxide reacts with DNA to give adducts in high yield (>98%). This interaction is characterized by a Kd of approximately 1.4 mM, intercalation between base pairs, and rapid reaction with the guanyl N7 atom (k approximately 40 s-1). A proton field on the periphery of DNA is postulated to catalyze hydrolysis and also conjugation. Rat and especially human epoxide hydrolase show very little rate acceleration of hydrolysis of AFB1 exo- or endo-8,9-epoxide. However, glutathione transferases (GSTs) can catalyze AFB1 exo-8,9-epoxide conjugation. Kinetic analysis indicates a range of ratios of kcat/Kd varying from 10 to 1700 s-1 M-1, with the polymorphic GST M1-1 having the highest activity of the human GSTs. Studies with human hepatocytes indicate a major role for GST M1-1 in AFB1 conjugation and that the model chemoprotective agent oltipraz can act by both inducing GSTs and inhibiting P450s.

Transgene expression of steel factor in the basal layer of epidermis promotes survival, proliferation, differentiation and migration of melanocyte precursors

Mutations at the murine dominant white spotting (KitW) and steel (MgfSl) loci, encoding c-Kit receptor kinase and its ligand respectively, exert developmental defects on hematopoietic cells, melanocytes, germ cells and interstitial cells of Cajal. The expression patterns of steel factor (SLF) observed in the skin and gonads suggest that SLF mediates a migratory or a chemotactic signal for c-Kit-expressing stem cells (melanocyte precursors and primordial germ cells). By targeting expression of SLF to epidermal keratinocytes in mice, we observed extended distribution of melanocytes in a number of sites including oral epithelium and footpads where neither melanocytes nor their precursors are normally detected. In addition, enlarged pigmented spots of KitW and other spotting mutant mice were observed in the presence of the SLF transgene. These results provide direct evidence that SLF stimulates migration of melanocytes in vivo. We also present data suggesting that SLF does not simply support survival and proliferation of melanocytes but also promotes differentiation of these cells. Unexpectedly, melanocyte stem cells independent of the c-Kit signal were maintained in the skin of the SLF transgenic mice. After the elimination of c-Kit-dependent melanoblasts by function-blocking anti-c-Kit antibody, these stem cells continued to proliferate and differentiate into mature melanocytes. These melanoblasts are able to migrate to cover most of the epidermis after several months. The SLF transgenic mice described in this report will be useful in the study of melanocyte biology.

Roles of CYP2A6 and CYP2B6 in nicotine C-oxidation by human liver microsomes

Nicotine C-oxidation by recombinant human cytochrome P450 (P450 or CYP) enzymes and by human liver microsomes was investigated using a convenient high-performance liquid chromatographic method. Experiments with recombinant human P450 enzymes in baculovirus systems, which co-express human nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH)-P450 reductase, revealed that CYP2A6 had the highest nicotine C-oxidation activities followed by CYP2B6 and CYP2D6; the Km values by these three P450 enzymes were determined to be 11.0, 105, and 132 microM, respectively, and the Vmax values to be 11.0, 8.2, and 8.6 nmol/min per nmol P450, respectively. CYP2E1, 2C19, 1A2, 2C8, 3A4, 2C9, and 1A1 catalysed nicotine C-oxidation only at high (500 microM) substrate concentration. CYP1B1, 2C18, 3A5, and 4A11 had no measurable activities even at 500 microM nicotine. In liver microsomes of 16 human samples, nicotine C-oxidation activities were correlated with CYP2A6 contents at 10 microM substrate concentration, whereas such correlation coefficients were decreased when the substrate concentration was increased to 500 microM. Contribution of CYP2B6 (as well as CYP2A6) was demonstrated by experiments with the effects of orphenadrine (and also coumarin and anti-CYP2A6) on the nicotine C-oxidation activities by human liver microsomes at 500 microM nicotine. CYP2D6 was found to have minor roles since quinidine did not inhibit microsomal nicotine C-oxidation at both 10 and 500 microM substrate concentrations. These results support the view that CYP2A6 has major roles for nicotine C-oxidation at lower substrate concentration and both CYP2A6 and 2B6 play roles at higher substrate concentrations in human liver microsomes.

Cloning and characterization of KAP3: a novel kinesin superfamily-associated protein of KIF3A/3B

We previously reported that KIF3A and KIF3B form a heterodimer that functions as a microtubule-based fast anterograde translocator of membranous organelles. We have also shown that this KIF3A/3B forms a complex with other associated polypeptides, named kinesin superfamily-associated protein 3 (KAP3). In the present study, we purified KAP3 protein by immunoprecipitation using anti-KIF3B antibody from mouse testis. Microsequencing was carried out, and we cloned the full-length KAP3 cDNA from a mouse brain cDNA library. Two isoforms of KAP3 exist [KAP3A (793 aa) and KAP3B (772 aa)], generated by alternative splicing in the carboxyl terminus region. Their amino acid sequences have no homology with those of any other known proteins, and prediction of their secondary structure indicated that almost the entire KAP3 molecule is alpha-helical. We produced recombinant KAP3 and KIF3A/3B using a baculovirus-Sf9 expression system. A reconstruction study in Sf9 cells revealed that KAP3 is a globular protein that binds to the tail domain of KIF3A/3B. The immunolocalization pattern of KAP3 was similar to that of KIF3A/3B in nerve cells. In addition, we found that KAP3 does not affect the motor activity of KIF3A/3B. KAP3 was associated with a membrane-bound form of KIF3A/3B in a fractional immunoprecipitation experiment, and since the KIF3 complex was found to bind to membranous organelles in an EM study, KAP3 may regulate membrane binding of the KIF3 complex.

Oxidation of aflatoxin B1 by bacterial recombinant human cytochrome P450 enzymes

Human cytochromes P450 (P450) 1A2 and P450 3A4 were expressed in Escherichia coli, purified, and used in reconstituted oxidation systems. The optimal system for P450 3A4 included a mixture of phospholipids, sodium cholate, cytochrome b5, GSH, and MgCl2. Relatively high catalytic activities were obtained with such a system for aflatoxin (AF) B1 3 alpha-hydroxylation and 8,9-epoxidation. P450 3A4 was more active than P450 1A2 in forming genotoxic AFB1 oxidation products. Analysis of the AFB1 products indicated that P450 3A4 formed AFQ1 and the exo-8,9-epoxide; P450 1A2 formed AFM1, a small amount of AFQ1, and both the exo- and endo-8,9-epoxides. The endo epoxide is essentially nongenotoxic in the umu test, as found previously in bacterial mutagenicity assays [Iyer, R. S., Coles, B. F., Raney, K. D., Thier, R., Guengerich, F. P., and Harris, T. M. (1994) J. Am. Chem. Soc. 116, 1603-1609]. 7,8-Benzoflavone (alpha-naphthoflavone, alpha NF) stimulated AFB1 (exo) 8,9-epoxidation and inhibited 3 alpha-hydroxylation in human liver microsomes and a reconstituted P450 3A4 system but was a potent inhibitor of all reactions catalyzed by P450 1A2. Plots of AFB1 3 alpha-hydroxylation and 8,9-epoxidation vs AFB1 concentration were sigmoidal in both human liver microsomes and the reconstituted P450 3A4 system. The results are consistent with the view that P450 3A4 is a major human liver P450 enzyme involved in AFB1 activation, although the in vivo situation may be more complex due to the presence of the enzyme in the gastrointestinal tract.

Selectivity of polycyclic inhibitors for human cytochrome P450s 1A1, 1A2, and 1B1

Human cytochrome P450s 1A1, 1A2, and 1B1 are known to have overlapping substrate specificities. All are regulated in part by the Ah locus; P450 1A2 is expressed essentially only in liver, but P450s 1A1 and 1B1 are both expressed in many extrahepatic tissues. Twenty-five polycyclic hydrocarbons, many containing acetylenic side chains, were examined as inhibitors of the three enzymes using 7-ethoxyresorufin O-deethylation as the enzyme assay in all cases. Several compounds were inhibitory at low nanomolar concentrations. 1-(1-Propynyl)pyrene and 2-(1-propynyl)phenanthrene nearly completely inhibited P450 1A1 at concentrations at which no P450 1B1 inhibition was observed. 2-Ethynylpyrene and alpha-naphthoflavone (7, 8-benzoflavone) nearly completely inhibited P450 1B1 at concentrations at which no P450 1A1 inhibition was noted. All four of the above compounds also inhibited P450 1A2. Several polycyclic hydrocarbons devoid of acetylenic groups were also inhibitory with respect to all three P450s. Some of the acetylenic compounds examined showed enhanced inhibition following preincubation with the P450s in the presence of cofactors NADPH and O2. However, of seven compounds (five acetylenes) tested with P450 1B1, only two [2-ethynylpyrene and 4-(1-propynyl)biphenyl] showed such evidence for mechanism-based inactivation. We conclude that (i) several polycyclic hydrocarbons and their oxidation products are very inhibitory with respect to human P450s 1A1, 1A2, and 1B1; (ii) of these inhibitors only some are mechanism-based inactivators; and (iii) some of the inhibitors are potentially useful for distinguishing between human P450s 1A1 and 1B1.

Elements of neural adhesion molecules and a yeast vacuolar protein sorting receptor are present in a novel mammalian low density lipoprotein receptor family member

Normal cell development depends to a large part on multifunctional proteins that have evolved by recombination of proven modular elements. We now have discovered and characterized in rabbit such a multi-domain protein, and classify it as novel member of the low density lipoprotein (LDL) receptor gene family. The extracellular portion of the approximately 250-kDa membrane protein, termed LR11, contains a cluster of 11 LDL receptor ligand binding repeats, a group of 5 LDL receptor "YWTD" repeats, a large hexarepeat domain of structural elements found in neural cell adhesion molecules, and a domain with similarity to a yeast receptor for vacuolar protein sorting, VPS10. The cytoplasmic domain exhibits features typical of endocytosis-competent coated-pit receptors. The mosaic, and presumably multifunctional, receptor is expressed abundantly in brain, in particular the hippocampus, dentate gyrus, and cerebral cortex, and is present at significant levels in liver, adrenal glands, and testis. Western blotting of tissues and ligand blotting of LR11-transfected cells demonstrated that the novel protein binds apolipoprotein E-containing lipoproteins. In contrast to the LDL receptor, hepatic expression of LR11 is unaffected by hyperlipidemia. The identification of this highly conserved and superbly complex protein offers the opportunity to gain new insights into the emergence of multifunctional mosaic proteins akin to the ever expanding LDL receptor gene family.

Kinesin superfamily protein 3 (KIF3) motor transports fodrin-associating vesicles important for neurite building

Kinesin superfamily proteins (KIFs) comprise several dozen molecular motor proteins. The KIF3 heterotrimer complex is one of the most abundantly and ubiquitously expressed KIFs in mammalian cells. To unveil the functions of KIF3, microinjection of function-blocking monovalent antibodies against KIF3 into cultured superior cervical ganglion (SCG) neurons was carried out. They significantly blocked fast axonal transport and brought about inhibition of neurite extension. A yeast two-hybrid binding assay revealed the association of fodrin with the KIF3 motor through KAP3. This was further confirmed by using vesicles collected from large bundles of axons (cauda equina), from which membranous vesicles could be prepared in pure preparations. Both immunoprecipitation and immunoelectron microscopy indicated the colocalization of fodrin and KIF3 on the same vesicles, the results reinforcing the evidence that the cargo of the KIF3 motor consists of fodrin-associating vesicles. In addition, pulse-labeling study implied partial comigration of both molecules as fast flow components. Taken together, the KIF3 motor is engaged in fast axonal transport that conveys membranous components important for neurite extension.

Inhibitory effects of amiodarone and its N-deethylated metabolite on human cytochrome P450 activities: prediction of in vivo drug interactions

AIMS

To predict the drug interactions of amiodarone and other drugs, the inhibitory effects and inactivation potential for human cytochrome P450 (CYP) enzymes by amiodarone and its N-dealkylated metabolite, desethylamiodarone were examined.

METHODS

The inhibition or inactivation potency of amiodarone and desethylamiodarone for human CYP activities were investigated using microsomes from B-lymphoblastoid cell lines expressing CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. The in vivo drug interactions of amiodarone and desethylamiodarone were predicted in vitro using the 1+Iu/Ki values.

RESULTS

Amiodarone weakly inhibited CYP2C9, CYP2D6, and CYP3A4-mediated activities with Ki values of 45.1-271.6 microm. Desethylamiodarone competitively inhibited the catalytic activities of CYP2D6 (Ki=4.5 microm ) and noncompetitively inhibited CYP2A6 (Ki=13.5 microm ), CYP2B6 (Ki=5.4 microm ), and CYP3A4 (Ki=12.1 microm ). The catalytic activities of CYP1A1 (Ki=1.5 microm, alpha=5.7), CYP1A2 (Ki=18.8 microm, alpha=2.6), CYP2C9 (Ki=2.3 microm, alpha=5.9), and CYP2C19 (Ki=15.7 microm, alpha=4.5) were inhibited by desethylamiodarone with mixed type. The 1+Iu/Ki values of desethylamiodarone were higher than those of amiodarone. Amiodarone inactivated CYP3A4, while desethylamiodarone inactivated CYP1A1, CYP1A2, CYP2B6, and CYP2D6.

CONCLUSIONS

The interactions between amiodarone and other drugs might occur via the inhibition of CYP activities by its N-dealkylated metabolite, desethylamiodarone, rather than by amiodarone itself. In addition, the inactivation of CYPs by desethylamiodarone as well as by amiodarone would also contribute to the drug interactions.

Roles of cytochromes P450 1A2 and 3A4 in the oxidation of estradiol and estrone in human liver microsomes

Of seven cDNA-expressed human cytochrome P450 (P450) enzymes (P450s 1A2, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4) examined, P450 1A2 was the most active in catalyzing 2- and 4-hydroxylations of estradiol and estrone. P450 3A4 and P450 2C9 also catalyzed these reactions although to lesser extents than P450 1A2. P450 1A2 also efficiently oxidized estradiol at the 16alpha-position but was less active in estrone 16alpha-hydroxylation; the latter reaction and also estradiol 16alpha-hydroxylation were catalyzed by P450 3A4 at significant levels. Anti-P450 1A2 antibodies inhibited 2- and 4-hydroxylations of these two estrogens catalyzed by liver microsomes of some of the human samples examined. Estradiol 16alpha-hydroxylation was inhibited by both anti-P450 1A2 and anti-P450 3A4, while estrone 16alpha-hydroxylation was significantly suppressed by anti-P450 3A4 in human liver microsomes. Fluvoxamine efficiently inhibited the estrogen hydroxylations in human liver samples that contained high levels of P450 1A2, while ketoconazole affected these activities in human samples in which P450 3A4 levels were high. alpha-Naphthoflavone either stimulated or had no effect on estradiol hydroxylation catalyzed by liver microsomes; the intensity of this effect depended on the human samples and their P450s. Interestingly, in the presence of anti-P450 3A4 antibodies, alpha-naphthoflavone was found to be able to inhibit estradiol and estrone 2-hydroxylations catalyzed by human liver microsomes. The results suggest that both P450s 1A2 and 3A4 have major roles in oxidations of estradiol and estrone in human liver and that the contents of these two P450 forms in liver microsomes determine which P450 enzymes are most important in hepatic estrogen hydroxylation by individual humans. P450 3A4 may be expected to play a more important role for some of the estrogen hydroxylation reactions than P450 1A2. Knowledge of roles of individual P450s in these estrogen hydroxylations has relevance to current controversies in hormonal carcinogenesis [Service, R. F. (1998) Science 279, 1631-1633].

Requirement of Sox2-mediated signaling for differentiation of early Xenopus neuroectoderm

From early stages of development, Sox2-class transcription factors (Sox1, Sox2 and Sox3) are expressed in neural tissues and sensory epithelia. In this report, we show that Sox2 function is required for neural differentiation of early Xenopus ectoderm. Microinjection of dominant-negative forms of Sox2 (dnSox2) mRNA inhibits neural differentiation of animal caps caused by attenuation of BMP signals. Expression of dnSox2 in developing embryos suppresses expression of N-CAM and regional neural markers. We have analyzed temporal requirement of Sox2-mediated signaling by using an inducible dnSox2 construct fused to the ligand-binding domain of the glucocorticoid receptor. Attenuation of Sox2 function both from the late blastula stage and from the late gastrula stage onwards causes an inhibition of neural differentiation in animal caps and in whole embryos. Additionally, dnSox2-injected cells that fail to differentiate into neural tissues are not able to adopt epidermal cell fate. These data suggest that Sox2-class genes are essential for early neuroectoderm cells to consolidate their neural identity during secondary steps of neural differentiation.

Skin antigens in the steady state are trafficked to regional lymph nodes by transforming growth factor-beta1-dependent cells

Antigen capturing in the skin and antigen trafficking into regional lymph nodes (LN) initiate immune responses. In this study, employing melanin granule (MG) as an easily traceable antigen in two mouse strains that carried steel factor or hepatocyte growth factor transgenes and had melanocytosis in the epidermis or in the dermis respectively, we investigated the mechanism of antigen trafficking from the skin. MG captured in the epidermis or dermis accumulated in the regional LN, but not other tissues. Only in alymphoplastic mice did MG-laden cells pass through the lymphatics and reached many tissues. Since inflammatory regions were not observed in the skin of either type of transgenic mouse, our developmental system enables us to investigate constitutive capturing and trafficking of insoluble antigens in the steady state. Both dendritic cells and macrophages were laden with MG in the regional LN. To determine which cells traffic antigens to the LN, we prepared double mutants that carried the transgenes and lacked transforming growth factor (TGF)-beta1, since mice lacking TGF-beta1 are reported to be deficient of Langerhans cells. Few MG were observed in the regional LN of these double-mutant mice. We also showed that signaling via macrophage colony stimulating factor receptor or Flt3/Flk2 is not essential for development of the cells for this antigen trafficking. These results indicate that antigens in the epidermis and dermis in the steady state are trafficked into regional LN only by TGF-beta1-dependent cells, which may be a dendritic cell lineage.

Oligodendroglial microtubular tangles in olivopontocerebellar atrophy

Oligodendroglial microtubular tangles (OMT), a distinctive oligodendroglial change, was observed in seven of eight cases of olivopontocerebellar atrophy (OPCA). This change was a well-defined glassy cytoplasmic structure showing intense argyrophilia. OMT were distributed in the brain stem, cerebellum, and basal ganglia where severe neurodegenerative changes were consistently observed. In 45 control cases, no OMT were found regardless of the presence or absence of neurological disorders. The OMT were immunostained by anti-tubulin antibodies, but no other antibodies reacted with them. Each OMT consisted of a meshwork of randomly oriented fibrils studded with granular and fuzzy material. The fibrillary elements were between 20 and 30 nm in diameter. It is suggested that OMT are primarily composed of altered microtubules, and are related to the neurodegenerative process of OPCA.

Lack of electron transfer from cytochrome b5 in stimulation of catalytic activities of cytochrome P450 3A4. Characterization of a reconstituted cytochrome P450 3A4/NADPH-cytochrome P450 reductase system and studies with apo-cytochrome b5

Many catalytic activities of cytochrome P450 (P450) 3A4, the major human liver P450 enzyme, require cytochrome b5 (b5) for optimal rates. The stimulatory effect of b5 on P450 reactions has generally been thought to be due to transfer of electrons from ferrous b5 to the ferrous P450-O2-substrate complex. We found that apo-b5, devoid of heme, could substitute for b5 in stimulating two prototypic activities, testosterone 6beta hydroxylation and nifedipine oxidation. The stimulatory effect was not seen with albumin, hemoglobin, catalase, or cytochrome c. Apo-b5 could not substitute for b5 in a testosterone 6beta hydroxylation system composed of NADH-b5 reductase and P450 3A4. Rates of electron transfer from NADPH-P450 reductase to ferric P450 3A4 were too slow (<2 min-1) to support testosterone 6beta hydroxylation ( approximately 14 min-1) unless b5 or apo-b5 was present, when rates of approximately 700 min-1 were measured. The oxidation-reduction potential (Em,7) of the ferric/ferrous couple of P450 3A4 was unchanged ( approximately -310 mV) under different conditions in which the kinetics of reduction were altered by the addition of substrate and/or apo-b5. Rapid reduction of P450 3A4 required substrate and a preformed complex of P450 3A4, NADPH-P450 reductase, and b5; the rates of binding of the proteins to each other were 2-3 orders of magnitude less than reduction rates. We conclude that b5 can facilitate some P450 3A4-catalyzed oxidations by complexing with P450 3A4 and enhancing its reduction by NADPH-P450 reductase, without directly transferring electrons to P450.

Cytochrome P450-dependent drug oxidation activities in liver microsomes of various animal species including rats, guinea pigs, dogs, monkeys, and humans

Levels of cytochrome P450 (P450 or CYP) proteins immunoreactive to antibodies raised against human CYP1A2, 2A6, 2C9, 2E1, and 3A4, monkey CYP2B17, and rat CYP2D1 were determined in liver microsomes of rats, guinea pigs, dogs, monkeys, and humans. We also examined several drug oxidation activities catalyzed by liver microsomes of these animal species using eleven P450 substrates such as phenacetin, coumarin, pentoxyresorufin, phenytoin, S-mephenytoin, bufuralol, aniline, benzphetamine, ethylmorphine, erythromycin, and nifedipine; the activities were compared with the levels of individual P450 enzymes. Monkey liver P450 proteins were found to have relatively similar immunochemical properties by immunoblotting analysis to the human enzymes, which belong to the same P450 gene families. Mean catalytic activities (on basis of mg microsomal protein) of P450-dependent drug oxidations with eleven substrates were higher in liver microsomes of monkeys than of humans, except that humans showed much higher activities for aniline p-hydroxylation than those catalyzed by monkeys. However, when the catalytic activities of liver microsomes of monkeys and humans were compared on the basis of nmol of P450, both species gave relatively similar rates towards the oxidation of phenacetin, coumarin, pentoxyresorufin, phenytoin, mephenytoin, benzphetamine, ethylmorphine, erythromycin, and nifedipine, while the aniline p-hydroxylation was higher and bufuralol 1'-hydroxylation was lower in humans than monkeys. On the other hand, the immunochemical properties of P450 proteins and the activities of P450-dependent drug oxidation reactions in dogs, guinea pigs, and rats were somewhat different from those of monkeys and humans; the differences in these animal species varied with the P450 enzymes examined and the substrates used. The results presented in this study provide useful information towards species-related differences in susceptibilities of various animal species regarding actions and toxicities of drugs and xenobiotic chemicals.

Megalin is an endocytic receptor for insulin

Renal clearance is a major pathway for regulating the levels of insulin and other low molecular weight polypeptide hormones in the systemic circulation. Previous studies have shown that the reabsorption of insulin from the glomerular filtrate occurs by binding to as yet unidentified sites on the luminal surface of proximal tubule cells followed by endocytosis and degradation in lysosomes. In this study, an insulin binding site was identified in renal microvillar membranes by chemical cross-linking procedures. By immunoprecipitation it was demonstrated that this binding site is megalin, the large multiligand binding endocytic receptor that is abundantly expressed in clathrin-coated pits on the apical surface of proximal tubule cells. Moreover, using cytochemical procedures, it was also shown that megalin is able to internalize insulin into endocytic vesicles. In ligand blotting assays, megalin also bound several other low molecular weight polypeptides, including beta2-microglobulin, epidermal growth factor, prolactin, lysozyme, and cytochrome c. These data suggest that megalin may play a significant role as a renal reabsorption receptor for the uptake of insulin and other low molecular weight polypeptides from the glomerular filtrate.

Comparative studies on the catalytic roles of cytochrome P450 2C9 and its Cys- and Leu-variants in the oxidation of warfarin, flurbiprofen, and diclofenac by human liver microsomes

S-Warfarin 7-hydroxylation, S-flurbiprofen 4'-hydroxylation, and diclofenac 4'-hydroxylation activities were determined in liver microsomes of 30 humans of which 19 were wild-type (Arg144.Ile359), 8 were heterozygous Cys (Cys144.Ile359), and 3 were heterozygous Leu (Arg144.Leu359) allelic variants of the cytochrome P450 2C9 (CYP2C9) gene. All of the human samples examined contained P450 protein(s) immunoreactive with anti-CYP2C9 antibodies in liver microsomes. Individuals with the Cys144 allele of CYP2C9 had similar, but slightly lower, activities for the oxidations of these substrates than those of wild-type CYP2C9. One of the three human samples heterozygous for the Leu359 allele had very low Vmax and high Km values for the oxidation of three substrates examined, while the other two individuals gave kinetic parameters comparable to those seen in the wild-type and Cys144 CYP2C9. Reverse transcriptase-polymerase chain reaction analysis, however, showed that all of the three human samples with the heterozygous Leu359 variant were found to express both Ile359 and Leu359 variants at relatively similar extents in liver RNA of three humans. These results suggest that the Cys144 variant of CYP2C9 catalyzes the CYP2C9 substrates at rates comparative to, but slightly lower than, those of wild-type CYP2C9, while the Leu359-allelic variant has slower rates for the oxidation of these drug substrates. Activities for the oxidation of these CYP2C9 substrates in humans with heterozygous Leu359 allele is likely to be dependent on the levels of expression of each of the wild- and Leu-variants in the livers. However, one of the humans with a heterozygous Leu allele was found to have very low activities towards the oxidation of CYP2C9 substrates. The basis of this defect in catalytic functions towards CYP2C9 substrates is unknown.