Production and release of proopiomelanocortin (POMC) derived peptides by human melanocytes and keratinocytes in culture: regulation by ultraviolet B

It is demonstrated that ultraviolet B (UVB) radiation stimulates increased expression of the proopiomelanocortin (POMC) gene which is accompanied by production and release of alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropin (ACTH) by both normal and malignant human melanocytes and keratinocytes. The production and release of both peptides are also stimulated by dibutyryl cyclic adenosine monophosphate (dbcAMP) and interleukin 1 alpha (IL-1 alpha) but not by endothelin-1 (ET-1) or tumor necrosis factor-alpha (TNF-alpha). N-acetyl-cysteine (NAC), a precursor of glutathione (GSH), an intracellular free radical scavenger, abolishes the UVB-stimulated POMC peptide production and secretion. Conclusions are as follows: (1) Cultured human cells of cutaneous origin, namely keratinocytes and melanocytes, can produce and express POMC; (2) POMC expression is enhanced by exposure to UVB, possibly through a cyclic AMP-dependent pathway; and (3) The action of UVB on POMC production may involve a cellular response to oxidative stress.

Protein phosphatase inhibitors potentiate Ca2+/calmodulin-dependent protein kinase II activity in rat pancreatic acinar cells

Cholecystokinin (CCK) is known to rapidly and transiently increase both [Ca2+]i and autonomous CaM kinase II activity in rat pancreatic acini. Because induction of autonomous activity may involve intramolecular autophosphorylation, the effects of protein phosphatase inhibitors were examined. None of the inhibitors tested (okadaic acid, calyculin A, and cyclosporin A) affected basal activity. Okadaic acid, a potent inhibitor of PP2A and weaker inhibitor of PP1, increased the peak autonomous activity by 30% over the level normally induced by CCK alone, while calyculin A, a potent inhibitor of both PP1 and PP2A, showed an even greater increase of 97%. Both inhibitors also delayed the decline of autonomous activity and calyculin A had a more potent effect than okadaic acid. Cyclosporin A, an inhibitor of PP2B, had no effect. The data indicate that PP1 may be involved in the dephosphorylation of CaMK II and decline of autonomous activity.

Contribution of an in vivo oxidized LDL to LDL oxidation and its association with dense LDL subpopulations

Oxidative modification of LDL is thought to be a radical-mediated process involving lipid peroxides. The small dense LDL subpopulations are particularly susceptible to oxidation, and individuals with high proportions of dense LDL are at a greater risk for atherosclerosis. An oxidatively modified plasma LDL, referred to as LDL-, is found largely among the dense LDL fractions. LDL- and dense LDL particles also contain much greater amounts of lipid peroxides compared with total LDL or the more buoyant LDL fractions. The content of LDL- in dense LDL particles appears to be related to copper- or heme-induced oxidative susceptibility, which may be attributable to peroxide levels. The rate of lipid peroxidation during the antioxidant-protected phase (lag period) and the length of the antioxidant-protected phase (lag time) are correlated with the LDL- content of total LDL. Once LDL oxidation enters the propagation phase, there is no relationship to the initial LDL- content or total LDL lipid peroxide or vitamin E levels. Beyond a threshold LDL- content of approximately 2%, there is a significant increase in the oxidative susceptibility of nLDL particles (ie, purified LDL that is free of LDL-), and this susceptibility becomes more pronounced as the LDL- content increases. nLDL is resistant to copper- or heme-induced oxidation. The oxidative susceptibility is not influenced by vitamin E content in LDL but is strongly inhibited by ascorbic acid in the medium. Involvement of LDL(-)-associated peroxides during the stimulated oxidation of LDL is suggested by the inhibition of nLDL oxidation when LDL- is treated with ebselen prior to its addition to nLDL. Populations of LDL enriched with LDL- appear to contain peroxides at levels approaching the threshold required for progressive radical propagation reactions. We postulate that elevated LDL- may constitute a pro-oxidant state that facilitates oxidative reactions in vascular components.

Characterization of human DNA topoisomerase II as an autoantigen recognized by patients with IDDM

Autoantibodies against several cytoplasmic autoantigens such as glutamic acid decarboxylase, heat shock protein 65, insulin, and carboxypeptidase H have been identified in the sera of patients with IDDM. To investigate whether type II DNA topoisomerase (TopII) is an autoantigen in IDDM patients, we have constructed a series of overlapping DNA TopII fragments that covered the entire length of this enzyme. These fragments were used as antigens to screen sera of IDDM patients. We have examined 195 Chinese IDDM patients (mean age 14.2 +/- 7.5 years, age at onset 9.2 +/- 6.4 years, duration of diabetes 4.6 +/- 3.4 years) and 51 nondiabetic individuals. The results showed that DNA TopII autoantibodies were detected in 49.2 and 47.2% of IDDM patients using purified TopII fragments and full-length TopII as antigens, respectively. The frequency of anti-TopII positivity was relatively stable irrespective of sex and disease duration. The patients were slightly older at onset and the prevalence of anti-thyroglobulin/anti-microsomal autoantibodies was twice that in the IDDM subgroup positive for anti-TopII than in IDDM patients who were negative for anti-TopII. We also characterized the epitopes of DNA TopII that were recognized by IDDM sera. Those epitopes resided mostly in three distinct domains. One resided in amino acid residues 1-147, another in amino acid residues 286-472, and the third in the COOH-terminal one-third of DNA TopII. Intriguingly, we found that these epitopes shared similarity (up to 36% identity and 63.6% homology) to previously identified epitopes of IDDM autoantigens.

The expression of proopiomelanocortin (POMC) and of corticotropin releasing hormone receptor (CRH-R) genes in mouse skin

The proopiomelanocortin (POMC) and the corticotropin releasing hormone receptor (CRH-R) genes are expressed in the skin of the C57BL/6 mouse throughout the entire hair cycle, and CRH immunoreactivity is present in the pilosebaceous unit of the hair follicle and the epidermis. These findings suggest that some components of hypothalamo-pituitary axis are operating in mouse skin.

Regulation of topoisomerase II expression during the VM-26 induced differentiation of IW32 murine erythroleukemia cells

The effect of VM-26, a topoisomerase II targeting drug, on IW32 murine erythroleukemia cells was investigated. The VM-26 induced IW32 cells to differentiate at a non-toxic but cytostatic concentration (0.01 microgram/ml). More than 40% of the cells were induced to synthesize hemoglobin, and cells were arrested in G2/M phase of the cell cycle. Levels of beta-globin mRNA also increased significantly. Cells became committed to erythroid maturation after 16 h of continuous drug exposure. Replacement with fresh VM-26 after 48 h of drug treatment further increased the hemoglobin containing cells to greater than 80%. Unlike other drug induced erythroleukemia cell differentiation, c-myc mRNA expression was not affected by VM-26. Inhibition of topoisomerase II activity was observed during the first 12 h of VM-26 treatment; however, elevated enzyme activity was found thereafter. Northern blot analysis showed significant increase in the expression of topoisomerase IIalpha mRNA at 12 and 24 h after VM-26 addition. These findings indicate that VM-26 inhibited the activity of topoisomerase II and promoted the committed differentiation of IW32 cells along the erythroid pathway. In addition, a parallel increase in mRNA and activity levels of topoisomerase II in differentiated cells suggests that regulation of the enzyme expression occurred in the VM-26 induced erythroid maturation.

Increased bcl-2 protein levels in prostatic adenocarcinomas are not associated with rearrangements in the 2.8 kb major breakpoint region or with p53 protein accumulation

bcl-2, an inhibitor of programmed cell death (apoptosis), is present in high levels in a subset of prostatic adenocarcinomas. In this study, 42 prostatic adenocarcinomas were analyzed to determine whether increased bcl-2 levels are associated with rearrangements in the 2.8-kb major breakpoint region, an association known to occur in certain follicular lymphomas featuring a t(14:18) translocation. Immunostaining for bcl-2 and p53 proteins was performed on formalin-fixed, paraffin-embedded tumor specimens using murine anti-human bcl-2 and p53 monoclonal antibodies in all 42 cases. Genomic DNA from paired frozen samples of each tumor was subjected to digestion with HindIII and EcoRI and the products analyzed on a Southern blot with a 2.8-kb-digoxigenin-labeled major breakpoint region probe. Comparisons between groups were evaluated with the Fisher exact test. Diffuse, strong cytoplasmic immunoreactivity for bcl-2 was present in the epithelial cells of tumor glands in 16 of 42 cases (38%), including 8 of 19 low grade (Gleason score 6 and below) and 8 of 23 high grade (Gleason score 7 and above) prostatic adenocarcinoma. Southern blotting demonstrated a normal 2.8-kb germline DNA fragment in every case, with no evidence of rearrangement. Nuclear p53 staining was present in 10 of 24 high grade and 0 of 18 low grade tumors (P < 0.001). Only four cases exhibited positivity for both bcl-2 and p53, and there was no association of bcl-2 positivity with co-expression of the p53 protein (P = 0.58). We conclude that aberrations in the function of either bcl-2 or p53 could possibly modify the apoptotic pathway resulting in the extended survival of tumor cells. Also, increased bcl-2 levels in prostatic adenocarcinomas occur in the absence of detectable rearrangements in the major breakpoint region.

Ultraviolet B and melanocyte-stimulating hormone (MSH) stimulate mRNA production for alpha MSH receptors and proopiomelanocortin-derived peptides in mouse melanoma cells and transformed keratinocytes

Cell lines of cutaneous origin, namely melanocytes and keratinocytes, were previously demonstrated to exhibit functional melanocyte-stimulating hormone (MSH) receptors that are up-regulated by ultraviolet (UV) radiation and by MSH itself. In this study, it is demonstrated that UVB irradiation, exposure to MSH, or exposure to N6,O2-dibutyryl cyclic adenosine monophosphate stimulates production of mRNAs for both alpha MSH receptors and proopiomelanocortin in cultured mouse Cloudman S91 melanoma cells, and that UVB stimulates production and release of MSH and adrenocorticotropin peptides in both melanoma cells and transformed PAM 212 mouse keratinocytes. The results add support to the hypothesis that the effects of UVB on cutaneous melanogenesis are mediated through a series of coordinated events in which MSH receptors and proopiomelanocortin-derived peptides play a central role.

Nanoscale complexity of phospholipid monolayers investigated by near-field scanning optical microscopy

Near-field scanning optical microscopy of phospholipid monolayers doped with fluorescent lipid analogs reveals previously undescribed features in various phases, including a concentration gradient at the liquid-expanded/liquid-condensed domain boundary and weblike structures in the solid-condensed phase. Presumably, the web structures are grain boundaries between crystalline solid lipid. These structures are strongly modulated by the addition of low concentrations of cholesterol and ganglioside GM1 in the monolayer.

Proopiomelanocortin, corticotropin releasing hormone and corticotropin releasing hormone receptor genes are expressed in human skin

Evidence is provided that human skin, the largest body organ exposed to multiple stressors, expresses proopiomelanocortin (POMC), corticotropin releasing hormone (CRH) and CRH-receptor (CRHR) genes in vivo. In vitro studies show that POMC and CRHR mRNAs are transcribed in melanocytes, cells derived from the neural crest, and in keratinocytes, cells derived from the ectoderm. CRH mRNA is transcribed in cultured melanocytes but not in keratinocytes. It is proposed that an equivalent of the 'hypothalamus-pituitary axis' composed of the CRH-CRHR-POMC loop is conserved in mammalian skin.

Characterization of endothelial cell injury by cholesterol oxidation products found in oxidized LDL

The present study describes the toxicity of oxidized LDL towards rabbit aortic endothelial cells in terms of its lipid components with specific attention to the cholesterol oxidation products (ChOx) found in oxidized LDL isolated from human plasma. Measurements of the major ChOx associated with freshly isolated unmodified LDL, those found in oxidized LDL isolated from human plasma and LDL subjected to oxidation in vitro are described. We have confirmed previous findings that most of the cytotoxicity of freshly isolated human LDL may be attributable to a minor fraction that appears to be oxidatively modified by several criteria. Moreover, this plasma-derived oxidized LDL (referred to as LDL) is highly enriched in ChOx, whereas the content of lipid peroxides or derived products (measured as conjugated dienes and thiobarbituric acid reacting products) are much lower, particularly when compared to copper-induced LDL oxidation. Much of the ChOx found in plasma are associated with LDL, however, the levels and proportions of the various ChOx found in LDL differ from those produced after extensive copper-induced oxidation but resemble those produced after moderate oxidation with copper. The species and concentrations of ChOx found in LDL when applied as a mixture exhibit considerably more toxicity than any individual ChOx alone. At non-toxic levels this ChOx mixture causes an increased influx of several ions, including calcium, an effect not seen with individual ChOx at comparable doses. Perturbations in ionic homeostasis, and particularly the sustained increase in intracellular calcium concentrations, are associated with much of the cytotoxicity, an effect attributable to the membrane disruptive action of ChOx leading to altered ion transporter activity. The effect of the ChOx mixture (but not any individual ChOx) on sodium and potassium flux appears to be due to enhanced Na+/K(+)-ATPase activity based on the complete inhibition produced by ouabain under all treatment conditions. These findings also show that the levels of cholesterol oxidation products found in normal LDL are not cytotoxic whereas those present in oxidized LDL exceed the toxic threshold for endothelial cells and account for most of the cytotoxicity produced by this modified lipoprotein.

A target-specific chimeric toxin composed of epidermal growth factor and Pseudomonas exotoxin A with a deletion in its toxin-binding domain

We have fused the epidermal growth factor (EGF) to the amino terminus of Pseudomonas exotoxin A (PE) to create a cytotoxic agent, designated EGF-PE, which preferentially kills EGF-receptor-bearing cells. In this study, we analyzed the effect of the Ia domain, the binding domain of PE on the cytotoxicity of EGF-PE towards EGF-receptor-bearing cells and tried to develop a more potent EGF-receptor-targeting toxin. EGF-PE molecules with sequential deletions at the amino terminus of PE were constructed and expressed in E. coli strain BL21(DE3). The cytotoxicity of these chimeric toxins was then examined. Our results show that the amino-terminal and carboxy-terminal regions of the Ia domain of PE are important for the cytotoxicity of a PE-based targeting toxin. To design a more potent PE-based EGF-receptor-targeting toxin, a chimeric toxin, named EGF-PE(delta 34-220), which had most of the Ia domain deleted but retained amino acid residues 1-33 and 221-252 of this domain, was constructed. EGF-PE(delta 34-220) has EGF-receptor-binding activity but does not show PE-receptor-binding activity and is mildly cytotoxic to EGF-receptor-deficient NR6 cells. As expected, EGF-PE(delta 34-220) is a more potent cytotoxic agent towards EGF-receptor-bearing cells than EGF-PE(delta 1-252), where the entire Ia domain of PE was deleted. In addition, EGF-PE(delta 34-220) was shown to be extremely cytotoxic to EGF-receptor-bearing cancer cells, such as A431, CE81T/VGH, and KB-3-1 cells. We also found that EGF-PE(delta 34-220) was highly expressed in BL21(DE3) and could be easily purified by urea extraction. Thus, EGF-PE(delta 34-220) can be a useful cytotoxic agent towards EGF-receptor-bearing cells.

p53 gene status in endometrial carcinomas showing diffuse positivity for p53 protein by immunohistochemical analysis

Although detection of p53 protein by immunohistochemical testing was originally thought to indicate p53 gene mutation, recent analyses of human malignancies have shown that high expression of p53 protein may occur without detectable gene mutation. Several explanations have been proposed for this phenomenon, including mutation out of "hot spot" regions, overexpression of wild-type protein, sampling error in molecular analyses, and conformational changes of wild-type p53 protein. As discussed, it is unlikely that the first two possibilities contribute significantly to the occurrence of this phenomenon, and the current study examined the possibility that sampling error in molecular analyses might account for a lack of concordance between immunohistochemical and molecular analyses. Such a possibility exists because immunohistochemical studies frequently report high expression when staining is only focal or regional and molecular analyses are based on the polymerase chain reaction, which is highly exponential in nature and may not detect mutation if the target gene segment is not amplified early in the chain reaction. In the current report, p53 protein expression was examined by immunohistochemical testing in 45 cases of endometrioid carcinoma, and all cases showing diffuse positivity were then examined by polymerase chain reaction in combination with single-strand conformational analysis for exons 4 to 9 with the use of a microdissection technique to separate malignant from benign cells. Of the 45 cases, diffuse staining was found in four cases, and only two of the four were found to show evidence of gene mutation.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of cyclin D1, DNA topoisomerase I, and proliferating cell nuclear antigen promoters during the cell cycle

Cyclin D1, DNA topoisomerase I, and proliferating cell nuclear antigen (PCNA) are three important cell cycle regulatory proteins. Recently, their promoters have been isolated, thus facilitating molecular analysis of transcriptional control mechanisms of these genes. Transcription of these three promoters in stable K562 transfectants during different cell cycle phases was analyzed after cell cycle synchronization. About 1 kb of 5' flanking region from either cyclin D1 or DNA topoisomerase I gene is sufficient to confer G1- or S-phase-specific transcription activity to chloramphenicol acetyltransferase (CAT) reporter genes, respectively. In contrast, 2.8 kb of 5' flanking sequences from the PCNA gene led to constitutive transcription, but the inclusion of a segment of the PCNA gene first intron, which contains evolutionarily conserved sequences, could enhance transcription in G1/S-enriched nuclei. This PCNA intron region contains a binding site recognized by the transcription factor E2F. To test whether this site is functional, we cotransfected PCNA-CAT genes with E2F-1 and DP-1 expression plasmids. Expression of the E2F-1/DP-1 heterodimer activated the CAT gene with the PCNA intron. Therefore, this intron region, involved in transcriptional activation at the cell cycle G1/S boundary, is also E2F inducible.

Report from the Maryland Epidemiology Schizophrenia Linkage Study: no evidence for linkage between schizophrenia and a number of candidate and other genomic regions using a complex dominant model

Our collaborative group has undertaken a linkage study of schizophrenia, using a systematic sample of patients admitted to Maryland hospitals. An initial sample of 39 families, each having two or more affecteds, was available for genotyping candidate genes, candidate regions, and highly polymorphic markers randomly distributed throughout the genome. We used a single complex dominant model (with a disease gene frequency of 0.005 and age-dependent penetrance for affected phenotype: for under 35, penetrance = .45; for 35 and older, penetrance = .85). We report here 130 markers, which met the exclusion criteria of LOD score < -2.00 at theta > 0.01 in at least 10 informative families, and no evidence for heterogeneity. We also report here markers that were tested as candidates for linkage to the schizophrenic phenotype. They were selected based on the following criteria: a) proximity to reported chromosomal rearrangements (both 5q and 11q), b) suggestions of linkage from other families (5q), or c) presence of a candidate gene (5q, 11q, 3q: Dopamine receptors 1, 2, and 3, respectively). We also tested for mutations of codon 717 in exon 17 of the amyloid precursor protein (APP) gene and were unable to detect the C to T substitution in our schizophrenic group.

Selective suppression of cytochrome P-450 gene expression by interleukins 1 and 6 in rat liver

Inflammatory stimuli suppress constitutive hepatic expression of the CYP2C11 and CYP2C12 genes in male and female rat livers, respectively. We have shown previously that injection of interleukin-1 (IL1), but not interleukin-6 (IL6), to female rats also suppresses CYP2C12. In the present study, we examined the effects of these cytokines on CYP2C12 expression in rat hepatocyte cultures, and their in vivo effects on expression of multiple cytochrome P-450 (P450) gene products in male rat livers. IL1 suppressed the expression of CYP2C12 mRNA and protein in hepatocytes cultured on Matrigel in the presence of growth hormone. No consistent effect of IL6 was observed. Maximal suppression of CYP2C12 mRNA after 24 h of IL1 treatment reached 12 and 32% of control levels in two separate experiments. The approximate ED50 for IL1 was 5 ng/ml. CYP2C12 protein was suppressed to 28% of control levels as early as 12 h after IL1 treatment. Injection of IL1, low doses of dexamethasone, or both, in male rats produced decreases in total P450, and in CYP3A2 and CYP2C11 mRNA and protein expression similar to effects previously seen for CYP2C12 expression in females. CYP2E1 mRNA and protein was significantly suppressed only by the combination of IL1 and dexamethasone. IL6 treatment of male rats down-regulated the CYP2C11 and CYP2E1 mRNAs at a dose of 4.5 micrograms/kg, which was lower than that required to induce haptoglobin mRNA, a prototype acute phase gene product. CYP2C11 protein content of the microsomes was also decreased by IL6 treatment, with a slower time-course than for suppression of its mRNA. No significant effects of IL6 treatment were seen on CYP3A2 mRNA or CYP3A2/1 proteins. These results demonstrate that IL1 and IL6 treatments in vivo differentially affect subsets of P450 gene products in rat liver.

Biochemical and cytotoxic characteristics of an in vivo circulating oxidized low density lipoprotein (LDL-)

Using ion exchange high pressure liquid chromatography, total plasma low density lipoprotein (LDL) from 30 hypercholesterolemic and 10 normocholesterolemic cynomolgus monkeys was subfractionated into unmodified LDL (n-LDL) and more negatively charged LDL (LDL-). In hypercholesterolemic monkeys, the absolute LDL-cholesterol level was 16.54 +/- 2.82 mg/dl (mean +/- SE) whereas in normocholesterolemic monkeys it was 2.39 +/- 0.12 mg/dl (P < 0.0001); the percentage of LDL- was 5.2 +/- 0.71% and 4.9 +/- 0.19% of the total LDL for hypercholesterolemic versus normocholesterolemic monkeys, respectively. LDL- averaged 5% and n-LDL 95% of the total plasma LDL cholesterol. To confirm and further elucidate the oxidative nature of LDL-, cholesterol and cholesterol oxide contents of LDL- and n-LDL were determined by capillary gas chromatography; 53.98 +/- 2.24% (mean +/- SE) of the LDL- cholesterol was oxidized whereas in n-LDL only 10.70 +/- 1.06% of the cholesterol was oxidized (P < 0.00001). The spectrum of oxysterols identified, which was similar for LDL- and n-LDL, suggested a free radical-mediated process for cholesterol oxidation. The principal oxysterols identified were: cholest-5-ene-3 beta, 7 alpha-diol, cholesta-3,5-diene-7-one, cholest-5-ene-3 beta, 7 beta-diol, 5,6 beta-epoxy-5 beta-cholestan-3 beta-ol, 5,6 alpha-epoxy-5 alpha-cholestan-3 beta-ol, 5 alpha-cholestan-3 beta,5,6 beta-triol, 3 beta-hydroxycholest-5-ene-7-one, and cholest-5-ene-3 beta,25-diol. To model one of the steps in the possible mechanism of atherogenesis, the cytotoxicity of LDL- was demonstrated to be greater against subconfluent than confluent aortic endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevalence and magnitude of perinatal substance exposures in California

BACKGROUND

Perinatal substance exposure has been linked to many neonatal and obstetrical complications. There have been few population-based epidemiologic studies to identify the prevalence and demographic profiles associated with drugs, alcohol, and smoking during pregnancy.

METHODS

We studied a fully probabilistic stratified-cluster sample to estimate the prevalence of perinatal substance exposures in California in 1992. Urine samples from 29,494 women presenting for delivery in 202 hospitals were coded and screened for toxins; the results of toxicology screening were later linked by code number to the subjects' demographic variables and their reported use of tobacco and prescribed drugs. Urinary toxicologic tests provide conservative estimates because they can detect only very recent substance use.

RESULTS

The weighted prevalence for perinatal substance exposure was 5.16 percent for the use of one or more drugs, 6.72 percent for alcohol (analyzed independently), and 8.82 percent for self-reported smoking. The percentage of women testing positive for any drug, including alcohol, was 11.35 percent. Estimates for racial and ethnic groups varied widely. Black women had the highest prevalence of total drug use (14.22 percent), alcohol use (11.58 percent), cocaine use (7.79 percent), and tobacco use (20.12 percent). Most drug exposures occurred among white non-Hispanic and Hispanic women. White non-Hispanic women had the second highest prevalence rate for the use of one or more drugs (6.79 percent) and self-reported tobacco use (14.82 percent). Hispanic women had the second highest prevalence rate for alcohol (6.87 percent).

CONCLUSIONS

In California in 1992, there were 67,361 estimated perinatal exposures to one or more drugs, including alcohol, and 52,346 self-reported exposures to tobacco. These findings have clinical and public health implications.

Increased synthesis and degradation of DNA topoisomerase I during the initial phase of human T lymphocyte proliferation

The regulation of DNA topoisomerase I was studied in human T lymphocytes following phytohemagglutinin (PHA) stimulation. As T lymphocytes began to enter the S phase 24 h after stimulation, there was a rapid increase in DNA topoisomerase I mRNA. The level of DNA topoisomerase I mRNA increased continuously over the next 18 h and peaked (> 50-fold increase) 42 h after stimulation with PHA. A concomitant increase in DNA topoisomerase I protein was also observed. However, the maximal increase in DNA topoisomerase I protein was only 6-fold. To explain the quantitative difference between the mRNA and protein levels, we investigated the change in the rates of DNA topoisomerase I protein synthesis versus degradation in human T lymphocytes following PHA stimulation. The increase in the mRNA parallels the increase in protein synthesis. However, the half-life of the enzyme protein was reduced to 9 h in proliferating T lymphocytes compared to a half-life of 36 h in resting lymphocytes. These results indicate that, in addition to the growth-regulated increase in the expression of DNA topoisomerase I, there was also a concomitant increase in the degradation of DNA topoisomerase I protein.

An EGF-pseudomonas exotoxin A recombinant protein with a deletion in toxin binding domain specifically kills EGF receptor bearing cells

We constructed two chimeric toxins; one composed of epidermal growth factor (EGF) and pseudomonas exotoxin A (PE), designated EGF-PE and the other composed of EGF and PE with a deletion of the Ia domain (cell-binding domain), designated EGF-PE (delta Ia). Both chimeric toxins reacted with anti-EGF and anti-PE antibodies. The cell-killing experiments showed that EGF-PE, but not EGF-PE (delta Ia), was cytotoxic to the murine fibroblast cell line NR6, which carried the PE receptor, but not the EGF receptor. However, after NR6 was transfected with DNA for the expression of human EGF receptor, the transfected cell line, designated NRHER5, over-expressed human EGF receptors and became sensitive to EGF-PE(delta IA). The cytotoxicity of EGF-PE(delta Ia), but not EGF-PE, to NRHER5 can be completely blocked by an excess amount of EGF. To completely reverse the cytotoxicity of EGF-PE on NRHER5, both the EGF receptor pathway and the PE receptor pathway need to be blocked. These results suggest that EGF-PE exhibits both EGF and PE binding activities, while EGF-PE(delta IA) possesses only EGF binding activity. Thus, EGF-PE(delta Ia) may be a better chimeric toxin than EGF-PE in terms of target specificity to EGF receptor bearing cells. We, therefore, examined the cytotoxicity of EGF-PE(delta Ia) to various human cancer cell lines. We find that human cancer cells containing more EGF receptors are more sensitive to EGF-PE(delta Ia).