Stimulators and Inhibitors of Hepatic Porphyrin Formation in Human Sera

Human sera were found to contain factors that stimulate and factors that inhibit porphyrin formation by cultured avian liver cells. The capacity of sera to stimulate or inhibit porphyrin formation varied in different hormonal states and in the porphyrias. Sera from 31 post partum women, eight of whom were not lactating, inhibited porphyrin formation to a mean level 30% below the level in control cultures and also inhibited drug and steroid stimulation of porphyrin formation. In contrast, mean porphyrin formation compared to control cultures was increased between 9 and 21% by sera from 52 normal subjects, 16 women on oral contraceptives, and 11 pregnant women. It was increased 193% by sera from nine subjects with acute intermittent porphyria and 172% by sera from 13 subjects with porphyria cutanea tarda. Heated sera or ethanol extracts of sera from all groups of subjects further increased the mean porphyrin stimulation by sera and, for the post partum subjects, eliminated the inhibitory effect. Ethanol extracts of sera from 28 oral contraceptive-treated women caused significantly greater mean stimulation of porphyrin formation than did extracts of sera from 30 normal women. While sera from 17 out of 22 porphyric subjects contained both stimulatory and inhibitory factors, 5 out of 22 had no evidence of an inhibitory component. There appeared to be heterogeneity in the occurrence of the factors among porphyrics.     

Regulation of cyclooxygenase by the heme-heme oxygenase system in microvessel endothelial cells.

Heme oxygenase (HO) is a microsomal enzyme that oxidatively cleaves heme to form biliverdin, with the release of iron and carbon monoxide (CO). HO not only controls the availability of heme for the synthesis of heme proteins but also is responsible for the generation of CO, which binds to the heme moiety of heme proteins thus affecting their enzymatic activity. Cyclooxygenase (COX) is a heme protein that catalyzes the conversion of arachidonic acid to prostaglandin H(2), the precursor of prostanoids that participate in the regulation of vascular function. The goal of the present study was to determine whether the heme-HO system regulates COX enzyme expression and activity in vascular endothelial cells. Endothelial cells stably transfected with the human HO-1 gene exhibited a severalfold increase in human HO-1 mRNA levels, which was accompanied by an increase in HO activity and a marked decrease in prostaglandin (PG) E(2) and 6-keto-PGF(1alpha) levels. Exposure of cells to CoCl(2), an inducer of HO-1 gene expression, resulted in increases in HO-1 protein levels and HO activity. The increase in HO activity was associated with a subsequent decrease in COX activity, which returned to normal levels following normalization of HO activity. The addition of heme resulted in an increase in COX activity with an increase in PGE(2) and 6-keto-PGF(1alpha) levels. The degree of HO-1 expression and, consequently, the level of cellular heme, were directly related to COX activity. These results demonstrate that the heme-HO system can function as a cellular regulator of the expression of vascular COX, thus influencing the generation of prostanoids, PGE(2) and PGI(2), known to play a role in vascular homeostasis.     

Hypothalamic mechanism for cobalt protoporphyrin-induced hypophagia and weight loss: inhibition of the feeding response to NPY.

The mechanism whereby neurally or peripherally administered cobalt-protoporphyrin (CoPP) leads to transient hypophagia and prolonged weight reduction in normal and genetically obese animals is unknown. Neuropeptide Y (NPY) is a known endogenous stimulator of feeding behavior and is elevated in the hypothalamus of food-deprived rats. Accordingly, we examined the interaction between CoPP and NPY in the central nervous system. Concentrations of NPY mRNA in the hypothalami of rats treated intracerebroventricularly with vehicle or CoPP responded to decreased food intake with comparable increases. However, intracerebroventricular infusions of NPY elicited increased intake of food in vehicle-treated rats but were without effect in CoPP-treated animals. The results suggest that CoPP acts, at least in part, by blocking the feeding response to NPY.     

Studies in porphyria. IV. Expression of the gene defect of acute intermittent porphyria in cultured human skin fibroblasts and amniotic cells: prenatal diagnosis of the porphyric trait

The gene lesion of the porphyrin-heme synthetic pathway in acute intermittent porphyria (AIP) is reflected in a deficient level of activity of the cytosol enzyme uroporphyrinogen I synthetase (URO-S). A marked URO-S deficiency has been demonstrated in the liver and in circulating erythrocytes of individuals with both active and latent AIP. This enzymic abnormality accounts for the excessive production and excretion into urine of the porphyrin precursors, lamda-aminolevulinic acid (ALA) and porphobilinogen (PBG) in AIP subjects. In this study, utilizing cell culture techniques, a marked URO-S deficiency has also been demonstrated in skin fibroblasts from AIP patients and in cells derived through aminocentesis from an approximately 17-wk old fetus. The prenatal diagnosis of the AIP trait in this fetus was confirmed postnatally by the demonstration in the child of a deficient level of erythrocyte URO-S activity which was comparable to those found in her AIP mother and affected sibling and which was approximately one-half the levels characterizing her normal father and aunt and a second unaffected sibling. The identification of the URO-S deficiency in cultured human fibroblasts from AIP patients was facilitated by a newly developed, sensitive assay for the enzyme activity. In this assay, the ability of such cells to convert ALA to protoporphyrin was quantitated; in the sequence of reactions involved in this transformation, URO-S is limiting so that the gene defect of AIP could be simply and precisely determined by appropriate spectrofluorometry of cell extracts. The technique described has distinct advantages over the direct enzymatic assay for URO-S activity in cultured human skin fibroblasts and permits clear differentiation of AIP carrier from normal individuals.     

Photophysical properties of Sn-porphyrins: potential clinical implications

The photophysical properties of Sn-protoporphyrin and two of its synthetic analogues, Sn-mesoporphyrin and Sn-diiododeuteroporphyrin, were examined. All three compounds are potent competitive inhibitors of heme oxygenase, the rate-limiting enzyme in the catabolism of heme to bilirubin, and can suppress completely or diminish significantly experimentally induced or naturally occurring forms of jaundice in animals or man. The results of these studies show that all three compounds have long-lived triplet states which are quenched by molecular oxygen both in solution and when incorporated in liposomes. However, the addition of quenching groups such as iodine to the porphyrin macrocycle results in a marked (approximately 60%) decrease in the triplet yield and a threefold decrease in the triplet lifetime. The triplet yield was shown to be independent of the excitation wavelength, and as a result, the metalloporphyrins were extremely poor photosensitizers when excited in the spectral region commonly used in phototherapy. In the presence of serum albumin, the triplet state of Sn-protoporphyrin was not quenched by oxygen. These results indicate that Sn-porphyrins can be custom designed with considerably reduced photosensitizing properties for potential clinical use as inhibitors of bilirubin production.     

Chlorcyclizine N-demethylase activity in chick embryo hepatic cell culture

Hepatic cells from chick embryo levers were grown in primary monolayer cell cultures. Homogenates of the cells were shown to contain cytochrome P-450 and chlorcyclizine N-demethylase in vitro. The addition of allylisopropylacetamide and phenobarbital increased the chlorcyclizine N-demethylase activity and cytochrome P-450 content, and this stimulation was prevented by the addition of inhibitors of protein and RNA synthesis.     

Selecting a specific pre- or postoperative adjuvant therapy for individual patients with operable gastric cancer

Although the very high locoregional recurrence rates reported with limited D0/D1 surgery can be reduced with extended D2 gastrectomy for operable gastric cancer, overall relapse and survival rates remain poor and can only be improved with adequate perioperative adjuvant treatment. However, despite intensive research, no regimen has been established as standard. Meta-analyses have demonstrated a marginal survival benefit with adjuvant chemotherapy. Two recent large randomized trials for operable gastric cancer, the MAGIC trial and the INT-0116 trial, provide evidence that some patients may benefit from perioperative chemotherapy and chemoradiation, respectively. However, while both trials suggest an overall survival benefit with adjuvant treatment, they don't provide the harm-benefit ratio for specific subsets of patients wih different extent of surgery (D1 or D2) and tumor stage (early [T1,2]/advanced [T3,4]). This lack of evidence complicates current therapeutic adjuvant decisions. Estimating the risk of local and distant recurrence (high, moderate or low) after D1 or D2 surgery in various tumor stages and the expected harm-benefit ratio, the authors provide useful information for decisions on adjuvant chemotherapy with or withour radiotherapy in individual patients. Research on newer cytotoxic and targeted agents may improve treatment efficacy. Simultaneously, advances with microarray-based gene-expression profiling signatures may improve individualized treatment decisions. However, the validation and translation of these genomic classifiers as biomarkers into a completed 'bench-to-bedside' cycle for tailoring treatment to individuals is a major challenge and limits inflated expectations.     

Cobalt inhibition of synthesis and induction of delta-aminolevulinate synthase in liver.

Cobalt has complex actions on the metabolism of heme in the liver. In this organ the metal potently induces heme oxygenase (EC 1.14.99.3), and decreases cellular heme and hemoprotein content. The metal also displays biphasic effects on hepatic heme synthesis. These effects are reflected in the ability of cobalt to initially inhibit synthesis of delta-aminolevulinate synthase [succinyl-CoA:glycine C-succinyltransferase (decarboxylating) EC 2.3.1.37], the rate limiting enzyme of the heme pathway, following which a later enhanced rate of formation of this enzyme occurs. In this study, cobalt was shown to block almost entirely the ability of the barbiturate analogue allylisopropylacetamide to induce delta-aminolevulinate synthase in liver. The blocking effect of cobalt on the otherwise potent enzyme inducing action of this drug was time-dependent; if the metal was injected 30 min prior to allylisopropylacetamide, inhibition of enzyme induction was complete. When the metal was administered 1.5 or more hours after allylisopropylacetamide, inhibition of enzyme induction was incomplete. Cobalt did not block the ability of the drug to directly degrade heme to "green pigment" thus the enzyme inducing action of allylisopropylacetamide and its degradative action on heme are separately mediated.