Porphyrias

Porfiria intermitente aguda como desencadenante de síndrome de Guillain-Barre

Introducción. La porfiria es un conjunto de enfermedades metabólicas que tienen como base fisiopatológica la acumulación de precursores tóxicos. Su similitud clínica con enfermedades como el síndrome de Guillain-Barre puede retrasar el diagnostico, aumentando la posibilidad de complicaciones.Presentación del caso. Paciente femenino quien presentó síntomas inespecíficos de porfiria y síndrome de Guillain-Barre. La mujer fue evaluada de manera integral y recibió tratamiento para ambas patologías, respondiendo de manera inusual.Conclusión. La respuesta farmacológica atípica encontrada y la relación causa-efecto entre ambas entidades se justifica a la luz de sus procesos fisiopatológicos y la respuesta inmune desencadenada por los mismos.

Erythropoietic protoporphyria: a functional analysis of the leader sequence of human ferrochelatase

Erythropoietic protoporphyria (EPP) results from an inherited partial deficiency of ferrochelatase, the terminal enzyme of haem biosynthesis. Excess protoporphyrin IX accumulates in erythrocytes, plasma, liver, and skin, which mediates a distinctive form of cutaneous photosensitivity that manifests during childhood. Ferrochelatase is synthesised on cytosolic ribosomes as a preprotein with a cleavable presequence at its amino-terminus. This leader sequence is thought to target ferrochelatase to mitochondria where it is cleaved to produce the active mature protein. In this study, we show that the 62 amino acid leader sequence is sufficient for targeting of a leader sequence-YFP fusion protein to mitochondria. A truncated fusion protein lacking the first 62 amino acids did not target to mitochondria, and formed punctate aggregates in the cytoplasm of cells. This suggests that all the information required for mitochondrial localisation resides within the first 62 amino acid presequence. A missense mutation, P62R, predicted to be located within the ferrochelatase presequence has been identified in a patient with EPP. We hypothesised that this mutation may exert its effect through defective targeting to mitochondria. Our data showed that this mutated full-length ferrochelatase successfully targeted to mitochondria. Interestingly, there was inhibited cleavage of YFP from wild-type and mutant leader sequence fusion proteins. Generation of leader sequence-YFP fusion proteins containing an additional 11 amino acids from the mature protein allowed proteolytic processing to occur. These data suggest that the first 62 amino acids allow targeting to mitochondria but do not contain sufficient information for efficient processing of the protein.

Heme: a versatile signaling molecule controlling the activities of diverse regulators ranging from transcription factors to MAP kinases

Heme (iron protoporphyrin IX) is an essential molecule for numerous living organisms. Not only does it serve as a prosthetic group in enzymes, it also acts as a signaling molecule that controls diverse molecular and cellular processes ranging from signal transduction to protein complex assembly. Deficient heme synthesis or function impacts the hematopoietic, hepatic and nervous systems in humans. Recent studies have revealed a series of heme-regulated transcription factors and signal transducers including Hap1, a heme-activated transcription factor that mediates the effects of oxygen on gene transcription in the yeast Saccharomyces cerevisiae; Bach1, a transcriptional repressor that is negatively regulated by heme in mammalian cells; IRR, an iron regulatory protein that mediates the iron-dependant regulation of heme synthesis in the bacterium Bradyrhizobium japonicum; and heme-regulated inhibitor, an eucaryotic initiation factor 2alpha kinase that coordinates protein synthesis with heme availability in reticulocytes. In this review, we summarize the current knowledge about how heme controls the activity of these transcriptional regulators and signal transducers, and discuss diseases associated with defective heme synthesis, degradation and function.

Heme deficiency suppresses the expression of key neuronal genes and causes neuronal cell death

Defective heme synthesis may cause acute porphyrias, which are associated with a wide array of neurological disturbances involving both the central and peripheral nervous systems. Thus, the understanding of the roles of heme in neuronal cell function may provide insights into the molecular events underlying the pathogenesis of neuropathies associated with defective heme synthesis. In this report, we use rat pheochromocytoma (PC12) clonal cells as a model system for studying the role of heme in neuronal cell survival. We examined the effects of inhibition of heme synthesis on signaling pathways and gene expression in nerve growth factor (NGF)-induced PC12 cells. We found that succinyl acetone-induced heme deficiency selectively caused apoptosis in NGF-induced PC12 cells. Further, we found that in succinyl acetone-treated, NGF-induced cells, the pro-survival Ras-ERK1/2 signaling pathway was inactivated and the pro-apoptotic JNK signaling pathway was activated. In these cells, the activation of caspase and the cleavage of nuclear poly (ADP-ribose) polymerase (PARP) were also evident. Importantly, microarray gene expression analysis showed that more than 20 key neuronal genes that were induced by NGF were suppressed by succinyl acetone. These genes include those encoding survival motor neuron protein, synaptic vesicle protein SVOP, and neural cell adhesion molecule NCAM. These results indicate that heme is important for neuronal cell signaling and the proper functioning of neuronal cells.

Heme deficiency causes apoptosis but does not increase ROS generation in HeLa cells

Mitochondria provide cellular energy supply via respiration and are the major sites for the generation of reactive oxygen species (ROS). Mitochondria also play a fundamental role in apoptosis. Heme is a key factor in mitochondrial function. Defective heme synthesis or altered heme metabolism is associated with numerous diseases. Here we investigated the molecular mechanism by which heme promotes HeLa cell growth and survival. We found that heme deficiency-induced apoptosis involves the release of cytochrome c and the activation of caspase 3. However, heme deficiency-induced apoptosis appears to occur by a unique mechanism distinct from those known to mediate mitochondrial-dependent apoptosis. Specifically, our data show that heme deficiency causes apoptosis in a pathway that is independent of ROS generation and the collapse of mitochondrial membrane potential. These results provide insights into how defective heme synthesis or altered heme metabolism causes diseases and how heme may control cell growth and cell death.

An examination of heme action in gene expression: heme and heme deficiency affect the expression of diverse genes in erythroid k562 and neuronal PC12 cells

The aim of this study was to identify key genes whose expression is altered by heme and heme deficiency in the human erythroleukemia K562 cells and in the NGF-induced rat pheochromocytoma neuronal PC12 cells, respectively. By quantitative RT-PCR, Northern blotting, and Western blotting analyses, we found that the expression of the CDK inhibitors p18 and p21 was upregulated at the early and late stages of heme-induced erythroid differentiation of K562 cells, respectively, while the expression of cyclin D1 was downregulated. Data from succinyl acetone and desferrioxamine treatments suggest that these effects of heme in K562 cells were specific. Further, by microarray expression analysis, we found that inhibition of heme synthesis by succinyl acetone in NGF-induced PC12 cells drastically altered the expression of several groups of important neuronal genes, including the structural genes encoding neurofilament proteins and synaptic vesicle proteins, regulatory genes encoding signaling components beta-arrestin and p38 MAPK, and stress-response genes encoding hsp70. These results show that heme and heme deficiency affect the expression of diverse genes in a cell-type specific manner in mammalian cells, and that heme, although needed at different levels, is critical for both erythropoiesis and neurogenesis. These studies provide insights into how heme may act to control diverse regulatory processes in mammals.

Non-alcoholic steatohepatitis induced by carbamazepine and variegate porphyria

A 42-year-old woman presented with acute bullous skin lesions and angio-oedema that had developed 3 months after initiation of treatment with carbamazepine for epilepsy. Chromatographic analysis of urinary porphyrins was compatible with variegate porphyria. This was manifested initially by neurological symptoms that were mistaken for epilepsy and later by cutaneous symptoms also. Histological findings excluded hepatic porphyria, but revealed severe fatty changes thought to be caused by idiosyncratic metabolism of carbamazepine. While the porphyrinogenicity of carbamazepine is well known, the presence of variegate porphyria has not been reported. The toxic hepatic effects of the drug on hepatic cytochrome P-450, which is involved in haem metabolism, could have aggravated the pre-existent porphyria, provoking the onset of skin lesions.