Introdução: A infecção crônica pelo vírus da hepatite C (VHC) está associada à presença da resistência insulínica e da esteatose hepática, independentemente dos fatores metabólicos do hospedeiro. A alteração na enzima MTHFR resulta em hiperhomocisteinemia, que altera o metabolismo intracelular dos lipídios e pode estar relacionada à esteatose hepática e à fibrose, em portadores do VHC. A redução da atividade hepática da MTP resulta em acúmulo de gordura nos hepatócitos, contribuindo para a severidade da esteatose hepática e da fibrose em portadores do VHC. Como objetivos foram estudados os polimorfismos 677 C/T do gene da MTHFR e -493 G/T do gene da MTP e sua relação com as variáveis clínicas, bioquímicas e histológicas em pacientes com infecção crônica pelo VHC. Métodos: 174 pacientes sem tratamento prévio com RNA do VHC positivo e com biópsia hepática foram genotipados para o polimorfismo 677C/T da MTHFR por Restriction Fragment Length Polymorfism-Polimerase Chain (PCRRFLP) e para -493G/T da MTP, por sequenciamento. Todos os pacientes tinham marcadores negativos para doença de Wilson, hemocromatose e doença autoimune, e também tinham baixa ingesta alcoólica, com menos de 100g/semana. Variáveis bioquímicas foram analisadas no momento da realização da biópsia hepática. Resultados: A frequência do genótipo TT do gene MTHFR foi de 9,8% nos pacientes com genótipo não 1 do VHC. No entanto, foi encontrada associação entre o genótipo TT x CT /CC do polimorfismo do gene MTHFR, com o grau de esteatose e fibrose em ambos os genótipos da hepatite C (p < 0,05). Uma diferença significativa foi encontrada em níveis plasmáticos de homocisteína em pacientes com esteatose (p = 0,03). A frequência do genótipo GG+GT do gene MTP foi de 56,8% nos pacientes com genótipo 1 do VHC com fibrose hepática grau 3+4 (OR 1,8, IC 95% 1,3-2,3). Foi observada uma associação direta entre a presença da esteatose hepática nos pacientes com VHC com o genótipo GG+GT do polimorfismo -493G/T do gene da MTP independentemente do genótipo do VHC (OR = 0,4, IC 95% 0,2-0,8, p = 0,01). Conclusões: o genótipo TT do polimorfismo C677T do gene da MTHFR foi mais frequente no genótipo não 1 do VHC, independentemente da classificação histopatológica, assim como a frequência do genótipo CT + TT na presença de fibrose grau 1+ 2 e da esteatose hepática. A hiperhomocisteinemia foi altamente prevalente em indivíduos com esteatose. Por outro lado, a presença do alelo G do do polimorfismo -493G/T do gene da MTP está associada a uma menor expressão da MTP hepática, protegendo contra a esteatose em pacientes com VHC do Nordeste do Brasil. Estudos adicionais em outras populações são necessários para avaliar melhor o papel desses polimorfismos em indivíduos infectados pelo VHC / Background: Chronic hepatitis C (CHC) infection has been shown to promote insulin resistance and hepatic steatosis independent of host metabolic factors. A lower MTHFR activity is associated to hiperhomocysteinemia and also may be related to steatosis and fibrosis in CHC. Futhermore a reduction on hepatic MTP activity resulting in fatty liver and could contribute to the severity of hepatic steatosis and fibrosis in CHC. The aim was to investigate this this polymorphism in the 677 C/T MTHFR and -493G/T MTP genes and there relation with metabolic and histological variables in patients with CHC. Methods: One hundred seven-four untreated patients with viral RNA and liver biopsy were genotyped for the 677C/T MTHFR and 493G/T MTP polymorphisms. The 677C/T polymorphism of the MTHFR gene was identified by Restriction Fragment Length Polymorfism- Polimerase Chain (PCRRFLP) and the 493 G/T polymorphism of the MTP gene was determined by direct sequencing of the polymerase chain reaction products. All patients were negative for markers of Wilsons disease, hemochromatosis and autoimmune diseases and had current and past daily alcohol intake less than 100g/week. A set of metabolic markers were also measured at the time of liver biopsies. Results: Among subjects infected with CHC genotype non-1 the frequency of MTHFR genotypes TT was 9.8%. Nevertheless, association was found between the MTHFR genotype TT x CT/CC polymorphism and the degree of steatosis and fibrosis in both hepatitis C genotype (p < 0.05). A significant difference was found on plasma homocysteine levels in patients with steatosis (p=0.03). Among subjects infected with CHC genotype 1 with fibrosis grade 3+4 the frequency of MTP genotypes GG+GT was 56.8% (OR 1.8; CI 95% 1.3-2.3). Observed an association with steatosis as dependent variable identified in genotypes GG+GT as independent protective factors against steatosis (OR=0.4, CI 95% 0.2-0.8, p = 0.01). Conclusion: The presence of genotype TT of MTHFR C677T polymorphism was more common in CHC genotype non-1 infected patient regardless of histopathological classification and genotype CT+TT frequencies were significant in the presence of fibrosis grade 1+2 and of steatosis. On the other hand the presence of the G allele of MTP 493G/T, which is possibly associated with a lower MTP hepatic expression, protects against steatosis in CHC patients from northeast of Brazil. Additional studies in other populations are needed to further assess the role of this polimorphysm in CHC

<p>BACKGROUND: Abetalipoproteinemia (ABL) is a rare autosomal recessive disorder caused by mutations in the gene coding for microsomal triglyceride transfer protein (MTTP).</p><p>AIM: To characterize the genetic basis of ABL in two unrelated patients.</p><p>RESULTS: In the first patient, the substitution c.1911C>T in exon 12 of the <em>MTTP</em> gene, resulting in the protein substitution p.P552L, was discovered using mutation screening. The parents are heterozygous and the proband is a homozygous carrier of this substitution. Using restriction fragment length polymorphism (RFLP), 100 control subjects were analyzed and none carried the substitution indicating that it is a novel <em>MTTP </em>mutation. Sequencing of the other ABL patient showed that the proband carried a homozygous single base insertion, at position  c.2342IVS16+2-3insT, located at the donor splice-site of intron 16 resulting in skipping of exon 16 and truncation of the protein. The proband's mother is heterozygous for the insertion while the father does not carry the insertion. Multiplex ligation-dependent probe amplification (MLPA) did not identify any deletion encompassing exon 16 in the proband, father or mother. Nonpaternity was excluded using polymorphic markers from several chromosomes. Haplotype analysis using markers spanning chromosome 4 revealed  heterodisomy (two homologous chromosomes) of 4p and the distal part of 4q, and isodisomy (duplication of one chromosome) of 4q12-4q26.</p><p>CONCLUSION: These data show that the cause of ABL in one of the patients is a missense mutation, p.P552L, while the cause of ABL in the other patient is due to uniparental disomy, probably resulting from non-disjunstion in meiosis I.</p>

BACKGROUND: Abetalipoproteinemia (ABL) is a rare autosomal recessive disorder caused by mutations in the gene coding for microsomal triglyceride transfer protein (MTTP). AIM: To characterize the genetic basis of ABL in two unrelated patients. RESULTS: In the first patient, the substitution c.1911C&gt;T in exon 12 of the MTTP gene, resulting in the protein substitution p.P552L, was discovered using mutation screening. The parents are heterozygous and the proband is a homozygous carrier of this substitution. Using restriction fragment length polymorphism (RFLP), 100 control subjects were analyzed and none carried the substitution indicating that it is a novel MTTP mutation. Sequencing of the other ABL patient showed that the proband carried a homozygous single base insertion, at position  c.2342IVS16+2-3insT, located at the donor splice-site of intron 16 resulting in skipping of exon 16 and truncation of the protein. The proband's mother is heterozygous for the insertion while the father does not carry the insertion. Multiplex ligation-dependent probe amplification (MLPA) did not identify any deletion encompassing exon 16 in the proband, father or mother. Nonpaternity was excluded using polymorphic markers from several chromosomes. Haplotype analysis using markers spanning chromosome 4 revealed  heterodisomy (two homologous chromosomes) of 4p and the distal part of 4q, and isodisomy (duplication of one chromosome) of 4q12-4q26. CONCLUSION: These data show that the cause of ABL in one of the patients is a missense mutation, p.P552L, while the cause of ABL in the other patient is due to uniparental disomy, probably resulting from non-disjunstion in meiosis I.

Introdução: A infecção crônica pelo vírus da hepatite C (VHC) está associada à presença da resistência insulínica e da esteatose hepática, independentemente dos fatores metabólicos do hospedeiro. A alteração na enzima MTHFR resulta em hiperhomocisteinemia, que altera o metabolismo intracelular dos lipídios e pode estar relacionada à esteatose hepática e à fibrose, em portadores do VHC. A redução da atividade hepática da MTP resulta em acúmulo de gordura nos hepatócitos, contribuindo para a severidade da esteatose hepática e da fibrose em portadores do VHC. Como objetivos foram estudados os polimorfismos 677 C/T do gene da MTHFR e -493 G/T do gene da MTP e sua relação com as variáveis clínicas, bioquímicas e histológicas em pacientes com infecção crônica pelo VHC. Métodos: 174 pacientes sem tratamento prévio com RNA do VHC positivo e com biópsia hepática foram genotipados para o polimorfismo 677C/T da MTHFR por Restriction Fragment Length Polymorfism-Polimerase Chain (PCRRFLP) e para -493G/T da MTP, por sequenciamento. Todos os pacientes tinham marcadores negativos para doença de Wilson, hemocromatose e doença autoimune, e também tinham baixa ingesta alcoólica, com menos de 100g/semana. Variáveis bioquímicas foram analisadas no momento da realização da biópsia hepática. Resultados: A frequência do genótipo TT do gene MTHFR foi de 9,8% nos pacientes com genótipo não 1 do VHC. No entanto, foi encontrada associação entre o genótipo TT x CT /CC do polimorfismo do gene MTHFR, com o grau de esteatose e fibrose em ambos os genótipos da hepatite C (p < 0,05). Uma diferença significativa foi encontrada em níveis plasmáticos de homocisteína em pacientes com esteatose (p = 0,03). A frequência do genótipo GG+GT do gene MTP foi de 56,8% nos pacientes com genótipo 1 do VHC com fibrose hepática grau 3+4 (OR 1,8, IC 95% 1,3-2,3). Foi observada uma associação direta entre a presença da esteatose hepática nos pacientes com VHC com o genótipo GG+GT do polimorfismo -493G/T do gene da MTP independentemente do genótipo do VHC (OR = 0,4, IC 95% 0,2-0,8, p = 0,01). Conclusões: o genótipo TT do polimorfismo C677T do gene da MTHFR foi mais frequente no genótipo não 1 do VHC, independentemente da classificação histopatológica, assim como a frequência do genótipo CT + TT na presença de fibrose grau 1+ 2 e da esteatose hepática. A hiperhomocisteinemia foi altamente prevalente em indivíduos com esteatose. Por outro lado, a presença do alelo G do do polimorfismo -493G/T do gene da MTP está associada a uma menor expressão da MTP hepática, protegendo contra a esteatose em pacientes com VHC do Nordeste do Brasil. Estudos adicionais em outras populações são necessários para avaliar melhor o papel desses polimorfismos em indivíduos infectados pelo VHC / Background: Chronic hepatitis C (CHC) infection has been shown to promote insulin resistance and hepatic steatosis independent of host metabolic factors. A lower MTHFR activity is associated to hiperhomocysteinemia and also may be related to steatosis and fibrosis in CHC. Futhermore a reduction on hepatic MTP activity resulting in fatty liver and could contribute to the severity of hepatic steatosis and fibrosis in CHC. The aim was to investigate this this polymorphism in the 677 C/T MTHFR and -493G/T MTP genes and there relation with metabolic and histological variables in patients with CHC. Methods: One hundred seven-four untreated patients with viral RNA and liver biopsy were genotyped for the 677C/T MTHFR and 493G/T MTP polymorphisms. The 677C/T polymorphism of the MTHFR gene was identified by Restriction Fragment Length Polymorfism- Polimerase Chain (PCRRFLP) and the 493 G/T polymorphism of the MTP gene was determined by direct sequencing of the polymerase chain reaction products. All patients were negative for markers of Wilsons disease, hemochromatosis and autoimmune diseases and had current and past daily alcohol intake less than 100g/week. A set of metabolic markers were also measured at the time of liver biopsies. Results: Among subjects infected with CHC genotype non-1 the frequency of MTHFR genotypes TT was 9.8%. Nevertheless, association was found between the MTHFR genotype TT x CT/CC polymorphism and the degree of steatosis and fibrosis in both hepatitis C genotype (p < 0.05). A significant difference was found on plasma homocysteine levels in patients with steatosis (p=0.03). Among subjects infected with CHC genotype 1 with fibrosis grade 3+4 the frequency of MTP genotypes GG+GT was 56.8% (OR 1.8; CI 95% 1.3-2.3). Observed an association with steatosis as dependent variable identified in genotypes GG+GT as independent protective factors against steatosis (OR=0.4, CI 95% 0.2-0.8, p = 0.01). Conclusion: The presence of genotype TT of MTHFR C677T polymorphism was more common in CHC genotype non-1 infected patient regardless of histopathological classification and genotype CT+TT frequencies were significant in the presence of fibrosis grade 1+2 and of steatosis. On the other hand the presence of the G allele of MTP 493G/T, which is possibly associated with a lower MTP hepatic expression, protects against steatosis in CHC patients from northeast of Brazil. Additional studies in other populations are needed to further assess the role of this polimorphysm in CHC

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

L’accumulation de triglycérides (TG) dans les hépatocytes est caractéristique de la stéatose hépatique non-alcoolique (SHNA). Cette dernière se produit dans diverses conditions dont le facteur commun est le métabolisme anormal des lipides. Le processus conduisant à l'accumulation des lipides dans le foie n’a pas encore été totalement élucidé. Toutefois, des lipides s'accumulent dans le foie lorsque les mécanismes qui favorisent leur exportation (oxydation et sécrétion) sont insuffisants par rapport aux mécanismes qui favorisent leur importation ou leur biosynthèse. De nos jours il est admis que la carence en œstrogènes est associée au développement de la stéatose hépatique. Bien que les résultats des études récentes révèlent l'implication des hormones ovariennes dans l'accumulation de lipides dans le foie, les mécanismes qui sous-tendent ce phénomène doivent encore être étudiés. En conséquence, les trois études présentées dans cette thèse ont été menées sur des rates ovariectomizées (Ovx), comme modèle animal de femmes post-ménopausées, pour étudier les effets du retrait des œstrogènes sur le métabolisme des lipides dans le foie, en considérant l'entraînement physique comme étant un élément positif pouvant contrecarrer ces effets. Il a été démontré que l'entraînement physique peut réduire l'accumulation de graisses dans le foie chez les rates Ovx. Dans la première étude, nous avons montré que chez les rates Ovx nourries à la diète riche en lipides (HF), les contenus de TG hépatiques étaient élevées (P < 0.01) comparativement aux rates Sham, 5 semaines après la chirurgie. Le changement de la diète HF par la diète standard (SD) chez les rates Sham a diminué l’accumulation de lipides dans le foie. Toutefois, chez les rates Ovx, 8 semaines après le changement de la HF par la SD le niveau de TG dans le foie était maintenu aussi élevé que chez les rates nourries continuellement avec la diète HF. Lorsque les TG hépatiques mesurés à la 13e semaine ont été comparés aux valeurs correspondant au retrait initial de la diète HF effectué à la 5e semaine, les niveaux de TG hépatiques chez les animaux Ovx ont été maintenus, indépendamment du changement du régime alimentaire; tandis que chez les rats Sham le passage à la SD a réduit (P < 0.05) les TG dans le foie. Les mêmes comparaisons avec la concentration des TG plasmatiques ont révélé une relation inverse. Ces résultats suggèrent que la résorption des lipides au foie est contrée par l'absence des œstrogènes. Dans cette continuité, nous avons utilisé une approche physiologique dans notre seconde étude pour investiguer la façon dont la carence en œstrogènes entraîne l’accumulation de graisses dans le foie, en nous focalisant sur la voie de l'exportation des lipides du foie. Les résultats de cette étude ont révélé que le retrait des œstrogènes a entraîné une augmentation (P < 0.01) de l’accumulation de lipides dans le foie en concomitance avec la baisse (P < 0.01) de production de VLDL-TG et une réduction l'ARNm et de la teneur en protéines microsomales de transfert des triglycérides (MTP). Tous ces effets ont été corrigés par la supplémentation en œstrogènes chez les rates Ovx. En outre, l'entraînement physique chez les rates Ovx a entraîné une réduction (P < 0.01) de l’accumulation de lipides dans le foie ainsi qu’une diminution (P < 0.01) de production de VLDL-TG accompagnée de celle de l'expression des gènes MTP et DGAT-2 (diacylglycérol acyltransférase-2). Des études récentes suggèrent que le peptide natriurétique auriculaire (ANP) devrait être au centre des intérêts des recherches sur les métabolismes énergétiques et lipidiques. Le ANP est relâché dans le plasma par les cellules cardiaques lorsque stimulée par l’oxytocine et exerce ses fonctions en se liant à son récepteur, le guanylyl cyclase-A (GC-A). En conséquence, dans la troisième étude, nous avons étudié les effets du blocage du système ocytocine-peptide natriurétique auriculaire (OT-ANP) en utilisant un antagoniste de l’ocytocine (OTA), sur l'expression des gènes guanylyl cyclase-A et certains marqueurs de l’inflammation dans le foie de rates Ovx. Nous avons observé une diminution (P < 0.05) de l’ARNm de la GC-A chez les rates Ovx et Sham sédentaires traitées avec l’OTA, tandis qu’une augmentation (P < 0.05) de l'expression de l’ARNm de la protéine C-réactive (CRP) hépatique a été notée chez ces animaux. L’exercice physique n'a apporté aucun changement sur l'expression hépatique de ces gènes que ce soit chez les rates Ovx ou Sham traitées avec l’OTA. En résumé, pour expliquer l’observation selon laquelle l’accumulation et la résorption de lipides dans le foie dépendent des mécanismes associés à des niveaux d’œstrogènes, nos résultats suggèrent que la diminution de production de VLDL-TG induite par une déficience en œstrogènes, pourrait être un des mecanismes responsables de l’accumulation de lipides dans le foie. L’exercice physique quant à lui diminue l'infiltration de lipides dans le foie ainsi que la production de VLDL-TG indépendamment des niveaux d'œstrogènes. En outre, l'expression des récepteurs de l’ANP a diminué par l'OTA chez les rates Ovx et Sham suggérant une action indirecte de l’ocytocine (OT) au niveau du foie indépendamment de la présence ou non des estrogènes. L’axe ocytocine-peptide natriurétique auriculaire, dans des conditions physiologiques normales, protègerait le foie contre l'inflammation à travers la modulation de l’expression de la GC-A. / Excessive accumulation of triglycerides (TGs) in hepatocytes is the characteristic of non-alcoholic hepatic steatosis (NAHS). NAHS occurs in various conditions in which abnormal fat metabolism is a common factor. The primary processes leading to lipid accumulation in the liver are not well understood. However, lipid in the form of TG accumulates within liver cells when mechanisms that promote their removal (by oxidation or secretion) cannot keep pace with mechanisms that promote lipid import or biosynthesis. Today, it is well accepted that estrogen deficiency is associated with the development of a state of hepatic steatosis. Although recent findings indicated the implication of ovarian hormones in liver lipid accumulation, mechanisms underlying this phenomenon need to be further investigated. Therefore, the three studies presented in this thesis have been conducted in ovariectomized (Ovx) rats, as animal model of post-menopausal women, to investigate the effects of estrogen withdrawal on liver fat metabolism and considering the effects of exercise training as a positive counteractive factor. It has been shown that exercise training can reduce liver fat accumulation in Ovx rats. In the first study, we showed that in high fat (HF) fed animals, liver TG content was higher (P < 0.01) in Ovx compared to Sham rats as soon as 5-week after the surgery. Switching from the HF to a standard (SD) diet resulted in a decrease in liver fat accumulation in Sham animals. However, 8 weeks after the diet switch, liver fat accumulation was as high in Ovx rats as those maintained on the HF diet. When liver TG content measured at week 13 was compared to initial pre-switching values (week 5), liver TG levels in Ovx animals were maintained at the same level independently of the diet switch, while in Sham rats switching to a SD diet reduced liver TG accumulation (P < 0.05). The same comparisons with plasma TG levels revealed an opposite relationship. These results may be taken as evidence that indeed liver fat resorption is hampered in the absence of estrogens. To go one step further, we used a physiological approach in our second study to investigate how estrogen deficiency affects liver fat accumulation putting an emphasis on the pathway of lipid exportation from the liver. Results of this study showed that estrogen withdrawal resulted in higher (P < 0.01) liver fat accumulation concomitantly with lower (P < 0.01) very low density lipoprotein-triglyceride (VLDL-TG) production and lower mRNA and protein content of hepatic microsomal triglyceride transfer protein (MTP). All of these effects in Ovx rats were corrected with estrogen supplementation. Moreover, exercise training in Ovx rats reduced (P < 0.01) liver fat accumulation and further reduced (P < 0.01) hepatic VLDL-TG production along with gene expression of MTP and diacylglycerol acyltransferase-2 (DGAT-2). A recent growing body of literature suggests that atrial natriuretic peptide (ANP) hormone should be the interest of new investigations in the field of energy and lipid metabolism. ANP is released from the heart into plasma by oxytocin (OT) stimulation and exerts its biological action by binding to its receptor, guanylyl cyclase-A (GC-A: ANP receptor). Therefore, in the third study, we investigated the effects of blocking the oxytocin-atrial natriuretic peptide (OT-ANP) system, using an OT antagonist (OTA), on the gene expression of hepatic guanylyl cyclase-A and some inflammatory markers in the liver of Ovx rats. Hepatic GC-A mRNAs were decreased (P < 0.05) in Ovx and Sham OTA-treated rats in the sedentary state, contrary to hepatic C-reactive protein (CRP) mRNA expression that increased in these animals (P < 0.05). Exercise training had no effect on hepatic expression of these genes in both Sham and Ovx rats receiving OTA. Overall, our results point to the interpretation that hepatic fat accumulation and resorption are dependent on mechanisms associated with a normal estrogenic status; indicating that a decrease in VLDL-TG production might be a contributing factor responsible for the hepatic fat accumulation induced by estrogen deficiency. Exercise training lowers liver fat accretion and VLDL-TG production independently of the estrogen levels. Moreover, hepatic expression of ANP receptors is decreased by OTA in both Sham and Ovx rats suggesting an indirect action of the OT system on the liver independently of the estrogenic status of the animal. Oxytocin-atrial natriuretic peptide axis may contribute to the protection of hepatic tissue under normal physiological conditions such as reducing inflammatory markers within the hepatocytes by exerting its role through guanylyl cyclase-A expression.

L’accumulation de triglycérides (TG) dans les hépatocytes est caractéristique de la stéatose hépatique non-alcoolique (SHNA). Cette dernière se produit dans diverses conditions dont le facteur commun est le métabolisme anormal des lipides. Le processus conduisant à l'accumulation des lipides dans le foie n’a pas encore été totalement élucidé. Toutefois, des lipides s'accumulent dans le foie lorsque les mécanismes qui favorisent leur exportation (oxydation et sécrétion) sont insuffisants par rapport aux mécanismes qui favorisent leur importation ou leur biosynthèse. De nos jours il est admis que la carence en œstrogènes est associée au développement de la stéatose hépatique. Bien que les résultats des études récentes révèlent l'implication des hormones ovariennes dans l'accumulation de lipides dans le foie, les mécanismes qui sous-tendent ce phénomène doivent encore être étudiés. En conséquence, les trois études présentées dans cette thèse ont été menées sur des rates ovariectomizées (Ovx), comme modèle animal de femmes post-ménopausées, pour étudier les effets du retrait des œstrogènes sur le métabolisme des lipides dans le foie, en considérant l'entraînement physique comme étant un élément positif pouvant contrecarrer ces effets. Il a été démontré que l'entraînement physique peut réduire l'accumulation de graisses dans le foie chez les rates Ovx. Dans la première étude, nous avons montré que chez les rates Ovx nourries à la diète riche en lipides (HF), les contenus de TG hépatiques étaient élevées (P < 0.01) comparativement aux rates Sham, 5 semaines après la chirurgie. Le changement de la diète HF par la diète standard (SD) chez les rates Sham a diminué l’accumulation de lipides dans le foie. Toutefois, chez les rates Ovx, 8 semaines après le changement de la HF par la SD le niveau de TG dans le foie était maintenu aussi élevé que chez les rates nourries continuellement avec la diète HF. Lorsque les TG hépatiques mesurés à la 13e semaine ont été comparés aux valeurs correspondant au retrait initial de la diète HF effectué à la 5e semaine, les niveaux de TG hépatiques chez les animaux Ovx ont été maintenus, indépendamment du changement du régime alimentaire; tandis que chez les rats Sham le passage à la SD a réduit (P < 0.05) les TG dans le foie. Les mêmes comparaisons avec la concentration des TG plasmatiques ont révélé une relation inverse. Ces résultats suggèrent que la résorption des lipides au foie est contrée par l'absence des œstrogènes. Dans cette continuité, nous avons utilisé une approche physiologique dans notre seconde étude pour investiguer la façon dont la carence en œstrogènes entraîne l’accumulation de graisses dans le foie, en nous focalisant sur la voie de l'exportation des lipides du foie. Les résultats de cette étude ont révélé que le retrait des œstrogènes a entraîné une augmentation (P < 0.01) de l’accumulation de lipides dans le foie en concomitance avec la baisse (P < 0.01) de production de VLDL-TG et une réduction l'ARNm et de la teneur en protéines microsomales de transfert des triglycérides (MTP). Tous ces effets ont été corrigés par la supplémentation en œstrogènes chez les rates Ovx. En outre, l'entraînement physique chez les rates Ovx a entraîné une réduction (P < 0.01) de l’accumulation de lipides dans le foie ainsi qu’une diminution (P < 0.01) de production de VLDL-TG accompagnée de celle de l'expression des gènes MTP et DGAT-2 (diacylglycérol acyltransférase-2). Des études récentes suggèrent que le peptide natriurétique auriculaire (ANP) devrait être au centre des intérêts des recherches sur les métabolismes énergétiques et lipidiques. Le ANP est relâché dans le plasma par les cellules cardiaques lorsque stimulée par l’oxytocine et exerce ses fonctions en se liant à son récepteur, le guanylyl cyclase-A (GC-A). En conséquence, dans la troisième étude, nous avons étudié les effets du blocage du système ocytocine-peptide natriurétique auriculaire (OT-ANP) en utilisant un antagoniste de l’ocytocine (OTA), sur l'expression des gènes guanylyl cyclase-A et certains marqueurs de l’inflammation dans le foie de rates Ovx. Nous avons observé une diminution (P < 0.05) de l’ARNm de la GC-A chez les rates Ovx et Sham sédentaires traitées avec l’OTA, tandis qu’une augmentation (P < 0.05) de l'expression de l’ARNm de la protéine C-réactive (CRP) hépatique a été notée chez ces animaux. L’exercice physique n'a apporté aucun changement sur l'expression hépatique de ces gènes que ce soit chez les rates Ovx ou Sham traitées avec l’OTA. En résumé, pour expliquer l’observation selon laquelle l’accumulation et la résorption de lipides dans le foie dépendent des mécanismes associés à des niveaux d’œstrogènes, nos résultats suggèrent que la diminution de production de VLDL-TG induite par une déficience en œstrogènes, pourrait être un des mecanismes responsables de l’accumulation de lipides dans le foie. L’exercice physique quant à lui diminue l'infiltration de lipides dans le foie ainsi que la production de VLDL-TG indépendamment des niveaux d'œstrogènes. En outre, l'expression des récepteurs de l’ANP a diminué par l'OTA chez les rates Ovx et Sham suggérant une action indirecte de l’ocytocine (OT) au niveau du foie indépendamment de la présence ou non des estrogènes. L’axe ocytocine-peptide natriurétique auriculaire, dans des conditions physiologiques normales, protègerait le foie contre l'inflammation à travers la modulation de l’expression de la GC-A. / Excessive accumulation of triglycerides (TGs) in hepatocytes is the characteristic of non-alcoholic hepatic steatosis (NAHS). NAHS occurs in various conditions in which abnormal fat metabolism is a common factor. The primary processes leading to lipid accumulation in the liver are not well understood. However, lipid in the form of TG accumulates within liver cells when mechanisms that promote their removal (by oxidation or secretion) cannot keep pace with mechanisms that promote lipid import or biosynthesis. Today, it is well accepted that estrogen deficiency is associated with the development of a state of hepatic steatosis. Although recent findings indicated the implication of ovarian hormones in liver lipid accumulation, mechanisms underlying this phenomenon need to be further investigated. Therefore, the three studies presented in this thesis have been conducted in ovariectomized (Ovx) rats, as animal model of post-menopausal women, to investigate the effects of estrogen withdrawal on liver fat metabolism and considering the effects of exercise training as a positive counteractive factor. It has been shown that exercise training can reduce liver fat accumulation in Ovx rats. In the first study, we showed that in high fat (HF) fed animals, liver TG content was higher (P < 0.01) in Ovx compared to Sham rats as soon as 5-week after the surgery. Switching from the HF to a standard (SD) diet resulted in a decrease in liver fat accumulation in Sham animals. However, 8 weeks after the diet switch, liver fat accumulation was as high in Ovx rats as those maintained on the HF diet. When liver TG content measured at week 13 was compared to initial pre-switching values (week 5), liver TG levels in Ovx animals were maintained at the same level independently of the diet switch, while in Sham rats switching to a SD diet reduced liver TG accumulation (P < 0.05). The same comparisons with plasma TG levels revealed an opposite relationship. These results may be taken as evidence that indeed liver fat resorption is hampered in the absence of estrogens. To go one step further, we used a physiological approach in our second study to investigate how estrogen deficiency affects liver fat accumulation putting an emphasis on the pathway of lipid exportation from the liver. Results of this study showed that estrogen withdrawal resulted in higher (P < 0.01) liver fat accumulation concomitantly with lower (P < 0.01) very low density lipoprotein-triglyceride (VLDL-TG) production and lower mRNA and protein content of hepatic microsomal triglyceride transfer protein (MTP). All of these effects in Ovx rats were corrected with estrogen supplementation. Moreover, exercise training in Ovx rats reduced (P < 0.01) liver fat accumulation and further reduced (P < 0.01) hepatic VLDL-TG production along with gene expression of MTP and diacylglycerol acyltransferase-2 (DGAT-2). A recent growing body of literature suggests that atrial natriuretic peptide (ANP) hormone should be the interest of new investigations in the field of energy and lipid metabolism. ANP is released from the heart into plasma by oxytocin (OT) stimulation and exerts its biological action by binding to its receptor, guanylyl cyclase-A (GC-A: ANP receptor). Therefore, in the third study, we investigated the effects of blocking the oxytocin-atrial natriuretic peptide (OT-ANP) system, using an OT antagonist (OTA), on the gene expression of hepatic guanylyl cyclase-A and some inflammatory markers in the liver of Ovx rats. Hepatic GC-A mRNAs were decreased (P < 0.05) in Ovx and Sham OTA-treated rats in the sedentary state, contrary to hepatic C-reactive protein (CRP) mRNA expression that increased in these animals (P < 0.05). Exercise training had no effect on hepatic expression of these genes in both Sham and Ovx rats receiving OTA. Overall, our results point to the interpretation that hepatic fat accumulation and resorption are dependent on mechanisms associated with a normal estrogenic status; indicating that a decrease in VLDL-TG production might be a contributing factor responsible for the hepatic fat accumulation induced by estrogen deficiency. Exercise training lowers liver fat accretion and VLDL-TG production independently of the estrogen levels. Moreover, hepatic expression of ANP receptors is decreased by OTA in both Sham and Ovx rats suggesting an indirect action of the OT system on the liver independently of the estrogenic status of the animal. Oxytocin-atrial natriuretic peptide axis may contribute to the protection of hepatic tissue under normal physiological conditions such as reducing inflammatory markers within the hepatocytes by exerting its role through guanylyl cyclase-A expression.