RESUMO
Squalene synthase (SS) is the first committed enzyme for cholesterol biosynthesis, located at a branch point in the mevalonate pathway. To examine the role of SS in the overall cholesterol metabolism, we transiently overexpressed mouse SS in the livers of mice using adenovirus-mediated gene transfer. Overexpression of SS increased de novo cholesterol biosynthesis with increased 3-hydroxy-3-methyglutaryl-CoA (HMG-CoA) reductase activity, in spite of the downregulation of its own mRNA expression. Furthermore, overexpression of SS increased plasma concentrations of LDL, irrespective of the presence of functional LDL receptor (LDLR). Thus, the hypercholesterolemia is primarily caused by increased hepatic production of cholesterol-rich VLDL, as demonstrated by the increases in plasma cholesterol levels after intravenous injection of Triton WR1339. mRNA expression of LDLR was decreased, suggesting that defective LDL clearance contributed to the development of hypercholesterolemia. Curiously, the liver was enlarged, with a larger number of Ki-67-positive cells. These results demonstrate that transient upregulation of SS stimulates cholesterol biosynthesis as well as lipoprotein production, providing the first in vivo evidence that SS plays a regulatory role in cholesterol metabolism through modulation of HMG-CoA reductase activity and cholesterol biosynthesis.
Assuntos
Colesterol/biossíntese , Farnesil-Difosfato Farnesiltransferase/metabolismo , Hipercolesterolemia/metabolismo , Fígado/metabolismo , Acil Coenzima A/genética , Acil Coenzima A/metabolismo , Adenoviridae/genética , Animais , Apoptose , Western Blotting , Peso Corporal , Colesterol/sangue , Farnesil-Difosfato Farnesiltransferase/genética , Hipercolesterolemia/enzimologia , Hipercolesterolemia/genética , Marcação In Situ das Extremidades Cortadas/métodos , Metabolismo dos Lipídeos , Lipídeos/sangue , Lipoproteínas/sangue , Lipoproteínas VLDL/sangue , Lipoproteínas VLDL/metabolismo , Fígado/crescimento & desenvolvimento , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Tamanho do Órgão , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Receptores de LDL/sangue , Receptores de LDL/genética , Reação em Cadeia da Polimerase Via Transcriptase ReversaRESUMO
Scavenger receptor expressed by endothelial cells I (SREC-I) is a novel endocytic receptor for acetylated low density lipoprotein (LDL). Here we show that SREC-I is expressed in a wide variety of tissues, including macrophages and aortas. Lipopolysaccharide (LPS) robustly stimulated the expression of SREC-I in macrophages. In an initial attempt to clarify the role of SREC-I in the uptake of modified lipoproteins as well as in the development of atherosclerosis, we generated mice with a targeted disruption of the SREC-I gene by homologous recombination in embryonic stem cells. To exclude the overwhelming effect of the type A scavenger receptor (SR-A) on the uptake of Ac-LDL, we further generated mice lacking both SR-A and SREC-I (SR-A(-/-);SREC-I(-/-)) by cross-breeding and compared the uptake and degradation of Ac-LDL in the isolated macrophages. The contribution of SR-A and SREC-I to the overall degradation of Ac-LDL was 85 and 5%, respectively, in a non-stimulated condition. LPS increased the uptake and degradation of Ac-LDL by 1.8-fold. In this condition, the contribution of SR-A and SREC-I to the overall degradation of Ac-LDL was 90 and 6%, respectively. LPS increased the absolute contribution of SR-A and SREC-I by 1.9- and 2.3-fold, respectively. On the other hand, LPS decreased the absolute contribution of other pathways by 31%. Consistently, LPS did not increase the expression of other members of the scavenger receptor family such as CD36. In conclusion, SREC-I serves as a major endocytic receptor for Ac-LDL in LPS-stimulated macrophages lacking SR-A, suggesting that it has a key role in the development of atherosclerosis in concert with SR-A.
Assuntos
Moléculas de Adesão Celular/metabolismo , Lipoproteínas LDL/metabolismo , Macrófagos/metabolismo , Receptores de LDL/metabolismo , Sequência de Aminoácidos , Animais , Aorta/metabolismo , Arteriosclerose/etiologia , Arteriosclerose/genética , Arteriosclerose/metabolismo , Sequência de Bases , Transporte Biológico Ativo , Antígenos CD36/genética , Antígenos CD36/metabolismo , Moléculas de Adesão Celular/deficiência , Moléculas de Adesão Celular/genética , Primers do DNA/genética , Endocitose , Expressão Gênica , Lipopolissacarídeos/farmacologia , Macrófagos/efeitos dos fármacos , Macrófagos Peritoneais/efeitos dos fármacos , Macrófagos Peritoneais/metabolismo , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Dados de Sequência Molecular , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Receptores de LDL/deficiência , Receptores de LDL/genética , Receptores Depuradores , Receptores Depuradores Classe A , Distribuição TecidualRESUMO
A 33-year-old man was referred to our hospital because of intractable cellulitis in his left lower leg. He was diagnosed with agammaglobulinemia at the age of 6 years and had been receiving gamma-globulin supplementation since then. Laboratory examination revealed a markedly reduced number of B cells, decreased protein amount of Bruton's tyrosine kinase (BTK) in monocytes, and a single base substitution of C994-->T(missense mutation of Arg288-->Trp) in BTK gene, confirming the diagnosis of X-linked agammaglobulinemia (XLA). The patient also had characteristic features of von Recklinghausen disease, such as numerous subcutaneous nodules, café-au-lait spots, Lisch nodules in the iris and spinal scoliosis. Biopsy of a subcutaneous nodule confirmed a neurofibroma. Although the influence of XLA on the development of von Recklinghausen disease is unknown for the moment, this is, to our knowledge, the first report of a patient with XLA who also developed von Recklinghausen disease.