Miyoko Okada

Gene expression in livers of BALB/C and C57BL/6J mice fed a high-fat diet.

We previously demonstrated that high-fat diet (HFD)–induced hepatic lipid accumulation is more severe in BALB/c mice than in C57BL/6J (B6) mice. To understand the changes in liver metabolism, we studied blood chemistry, gene expression, and histopathological changes of the liver in nine-week HFD-fed BALB/c and B6 mice and one- or four-week HFD-fed BALB/c mice. Serum total cholesterol and triglyceride levels were significantly increased in all HFD-fed groups, and one- and four-week HFD-fed BALB/c groups, respectively. Histopathology revealed that vacuolation of hepatocytes was severe in nine-week HFD-fed BALB/c mice, although it was less severe in the other groups. Microarray analysis of mRNA expression of nine-week HFD-fed BALB/c mice showed up-regulation of genes involved in fatty acid uptake and biosynthesis, such as Cd36, Acaca, Acly, and Fasn. Some changes were observed in the one- and four-week HFD-fed BALB/c groups and the nine-week HFD-fed B6 group, however these changes in mRNA expression were not so marked. In conclusion, the fatty accumulation observed in BALB/c mice may be caused, at least in part, by up-regulation of fatty acid uptake and biosynthesis. Cd36, Acaca, Acly and Fasn may be involved in these metabolic processes.

Chimeric Mice with Hepatocyte-humanized Liver as an Appropriate Model to Study Human Peroxisome Proliferator–activated Receptor-α

Peroxisome proliferator (PP)–activated receptor-α (PPARα) agonists exhibit species-specific effects on livers of the rodent and human (h), which has been considered to reside in the difference of PPARα gene structures. However, the contribution of h-hepatocytes (heps) to the species-specificity remains to be clarified. In this study, the effects of fenofibrate were investigated using a hepatocyte-humanized chimeric mouse (m) model whose livers were replaced with h-heps at >70%. Fenofibrate induced hepatocellular hypertrophy, cell proliferation, and peroxisome proliferation in livers of severe combined immunodeficiency (SCID) mice, but not in the h-hep of chimeric mouse livers. Fenofibrate increased the expression of the enzymes of β- and ω-hydroxylation and deoxygenation of lipids at both gene and protein levels in SCID mouse livers, but not in the h-heps of chimeric mouse livers, supporting the studies with h-PPARα-transgenic mice, a hitherto reliable model for studying the regulation of h-PPARα in the h-liver in most respects, except the induction of the peroxisome proliferation. This study indicates the importance of not only h-PPARα gene but also h-heps themselves to correctly predict effects of fibrates on h-livers, and, therefore, suggests that the chimeric mouse is a currently available, consistent, and reliable model to obtain pharmaceutical data concerning the effects of fibrates on h-livers.

Skeletal Muscle Susceptibility to Clofibrate Induction of Lesions in Rats

Morphological changes induced by clofibrate in type-1 predominant soleus, type-2 predominant tensor fasciae latae, and type-1 and -2 mixed biceps femoris muscles and diaphragm in rats were investigated. Administration of the agent at 500 or 750 mg/kg/day by oral gavage for 14 or 28 days caused lesions in the soleus muscle and diaphragm, bur no changes in the tensor fasciae latae and biceps femoris muscles. In soleus muscle, vacuolation of muscle fibers was observed in all animals treated with clofibrate, and degeneration of muscle fibers and infiltration of leukocytes were noted at 750 mg/kg/day. In diaphragm, vacuolation of muscle fibers was also observed in all animals treated with clofibrate, and these lesions were located in type-1 skeletal muscles densely stained with NADH-TR. The vacuoles seen in soleus muscle and diaphragm were positive for oil red O staining. In addition, increase of lipid droplets and mitochondrial hypertrophy was seen in soleus muscle, ultrastructurally. These data suggest that sensitivity to clofibrate-induced muscle toxicity differs among muscles, with type-1 fibers being susceptible.

Fenofibrate-Induced Muscular Toxicity Is Associated with a Metabolic Shift Limited to Type-1 Muscles in Rats

Morphological changes and mRNA expression levels in type-1 predominant soleus and type-2 predominant tensor fasciae latae muscles of rats treated with fenofibrate were investigated. After fenofibrate by oral gavage at 300 mg/kg/day for 28 days, degeneration/necrosis and regeneration of muscle fibers, cellular infiltration, and fibrosis were seen in soleus muscle. Additionally, expression of PDK4, CPT1-M, CPT2, and FACO mRNAs was increased. In contrast, no morphological changes or mRNA induction were apparent in tensor fasciae latae muscle. These data suggest that sensitivity to fenofibrate-induced muscle toxicity differs among muscles, with only type-1 fibers being susceptible. The up-regulation of PDK4, CPTs and FACO mRNA expression in soleus muscle indicates that the energy source is switched from glucose to fatty acids, and this might be related to the observed fenofibrate-induced muscular toxicity.

Skeletal Myopathy in Transgenic Mice Carrying Human Prototype c-Ha-ras Gene

Skeletal myopathy was found in almost all-transgenic mice carrying the human prototype c-Ha-ras gene (rasH2 mouse). Microscopically, variation of the muscle fiber size, centrally placed nuclei, regenerating fibers, and interstitial fibrosis were evident; hyalinization and necrosis were sometimes observed in the skeletal muscle (femoralis and pectoralis) of the rasH2 mice. Inflammatory changes in the skeletal muscle or abnormality of adjacent peripheral nerve were not observed. The features were essentially similar to those of muscular dystrophy. Although the severity was relatively mild compared to 34-week-old rasH2 mice, the skeletal myopathy was also observed in younger male (10 weeks of age) rasH2 mice. In nontransgenic littermates, skeletal myopathy was not observed. The mRNA of human c-Ha-ras product was detected in femoral muscle from the rasH2 mice by RT-PCR. In conclusion, these data suggest that skeletal myopathy is occurring in almost all rasH2 mice. Integration of c-Ha- ras gene is thought to be crucial to pathogenesis of skeletal myopathy in the rasH2 mice. Further characterization of the muscular lesion and its pathogenesis are needed to explore the possibility of rasH2 mouse becoming a new model for muscular dystrophy.

Spontaneous Poorly Differentiated Carcinoma with Cells Positive for Vimentin in a Salivary Gland of a Young Rat

Spontaneous salivary gland tumors in rats are rare. The authors report a poorly differentiated carcinoma of a submandibular gland in a ten-week-old rat that was positive for vimentin. Microscopically, the neoplastic cells showed a diffuse growth pattern in most areas of the tumor mass and a nestlike structure in a part of the peripheral area. Immunohistochemically, the cells were positive for keratin and vimentin but not for α-smooth muscle actin. Ultrastructurally, desmosome-like structures were observed. Based on these findings, the tumor was diagnosed as a poorly differentiated carcinoma. The origin of the neoplastic cells would be either acinar or ductal cells. This suggests that acinar or ductal cells have the potential to transform into vimentin-expressing cells.