Lipin Expression Is Attenuated in Adipose Tissue of Insulin-Resistant Human Subjects and Increases With Peroxisome Proliferator–Activated Receptor γ Activation

Aiwei Yao-Borengasser 2 Neda Rasouli 2 Vijayalakshmi Varma 2 Leslie M. Miles 2 Bounleut Phanavanh 2 Tasha N. Starks 2 Jack Phan 1 Horace J. Spencer III 0 Robert E. McGehee Jr. 3 Karen Reue 1 Philip A. Kern 2 0 Department of Biostatistics, University of Arkansas for Medical Sciences , Little Rock, Arkansas; and the 1 Departments of Medicine and Human Genetics, VA Greater Los Angeles Healthcare System, University of California at Los Angeles , Los Angeles, California; the 2 Department of Medicine, Division of Endocrinology, Central Arkansas Veterans Healthcare System, University of Arkansas for Medical Sciences , Little Rock, Arkansas; the 3 Department of Pediatrics, University of Arkansas for Medical Sciences , Little Rock , Arkansas. Associate Chief of Staff, Research, Central Arkansas Veterans Healthcare System , 598/151 LR, 4300 West 7th St., Little Rock, AR 72205 Lipin- and - are the alternatively spliced gene products of the Lpin1 gene, whose product lipin is required for adipocyte differentiation. Lipin deficiency causes lipodystrophy, fatty liver, and insulin resistance in mice, whereas adipose tissue lipin overexpression results in increased adiposity but improved insulin sensitivity. To assess lipin expression and its relation to insulin resistance in humans, we examined lipin- and - mRNA levels in subjects with normal or impaired glucose tolerance. We found higher expression levels of both lipin isoforms in lean, insulinsensitive subjects. When compared with normal glucosetolerant subjects, individuals with impaired glucose tolerance were more insulin resistant, demonstrated higher levels of intramyocellular lipids (IMCLs), and expressed 50% lower levels of lipin- and -. In addition, there was a strong inverse correlation between adipose tissue lipin expression and muscle IMCLs but no evidence for an increase in muscle lipid oxidation. After treatment of the impaired glucose-tolerant subjects with insulin sensitizers for 10 weeks, pioglitazone (but not metformin) resulted in a 60% increase in the insulin sensitivity index (Si) and a 32% decrease in IMCLs (both P < 0.01), along with an increase in lipin- (but not lipin-) expression by 200% (P < 0.005). Lipin expression in skeletal muscle, however, was not related to obesity or insulin resistance. Hence, high adipose tissue lipin expression is found in insulin-sensitive subjects, and lipin- expression increases following treatment with pioglitazone. These results suggest that increased adipogenesis and/or lipogenesis in subcutaneous fat, mediated by the LPIN1 gene, may prevent lipotoxicity in muscle, leading to improved insulin sensitivity. Diabetes 55:2811-2818, 2006 - A is a parallel increase in diabetes (13). With long with the increasing prevalence of obesity progressive obesity, there is a significant worsening of insulin resistance and the development of features of the metabolic syndrome (4), which include abnormalities in insulin secretion, hepatic steatosis, dyslipidemia, and atherosclerosis (5,6). A common element in obesity and its complications is the accumulation of lipids in adipose tissue and in other organs, such as liver, islets, and muscle (79). However, the pathophysiology of these relationships is not well understood. Important insight into the role of adipose tissue in metabolic syndrome comes from studies of lipodystrophic animals and humans. Whereas excess adipose tissue in the form of obesity leads to insulin resistance, the absence of adipose tissue leads to a severe syndrome characterized by ectopic deposition of lipids in other organs (10). This condition can be treated and partially reversed by the transplantation of adipose tissue in mice (11) or by the treatment of mice or humans with recombinant leptin (12,13). These studies suggest that adipose tissue secretory proteins may be important for normal glucose and lipid homeostasis and also suggest that the absence of adipose tissue deprives the animal of an important lipid storage depot, resulting in inappropriate lipid storage and lipotoxicity in other organs. Indeed, several studies (9,14,15) in humans have demonstrated a significant relationship between muscle lipid accumulation and peripheral insulin resistance. Further insight into the role of adipose tissue in insulin resistance comes from studies using thiazolidinediones (TZDs). These drugs are high-affinity ligands for peroxisome proliferatoractivated receptor (PPAR), a transcription factor activated early in the process of adipocyte differentiation (16), and result in improved insulin sensitivity, a process that predominantly involves glucose uptake in skeletal muscle (17,18). In recent studies, the treatment of subjects with the TZD pioglitazone resulted in an increase in subcutaneous adipose tissue, a decrease in intramyocellular lipids (IMCLs), along with an improvement in insulin sensitivity (19). Such studies with TZDs suggest that the redistribution of lipids into adipose tissue results in the loss of ectopic lipids from other organs and thus improves insulin action due to a relief of lipotoxicity. The fatty liver dystrophy (fld) mouse has features of human lipodystrophy, and this phenotype results from a mutation in the Lpin1 gene, whose gene product is lipin (20). Lipin expression is required for normal adipocyte differentiation, such that lipin-deficient mice fail to develop lipid-filled mature adipocytes (21). In contrast, transgenic mice with enhanced lipin expression in adipose tissue become obese, exhibiting increased lipogenic gene expression and hypertrophic adipocytes (22). Despite increased adipose tissue accumulation, the lipin transgenic mice remain insulin sensitive, likely due to a redistribution of lipids from ectopic sites, such as liver and skeletal muscle, into adipose tissue. On the other hand, lipin overexpression in muscle yielded a mouse that was obese, insulin resistant, and with decreased muscle lipid oxidative capacity (22). In recent studies, the Lpin1 gene has been shown to generate two alternatively spliced mRNA forms, referred to as lipin- and -, which differ by 33 amino acids encoded by an alternatively spliced exon. The two lipin isoforms appear to have distinct roles in the adipocyte in terms of expression dynamics, subcellular localization, and induction of adipogenic and lipogenic gene expression (23). Thus, lipin- is primarily present in the nucleus and stimulates expression of genes involved in adipocyte differentiation such as PPAR and CCAAT/enhancer-binding protein . On the other hand, lipin- is predominantly localized to the cytoplasm and is associated with the expression of genes involved in lipogenesis and triglyceride storage in adipocytes, including fatty acid synthase and diacylglycerol acyltransferase (23). The molecular function of lipin is not known, but studies of yeast lipin homologs have revealed interactions with proteins involved in nuclear transport and chromosome segregation (24) and a potential role for l (...truncated)