Alternatively Spliced Lipin Isoforms Exhibit Distinct Expression Pattern, Subcellular Localization, and Role in Adipogenesis

Abstract

We recently identified mutations in the Lpin1 (lipin) gene to be responsible for lipodystrophy in the fatty liver dystrophy (fld) mouse strain. Previous studies revealed that lipin plays a critical role in adipogenesis, explaining the adipose-deficient phenotype of the fld mouse. In the current study, we demonstrate that alternative mRNA splicing generates two lipin isoforms, lipin-␣ and lipin-␤, which are differentially expressed during adipocyte differentiation. Lipin-␣ expression peaks at day 2 of 3T3-L1 cell differentiation, after which its levels gradually decrease. In contrast, lipin-␤ expression is transiently elevated at 10 h, followed by a drop to background levels at 20 h and a gradual increase between days 2 and 6 of differentiation. The two lipin isoforms also exhibit differences in subcellular localization. Lipin-␣ is predominantly nuclear, whereas lipin-␤ is primarily located in the cytoplasm of 3T3-L1 adipocytes, suggesting distinct cellular functions. Using primary mouse embryonic fibroblasts expressing either lipin-␣ or lipin-␤, we demonstrate functional differences between the two isoforms. Whereas lipin-␣ is required for adipocyte differentiation, the predominant effect of lipin-␤ expression is the induction of lipogenic genes. In vivo, overexpression of lipin-␤ specifically in mature adipocytes leads to elevated expression of lipogenic genes and adipocyte hypertrophy, confirming a role of lipin-␤ in the regulation of lipogenesis. In conclusion, our data suggest that the two lipin isoforms have distinct, but complementary, functions in adipogenesis, with lipin-␣ playing a primary role in differentiation and lipin-␤ being predominantly involved in lipogenesis.Lipodystrophies are a group of disorders characterized by loss of adipose tissue and the ensuing development of metabolic abnormalities of lipid and glucose metabolism (1). We previously isolated a novel gene, Lpin1 (lipin), which is responsible for lipodystrophy in a naturally occurring mouse model of this disease, the fatty liver dystrophy (fld) 2 mouse strain (2). Lpin1 mutations in fld mice lead to dramatically reduced body fat, insulin resistance, and increased susceptibility to dietinduced atherosclerosis (3). Lipin is a novel protein without any known protein domains other than a nuclear localization signal. However, it contains two evolutionarily conserved regions (NLIP and CLIP domains), which identify a novel gene family including two additional members in mammals, Lpin2 and Lpin3, and several orthologous genes in invertebrates, plants, and fungi (2). Recent studies in Schizosaccharomyces pombe revealed interactions between the NLIP domain and several nuclear proteins (4). Although the molecular function of lipin remains to be identified, studies in yeast suggest a role in the maintenance of nuclear envelope structure and nucleocytoplasmic transport.Consistent with the effects of lipin mutations on adipose tissue function, lipin is prominently expressed in adipose tissue (2). Lipin is phosphorylated in response to insulin stimula...

Identification of the transcript isoforms and expression characteristics for chicken Lpin1

“…It was found that the PAP of Lpin1-a, -b and -g isoforms were dependent on Mg 21 or Mn 21 ions (Han and Carman, 2010). Lpin1-a and Lpin1-b had different and complementary function (Peterfy et al, 2005;van Harmelen et al, 2007;Reue and Zhang, 2008). Here we identified a novel isoform Lpin1-d, which has an insertion between coding exons 12 and 13 of chicken Lpin-a, different from the reported transcript variants Lpin1-b and Lpin1-g of human/mouse, which have an insertion between coding exons 4 and 5, and coding exon 10 and 11 of Lpin1-a sequence, respectively (Han and Carman, 2010).…”

“…Lpin1 gene could generate multiple alternatively spliced mRNA forms. Transcript variants Lpin1-a, -b and -g have been identified in human and mouse (Peterfy et al, 2005;Han and Carman, 2010). It was found that the PAP of Lpin1-a, -b and -g isoforms were dependent on Mg 21 or Mn 21 ions (Han and Carman, 2010).…”

Endoplasmic reticulum stress suppresses lipin-1 expression in 3T3-L1 adipocytes

“…1E). In adipocytes, studies with these lipin-1 isoforms characterized that lipin-1A is localized mainly in nucleus and induces adipogenic genes, whereas lipin-1B is localized mainly in cytosol and induces lipogenic genes [28]. Because ER is located in cytosol and is the site of triacylglycerol synthesis, the prominent decrease in lipin-1B expression by ER stress might be relevant to ER functions, although it requires further investigation.…”

LPIN1 genetic variation is associated with rosiglitazone response in type 2 diabetic patients

“…Lipin1 protein has two isoforms (a and b) produced by alternative splicing [17]. Lipin a is mainly localized to the nucleus and plays a role in adipocyte differentiation whereas lipin b is localized to the cytoplasm and stimulates lipogenesis and triglyceride synthesis [17].…”

“…Lipin a is mainly localized to the nucleus and plays a role in adipocyte differentiation whereas lipin b is localized to the cytoplasm and stimulates lipogenesis and triglyceride synthesis [17]. Both lipin a and b ex pression levels are reduced in adipose tissue of insulin-resistant subjects [6].…”