Abstract
Perilipin2 (Plin2), also known as adipose differentiation-related protein (ADRP), or adipophilin, is a member of the PAT family involved in lipid droplet (LD) formation in the liver and peripheral tissues. Although Plin2 was originally identified as a highly expressed gene in adipocytes, its physiological role in mature adipocytes is largely unknown. In this report, we investigated the regulation of Plin2 expression and its function in differentiated adipocytes of mouse embryonic fibroblasts (MEFs). Plin2 mRNA levels increased during adipocyte differentiation whereas protein levels did not. Plin2 was degraded through the ubiquitin-proteasome pathway but was inhibited by lipolytic inducers. Furthermore, lentiviral-mediated Plin2 knockdown attenuated lipolysis in differentiated MEFs in a time-dependent manner. Oleic acid-induced LD formation enhanced Plin2 protein stability when it was localized to LDs. Furthermore, a mutational analysis revealed that the ubiquitination and degradation of Plin2 required both the second and third alanine in the N-terminal region. These results suggest that Plin2 is degraded in the cytosol in its N-terminal amino acid sequence-dependent manner and instead becomes stable when localized on LDs. Our findings highlight the relationship between protein stability and a previously unnoticed function of Plin2 during lipolysis in adipocytes.
Highlights
Step of lipolysis[16,17]
We and other groups have previously shown that 3T3-L1 cells, which are a widely used model for adipogenesis studies, do not express some important genes which are expressed in vivo white adipose tissues (WATs), such as estrogen receptor α (ERα )[28], Plin[520], and sterol regulatory element binding protein-1c (SREBP-1c)[29]
We have shown that these genes are expressed in adipocyte-differentiated mouse embryonic fibroblasts (MEFs) or stromal vascular cells (SVCs)[30]
Summary
Step of lipolysis[16,17]. the molecular mechanisms involved in the lipolytic cascade remain unknown. Plin[2] is ubiquitously expressed and reported to be involved in LD formation[18,19] It is induced in the liver during fasting[20] and contributes to the development of fatty liver diseases, which has been proven by knockout mice experiments[21], the mice still produce an N-terminally truncated form of Plin[2], which possesses biological activities[22]. To elucidate the precise roles of Plin[2] in adipocytes, we investigated the mechanism regarding its expression and function during LD formation using either an adipocyte differentiation model or a simplified LDs formation model by loading bovine serum albumin (BSA)-conjugated oleic acids (OAs) on non-adipogenic cells. While the Plin[2] in the LDs fraction was protected from protein degradation, cytosolic Plin[2] was susceptible to ubiquitin-proteasomal degradation
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