Abstract
Genetic knockout of hormone-sensitive lipase in mice has implicated the presence of other intracellular triacylglycerol (TAG) lipases mediating TAG hydrolysis in adipocytes. Despite intense interest in these TAG lipases, their molecular identities thus far are largely unknown. Sequence data base searches for proteins containing calcium-independent phospholipase A2 (iPLA2) dual signature nucleotide ((G/A)XGXXG) and lipase (GXSXG) consensus sequence motifs identified a novel subfamily of three putative iPLA2/lipase family members designated iPLA2epsilon, iPLA2zeta, and iPLA2eta (previously named adiponutrin, TTS-2.2, and GS2, respectively) of previously unknown catalytic function. Herein we describe the cloning, heterologous expression, and affinity purification of the three human isoforms of this iPLA2 subfamily in Sf9 cells, and we demonstrate that each possesses abundant TAG lipase activity. Moreover, iPLA2epsilon, iPLA2zeta, and iPLA2eta also possess acylglycerol transacylase activity utilizing mono-olein as an acyl donor which, in the presence of mono-olein or diolein acceptors, results in the synthesis of diolein and triolein, respectively. (E)-6-(Bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one, a mechanism-based suicide substrate inhibitor of all known iPLA2s, inhibits the triglyceride lipase activity of each of the three isoforms similarly (IC50=0.1-0.5 microm). Quantitative PCR revealed dramatically increased expression of iPLA2epsilon and iPLA2zeta transcripts during the hormone-induced differentiation of 3T3-L1 cells into adipocytes and identified the presence of all three iPLA2 isoforms in human SW872 liposarcoma cells. Collectively, these results identify three novel TAG lipases/acylglycerol transacylases that likely participate in TAG hydrolysis and the acyl-CoA independent transacylation of acylglycerols, thereby facilitating energy mobilization and storage in adipocytes.
Highlights
Triacylglycerol/fatty acid recycling is an important mechanism by which adipocytes modulate fatty acyl flux in response to changing metabolic conditions (9 –12)
Despite the obvious critical role of these non-Hormone-sensitive lipase (HSL) TAG lipases in adipocyte signaling and lipid homeostasis, virtually nothing is known about their molecular identities, catalytic properties, or mechanisms of regulation
Progress in understanding adipocyte triglyceride homeostasis has been hindered by the difficulty in determining the diversity and chemical identities of non-HSL TAG lipases present in adipocytes [24, 26, 27]
Summary
Triacylglycerol/fatty acid recycling is an important mechanism by which adipocytes modulate fatty acyl flux in response to changing metabolic conditions (9 –12). In the process of searching for novel calcium-independent phospholipases A2 by protein sequence homology searches for candidate enzymes containing the iPLA2 dual signature nucleotide ((G/A)XGXXG) and active site lipase (GXSXG) sequence motifs, we identified a subfamily of putative iPLA2 enzymes (previously named adiponutrin, TTS-2.2, and GS2) of previously unknown catalytic function, each of which contain an N-terminal patatin (iPLA2␣) homology domain as determined by protein family analysis (Fig. 1). One of these proteins, adiponutrin, has received much attention as an adipocyte-specific protein that is down-regulated by either fasting [30] or treatment with thiazolidinediones [31] and is acutely up-regulated by re-feeding a high carbohydrate [30, 32] or high protein diet [33]. These results identify a novel class of triglyceride lipase/ transacylase enzymes that likely participate in adipocyte triglyceride fatty acyl liberation, recycling, and lipid homeostasis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
