Biology Reference
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et al., 2004; Villena, Roy, Sarkadi-Nagy, Kim, &Sul, 2004; Zimmermann et al., 2004
).
Knockout and transgenic mouse models of ATGL have provided valuable information
that helps to establish ATGL as a key TG lipase in both adipose and nonadipose tissues
(
Ahmadian et al., 2009; Haemmerle et al., 2006, 2011; Ong, Mashek, Bu, Greenberg, &
Mashek, 2011; Reid et al., 2008;Wu et al., 2011
). Despite the obvious advantages, how-
ever, there are several limitations to the use of mouse models for studying adipose
lipolysis. Firstly, it routinely takes 1-2 years to construct such an animal model, espe-
cially knockout mice. The process can be longer in the case of creation of an adipose
tissue specific knockout mouse. Secondly, hormone-stimulated lipolysis only occurs in
mature adipocytes, which originate from preadipocytes via adipogenic differentiation.
It is possible that the effect of loss or gain of a particular gene of interest on lipolysis as
revealed in a mouse model may be secondary to that on adipocyte differentiation other
than lipolysis
per se
. Thirdly, the phenotypic changes observed in transgenic or knock-
out mice often reflect a chronic and systemic influence of a protein on lipolysis. Impact
on lipolysis could be a result of complementation during the course of development and
thus indirect. Lastly, lipolysis is mediated via a coordinated action of multiple enzymes
and regulators. Given that they are usually separate models for one individual gene or
protein, knockout and transgenic mice are not suitable for studies of interplay between
two or more proteins involved in the lipolytic control. Though technically possible, it is
costly and time-consuming to manipulate expression of more than one gene in mice.
Accordingly, a new cell-based model is needed to provide a setting for studying the
function of multiple proteins in adipocyte lipolysis.
HeLa cells loaded with exogenous sources of FAs (oleate or palmitate) can form
lipid droplets mimicking those in adipocytes (
Smirnova et al., 2006; Soni et al., 2009;
Yang et al., 2010
). Due to their readiness for transfection with exogenous DNA or
siRNA, these cells have become a widely employed model for studying the roles of
proteins involved in lipid droplet turnover. However, despite valuable information
derived from these studies, the protein makeup of lipid droplets in HeLa cells does
not allow hormone-responsive lipolysis like the one observed in adipocytes. For
example, ATGL, HSL, perilipin, and other proteins essential for adipocyte lipolysis
are expressed at low levels endogenously in HeLa cells (
Miyoshi, Perfield, Obin, &
Greenberg, 2008; Yang, Heckmann, Zhang, Smas, & Liu, 2013
). In addition, in
HeLa cells ATGL is constitutively located at the surface of lipid droplets
(
Smirnova et al., 2006; Soni et al., 2009
), whereas in adipocytes it resides in cytosol
in the basal state and translocates to lipid droplets in response to
-adrenergic stim-
ulation (
Bezaire et al., 2009; Wang et al., 2011; Yang et al., 2010
).
3T3-L1 cell line, a highly adipogenic clone derived from Swiss mouse embryo
tissue (
Green &Meuth, 1974; Vogel & Pollack, 1973
), is one of the most commonly
used model for the studies of WAT. 3T3-L1 preadipocytes can differentiate into
mature adipocytes when stimulated with an appropriate hormonal regimen
(
Madsen et al., 2003; Sadowski, Wheeler, & Young, 1992
). During the process of
adipocyte conversion, these preadipocytes lose their primitive mesenchymal charac-
ter, accumulate TG-containing lipid droplets, and acquire adipocyte-like phenotypes
(
Ducharme & Bickel, 2008
). In particular, differentiated 3T3-L1 cells are known to
possess the complete LDs and protein machinery that support hormone-stimulated
b
