Rab18 Dynamics in Adipocytes in Relation to Lipogenesis, Lipolysis and Obesity

Abstract

Lipid droplets (LDs) are organelles that coordinate lipid storage and mobilization, both processes being especially important in cells specialized in managing fat, the adipocytes. Proteomic analyses of LDs have consistently identified the small GTPase Rab18 as a component of the LD coat. However, the specific contribution of Rab18 to adipocyte function remains to be elucidated. Herein, we have analyzed Rab18 expression, intracellular localization and function in relation to the metabolic status of adipocytes. We show that Rab18 production increases during adipogenic differentiation of 3T3-L1 cells. In addition, our data show that insulin induces, via phosphatidylinositol 3-kinase (PI3K), the recruitment of Rab18 to the surface of LDs. Furthermore, Rab18 overexpression increased basal lipogenesis and Rab18 silencing impaired the lipogenic response to insulin, thereby suggesting that this GTPase promotes fat accumulation in adipocytes. On the other hand, studies of the β-adrenergic receptor agonist isoproterenol confirmed and extended previous evidence for the participation of Rab18 in lipolysis. Together, our data support the view that Rab18 is a common mediator of lipolysis and lipogenesis and suggests that the endoplasmic reticulum (ER) is the link that enables Rab18 action on these two processes. Finally, we describe, for the first time, the presence of Rab18 in human adipose tissue, wherein the expression of this GTPase exhibits sex- and depot-specific differences and is correlated to obesity. Taken together, these findings indicate that Rab18 is involved in insulin-mediated lipogenesis, as well as in β-adrenergic-induced lipolysis, likely facilitating interaction of LDs with ER membranes and the exchange of lipids between these compartments. A role for Rab18 in the regulation of adipocyte biology under both normal and pathological conditions is proposed.

Nutritional, hormonal, and depot-dependent regulation of the expression of the small GTPase Rab18 in rodent adipose tissue

“…fasting and diet-induced or genetic obesity), and thyroid function. These observations, mostly based on mRNA data, are in agreement with previous functional results supporting the participation of Rab18 in the regulation of lipid metabolism in adipose tissue (Pulido et al 2011). Effects of fasting on WAT Rab18 expression.…”

“…Consistent with these early studies, we recently demonstrated that RAB18 overexpression increased basal lipolysis and RAB18 silencing-impaired forskolin-stimulated lipolysis in 3T3-L1 cells (Pulido et al 2011). Intriguingly, we also observed that insulin, like the b-adrenergic receptor agonist isoproterenol, induced the recruitment of RAB18 to the surface of LDs in 3T3-L1 cells, where silencing of RAB18 impaired insulin-stimulated lipogenesis and its overexpression enhanced lipogenesis (Pulido et al 2011). Together, these data suggest that RAB18 plays a role in both lipolysis and lipogenesis in 3T3-L1 adipocytes.…”

“…Among those best documented is RAB18, which was first shown to associate with the surface of LDs in 3T3-L1 adipocytes in response to b-adrenergic stimulation and was therefore proposed to be involved in the release of lipids from LDs (Brasaemle et al 2004, Martin et al 2005, Ozeki et al 2005. Consistent with these early studies, we recently demonstrated that RAB18 overexpression increased basal lipolysis and RAB18 silencing-impaired forskolin-stimulated lipolysis in 3T3-L1 cells (Pulido et al 2011). Intriguingly, we also observed that insulin, like the b-adrenergic receptor agonist isoproterenol, induced the recruitment of RAB18 to the surface of LDs in 3T3-L1 cells, where silencing of RAB18 impaired insulin-stimulated lipogenesis and its overexpression enhanced lipogenesis (Pulido et al 2011).…”

“…This age-related increase in RAB18 expression coincides with the period of highest adipocyte hypertrophy in rat WAT, which occurs between P40 and P80 and is characterized by an increase in the lipogenic activity and a decline in the lipolytic rate of adipocytes (Tsujikawa & Kimura 1980, Herrera & Amusquivar 2000. Accordingly, it has been reported that Rab18 mRNA expression and protein content increased in 3T3-L1 adipocytes as these cells accumulated lipids during differentiation (Pulido et al 2011). This may account for the presence of Rab18 immunoreactivity in the SVF of both subcutaneous and visceral fat, as this component of WAT contains a population of preadipocytes (Frayn et al 2003).…”

“…Glucocorticoids do not appear to play a major role in the regulation of Rab18 expression in WAT as much as administration of dexamethasone to either adrenalectomized or sham-operated animals did not alter the expression levels of this GTPase. Additionally, exposure of 3T3-L1 adipocytes to dexamethasone had no effect on Rab18 mRNA levels (Pulido et al 2011). On the other hand, the b-adrenergic agonist isoproterenol, besides increasing the association of this protein to the LD surface, enhanced Rab18 transcript and protein content in 3T3-L1 cells (Pulido et al 2011).…”