Members of the ORPs family are thought to act as sterol sensors that relay information to the cellular machineries maintaining lipid homeostasis, energy balance and cell signaling. In the present study, we describe OSBP/ORP expression patterns in human adipose depots and SGBS preadipocytes/adipocytes. Furthermore, we present evidence for a functional impact of ORPs on the adipocyte phenotype.
OSBP/ORPs displayed a similar expression pattern in subcutaneous and visceral adipose tissues from morbidly obese patients. However, the expression profiles across different ORPs were markedly uneven, and resembled that in SGBS adipocytes. This suggests that the distinct ORP expression pattern observed in the human adipose tissues mainly reflects the adipocytic ORP profile, and is not strongly disturbed by other cell types present in the fat tissues, such as fibroblasts, macrophages, and endothelial cells 
. Bioinformatic analysis suggested that, despite a relatively low mRNA copy number, ORP11 has an expression hotspot in adipocytes, unlike the other ORPs. In keeping with this, ORP11 mRNA and protein concentrations were up-regulated upon SGBS cell adipocytic differentiation while ORP2, ORP3, ORP4, ORP7 and ORP8 mRNAs (verified for ORP3 and ORP8 also at the protein level) were found to be down-regulated in the SGBS adipocytes as compared to preadipocytes.
ORP11 was reported to be up-regulated in the visceral adipose tissue of obese Canadian men with metabolic syndrome compared to men without the metabolic syndrome 
. We have previously found ORP11 to localize at the Golgi-late endosome interface in HEK293 cells, and demonstrated its interaction with the related ORP9 
, a protein implicated in sterol sensing or transport between the endoplasmic reticulum and the Golgi complex 
. However, the precise function of ORP11 remains poorly understood. Knock-down of ORP11 in the SGBS cell model resulted in decreased expression of adiponectin and aP2. Adiponectin is an adipokine highly expressed in the white adipose tissue. Because of its protective role against chronic inflammation, insulin resistance, weight gain, obesity and cardiovascular disease, adiponectin is evaluated as an important potential target for therapy development 
. aP2, also designated as FABP4, is a 15-kD cytosolic fatty acid binding protein present in adipocytes and macrophages. The protein plays a key role in glucose and lipid homeostasis, and its expression is regulated by fatty acids and insulin 
. The expression level of aP2 was suggested to predict the risk of metabolic syndrome, type 2 diabetes and the development of atherogenic dyslipidemia 
. In addition to impacts on adiponectin and aP2 expression, silencing of ORP11 resulted in an impairment of triglyceride storage in the SGBS adipocytes. During the SGBS differentiation time course, ORP11 was induced at late time points (16–22 days). Therefore, ORP11 most likely plays a role during late stages of adipogenic differentiation and possibly in the maintenance of the mature adipocyte phenotype.
ORP8 is abundantly expressed in CD14(+) monocytes (Fig. S1
) and in tissue macrophage 
, bringing up the possibility that its expression in the adipose depots of the patients studied could in part derive from monocyte-macrophages abundant in the inflamed adipose tissue of obese subjects 
. However, the abundant expression of ORP8 mRNA we detected in SGBS adipocytes and the data at Human U133A/GNF1H Gene Atlas suggest that the human adipose depot ORP8 mRNA derives to a large extent from adipocytes. Together with our recent data on ORP8 function 
, these observations suggest that the protein mediates lipid signals and fine-tunes the adipocyte lipid metabolism. Consistent with this notion, ORP8 overexpression in adipocytes significantly reduced the mRNA expression of aP2 and attenuated cellular triglyceride storage. Interestingly, Jordan et al. 
reported that ORP8, as a target of miR-143, has a function in insulin signaling in mouse hepatocytes. These observations are consistent with the idea that ORP8 manipulation may, via modification of membrane microdomain organization, impact signal transduction events. We therefore envision that ORP8 overexpression might attenuate insulin stimulation of the adipogenic process. Further studies on the putative connection of ORP8 with the insulin signaling pathways in adipocytes are therefore warranted.
ORP3 overexpression did not significantly alter the adipocyte differentiation marker mRNA levels or the triglyceride storage in SGBS adipocytes. This speaks against an important role of ORP3 in the control of adipocyte differentiation. We previously characterized ORP3 as a phosphoprotein involved in cell adhesion, capable of modifying integrin activity, the actin cytoskeleton, and phagocytosis 
. In contrast to the down-regulation of ORP3 we observed upon SGBS adipocyte differentiation, Lee et al. 
found the protein up-regulated in adipocytes differentiated from human mesenchymal stem cells. The apparent inconsistency between their and our findings is most likely due to the use of different cell models: Consistent with the data in 
, ORP3 was also in our study among the ORPs expressed abundantly in the s.c. and visceral adipose depots, as well as in the SGBS adipocytes, even though we detected its moderate down-regulation during SGBS preadipocyte conversion into adipocytes. We thus find it likely that induction of ORP3 occurs in the adipocyte lineage between the mesenchymal stem cell and the preadipocyte stages 
. Both ORP3 and ORP8 were down-regulated on the very first days of SGBS cell adipocytic differentiation. This may relate to the cessation of cell proliferation occuring at this stage – In other words, higher ORP3/ORP8 expression in the preadicytes could be associated with the active proliferation of these cells.
ORP2 was recently shown to have the capacity to facilitate cholesterol transfer from the plasma membrane to the endoplasmic reticulum and lipid droplets 
. Moreover, we have earlier demonstrated localization of ORP2 on cytoplasmic lipid droplets 
. However, in the present study we found the ORP2 mRNA down-regulated upon SGBS cell adipogenic differentiation, suggesting that elevated ORP2 expression is not essential for adipocyte lipid storage.
In summary, we report the expression patterns of OSBP/ORPs in human s.c. and visceral adipose depots and present, by using the SGBS preadipocyte/adipocyte model, the first data on the functional role of ORP proteins in adipocyte differentiation.