Obesity is increasing at an alarming rate in both the genders of all age groups around the world including United States [54
]. According to the National Bureau of Economic Research report, the prevalence of obesity will rise to 40% in men and 43% in women by 2020 [55
]. The major factors contributing to obesity are sedentary life style, inactivity, over eating and disturbances in the metabolism of fatty acids [56
]. Various studies indicated that obese humans are at increased risk of developing diabetes, hypertension, cancer and atherosclerosis [57
]. Thus, adipose tissue is recognized as a major secretory organ, releasing a variety of adipocytokines (e.g. adiponectin, leptin, resitin and others) which provide the link between obesity, insulin resistance and inflammatory disorders [60
]. Therefore, new therapeutic agents that can regulate adipogenesis and glucose uptake can be employed to control obesity and type 2 diabetes.
In the present study, we evaluated the effect of ReishiMax (RM), dietary supplement containing triterpenes and polysaccharides isolated from medicinal mushroom Ganoderma lucidum
, on adipocyte differentiation and glucose uptake in adipocytes. Here, we show that RM inhibited adipocyte differentiation/lipid accumulation through: a) the down-regulation of expression of transcription factors PPAR-γ, SREBP-1c and C/EBP-α, and b) the suppression of expression of genes responsible for lipid synthesis (FAS, ACS1), lipid transport (FABP4, FATP1) and lipid storage (perillipin). However, we did not observe any changes in the expression of LPL, which expression is also controlled by PPAR-γ, SREBP-1c and C/EBP-α. In agreement with our study, hydroxytorosol from olive oil inhibited lipid accumulation during adipocyte differentiation and inhibited all tested genes except LPL [61
]. Nevertheless, others demonstrated down-regulation of LPL expression associated with the adipocyte differentiation [62
]. These contradictory results might be explained by the use of different cell models: mouse 3T3-L1 (our study and [63
]), C3H10 T1/2 [61
] or human pre-adipocytes [62
] or activation of β-catenin pathway [63
]. Finally, RM activated AMPK resulting in the increased glucose uptake by adipocytes (Figure ).
On the molecular level, C/EBP-α, PPAR-γ and SREBP-1c are induced during adipocyte differentiation [12
]. Specifically, PPAR-γ and SREBP-1c mRNAs start to be expressed at the very early/early stage (day 1-2 post-confluence) followed by the expression of C/EBP-α at the intermediate stage (day 4 post-confluence) [50
]. These transcription factors further controls expression of adipocyte-specific genes (e.g. FAS, ACS1, FABP4, FATP1 and perilipin) at the late stage (day 5 post-confluence) leading to the fat droplet formation [64
]. Therefore, the inhibition of PPAR-γ, SREBP-1c and C/EBP-α, by RM at the early/intermediate stages results in the suppression of expression of adipocyte specific genes and lipid formation at the late stage of adipocyte differentiation.
The inhibition of adipogenesis by RM is in an agreement with recent studies demonstrating anti-adipogenic effects of white tea or bitter melon extracts or other isolated phytochemicals [65
]. Moreover, activation of AMPK was previously associated with the inhibition of adipocyte differentiation [70
] or increased glucose uptake. Indeed, our data are in an agreement with recent reports by Ha et al
] demonstrating an increase of glucose uptake and inhibition of adipocyte differentiation through the activation of AMPK in 3T3-L1 cells.
During the preparation of the present manuscript, Lee et al
. demonstrated that some purified triterpenes from G. lucidum
inhibit adipocyte differentiation, decrease lipid accumulation and down-regulate expression of PPAR-γ, SREBP-1c, C/EBP-α, FAS and ACC [73
]. Although the identification of biologically active components from G. lucidum
is important for their possible therapeutic use, and more than 130 triterpenes have been isolated and new triterpenes continue to be identified [76
], the yield of a majority of purified triterpenes is generally low. Therefore, the possible therapeutic use of purified triterpenes is currently limited. On the other hand, in our study we used standardized dietary supplement ReishiMax (RM) containing 6% triterpenes and 13.5% polysaccharides. Moreover, we have previously identified some of the triterpenes in RM (e.g. ganoderic acids A, F, H, Mh, S, lucidenic acid B, D, D1, E1, L and methyl lucidenate G) [77
], that can be used for the standardization of the active supplements or G. lucidum
extracts. Our study is with agreement with previous study demonstrating stimulation of glucose uptake and activation of AMPK in rat muscle cells [78
]. However, the authors used uncharacterized extract prepared from G. lucidum
purchased from the Korean market [78
]. On the other hand G. lucidum
extract (GE), containing approximately 10% of ganoderic acid A, induced adipocyte differentiation and expression PPAR-γ [79
]. Although GE activated PPAR-γ, ganoderic acid A itself did not show any effect on PPAR-γ, suggesting that other compounds in GE are responsible for the GE activity [79
]. Finally, standardized G. lucidum
extract, containing polysaccharides, adenosine and ganoderic acid A, demonstrated mild anti-diabetic effects in a controlled human trial [80
]. Since the presence and the amount of specific biologically active compounds in G. lucidum
extracts depends on the source of G. lucidum
, cultivation conditions, storage and extraction process [81
], it is not surprising that extracts from the same mushroom could have different or opposite activities.
In summary, only standardized and characterized dietary supplements/extracts should be considered for the use in alternative medicine. Since the use of dietary supplements is not regulated and they are freely accessible, the proper characterization must be performed and associated with the specific activity.