Obesity is a growing epidemic around the world and dramatically increases the risk of developing a number of chronic diseases (Gambero and Ribeiro, 2015). The currently available anti-obesity drugs contain numerous adverse effects (Li and Cheung, 2011), having a great interest in natural products as therapeutics since they are considered reliable than synthetic medicines (Vermaak et al, 2011). To the best of our knowledge, no scientific report exists on the adipogenic activities of Citrullus colocynthis seed extracts and its bioactive molecules in in vitro model. Therefore, the current experiment was designed to evaluate such biological properties on controlling the adipogenesis process in 3T3-L1 preadipocytes.
The UPLC-ESI-MS/MS analysis of the hydro ethanolic extract of C. colocynthis seeds (SCEE) revealed the isolation and characterization of ten compounds: Quinic acid, Isovitexin, Scoparin, Vitexin-2’’-O-rhamnoside, Reserpine, Digitoxin, Triprolidine, Naringenin, Linoleic acid and Oleic acid. Our current study, demonstrated that the addition of SCEE to 3T3-L1 cells suppressed the intracellular lipid accumulation in the early stage (days 0–2) but not at middle (days 2–4) and late (days 4–8) stages of adipogenesis (Fig. 5) without being cytotoxic (Fig. 4). In the early stage, SCEE decreased the levels of GPDH (Fig. 6) which is a triglyceride accumulation-independent marker of adipocyte differentiation (Bae et al, 2014). These results are in agreements with numerous previous works showing that arresting or delaying the cell cycle of 3T3-L1 cells by phenolic compounds during the first 2 days of differentiation, decreased cell number and inhibited the differentiation rate of adipocytes (Lee et al, 2009; Kim et al, 2008). Further, UPLC–MS/MS results revealed that Isovitexin, Scoparin, Vitexin-2’’-O-rhamnoside, Naringenin are the most abundant active component in SCEE which presents anti-adipogenic effects. Several studies in 3T3-L1 (Richard et al, 2013, Kim et al, 2010) and human adipocytes (Claussnitzer et al. , 2013) indicate that vitexin, isovetexin and naringenin have the ability to impair adipogenesis or lipid accumulation in differentiating adipocytes. Furthermore, apart from the effects on adipogenesis regulators, SCEE has been demonstrated to act on the expression of genes related to adipogenesis. Adipogenesis implicates a series of sequential events, such as cell cycle arrest, clonal expansion and differentiation. These events results from a series of programmed changes transcription factors and their target genes involved in adipogenesis, such as PPARγ, C/EBPα and SREBP1. (Kim and Kong 2010; Ko et al. 2015).
The process probably starts with the activation of C/EBPα, C/EBPβ and C/EBPδ and is followed by PPARγ activation, which acts directly on different genes associated with adipogenesis (Gambero and Ribeiro, 2015). C/EBPs can affect adipogenesis by coordinating and regulating a cascade of transcription factors leading to the establishment of the differentiated cells. For example, C/EBP beta and delta have been shown to be expressed in adipogenesis at an early stage, while C/EBP alpha is expressed much later. The vital role played by C/EBP beta and delta in adipogenesis was demonstrated by the action of both genes, which prevents the normal development of adipose tissue (Chen et al, 2009). In our in vitro study, we found that SCCE inhibited C/EBPβ and C/EBPδ mRNA in dose dependant manner versus control in 24, 48 and 72H (Fig. 7). Our results are in accordance with studies cited by Moseti et al, 2016 (Moseti et al. , 2016). Then the main transcriptional factors are PPAR-γ and C/EBP-α. PPAR-γ is an indispensable factor for adipocyte differentiation and is involved in a positive feedback loop with C/EBP-α to sustain expression mutually (Koh et al. 2008).
In this study, SCEE considerably reduced the mRNA expression levels of PPAR-γ and C/EBP-α in 3T3-L1 adipocytes which lead to a reduction in fat deposition (Fig. 8A). These data suggest that the principal mechanism responsible for the anti-adipogenesis effects of SCEE occurs through down-regulation of the activation of the key transcriptional regulators C/EBPα and PPARγ in 3T3-L1 adipocytes. Recent reports have described that obesity represents an inflammatory disease that causes insulin resistance in adipose tissue (Weisberg et al, 2003; Xu et al, 2003). Obese adipose tissues are characterized by elevated content of macrophages. A paracrine loop involving adipocyte is believed to derive free fatty acids (Kanada et al, 2006). IL-6, resistin, and TNF-α are associated strongly with obesity-induced inflammation and obesity-related diseases. In this study, SCEE suppressed these gene mRNA expressions. These results suggest that SCEE may exert anti-adipogenic effects through multiple mechanistic routes and attenuates the inflammatory response in adipocytes. A previous study reported that vitexin reduced lipid droplet accumulation in 3T3-L1 cells by 37% at 100 µM and decreased C/EBPα and PPARγ protein expression level (Kim et al, 2010).
Moreover, Vitexin and isovitexin were shown to be involved in the regulation of inflammation induced lipogenesis, insulin resistance, oxidative stress, and lipid synthesis. Kang et al showed that vitexin significantly decreased the inflammation-induced adipogenesis and lipogenesis with in vitro and in vivo study (Kang et al, 2015). Adiponectin and leptin are recognized as adipokines with health promoting effects. Adiponectin has anti-diabetic, anti-artherogenic and anti-inflammatory (Pajvani and Scherer, 2003). Decreased expression of adiponectin is usually associated with obesity and insulin resistance. Increase of adiponectin mRNA expression in the adipocyte represents one of the major criteria for lipid accumulation in the adipocyte. Our finding strongly supports the above said suggestion, in that SCEE treatment decreased lipid droplet and glycerol accumulation in adipocyte may be due to decreasing of adiponectin.
This data suggest that the triglyceride highly limit the adipogenesis process through inhibition of adiponectin by PPAR-γ and C/EBP-α signaling pathway. Leptin, previously named the ‘satiety or anti-obesity hormone’, blocks food intake and stimulates thermogenesis (Ahima and Flier, 2000). The present study, adipocyte supplemented with different concentration of SCEE downregulates leptin mRNA expression. Resistin represents one of secreted proteins (resistin like molecules) that share an unusual cysteine-rich motif at the aminoterminal ending. Secretion of resistin has also been shown to dramatically enhanced during adipogenesis of the 3T3-L1 murine clonal cell line and of primary cultured stromal cells derived from rat adipose tissue. Expression of the resistin gene in the 3T3-L1 cell line sounds to be controlled by mechanisms involved protein kinase A-dependent. (Janke et al, 2002). Our data showed that SCEE also decreased the expression of Resistin in dose dependent manner.
On the other hand, C/EBP and PPAR expression depends on other genes, which are also concerned in adipogenesis process such us cAMP responsive element binding protein 1 (SREBP-1c). The SREBP family has been demonstrated to directly regulate a group of genes implicated in triglycerides and cholesterol synthesis (Horton et al, 2003). In previous studies, dominant negative SREBP-1c expression was found to block preadipocyte differentiation and HLH over expression to prove the adipogenic activity of PPARγ (Kim and Spiegelman, 1996). Moreover, the SREBP-1c isoform seems to be mainly regulated at the transcriptional level by insulin (Eberlé et al, 2004). In our study, we demonstrated that SCEE decreased the expression of SREBP-1c and its downstream target gene FAS in a dose-dependent manner (Fig. 5B). Our data suggest that SCEE inhibit adipogenesis in both pathways PPARγ, C/EBPα and SREBP.
Fatty acid synthase (FAS), which an enzyme that catalyzes the de novo synthesis of palmitate from acetyl CoA and maloinyl CoA, play an important role in the long term regulation of lipogenesis (Wong and Sul 2010). GLUT-4 is the principal protein of transporting glucose into adipose tissue and muscle (McGowan et al. 1995) and then the glucose molecule converted into acetyl CoA which is the fundamental precursor for fatty acid synthesis. The fatty acid synthase and GLUT-4 mRNA expression ameliorates the adipocyte differentiation via the metabolism of fatty acid and glucose uptake by the cells.
Therefore, we planned to find out the inhibitory mechanism of SCEE on fatty acid synthase and GLUT-4 mRNA expression in the adipocyte. In the present study, we demonstrated that 3T3-L1 cell incubated with different concentration of SCEE decreased expression of FAS mRNA as compared with control cells. This result suggested that the SCEE has anti-adipogenic activity by inhibiting FAS mRNA expression. FAS is a primary enzyme that are regulating fatty acid synthesis. The findings of the present study revealed that FAS and expression were inhibited by SCEE treatment. This suggested that SCEE inhibits preadipocytes differentiation and lipid accumulation through inhibiting the expression or activity of adipogenic transcriptional factors and their related genes.
In conclusion, our data demonstrate that Citrullus colocynthis seeds have the inhibitory effects on adipogenesis in 3T3-L1 adipocytes as indicated by a significant reduction in triglyceride accumulation without cytotoxicity effect. The inhibitory effects of SCEE may be mediated through the suppression of PPAR-γ and C/EBP-α as well as their target genes. In vivo investigations are necessary to clarify the importance of these results for human therapeutic use of Citrullus colocynthis’s seeds extract as a treatment for obesity.
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