Bactrian Camel (Camelus bactrianus) Milk Exosomes Promote Glucose Consumption in L6 Cells

In recent decades, type 2 diabetes mellitus (T2DM) has become a global pandemic, which has brought a heavy burden to the global health and economy (Reed et al., 2021). The etiology of T2DM is mainly due to insufficient insulin secretion of pancreatic β cells and insulin resistance (IR) of peripheral organs (Galicia-Garcia et al., 2020). IR in peripheral organs, including skeletal muscle and adipose tissue, results in decreased glucose uptake and utilization (Laakso 2001). Therefore, alleviating IR in peripheral organs plays a very important role in the treatment of T2DM (Hao et al., 2011). At present, there are contraindications and adverse reactions of various drugs used in clinical treatment of diabetes and long-term application can cause secondary failure (Wang et al., 2019). Recent studies have found that milk exosomes have various biological functions and can be used as drug carriers for targeted therapy of diseases (Galley and Besner 2020).

Bactrian camels feature two humps on their backs and are adapted to live in desert areas (Bai et al., 2020). Studies have proved that camel milk has antioxidant, anti-infection, anti-diabetes, anti-cancer and anti-hypertensive effects (Sharma et al., 2022; Singh et al., 2019). Studies have found that camel milk has the effects of lowering blood sugar and IR and can be used as an adjuvant in the treatment of diabetes. However, the exact anti-diabetic components contained in camel milk and the molecular mechanisms are still not fully understood (Ayoub et al., 2018).

Exosomes are nanoscale vesicles released from various cells into the extracellular space. The nucleic acids, proteins and lipids contained in exosomes can be taken up by adjacent or distant cells and regulate the function of recipient cells (Zhang et al., 2020; Zhao and Zhao 2015). Mammalian milk exosomes have been isolated and identified (Admyre et al., 2007; Yun et al., 2021). Milk-derived exosomes contain non-coding RNA, including miRNA, lncRNA and circular RNA, which are involved in regulation of immune response and development of metabolic diseases such as obesity and diabetes (Jiang et al., 2021). In addition, it has been reported that camel milk exosomes have anti-tumor and anti-oxidative stress effects (Badawy et al., 2018; Ibrahim et al., 2019). Now studies have shown that miRNAs in milk exosomes play an important role in anti-inflammatory, immune regulation and metabolic regulation (Chen et al., 2020; Melnik et al., 2021). However, the function of miRNAs in exosomes of camel milk remains to be studied and developed.

Based on the above, we hypothesized that camel milk exosomes could regulate glucose metabolism. We detected the changes of glucose consumption and DEGs in L6 cells treated with camel milk exosomes. The results suggested that camel milk exosomes promoted glucose consumption by inhibiting mitochondrial respiratory chain complex I.

Experiment
 
The study was conducted at college of veterinary medicine of Inner Mongolia Agricultural University and Novogene company from 2021 to 2022. Camel milk was collected from Siziwang Banner, Inner Mongolia.
 
Exosomes isolation and characterization
 
Fresh camel milk was collected from female Bactrian camels at mid lactation period. The camel milk exosomes were isolated by differential ultracentrifugation. Camel milk was centrifuged at 8000 g at 4°C for 30 min to remove fat, casein and cell debris. The supernatant was taken and centrifuged at 13,000 g at 4°C for 1 h to remove the remaining fat and cell debris. The supernatant of skim milk was ultracentrifuged at 120,000 g at 4°C for 120 min to remove the supernatant and obtain exosome precipitation. The exosome particles were suspended in phosphate buffered saline (PBS) to obtain homogenous suspension. The milk exosomes were filtered by a 0.22 µm filter and then stored in a refrigerator at -80°C. The total protein concentration of milk exosomes was determined by Bicinchoninic Acid (BCA) method. The exosomes were fixed in 2.5% glutaraldehyde in cacodylate buffer at 20°C for 1 h and stained with 2% uranyl acetate. The exosomes were identified by transmission electron microscopy (TEM) (JEM2100, Joel Inc., Japan).
 
Materials
 
The rat skeletal muscle cell line L6 myoblasts were purchased from Chinese Type Culture Collection(CTCC). Dulbecco’s modifified Eagle’s medium (DMEM), fetal bovine serum (FBS) and penicillin-streptomycin were purchased from Gibco (USA). BCA protein quantitation assay kit was purchased from Keygen Biotech (China). CCK-8 assay kit was purchased from Meilunbio (China). Glucose oxidase activity assay kit was purchased from Applygen Technologies (China).
 
Cell culture and CCK-8 assay
 
L6 myoblasts were cultured in DMEM supplemented with 10% FBS and 100U/mL penicillin-streptomycin at 37°C in a humidified atmosphere containing 5% CO₂.

L6 myoblasts were seeded in 96-well plates at the density of 3 × 103 per well. The cells were treated with medium containing different doses of camel milk exosomes. The treatment doses were 0, 3, 6, 12 and 24 ng/µL, respectively and the treatment time was 24 h.10 μL of CCK-8 solution was added to each well and continue to culture for 0.5 h. The OD values were read at 450 nm.
 
Detection of glucose consumption
 
L6 myoblasts were seeded in 96-well plates at a density of 5 × 103 per well. The cells were washed three times with PBS. The cells were treated with medium containing different doses of camel milk exosomes. The treatment doses were 0, 3, 6, 12 and 24 ng/µL, respectively, metformin (2 mM) was used as the positive control and the treatment time was 24 h. Glucose concentration of cell culture supernatant was detected by Glucose oxidase activity assay kit according to the manufacturer’s instructions and calculated the glucose consumption.
 
RNA sequence
 
L6 myoblasts were seeded in 25 cm² cell culture flask. L6 cells were cultured with medium containing 12 ng/µL camel milk exosomes for 24 h and the control group was not added with exosomes. There were three replicates in each treatment group and control group. Trypsinized cells were frozen at -80°C for RNA sequencing.

The samples were sequenced on the Illumina Hiseq 2500 platform (Novogene, Beijing, China). Data quality was checked using the fastq software. The reads were compared with rat genome and treated by Hisat2. Differential gene expression analysis was performed by the DESeq2 R package. Genes with p<0.05 and |log2Fold change| >1 are defined as DEGs. The statistical enrichment of DEGs in GO terms and KEGG pathways was tested by clusterProfiler R package. GO terms with corrected p<0.05 were considered significantly enriched by DEGs. KEGG pathway with corrected p<0.05 were significantly enriched by DEGs.
 
Statistical analysis
 
Statistical analysis was performed by ANOVA (GraphPad Prism 9). The data are presented as means±SD. Significant differences between or among groups are indicated by *p<0.05 and **p<0.01.

TEM showed that the diameter of camel milk exosomes ranged from 30 nm to 100 nm (Fig 1). The size of camel milk exosomes observed by TEM in this study is consistent with the results previously reported (Ibrahim et al., 2019).



In this study, CCK-8 assay was used to detect the effect of different concentrations of camel milk exosomes on the viability of L6 cells. The results of CCK-8 assay showed that there was no significant change in the cell viability after 3, 6 and 12 ng/µL camel milk exosomes treated L6 cells for 24 h (P>0.05). The viability of cells treated with 24 ng/µL was significantly decreased for 24 h (P<0.05) (Fig 2).



In this study, the effect of camel milk exosomes on glucose consumption in L6 cells was detected. The results showed that 6, 12 and 24 ng/µL camel milk exosomes significantly increased the glucose consumption of L6 cells for 24 h (P<0.05) (Fig 3). Camel milk can be used to treat diseases such as diabetes, cancer and gastrointestinal disease (Sabha et al., 2020). Furthermore, camel milk exosomes have been shown to have anti-cancer effects by inhibiting inflammation and oxidative stress (Badawy et al., 2018). These results suggest that camel milk exosomes have a potential role in treating T2DM.



We sequenced 6 cDNA libraries from three adipose depots from the control group and treatment group. We obtained 42.39-46.92 million raw reads by high-throughput sequencing. The raw reads were filtered to obtain clean reads, which were then aligned to the rat reference genome using Hisat2. 95.14%~95.64% of the total sequenced fragments could be mapped to the reference genome (Table 1).



A total of 401 DEGs were identical between control group and treatment group, of which 135 DEGs were upregulated and 266 DEGs were downregulated in treatment vs. control. 135 upregulated genes were not significantly enriched in GO terms. 266 downregulated genes were significantly enriched in respiratory chain, mitochondrial respiratory chain complex I and NADH dehydrogenase complex for cellular component (CC) category; NADH dehydrogenase activity for molecular function (MF) category (Table 2).



Pathway annotation of DEGs was performed using the KEGG database. 135 upregulated genes were not significantly enriched in KEGG terms. 266 downregulated genes were significantly enriched in oxidative phosphorylation and so on (Table 3).



From the functional analysis of DEGs, we found that downregulated differential genes were significantly enriched in respiratory chain complex I (GO analysis) and oxidative phosphorylation (KEGG analysis). From these results, we speculated that the promotion of glucose consumption by camel milk exosomes may be due to the inhibition of mitochondrial respiratory chain complex I. It has been reported that metformin, the first-line anti-diabetic drug, exerts anti-diabetic effects by inhibiting mitochondrial respiratory chain complex I (Owen et al., 2000). In addition, the research has proposed that mitochondrial respiratory chain complex I can be used as a target for the treatment of diabetes (Hou et al., 2018).

Milk exosomes contain miRNAs (Melnik et al., 2021), they act a key player for intracellular communication by carrying their contents (e.g., miRNA) to target cells (Yun et al., 2021). we hypothesized that miRNAs released from the exosomes of camel milk after their entry into cells inhibited the mitochondrial respiratory chain complex I and thus promoted the glucose consumption of cells.

This study demonstrated for the first time that camel milk exosomes can promote glucose consumption in L6 cells. The reason why camel milk exosomes promote cellular glucose consumption may be due to the inhibition of mitochondrial respiratory chain complex I. Therefore, camel milk exosomes may be used as adjuvant in the treatment of T2DM. However, further studies are needed to clarify the mechanism behind it.

This study was funded by Research Program of science and technology at Universities of Inner Mongolia Autonomous Region (NJZY22486) and Inner Mongolia agricultural university high-level talents research initiation fund project (No. NDYB2018-27). 

The authors declare that there is no conflict of interests regarding the publication of this article.