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  • DGK is highly expressed in skeletal muscle which is


    DGKδ is highly expressed in skeletal muscle [7], which is a primary target of insulin for glucose disposal [14]. Notably, Chibalin et al. reported that DGKδ contributes to hyperglycemia-induced peripheral insulin resistance and that decreased protein levels of DGKδ in skeletal muscle attenuated glucose uptake, which is critical for the pathogenesis of type 2 diabetes [15]. In addition, Miele et al. demonstrated that acute exposure (within 5 min) to high glucose levels augmented DGKδ activity in L6 myotubes, which was followed by a decrease in protein kinase C (PKC) α activity and an increase of insulin receptor activity [16]. We observed that DGKδ translocated from the cytoplasm to the plasma membrane in C2C12 myoblasts within 5 min of short term exposure to a high glucose concentration [17]. Moreover, we recently found that DGKδ preferentially utilizes palmitic SKF 83566 hydrobromide weight (the 16-carbon saturated fatty acid (16:0))-containing DG species such as 14:0/16:0-, 16:0/16:0-, 16:0/16:1-, 16:0/18:0- and 16:0/18:1-DG that are likely supplied from the phosphatidylcholine-specific phospholipase C-dependent (D609-sensitive) pathway in C2C12 cells in response to extracellular high glucose concentration [18,19]. Therefore, an understanding of the regulation of the DGKδ protein level is essential for revealing the pathogenic and exacerbation mechanisms of type 2 diabetes and for its treatment and prevention. Because decreased levels of DGKδ in skeletal muscle are critical for the pathogenesis of type 2 diabetes, as described above, the up-regulation of the DGKδ protein level is crucial for treatment and prevention of the disease. Intriguingly, we recently found that myristic acid (14:0) significantly increased DGKδ protein levels in C2C12 myotubes, whereas other free fatty acids that were examined, such as lauric (12:0), palmitic (16:0), stearic (18:0), palmitoleic (16:1) and oleic (18:1) acids, did not [20]. Moreover, free myristic acid significantly increased glucose uptake in C2C12 myotubes in a DGKδ-dependent manner [21]. Notably, chronic oral administration of myristic acid improved glucose tolerance and reduced the insulin-responsive blood glucose level in Nagoya-Shibata-Yasuda congenital type 2 diabetic mice in comparison to mice treated with control substances (vehicle and palmitic acid) at 24–30 weeks of age, during which the severity of type 2 diabetes was exacerbated [22]. Myristic acid also reduced body weight increases in Nagoya-Shibata-Yasuda mice. Moreover, the presence of the fatty acid increased DGKδ expression in skeletal muscle. However, it remains unclear how myristic acid regulates DGKδ protein levels.
    Materials & methods
    Discussion We previously demonstrated that free myristic acid markedly increased the protein level of DGKδ2 in C2C12 myotubes [20,21]. However, it has previously been unclear how myristic acid regulates DGKδ2 protein levels. In the present study, we demonstrated for the first time that myristic acid stabilizes DGKδ2 protein. Interestingly, we found that myristic acid-dependent DGKδ2 protein stabilization possesses three kinds of specificity. First, the DGKδ2 protein stabilization has fatty acid specificity. Our previous reports showed that only myristic acid (14:0), but not other free fatty acids, such as palmitic acid (16:0), increased the levels of DGKδ2 protein in C2C12 myotube cells [20,21] and mouse skeletal muscle [22]. We also confirmed that only myristic acid, but not palmitic acid, blocked DGKδ2 protein degradation in the presence of cycloheximide (Fig. 2, Fig. 3). Because myristic acid- and palmitic acid-uptakes into myocytes were reported to be essentially equivalent [27], it is likely that myristic acid has specific effects, which are different from those of palmitic acid, in myotubes. Second, myristic acid-dependent DGKδ2 protein stabilization has protein specificity. DGKη is a type II DGK that is most structurally similar to DGKδ. However, DGKη protein was not stabilized by myristic acid (Fig. 4). DGKζ has also been reported to be involved in insulin resistance [24]. However, the protein levels of DGKζ were not altered by myristic acid-treatment (Fig. 4). The stabilities of other type 2 diabetes-related proteins, such as PKCα, PKCζ, Akt and GSK3β, also failed to be affected by myristic acid (Fig. 4).