Functional relationship between sphingomyelin synthase‐related protein and diacylglycerol kinase δ
Authors: Murakami Chiaki , Hoshino Fumi , Sakai Hiromichi , Hayashi Yasuhiro , Yamashita Atsushi , Sakane Fumio 
Published: April 2020

IntroductionThe δ isozyme of diacyglycerol kinase (DGKδ) plays critical roles in lipid signaling by converting diacylglycerol (DG) to phosphatidic acid (PA). Intriguingly, we recently demonstrated that DGKδ preferably metabolized palmitic acid (16:0)‐ and/or palmitoleic acid (16:1)‐containing DG molecular species in response to high glucose stimulation in myoblasts. However, it is still unclear where 16:0‐ and/or 16:1‐containing DG molecular species come from, although another species arachidonic acid (20:4)‐containing DGs are derived from phosphatidylinositol (PI) turnover. Sphingomyelin synthase (SMS) 1 and SMS‐related protein (SMSr) synthesize the sphingomyelin and ceramide‐phosohoethanolamine by the transfer of phosphocholine/phosphoethanolamine to ceramide, respectively. These are also assumed to function as DG‐generating enzymes. SMS1, SMSr and DGKδ contain a sterile α motif domain (SAM), which is a putative protein interaction module. Interestingly, we found that DGKδ interacted with SMSr via their SAMs directly. In the present study, we investigated the functional relationship between DGKδ and SMSr.Results and ConclusionWe first investigated the changes in the amounts of PA molecular species in COS‐7 cells overexpressing DGKδ and/or SMSr using LC‐MS/MS. We found that the overexpression of SMSr and DGKδ significantly enhanced the production of 16:0‐ and/or 16:1‐containing PA species such as 16:0/16:1‐, 16:0/16:0‐, 16:0/18:1‐ and 16:0/18:2‐PA in COS‐7 cells. It is possible that DGKδ and SMSr are functionally linked and that SMSr acts in an upstream pathway of DGKδ by providing 16:0‐ and/or 16:1‐DG. To support the possibility, we determined DG levels in SMSr‐overexpressing cells. Compared with control cells, total DG levels in SMSr‐overexpressing cells were significantly increased. In particular, 16:0‐ and/or 16:1‐containig DG species, such as 14:0/16:0 (30:0)‐, 16:0/16:1 (32:1)‐, 16:1/18:2 (34:3)‐, and 16:1/18:1 (34:2)‐DG were significantly increased. These results further suggest the functional relationship (SMSr supplies DG to DGKδ) between DGKδ and SMSr. To examine whether forming SMSr‐DGKδ heteromeric complex affects DGKδ activity, we measured purified DGKδ activity in vitro using DG and ATP as substrates. Interestingly, when presence of purified SMSr with purified DGKδ, DGK activity was significantly increased. However, the deletion of SAM of SMSr did not significantly augment the DGKδ activity. Taken together, these results suggest that SMSr is one of the candidates of upstream DG‐providing enzymes of DGKδ and that SMSr enhances DGKδ activity by forming heteromeric complex via their SAMs.Support or Funding InformationThis work was supported in part by grants from MEXT/JSPS KAKENHI (Grant Numbers: JP18J20003 to C.M.); The Chiba Bank (to C.M.); the Chiba University Venture Business Laboratory (to C.M.); Chiba University Open Recruitment for International Exchange Program at the Institute for Global Prominent Research (to C.M.); and the American Society for Biochemistry and Molecular Biology (ASBMB 2019 Graduate/Postdoctoral Travel Award to C.M)