Alternative splicing of the human diacylglycerol kinase 8 gene generates two isoforms differing in their expression patterns and in regulatory functions

Diacylglycerol kinase (DGK) plays an important role in signal transduction through modulating the balance between two signaling lipids, diacylglycerol and phosphatidic acid. DGKdelta (type II isozyme) contains a pleckstrin homology domain at the N terminus and a sterile a motif domain at the C terminus. We identified another DGKdelta isoform (DGKdelta2, 135 kDa) that shared the same sequence with DGKdelta previously cloned (DGKdelta1, 130 kDa) except for the 52 residues N-terminally extended. Analysis of panels of human normal and tumor tissue cDNAs revealed that DGKdelta2 was ubiquitously expressed in all normal and tumor tissues examined, whereas the transcript of DGKdelta1 was detected only in ovary and spleen, and in a limited set of tumor-derived cells. The expression of DGKdelta2 was induced by treating cells with epidermal growth factor and tumor-promoting phorbol ester. In contrast, the levels of mRNA and protein of DGKdelta1 were suppressed by phorbol ester treatment. It thus becomes clear that the two DGKdelta isoforms are expressed under distinct regulatory mechanisms. DGKdelta1 was translocated through its pleckstrin homology domain from the cytoplasm to the plasma membrane in response to phorbol ester stimulation, whereas DGKdelta2 remained in the cytoplasm even after stimulation. Further experiments showed that the delta2-specific N-terminal sequence blocks the phorbol ester-dependent translocation of this isoform. Co-immunoprecipitation analysis of differently tagged DGKdelta1 and DGKdelta2 proteins showed that they were able to form homo- as well as hetero-oligomers. Taken together, alternative splicing of the human DGKdelta gene generates at least two isoforms, differing in their expressions and regulatory functions.