Mechanism of increased PLD1 gene expression during early adipocyte differentiation process of mouse cell line 3T3-L1.

A mouse cell line 3T3-L1 is differentiated into adipocytes when treated with an inducer cocktail (IDX) (insulin, dexametahsone, and a cAMP phosphodiesterase inhibitor of isobutyl-methylxanthine (IBMX)). Here, we report that PLD1, but not PLD2, mRNA and protein increased during the early differentiation process. Our analysis shows that IDX resulted in a sequential induction of C/EBPbeta, PLD1, and C/EBPalpha which is a key transcription factor of late adipocyte differentiation. Among the three inducers, IBMX + any other inducer induced mild adipocyte differentiation, whereas insulin + dexamethasone did not. IBMX increased PLD1 but not PLD2 mRNA. Forskolin, an adenylate cyclase activator, and dbcAMP also increased PLD1 mRNA, suggesting the cellular cAMP as the inducer of both adipocyte differentiation and PLD1 transcription. We focused on the regulatory mechanism of PLD1 transcription during this differentiation process. IDX or a combination of inducers including IBMX increased PLD1 promoter activity, which is consistent with mRNA analysis. Promoter analysis identified two adjacent C/EBP motifs located between -338 and -231 bp from the first exon as the IBMX responsive elements. Furthermore, overexpression of C/EBPbeta, but not C/EBPalpha, increased PLD1 mRNA and PLD1 5' promoter activity. EMSA and chromatin immunoprecipitation assay confirmed the direct binding of C/EBPbeta, but not C/EBPalpha, to these C/EBP motifs of PLD1 5' promoter. Our results show that PLD1 is a target gene of C/EBPbeta through the increased cellular cAMP during early adipocyte differentiation of 3T3-L1 cells.

Mutated ras induced PLD1 gene expression through increased Sp1 transcription factor.

The underlying mechanisms of oncogene-induced phospholipase D (PLD) activation have not been fully elucidated. The effect of the mutated-ras on PLD mRNA was examined using colon cancer cell lines as well as mock- and mutated ras-transfected NIH3T3 cells. Ras-mutation and activation were correlated, and cells with enhanced ras-activation showed increased PLD1 mRNA and protein. Analysis of the 5' PLD1 promoter using a representative cell line, DLD-1 and also mutated ras-NIH3T3, showed one Sp1-site as the important ras-responsible motif. Spl inhibition with mithramycin A and Spl siRNA inhibited PLD1 protein expression and its promoter activity. Sp1 but not Sp3 protein level and increased Sp1-motif binding activity were correlated with ras activation. Furthermore, overexpression of Sp1 in drosophila SL2 cells lacking Sp family proteins increased PLD1 promoter activity. EMSA and chromatin immunoprecipitation assay confirmed the importance of Sp1 protein binding to the Sp1-motif in ras-induced PLD1 mRNA expression.