Activation mechanism of phospholipase D involved in the generation of lipid mediators in cultured Madin-Darby canine kidney cells

Endogenous 15-deoxy-Delta(12,14)-prostaglandin J(2) synthesized by adipocytes during maturation phase contributes to upregulation of fat storage

15-Deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) has been identified as a natural ligand for peroxisome proliferator-activated receptor (PPAR) gamma to promote adipogenesis. However, it remains elusive about the ability of PPARgamma-expressing adipocytes to produce PGJ(2) series and the role in the life cycle of adipocytes. Here, we developed an enzyme-linked immunosorbent assay specific for 15d-PGJ(2). The analysis using this method revealed the increase in the endogenous synthesis of immunoreactive 15d-PGJ(2) in cultured adipocytes during the maturation phase. Further studies using cyclooxygenase inhibitors clarified the contribution of endogeous 15d-PGJ(2) produced by mature adipocytes to upregulation of fat storage in an autocrine manner.

Gene expression of arachidonate cyclooxygenase pathway leading to the delayed synthesis of prostaglandins E2 and F2α in response to phorbol 12-myristate 13-acetate and action of these prostanoids during life cycle of adipocytes

Several types of prostaglandin (PG)s are synthesized in adipocytes and involved differently in the control of adipogenesis. To elucidate how the PG synthesis is regulated at different stages in the life cycle of adipocytes, we examined the gene expression of arachidonate cyclooxygenase (COX) pathway leading to the delayed synthesis of PGE2 and PGF2alpha and their roles in adipogenesis after exposure of cultured cells to phorbol 12-myristate 13-acetate (PMA), which is a useful system for monitoring mitogen-induced changes. While the expression of COX-1 remained constitutive, mRNA and protein levels of COX-2 were up-regulated by treatment with PMA. Preadipocytes exhibited higher gene expression of cytosolic phospholipase A2alpha (cPLA2alpha) and PGF synthase. In contrast, three isoforms of PGE synthase are expressed constitutively during all phases. The delayed synthesis of PGE2 and PGF2alpha following the stimulation for 24 with a mixture of PMA and calcium ionophore A23187 was the highest in preadipocytes, reflecting the increased expression levels of cPLA2alpha and COX-2. Cultured cells treated with PMA during the differentiation phase and then exposed to the maturation medium, or cells treated with PMA in the maturation medium after the differentiation phase showed the suppression of adipogenesis in adipocytes. The attenuating effect of PMA was additionally enhanced when the cell were treated along with A32187 during the differentiation phase, suggesting the involvement of endogenous PGs. The cells at the stages of the differentiation and maturation phases were highly sensitive to exogenous PGE2 and PGF2alpha, respectively, resulting in the marked suppression of the stored fats in adipocytes. Taken together, these results provided the evidence for the distinct gene expression of isoformic enzymes in the COX pathway leading to the synthesis of PGE2 and PGF2alpha and the specific action of these prostanoids at different cycle stages of adipocytes.

Prostaglandin F2α is protective for apoptosis stimulated synergistically with 12-O-tetradecanoyl phorbol-β-acetate and nordihydroguaiaretic acid in Madin-Darby canine kidney cells

We recently found that 12-O-tetradecanoyl phorbol-beta-acetate (TPA) induced apoptosis in cultured Madin-Darby canine kidney (MDCK) cells. The present study shows that the apoptosis was mediated by the activation of caspases including caspase-3 and -7. Moreover, nordihydroguaiaretic acid (NDGA), a general lipoxygenase (LOX) inhibitor, synergistically stimulated the TPA-induced apoptosis despite no activation with NDGA alone. TPA preferentially increased the transcription of cyclooxygenase (COX)-2 in MDCK cells, whereas the expression of LOXs was almost negligible. These findings suggested that the effect of NDGA was independent of the inhibition of LOXs. The study using a cell-permeable 2',7'-dichlorofluorescin diacetate confirmed the more remarked production of reactive oxygen species at 6 h after the cells were treated with a mixture of TPA and NDGA. Calcium ionophore A23187 was markedly effective to attenuate the TPA-induced apoptosis, indicating that elevated endogenous prostaglandins (PGs) served as survival factors through not only the activation of phospholipase A(2) by A23187 but also the induction of COX-2 by TPA. Consistent with this indication, exogenous addition of PGF(2alpha), a predominant prostanoid in MDCK cells, was the most potent to protect the cells from the apoptosis induced by a mixture of TPA and NDGA.

Phospholipase D, tumor promoters, proliferation and prostaglandins

Phosphatidylcholine hydrolysis by phospholipase D is a widespread response to cellular stimulation. However, the downstream signaling events subsequent to phosphatidylcholine hydrolysis are just beginning to be determined. Initially it was proposed that diglyceride formation by phospholipase D and phosphatidate phosphohydrolase resulted in long-term stimulation of protein kinase C. However, recent studies indicate that phosphatidic acid is the relevant signaling molecule in some signaling pathways. The present review will summarize studies of phospholipase D in the response of cells to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, which causes cells to mimic the phenotype of oncogenic transformation. The role of phospholipase D in stimulation of Raf-1 and prostaglandin H synthase type-2 is emphasized.