構成細胞膜的主要成份是磷脂質,其中磷脂醯肌醇4,5二磷酸(Phosphatidylinositol (4,5)-bisphosphate , PtdIns(4,5)P2)可作為磷脂醯肌醇激酶(phosphatidylinositol 3-kinase,PI3K)和磷脂醯肌醇特異性磷酸脂肪酶(phosphoinositide specific phospholipase C,PI-PLC)的受質,當PtdIns(4,5)P2作為PI3K的受質時,PI3K會將PtdIns(4,5)P2磷酸化生成磷脂醯肌醇3,4,5三磷酸(Phosphatidylinositol (3,4,5)-triphosphate,PtdIns (3,4,5)P3)。當PtdIns(4,5)P2 作為PI-PLC的受質時,PI-PLC會將PtdIns(4,5)P2水解為肌醇-1,4,5-三磷酸(inositol 1,4,5-trisphosphate,Ins(1,4,5)P3)和二醯甘油(diacylglycerol,DG,DAG)。我們的研究主要是探討PI3K抑制劑wortmannin增強誘導型一氧化氮合成酶(induceble nitric oxide synthase, iNOS)表現的分子機制。首先,經由wortmannin處理的巨噬細胞,其Ins(1,4,5)P3產物和iNOS的表現均會增加。而轉殖p85 siRNA(small interfering RNA)的巨噬細胞,因為PI3K的活性消失,也會增強iNOS的表現;相反的,用表現質體ras-p110轉殖的巨噬細胞,因為PI3K的過度活化,使得原本增強的iNOS卻顯著的下降了。此外,在脂質多醣體(lipopolysaccharide, LPS)活化的巨噬細胞裡,過度表現wild-type Akt及dominant-negative Akt,對於增強iNOS的表現並沒有影響。如果使用PI-PLC的抑制劑U73122處理在LPS活化的巨噬細胞,除了iNOS的表現會下降之外,並且也會降低ERK(extracellar signal-regulated kinase,)、JNK(c-Jun N-terminal kinase)、p38 (protein 38)的磷酸化及AP-1(activating protein -1,)的轉錄活性。但是不論「有」和「無」wortmannin處理下,在LPS活化的巨噬細胞,其kappa B 激酶抑制子(inhibitor kappa B, IkappaB)的降解、核因子kappa B(nuclear factor kappa B, NF-kappa B)的結合活性(EMSA)、NF-kappa B的轉錄活性(luciferase reporter assay)均沒有變化;但有wortmannin處理的巨噬細胞其AP-1的結合活性(EMSA)及AP-1的轉錄活性(luciferase reporter assay)均明顯增加。因此我們提出:使用PI3K的抑制劑wortmannin處理的巨噬細胞,其所增強iNOS的表現是透過PI-PLC下游的訊號及轉錄因子AP-1轉錄活性的增強,因而促進iNOS的表現增加。換言之;PI3K的抑制劑wortmannin處理的巨噬細胞,所增強的iNOS表現,並不依賴Akt路徑、及NF-kappa B轉錄因子。
Phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P2) has been known to serve as substrate for phosphatidylinositol 3-kinase (PI3K) and phosphoinositide-specific phospholipase C (PI-PLC), and that can produce PtdIns(3,4,5)P3 and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) & diacylglycerol (DAG), respectively. To elucidate the role of PI-PLC during activated-macrophages treated with PI3K inhibitor wortmannin, we examined the effects of wortmannin on the enhancement of PI-PLC downstream signals and that leading to increase of inducible nitric oxide synthase (iNOS) expression in lipopolysaccharide (LPS)-activated macrophages. Here, we report that wortmannin inhibited PI3K activity but enhanced PI-PLC downstream signals, and subsequently increased iNOS expression independently of Akt & NF-kB pathways. First, wortmannin treatment enhanced Ins(1,4,5)P3 production and iNOS expression in LPS-activated macrophages. Inhibition of PI3K by p85 siRNA also showed an enhancement of iNOS expression. On the other hand, overexpression of PI3K by ras-p110 expression plasmid significantly decreased iNOS expression in LPS-activated macrophages. In addition, overexpression of wild-type or dominant-negative Akt expression plasmid did not affect the iNOS expression in LPS-activated macrophages. Second, treatment of PI-PLC inhibitor U73122 reversed the enhancement of iNOS expression, the increase of phosphorylation level of ERK, JNK & p38, and the increase of AP-1-dependent gene expression in wortmannin-treated & LPS-activated macrophages. However, I kappa B degradation, NF-kappaB binding activity by EMSA assay, and NF-kappa B-dependent gene expression by luciferase reporter assay did not change during LPS-activated macrophages with or without wortmannin. We propose that the inhibition of PI3K by wortmannin in mouse macrophages enhances the PI-PLC downstream signals, and subsequently increases the LPS induction of iNOS expression independently of Akt pathway, and AP-1 but not NF-kappa B mediates the enhancement of iNOS expression.