Class I and III Phosphatidylinositol 3′-Kinase Play Distinct Roles in TLR Signaling Pathway

PI3K involvement has been implicated in the TLR signal pathway. However, the precise roles of the different classes of PI3K in the pathway remain elusive. In this study, we have explored the functions of class I and class III PI3K in the TLR signal pathway using specific kinase mutants and PI3K lipid products. Our results reveal that class III PI3K specifically regulates CpG oligodeoxynucleotide (ODN)-induced cytokine and NO production as well as NF-kappaB activation, whereas class I PI3K regulates both CpG ODN- and LPS-induced IL-12 production and NF-kappaB activation. Additional studies of CpG ODN uptake with flow cytometric analysis show that class III PI3K, but not class I, regulates cellular CpG ODN uptake. Furthermore, experiments with MyD88-overexpressing fibroblast cells transfected with dominant-negative mutants of PI3K demonstrate that class III PI3K regulates CpG ODN-mediated signaling upstream of MyD88, while class I PI3K regulation is downstream of MyD88. These results suggest that class I and class III PI3K play distinct roles in not only the uptake of CpG ODN, but also responses elicited by CpG ODN and LPS.