Class I phosphoinositide 3-kinase (PI3K) signaling pathways
regulate several important cellular functions, including cellular
growth, division, survival, and movement. Class IB PI3K (also
known as PI3K

) links heterotrimeric GTP-binding protein–coupled
receptors to these pathways. Activation of class IB PI3K results
in the rapid synthesis of phosphatidylinositol-3,4,5-trisphosphate
[PtdIns(3,4,5)P
3] and its dephosphorylation product PtdIns(3,4)P
2 in the plasma membrane. These two lipid messengers bind to pleckstrin
homology domain–containing effectors that regulate a complex
signaling web downstream of receptor activation. Characteristic
features of this pathway are the regulation of protein kinases
and the regulation of small guanosine triphosphatases that control
cellular movement, adhesion, contraction, and secretion. Most
of the ligands that activate class IB PI3K are involved in coordinating
the body's response to injury and infection, and recent studies
suggest that small molecule inhibitors of this enzyme may represent
a novel class of anti-inflammatory therapeutic agents.