FIGURE 7.

Parallel processing of the Ca²⁺ microdomain by Syk. Local Ca²⁺ entry is detected by Syk, which then activates two distinct signaling pathways. Syk recruits protein kinase C and the MEK/ERK cascade, resulting in activation of cPLA₂ and 5-lipoxygenase. This generates the intracellular messenger arachidonic acid (AA) and the proinflammatory paracrine signal leukotriene C₄ (LTC₄) (18). At the same time, Syk phosphorylates the transcription factor STAT5, which then migrates to the nucleus, where it increases c-fos expression. The Ca²⁺ microdomain can therefore be processed by Syk through two parallel pathways into the activation of temporally distinct cellular responses.