On/Off-regulation of phospholipase C-γ1-mediated signal transduction

Alterations in the production and regulation of lipid second messengers can give rise to key molecular lesions that trigger tumorigenesis and cancer progression. Especially, the hydrolysis of membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP2), is mediated by a specific phospholipase C (PLC), which plays important roles in the regulation of various cell functions. PLC generates two intracellular messengers, diacylglycerol and inositol 1,4,5-trisphosphate, which mediate the activation of protein kinase C (PKC) and intracellular Ca²⁺ release, respectively. Among the PLC isozymes, PLC-γ1 contains two src homology (SH) 2 domains and one SH3 domain between the X and Y catalytic domains. The SH2 domains of PLC-γ1 have been implicated in the association between PLC-γ1 and activated receptor tyrosine kinases, and the SH3 domain of PLC-γ1 has been reported to be responsible for the mitogenic effect of PLC-γ1, suggesting that PLC-γ1 exerts other actions that are independent of its lipase activity and appears to be involved in the SH domains. However, the physiological role of SH domains in the regulation of PLC-γ1 is still unclear. We have recently characterized the regulation mechanism of PLC-γ1 through protein-protein interactions. We have elucidated that PLC-γ1 can serve as a guanine nucleotide exchange factor (GEF) for dynamin-1 through direct interaction via its SH3 domain. These results indicate that GEF function of PLC-γ1 for dynamin-1 may link with PLC-γ1's mitogenic actions. The SH3 domain of PLC-γ1 can mediate Sos and PIKE activation by acting as GEF, suggesting that PLC-γ1 plays an essential role on cellular proliferation through protein–protein interaction independent of its enzymatic activity. On the other hand, we have found that Cbl directly interacts with the SH3 domain of PLC-γ1 and inhibits its tyrosine phosphorylation and enzymatic activity, suggesting that PLC-γ1 can be off-regulated by protein–protein interaction. In addition, we demonstrate that Grb2 interacts with tyrosine phosphorylated PLC-γ1 and acts as an inhibitor on PLC-γ1-mediated signaling. These results suggest that Grb2, one of the key regulators of Ras/Raf/MAPK signaling pathway, may participate in the regulation of PLC-γ1. Lastly, PLC-γ1 forms a ternary complex with Jak2 and PTP-1B and negatively regulates GH-induced Jak2 phosphorylation. Taken together, our data strengthen the hypothesis that the interaction between PLC-γ1 and effector proteins plays a key role in on- or off-regulating PLC-γ1-mediated cellular proliferation independent its enzymatic activity. These results can provide novel insights to understand how PLC-γ1 is regulated and involved in cellular growth and proliferation.