Golgi retention of KIT in gastrointestinal stromal tumour cells is phospholipase D activity-dependent

A constitutively active mutant of the receptor protein-tyrosine kinase KIT is a major cause of gastrointestinal stromal tumours (GISTs). Recently, we discovered that during biosynthetic transport, the KIT mutant (KITmut) is retained in the Golgi/trans-Golgi network (TGN), where it activates downstream molecules. This retention is dependent on the phospholipase C{gamma}2-protein kinase D2-PI4 kinase III{beta} (PLC{gamma}2-PKD2-PI4KIII{beta}) pathway, which KITmut activates at the Golgi/TGN. The activated cascade aberrantly recruits GGA1 and the {gamma}-adaptin subunit of AP1, resulting in KITmut retention in the Golgi/TGN. However, the precise mechanisms, including the mediators and effectors in the pathway, remain unclear. In humans, the phosphatidic acid-generating enzymes, phospholipase D1 (PLD1) and PLD2, are known downstream proteins of PKD. In the presence of the PLD inhibitor CAY10594, KITmut is released from the Golgi/TGN and subsequently degraded in lysosomes, leading to signal inactivation. Knockdown experiments indicated that PLD2 plays a role in KITmut retention. KITmut activates PLD2 through PKD2, but not PI4KIII{beta}, for Golgi/TGN retention. PLD activity is required for the association of {gamma}-adaptin with GGA1. Therefore, the KIT-PLC{gamma}2-PKD2 pathway separately activates PLD2 and PI4KIII{beta} to recruit {gamma}-adaptin and GGA1. Collectively, these results suggest that KITmut retention is dependent on the activation of the PLC{gamma}2-PKD2-PLD2 cascade in GIST.