Requirement of tyrosine-phosphorylated Vav for morphological differentiation of all-trans-retinoic acid-treated HL-60 cells.

Our previous data demonstrated that cellular and nuclear tyrosine-phosphorylated Vav associate with phosphoinositide 3-kinase during all-trans-retinoic acid-dependent granulocytic differentiation of HL-60 cells. In this study, aimed to analyze the mechanism by which Vav is recruited and activated, we report that the Src homology 2 domain of Vav interacts with tyrosine-phosphorylated proteins in a differentiation-dependent manner. Two adaptor proteins, Cbl and SLP-76, were identified, showing a discrete distribution inside the cells, with Cbl absent from the nuclei and SLP-76 particularly abundant in the nuclear compartment. Of note, Vav interacts with the tyrosine kinase Syk, which is also present in the nuclear compartment and may phosphorylate Vav in vitro when cells differentiate. Inhibition of Syk activity by piceatannol prevents both in vitro and in vivo Vav tyrosine phosphorylation, its association with the regulatory subunit of phosphoinositide 3-kinase, and the nuclear modifications typically observed during granulocytic differentiation of this cell line. These findings suggest that tyrosine-phosphorylated Vav and its association with phosphoinositide 3-kinase play a crucial role in all-trans-retinoic acid-induced reorganization of the nucleoskeleton, which is responsible for the changes in nuclear morphology observed during granulocytic differentiation of HL-60 cells.