A CALCIUM-DEPENDENT TYROSINE KINASE SPLICE VARIANT IN HUMAN MONOCYTES- ACTIVATION BY A 2-STAGE PROCESS INVOLVING ADHERENCE AND A SUBSEQUENT INTRACELLULAR SIGNAL

Freshly isolated human monocytes do not express p125(FAK) but upon adh erence to substrata activate the highly related calcium-dependent tyro sine kinase (CADTK), also known as Pyk2, CAK beta, RAFTK, and FAK2, Th e monocyte CADTK was 5 kDa smaller than protein from epithelial cells; isolation and sequencing of the monocyte CADTK cDNA revealed a predic ted 42-amino acid deletion between the two proline-rich domains of the enzyme. The nucleic acid sequence suggests that the deletion is cause d by alternative RNA splicing. This species was also found in T and B lymphocytes and appears to be the predominant form of cytoskeletal ass ociated tyrosine kinase in non-neoplastic, circulating, hematopoietic cells. CADTK was not activated when monocytes maintained in suspension were treated with agents that produce an intracellular calcium (thaps igargin) or protein kinase C (phorbol 12-myristate 13-acetate) signal including a chemokine, RANTES, that binds to the HIV co-receptor, CCK5 , In contrast, monocyte adherence to tissue culture plastic-stimulated CADTK tyrosine phosphorylation, a process that was enhanced by thapsi gargin, phorbol 12-myristate 13-acetate, and RANTES but that was compl etely blocked by preincubation with cytochalasin D. When compared with plastic, adherence to fibronectin-or collagen-coated surfaces produce d only minimal CADTK activation but permitted significant stimulation by added thapsigargin, These data suggest that in a cell type that lac ks p125(FAK), CADTK plays an early role in post-adherence signaling. I ts activation involves two stages, cytoskeletal engagement, which is p ermissive, and co-stimulatory signals (calcium or protein kinase C) ge nerated by extensive cell surface engagement, agonists, or inflammator y chemokines.