THE CA2-2-INTEGRIN ON HLGO-GRANULOCYTIC CELLS IS ABROGATED FOLLOWING PHOSPHORYLATION OF ITS CD18-CHAIN - RELATION TO IMPAIRED PROTEIN-TYROSINE PHOSPHORYLATION( SIGNALING CAPACITY OF THE BETA)

The phosphorylation state of the CD18-chain of beta(2)-integrins have been shown not to mediate changes in the avidity of these receptors (i .e., inside-out signaling); however, no alternative functional signifi cance has been proposed. Our study focused on how changes in the phosp horylation state of beta(2)-integrin-receptors on HL60-granulocytic ce lls are related to its intracellular signal transduction properties (i .e., outside-in signaling). Engagement of beta(2)-integrins on differe ntiated HLBO cells induced a transient increase in the cytosolic free Ca2+ concentration and an increased tyrosine phosphorylation of three major protein bands (70, 115, and 140 kDa). These signaling events occ urred without any detectable phosphorylation of the CD 18-chain. Howev er, a strong phosphorylation of the CD18-chain by preexposure to phorb ol myristate acetate (PMA) coincided with an abolishment of both the b eta(2)-integrin-induced Ca2+ signal and the protein tyrosine phosphory lations. By comparison, none of these effects were exhibited by 4-alph a-PMA, an analogue that does not activate protein kinase C. Thus, phos phorylation of the CD18-chain of beta(2)-integrins is not required for outside-in signal transduction by these receptors, but it could const itute an effective mechanism by which the signaling properties of beta (2)-integrins can be modulated by exogenous factors and possibly also by intracellular signals induced by other receptors. The fact that bot h the cytosolic free Ca2+ signal and protein tyrosine phosphorylations were abrogated by PMA suggests an intimate relationship between these two intracellular signals. To explore this possible relationship, we chelated the beta(2)-integrin-induced Ca2+ signal with BAPTA. The beta (2)-integrin-induced protein tyrosine phosphorylations were blocked by BAPTA but not by abolishment of the Ca2+ signal due to chelation with MAPT or by pretreatment with thapsigargin. These findings and the obs ervation that pretreatment of cells with methyl-2,5-dihydroxycinnamate (a tyrosine kinase inhibitor) blocked the beta(2)-integrin- but not t he fMet-Leu-Phe-induced Ca2+ signal suggest that beta(2)-integrin-indu ced tyrosine kinase activation occurs prior to and is a prerequisite f or the subsequent Ca2+ signal. (C) 1995 Academic Press, Inc.