The Ca2+ signaling capacity of the beta 2-integrin on HL60-granulocytic cells is abrogated following phosphorylation of its CD18-chain: relation to impaired protein tyrosine phosphorylation.
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 significance has been proposed. Our study focused on how changes in the phosphorylation state of beta 2-integrin-receptors on HL60-granulocytic cells are related to its intracellular signal transduction properties (i.e., outside-in signaling). Engagement of beta 2-integrins on differentiated HL60 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 occurred without any detectable phosphorylation of the CD18-chain. However, a strong phosphorylation of the CD18-chain by preexposure to phorbol myristate acetate (PMA) coincided with an abolishment of both the beta 2-integrin-induced Ca2+ signal and the protein tyrosine phosphorylations. By comparison, none of these effects were exhibited by 4-alpha-PMA, an analogue that does not activate protein kinase C. Thus, phosphorylation of the CD18-chain of beta 2-integrins is not required for outside-in signal transduction by these receptors, but it could constitute 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 both 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 observation that pretreatment of cells with methyl-2,5-dihydroxycinnamate (a tyrosine kinase inhibitor) blocked the beta 2-integrin- but not the fMet-Leu-Phe-induced Ca2+ signal suggest that beta 2-integrin-induced tyrosine kinase activation occurs prior to and is a prerequisite for the subsequent Ca2+ signal.
Hellberg
C
C
Department of Cell Biology, Linköping University, Sweden.
Eierman
D
D
Sjölander
A
A
Andersson
T
T
D018821:CD18 Antigens / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D002118:Calcium / Q000378:metabolism; D002454:Cell Differentiation; D002460:Cell Line; D003600:Cytosol / Q000378:metabolism; D016023:Integrins / Q000737:chemistry / Q000502:physiology*; D009504:Neutrophils / Q000378:metabolism / Q000502:physiology*; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D019000:Phosphotyrosine; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D013329:Structure-Activity Relationship; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014443:Tyrosine / Q000031:analogs & derivatives / Q000378:metabolism
D018821:CD18 Antigens; D016023:Integrins; D010750:Phosphoproteins; D019000:Phosphotyrosine; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium