Role of guanine nucleotide regulatory protein in polyphosphoinositide degradation and activation of phagocytic leukocytes by chemoattractants

Leukocyte activation by Chemoattractants provides an important model to study the biochemical mechanisms of stimulus‐response coupling in these cells. Well‐defined chemotactic factors induce readily quantifiable responses in phagocytic leukocytes. These include directed migration and the production and release of toxic substances including oxygen radicals and lysosomal enzymes. The development of radiolabeled synthetic oligopeptides with potent chemotactic activity allowed the demonstration of chemoattractant receptors on polymorphonuclear leukocytes (PMNs) as well as macrophages. In membrane preparations from these cells, these receptors exist in high‐ and low‐affinity states which are regulated by guanosine di‐ and triphosphates. This suggested that chemoattractant receptors interact with guanine nucleotide regulatory proteins (N or G proteins). Although Chemoattractants elicit a rapid but transient increase in intracellular cAMP levels, they neither stimulate nor inhibit membrane‐bound adenylate cyclase, suggesting a novel role for N proteins in certain receptor‐transduction mechanisms. Stimulation of phagocytes by Chemoattractants is also associated with a rapid increase in cytosolic Ca²⁺ concentrations ([Ca²⁺]_i) which appears to result from the production of inositol 1,4,5‐triphosphate (IP₃) as a consequence of the diesteric cleavage of phosphatidylinositol 4,5‐bisphosphate (PIP₂). Treatment of phagocytes with pertussis toxin (PT), which ADP‐ribosylates and thereby inactivates certain N proteins, abolishes the cells' responsiveness to chemoattractants. More direct evidence for a role of a PT‐sensitive N protein in leukocyte activation was provided by the demonstration that chemoattractants stimulate the hydrolysis of PIP_i in PMN membranes only in the presence of GTP. This receptor‐mediated hydrolysis of PIP_i is not observed in plasma membranes prepared from PT‐treated PMNs. Therefore, these studies suggest that occupancy of chemoattractant receptors activates a PT‐sensitive N protein. The activated N protein shifts the Ca²⁺ requirement for phospholipase C activity from supraphysiological levels to ambient cytosolic Ca²⁺ concentrations. Cleavage of PIP₂ results in the formation of the second messenger molecules, IP₃ and 1,2‐diacylglycerol, which can initiate cellular activation. These messengers also seem to activate responses which feed back to attenuate receptor stimulation of phospholipase.