Characterization of protein serine/threonine phosphatase activities in human lung mast cells and basophils

The use of okadaic acid to elucidate the intracellular role(s) of protein phosphatase 2A: Lessons from the mast cell model system

In recent years a heightened appreciation has emerged for the role(s) that phosphatases play in regulating signal transduction pathways and other cellular processes. The tumor-promoting agent okadaic acid (OA) has been an invaluable tool in efforts aimed at delineating the contributions of the most abundant mammalian serine/threonine phosphatase, protein phosphatase 2A (PP2A), to intracellular signaling and cell function. PP2A, which is ubiquitous and vital in virtually every cell system studied, continues to be the focus of much research on phosphorylation control machinery. Mast cells represent an excellent in vitro model for the study of protein phosphorylation events because they possess a number of distinct signaling pathways that lead to the production and/or release of discreet mediators in response to different stimuli. The utility of OA in analyzing PP2A function has been demonstrated in mast cells across several species. Results of these studies have contributed to the current recognition that PP2A plays a crucial role in the biology of mast cells and other cell types.

Regulation of immunoglobulin E-mediated secretion by protein phosphatases in human basophils and mast cells of skin and lung

A wide range of serine/threonine protein phosphatase (PP) inhibitors were studied for effects on the immunoglobulin E (IgE)-mediated release of histamine from human lung mast cells, human skin mast cells and basophils. Okadaic acid (OA) inhibited the release of histamine from all three cell types in a concentration-dependent manner. Two structural analogues of okadaic acid, okadaol and okadaone, known to be less active than the parent molecule as inhibitors of PP, were less active than okadaic acid as inhibitors of histamine release in these three cell types. A number of PP inhibitors, showing differences in selectivity for PP1 and PP2A, were also evaluated. Calyculin, which is roughly equipotent as a PP1 and PP2A inhibitor, attenuated the release of histamine from all three cell types. Similarly, tautomycin (TAU), which shows greater selectivity for PP1 over PP2A, was also effective at inhibiting histamine release in all three cell types. In contrast, fostriecin, which is very much more potent as an inhibitor of PP2A over PP1, was ineffective as an inhibitor in all three cell types. These data indicate that the regulation of mediator release by PPs is similar in lung mast cells, skin mast cells and basophils. Moreover, the data suggest that PP1 is important in the control of cellular activity.

Role of protein phosphatases in the regulation of human mast cell and basophil function

Many extracellular stimuli mediate physiological change in target cells by altering the phosphorylation state of proteins. These alterations result from the dynamic interplay of protein kinases, which mediate phosphorylations, and protein phosphatases, which catalyse dephosphorylations. The antigen-mediated aggregation of high-affinity receptors for IgE on mast cells and basophils triggers rapid changes in the phosphorylation of many proteins and culminates in the generation of inflammatory mediators involved in allergic inflammatory diseases such as asthma. Although protein kinases have an established role in this process, less is known about the involvement of protein phosphatases. This imbalance has been redressed in recent years by the availability of phosphatase inhibitors, such as okadaic acid, that facilitate investigations of the role of protein phosphatases in intact cells. Here we review a number of studies in which inhibitors of protein phosphatases have been used to shed light on the potential importance of these enzymes in the regulation of human mast cell and human basophil function.