Dual regulation of mast cell degranulation through IgE receptor-mediated modulation of M₂-type pyruvate kinase.

It was reported that mast cell degranulation is inversely related to the enzymatic activity of M₂-type pyruvate kinase (M₂PK). This study shows that activation of high-affinity IgE receptor (FcεRI) evokes a sequential dual regulation of M₂PK, i.e., an immediate decrement followed by slow phase increment of enzymatic activities. Changes in the activities of M₂PK and mast cell degranulation showed similar time course after antigenic stimulation of FcεRI. The immediate inhibition of M₂PK involved tyrosine phosphorylation, and subsequently led to a cellular accumulation of glycolytic intermediates, including fructose 1,6-biphosphate (FBP), a feedforward activator of M₂PK. As the cellular levels of FBP were increased, both the enzymatic acitivity of M₂PK and mast cell degranulation slowly returned to near basal levels. A-Raf, when exogenously introduced into RBL-2H3 cells, phosphorylated M₂PK on the serine residues, elevated enzyme activities of M₂PK, and resulted in the inhibition of degranulation. These results suggest that dual regulation of M₂PK which involves the phosphorylation of M₂PK and accumulation of a feedforward activator of M₂PK plays important roles in the control of mast cell degranulation.