The modulation of phosphoinositide 3-kinase (PI3K) activity influences the quality Of Cellular responses triggered by various receptor tyrosine kinases. Protein kinase C (PKC) has been reported to phosphorylate signalling molecules upstream of PI3K and thereby it may affect the activation of PI3K. Here, we provide the first evidence for a direct effect of a PKC isoenzyme oil the activity of PI3K. PKC alpha but not PKC epsilon phosphorylated the catalytic subunit of the p110 alpha/p85 alpha PI3K in vitro in a manner inhibited by the PKC inhibitor bisindolylmaleimide I (BIM I). The incubation of PI3K with active PKC alpha resulted in a significant decrease in its lipid kinase activity and this effect was also attenuated by BIM I. We conclude that PKC alpha is able to modulate negatively the lipid kinase activity of the p110 alpha/p85 alpha PI3K through the phosphorylation of the catalytic Subunit. (c) 2005 Elsevier Inc. All rights reserved.