Phosphorylation of protein kinase C (PKC) provides an amplitude control that operates in conjunction with allosteric effectors. Under many conditions, PKC isotypes appear to be highly phosphorylated; however, the cellular inputs that maintain these phosphorylations are not characterized. In the present work, it is shown that there is a differential phosphorylation of PKCδ in adherent versus suspension cultures of transfected HEK-293 cells. It is established that integrin activation is sufficient to trigger PKCδ phosphorylation and that this signals through phosphoinositide 3-kinase (PI3-kinase) to stimulate the phosphorylation of two sites, T505 and S662. The loss of signal input to PKCδ in suspension culture is dependent on the tumour suppressor gene PTEN, which encodes a bi-functional phosphotyrosine/phosphoinositide 3-phosphate phosphatase. In the PTEN-/- UM-UC-3 bladder carcinoma cell line grown in suspension, transfected PKCδ no longer accumulates in a dephospho-form on serum removal. By contrast, in a UM-UC-3-derivative cell line stably expressing PTEN, PKCδ does become dephosphorylated under these conditions. Employing the PTEN Gly129 → Glu mutant, which is selectively defective in lipid phosphatase activity, it was established that it is the lipid phosphatase activity that controls PKCδ phosphorylation. The evidence indicates that PKCδ phosphorylation and its latent activity are maintained in serum-deprived adherent cultures through integrin–matrix interactions. This control acts through a pathway involving a lipid product of PI3-kinase in a manner that can be suppressed by PTEN.

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