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A more recent version of this article appeared on July 1, 2005
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Submitted on February 10, 2005
Revised on April 11, 2005
Accepted on April 25, 2005
1
Endocrine Unit, Veterans Affairs Medical Center, Northern California Institute for Research and Education and University of California at San Francisco, San Francisco, CA 94121
Monitoring Editor: M. Bishr Omary
We have previously demonstrated that phospholipase C-
1 (PLC-1) is required for calcium-induced human keratinocyte differentiation. In the present study, we investigated whether the activation of PLC-1 by nonreceptor kinases such as src and fyn plays a role in mediating this process. Our results showed that the combination of dominant negative src and fyn blocked calcium-stimulated PLC-1 activity and human keratinocyte differentiation, whereas each separately has little effect. However, unlike the activation of PLC-1 by epidermal growth factor (EGF), calcium-induced activation of PLC-1 was not a result of direct tyrosine phosphorylation. Therefore, we examined an alternative mechanism, in particular phosphatidylinositol 3,4,5-triphosphate (PIP3) formed as a product of phosphatidylinositol 3-kinase (PI3K) activity. PIP3 binds to and activates PLC-1. The combination of dominant negative src and fyn blocked calcium-induced tyrosine phosphorylation of the regulatory subunit of PI3K, p85
, and the activity of the catalytic subunit of PI3K. PI3K inhibitors blocked calcium activation of PLC-1 as well as the induction of keratinocyte differentiation markers involucrin and transglutaminase. These data indicate that calcium activates PLC-1 via increased PIP3 formation mediated by c-src and fyn activated PI3K. This activation is required for calcium-induced human keratinocyte differentiation.
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