Calphostin C induces tyrosine dephosphorylation/inactivation of protein kinase Fa/GSK-3α in a pathway independent of tumor promoter phorbol ester-mediated down-regulation of protein kinase C
✍ Scribed by Shan-Chih Lee; Shiaw-Der Yang
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 950 KB
- Volume
- 60
- Category
- Article
- ISSN
- 0730-2312
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✦ Synopsis
The signal transduction mechanism of protein kinase FA/GSK-3a by tyrosine phosphorylation in A431 cells was investigated using calphostin C as an inhibitor for protein kinase C (PKC). Kinase F A / C S K -~~ could be tyrosine-dephosphorylated and inactivated to -10% of control in a concentration-dependent manner by 0.1-1 0 p M calphostin C (IC50, -1 pM), as demonstrated by immunoprecipitation of kinase F A / G S K -~~ from cell extracts, followed by phosphoamino acid analysis and by immunodetection in an antikinase FAIGSK-3a immunoprecipitate kinase assay.
In sharp contrast, down-regulation of PKC by 0.05 p M calphostin C (IC50, -0.05 p M for inhibiting PKC in cells) or by tumor promoter phorbol ester TPA was found to have stimulatory effect on the cellular activity of kinase F A I G S K -~~, when processed under identical conditions. Furthermore, TPA-mediated down-regulation of PKC was tound to have no effect on calphostin C-mediated tyrosine dephosphorylationiinactivation of kinase F A I G S K -~~. Taken together, the results provide initial evidence that the PKC inhibitor calphostin C may induce tyrosine dephosphorylation/inactivation of kinase F A / G S K -~~ in a pathway independent of TPA-mediated down-regulation of PKC, representing a new mode of signal transduction for the regulation of this multisubstrate/multifunctional protein kinase by calphostin C in cells. Since kinase F A / G S K -~~ is a possible carcinoma dedifferentiation/progression-promoting factor, the results further suggest calphostin C as a potential anticancer drug involved in blocking carcinoma dedifferentiation/progression, possibly via inactivation of protein kinase F A / G S K -~~ in tumor cells.