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Induction of Senescence in Diterpene Ester–Treated Melanoma Cells via Protein Kinase C–Dependent Hyperactivation of the Mitogen-Activated Protein Kinase Pathway

Melanoma Genomics Group, Queensland Institute of Medical Research, Brisbane, Queensland, Australia

Requests for reprints: Glen M. Boyle, Queensland Institute of Medical Research, Post Office Royal Brisbane Hospital, Brisbane, Queensland 4029, Australia. Phone: 61-7-3362-0319; Fax: 61-7-3845-3508; E-mail: Glen.Boyle{at}qimr.edu.au.

The diterpene ester PEP005 is a novel anticancer agent that activates protein kinase C (PKC) and induces cell death in melanoma at high doses. We now describe the in vitro cytostatic effects of PEP005 and the diterpene ester phorbol 12-myristate 13-acetate, observed in 20% of human melanoma cell lines. Primary cultures of normal human neonatal fibroblasts were resistant to growth arrest, indicating a potential for tumor selectivity. Sensitive cell lines were induced to senesce and exhibited a G₁ and G₂-M arrest. There was sustained expression of p21^WAF1/CIP1, irreversible dephosphorylation of the retinoblastoma protein, and transcriptional silencing of E2F-responsive genes in sensitive cell lines. Activation of mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase (MEK) 1/2 by PKC was required for diterpene ester–induced senescence. Expression profiling revealed that the MAP kinase inhibitor HREV107 was expressed at a higher transcript level in resistant compared with sensitive cell lines. We propose that activation of PKC overstimulates the RAS/RAF/MEK/ERK pathway, resulting in molecular changes leading to the senescent phenotype. (Cancer Res 2006; 66(20): 10083-91)

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