The induction of senescence-like growth arrest by protein kinase C-activating diterpene esters in solid tumor cells.

We previously identified the induction of senescence in melanoma cell lines sensitive to diterpene esters, indicating a therapeutic potential. Here we compared the cytostatic effects of two diterpene esters: the prototypic PKC-activating drug TPA (12-O-tetradecanoylphorbol-13-acetate), and the novel compound PEP008 (20-O-acetyl-ingenol-3-angelate) in cell lines derived from melanoma, breast cancer and colon cancer. The diterpene esters induced permanent growth arrest with characteristics of senescence in a subset of cell lines in all three solid tumor models at 100-1000 ng/ml. Use of the PKC inhibitor bisindolylmaleimide-l demonstrated that activation of PKC was required for growth arrest. Full genome expression profiling identified pivotal genes involved in DNA synthesis and cell cycle control down-regulated by treatment in all three sensitive tumor models. At the protein level, prolonged down-regulation of E2F-1 and proliferating cell nuclear antigen (PCNA), sustained expression of p21(WAF1/CIP1) and dephosphorylation of retinoblastoma (Rb) occurred in the sensitive cells. Additionally, the type II tumor suppressor HRASLS3, which has a role in mitogen-activated protein kinase (MAPK) pathway suppression, was constitutively elevated in cell lines resistant to the senescence effects compared to their sensitive counterparts. Together, these results demonstrate that both TPA and the novel PKC-activating drug PEP008 induce growth arrest with characteristics of senescence in solid tumor cell lines derived from a variety of tissue types, and by a similar mechanism. PKC-activating diterpene esters may therefore have therapeutic potential in a subset of breast cancer, colon cancer and melanoma tumors.