Cellular and transcriptional modulation of the DNA damage response in TK6 cells pretreated with protein kinase C activating tumor promoters

Tumor promoting chemicals enhance the rate of cancer formation through modulation of cell signaling circuitry. 12-O-tetradecanoylphorbol-13-acetate (TPA) has been used to model the process of tumor promotion in mouse skin for over 75 years. TPA belongs to the class of tumor promoting chemicals known to activate protein kinase C which is considered their primary mode-of-action. However, PKC-activation alone is not sufficient to augment tumorigenesis. These chemicals cause myriad pro-tumorigenic changes to the cell and surrounding microenvironment that ultimately facilitates the outgrowth of neoplastic cells. Despite years of research, the process of tumor promotion remains an enigma, particularly in the context of human health and exposure to potential carcinogenic agents in the environment. It is assumed that greater than 80% of cancer cases can be attributed to tumor promoting environmental conditions such as smoking, diet, inflammation, radiation and carcinogenic chemicals. In order to expand our knowledge on the process of tumor promotion, it is important to build models to characterize the biochemical changes that occur in the tumor microenvironment. Here we established a model system to analyze the effects of tumor promoting agents on the DNA damage response (DDR) in a human cell line. The DDR is a major tumor suppressive pathway that becomes constitutively activated in early tumors. Previous studies have shown this pathway to be modulated by TPA; however, limited conclusions can be drawn due to controversial findings and high variability between models. We analyzed the effect of PKC-activating tumor promoters on non-tumor derived human lymphoblastoid cells and analyzed the cellular and transcriptional modulation of the DDR in response to UVC-irradiation. Here we show how TPA-pretreated TK6 cells undergo synergistic increases in apoptosis upon UVC-irradiation in addition to delayed resolution of gammaH2AX, a common marker for the DDR. The synergistic effects were also apparent at the transcriptional level and a gene signature associated with TPA+UVC treatment was elucidated. The TPA+UVC gene signature was reflective of genes and pathways activated in the tumor microenvironment where tumor suppressing and tumor promoting pathways converge.