Photodynamic Therapy (PDT)-Mediated Inhibition of the Transcription Factor Yin Yang 1 (YY1) That Regulates Resistance In Lymphoma

Abstract 5113

Photodynamic therapy (PDT) is a cancer therapeutic treatment that uses a compound called the “photosensitizer” and a particular type of visible light. When photosensitizers are exposed to a specific wavelength of light (600-800 nm), cytotoxic oxygen species are generated that kill cells (Dougherty, TJ et al., JNCI 90:889, 1998). Several clinical trials are currently underway to evaluate the use of PDT for a variety of cancers. A phase II study has been completed with photodynamic therapy in the treatment of patients with lymphoma or chronic lymphocytic leukemia. (NCT00054171). Recently, we have focused our attention about the properties of the photosensitizer Pheophorbide a (Pba), a chlorine, and its effects on different types of solid tumor cells (Rapozzi, V et al., Cancer Biol Ther 14:1318, 2009). The objective of the present study is to investigate the biochemical and molecular mechanisms by which PDT signals the B-NHL Raji lymphoma cell line (as model) and rendering the cells susceptible to both the cytotoxic mechanism of the tumor microenvironment in vivo or to the response to cytotoxic agents in vitro. We hypothesized that treatment of Raji cells with Pba/PDT in our in vitro system may result in the inhibition of resistance factors that regulate tumor cell responses to both chemotherapeutic and immunotherapeutic drugs. Our recent findings demonstrated that the constitutively overexpressed transcription factor Yin Yang 1 (YY1) regulates, in part, tumor cell resistance in lymphoma (Vega, MI et al., J Immun 175:2174, 2005). Accordingly, we examined whether treatment of Raji lymphoma cells with Pba/PDT will also result in the downregulation of YY1 expression and reverse resistance. The Raji cells were seeded at a cell density of 2×10⁵/ml in Petri dishes. When the cells reached a 70% confluency, they were treated with different concentration (80-160-240 nM) of Pba for three hours in the dark and were then irradiated by an LED light source (640 nm at 12,7 mW for 9 min; 6.7 J/cm²). Following the light treatment, the cells were harvested at different times of incubation (18-36h) to assess apoptosis by the activation of caspase 3 using flow cytometry. In addition, different aliquots of cells were used to prepare slides for immunohistochemistry analyses. The results demonstrate that, indeed, treatment with Pba/PDT resulted in the inhibition of YY1 protein expression in Raji cells. By immunohistochemistry, PDT inhibited the basal nuclear and cytoplasmic expression of YY1 and resulted in weak cytoplasmic YY1 expression. The mechanism of YY1 inhibition might have been the result of PDT-mediated inhibition of NF-κB activity (Karmakar, S. et al., Neurosci lett 415: 242, 2007) since YY1 is transcriptionally regulated by NF-κB (Wang, H et al., Mol Cell Biol 67:4374, 2007). In addition, our preliminary findings demonstrate that treatment of drug-resistant tumor cells with PDT sensitizes the cells to drug-induced apoptosis. Overall, the data suggest that YY1 may be considered as a novel therapeutic target in PDT. Based on the findings here, we are currently examining the role of PDT in the dysregulation of the NF-κB/YY1/Snail/RKIP loop (Wu, K and Bonavida, B. Crit Rev Immun 29:241, 2009) that regulates cell survival and proliferation and resistance in lymphoma. (We acknowledge Doctors Oscar Stafsudd and Romaine Saxton for their assistance.)