Effect of PP2A on p34(SEI-1) expression in response to ionizing radiation in MCF-7 human breast cancer cells

Abstract

Breast cancer is one of the most common cancers in women and it is highly treatable by radiotherapy and/or radiochemotherapy. A global analysis of the protein expression pattern was performed to identify radiation-responsive proteins in MCF-7 breast cancer cells using 2D-PAGE coupled with MALDI-TOF-MS. When MCF-7 cells were exposed to ionizing radiation (IR) such as gamma-rays, eight proteins (GH2, RGS17, BAK1, CCNH, TSG6, RADS1B, IGFBP1, and CASP14) were up-regulated and three proteins (C1QRF, PLSCR2, and p34(SEI-1)) were down-regulated. In an effort to find what mechanisms are responsible for these changes, we initially focused on p34(SEI-1), which is known as a transcriptional regulator and oncogene. Our results show that p34(SEI-1) expression is significantly decreased only at the protein level but not at the transcriptional level after IR treatment. We suggest that the B55 regulatory subunit of PP2A., a positive regulator of p34(SEI-1), is at least partly responsible for the decreased p34(SEI-1) expression, in which the B55 regulatory subunit of PP2A was down-regulated at the protein level as a cellular response to IR. We, therefore, propose that inactivated PP2A resulting from the absence of the B55 subunit may not be able to dephosphorylate p34(SEI-1) and therefore increase the phosphorylated form of p34(SEI-1) with low stability. Our further extended study shows that the p34(SEI-1) expression level was not changed after H2O2 treatment at either protein or transcriptional levels. This result implies that MCF-7 cells seem to use different signaling pathways in response to IR and H2O2 stresses although both of them. belong to the same DNA damage inducing stimuli of reactive oxygen species (ROS).