Prognostic Significance of Both the Cytoplasmic and Nuclear Overexpression of Yin-Yang 1 (YY1) among Patients with Multiple Myeloma (MM)

Objective and Rationale Yin-Yang 1 (YY1) is a multi-functional DNA-binding protein, which can activate, repress or initiate transcription depending on the context in which it binds. In addition, YY1 can modulate protein levels or activity through protein-protein interaction. YY1 has been identified as a potential repressor factor for several genes. We have reported that YY1 can act as a transcription repressor for both Fas and TRAIL DR5. In addition to YY1-mediated regulation of tumor cell resistance to cytotoxic immunotherapy, it also has been shown to regulate resistance to chemotherapy [Baritaki et al., J Immunol 80:6199,2008]. Overexpression of YY1 has been shown to be of prognostic significance in prostate cancer [Seligson et al., Int J Oncol 27:131,2005].

Hypothesis In this study, we have examined the expression of YY1 in MM (cell lines and patients’ bone marrow [BM]) by hypothesizing that the resistance of MM cells to various cytotoxic agents may be, in part, regulated by overexpression of YY1 and that YY1 may also be of prognostic significance.

Designs and Methods MM cell lines and fresh BM samples from MM patients (n=21) were examined for YY1 expression, cytoplasmic and nuclear, by immunohistochemistry and by Western. The specificity of the anti-YY1 antibody was demonstrated by the competitive inhibition with a peptide used for immunization. Control isotype IgG did not show any staining on the cells.

Results First, we found that various MM cell lines (RMPI 8226, IM-9, U266) overexpress YY1 both in the cytoplasm and the nucleus. Next, we examined the expression of YY1 in BM derived from MM patients by immunohistochemistry. BM from MM patients showed overexpression of YY1 as compared to normal bone marrow samples. Furthermore, analysis of both the intensity and frequency of cells expressing YY1 in both the cytoplasm and nucleus was shown to be significantly higher among patients with progressive disease as compared to patients with stable or responsive disease.

Conclusions and Implications These findings show that the expression of YY1 among patients with MM may correlate with progression and also suggest the prognostic significance of YY1 in MM patients. Inhibition of YY1 by various agents (example: low-dose chemotherapy, proteasome inhibitors, NO donors, NF-kB inhibitors, etc.) all result in the reversal of resistance to various cytotoxic agents (eg CDDP, TRAIL) therefore, the findings also imply that agents that can inhibit YY1 expression in MM patients may be of therapeutic potential when used in combination with conventional therapeutics.