Abstract LB-24: Nuclear localization of PDC-E2, a mitochondrial enzyme, and its role in STAT5-dependent gene transcription

STAT (signal transducer and activator of transcription) proteins play a critical role in cellular response to a wide variety of cytokine and growth factors by regulating specific nuclear genes. Many STAT-target genes are involved in cell proliferation, resistance to apoptosis, angiogenesis, and immune evasion. As a result, numerous cancer cells exhibit constitutive activation of STAT proteins. Among seven STAT family members, STAT3, STAT5a and STAT5b are most widely implicated in tumorigenesis. STAT-dependent gene transcription can be finely tuned through the association with co-factors in the nucleus. Therefore, detailed understanding of how different STAT family member acts in concert with distinct co-factor in the nucleus is essential in elucidating the mechanisms underlying STAT-dependent oncogenesis. We showed previously that STAT5 (including 5a and 5b), but not STAT3, specifically interacts with PDC-E2 (E2 subunit of pyruvate dehydrogenase complex), which is a nuclearly encoded mitochondrial enzyme. However, the functional significance of this novel association remains unknown. Here we report that PDC-E2 may function as a co-activator in STAT5-dependent gene transcription. Our earlier studies indicated that interleukin-3 (IL-3) induced mitochondrial translocation of STAT5 in BaF3, an IL-3-dependent mouse pro-B cell line. Further subcellular fractionation analysis revealed that a substantial amount of PDC-E2 could be detected in the nucleus in addition to mitochondrion. In contrast to IL-3-induced STAT5 nuclear translocation, the nuclear presence of PDC-E2 was constitutive and independent of IL-3 stimulation. This finding was confirmed by immunofluoresence analysis showing nuclear localization of PDC-E2 and its co-localization with STAT5 after IL-3 stimulation. Overexpression of PDC-E2 in BaF3 cells augmented IL-3-induced STAT5 activity as measured by reporter assay with consensus STAT5-binding sites. Consistent with the reporter data, PDC-E2 overexpression in BaF3 cells led to elevated mRNA levels of endogenous SOCS3 (suppressor of cytokine signaling 3) gene, a known STAT5 target. We further identified two functional STAT5-binding sites in the SOCS3 gene promoter important for its IL-3-inducibility. The observation that both cis-acting elements were essential for PDC-E29s synergistic effect strongly supports the role of PDC-E2 in up-regulating the transactivating ability of STAT5. All together, our results clearly demonstrate a unique function of PDC-E2 in the nucleus through STAT5. Functional interaction between STAT5 and PDC-E2 in the nucleus, therefore, may represent a novel molecular target in modulating STAT5 activity in tumor cells and shed new light on targeted cancer therapy. Furthermore, these findings will provide important insights into dysregulated nuclear-mitochondrial crosstalk in human cancers. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-24.