Aberrant regulation of ribosomal RNA (rRNA) synthesis and translation control can facilitate tumorigenesis. The ErbB2 growth factor receptor is overexpressed in many human tumors and has been detected in the nucleus, but the role of nuclear ErbB2 is obscure. In this study, we defined a novel function of nuclear ErbB2 in enhancing rRNA gene transcription by RNA polymerase-I (RNA Pol I). Nuclear ErbB2 physically associates with β-actin and RNA Pol I, coinciding with active RNA Pol I transcription sites in nucleoli. RNAi-mediated knockdown of ErbB2 reduced pre-rRNA and protein synthesis. In contrast, wild-type ErbB2 augmented pre-rRNA level, protein production and cell size/cell growth, but not by an ErbB2 mutant which is defective in nuclear translocation. Chromatin immunoprecipitation assays revealed that ErbB2 enhances binding of RNA Pol I to rDNA. Additionally, ErbB2 associated with rDNA, RNA Pol I and β-actin, suggesting how it could stimulate rRNA production, protein synthesis and increased cell size and cell growth. Lastly, ErbB2-potentiated RNA Pol I transcription could be stimulated by ligand and was not substantially repressed by inhibition of PI3-K and MEK/ERK, the main ErbB2 effector signaling pathways. Together, our findings indicate that nuclear ErbB2 functions as a regulator of rRNA synthesis and cellular translation, which may contribute to tumor development and progression.

We have demonstrated that ErbB-2 translocates into the nucleus through importin β1—mediated nuclear import by endosomal sorting, and transactivates transcription of cyclooxygenase-2 which has been known to be involved in tumor progression and metastasis, suggesting nuclear ErbB-2 may also contribute to the tumor malignancy. However, how the membranebound ErbB-2 can be released from the lipid bilayer and translocates into the nucleus; and the biological functions of the nuclear ErbB-2 are not yet completely understood. Here, we have identified novel ErbB-2-interacting nuclear proteins. The interplay between nuclear ErbB-2 and interacting protein was investigated, which may elucidate a novel function of nuclear ErbB-2 in contributing to tumorigenesis. In addition, ErbB-2 interacts with endoplasmic reticulum (ER) proteins, Sec61α, a subunit of Sec61 channel, and EDEM. Sec61α and EDEM are known to mediate retrotranslocation of misfolded proteins in the ER to the cytosol (called ER-associated degradation pathway). We have demonstrated that Sec61α and EDEM also play important roles in retrotranslocation of ErbB-2 to the cytosol and to the nucleus, suggesting part of ER retrotranslocation pathway is likely involved in nuclear trafficking of ErbB-2. [This work was supported by National Health Research Institutes NHRI-EX98-9603BC, National Science Council NSC952311-B-039-002 and NSC 97-3111-B-039-004 and Department of Health DOH97-TD-I-111-TM003 and DOH98-TD-I-111-TM002 (to LYL)]. Citation Information: Cancer Res 2009;69(23 Suppl):B14.

Accumulating evidence shows that deregulation of ribosomal RNA (rRNA) synthesis and translation control can facilitate tumorigenesis. The ErbB2 growth factor receptor is overexpressed in many human tumors and has been detected in the nucleus, but the role of nuclear ErbB2 is obscure. Here, we demonstrated a novel function of nuclear ErbB2 in enhancing rRNA gene transcription by RNA polymerase-l (RNA Pol I). Nuclear ErbB2 physically associates with β-actin and RNA Pol I, coinciding with active RNA Pol I transcription sites in nucleoli. RNAi-mediated knockdown of ErbB2 reduced pre-rRNA and protein synthesis. In contrast, wild-type ErbB2 augmented pre-rRNA level, protein production and cell size/cell growth, but not by an ErbB2 mutant which is defective in nuclear translocation. Chromatin immunoprecipitation assays revealed that ErbB2 enhances binding of RNA Pol I to rDNA. Additionally, ErbB2 associated with rDNA, RNA Pol I, and β-actin, suggesting how it could stimulate rRNA production, protein synthesis, and increased cell size and cell growth. Lastly, ErbB2-potentiated RNA Pol I transcription could be stimulated by ligand and was not substantially repressed by inhibition of PI3-K and MEK/ERK, the main ErbB2 effector signaling pathways. Together, our findings indicate that nuclear ErbB2 functions as a regulator of rRNA synthesis and cellular translation, which may contribute to tumor development and progression. This work was supported by National Health Research Institutes NHRI-EX100-9603BC, National Science Council NSC95-2311-B-039-002 and NSC99-3111-B-039-001 and Department of Health DOH99-TD-I-111-TM026 and DOH100-TD-C-111-005 (to LYL), Taiwan. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr A57.