Carcinogenesis Advance Access originally published online on August 19, 2008
Carcinogenesis 2008 29(11):2078-2088; doi:10.1093/carcin/bgn197
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Akt phosphorylates the TR3 orphan receptor and blocks its targeting to the mitochondria
Key Laboratory of the Ministry of Education for Cell Biology and Tumor Cell Engineering, School of Life Sciences, Xiamen University, Xiamen 361005, Fujian Province, China
* To whom correspondence should be addressed. Department of Biomedical Sciences, School of Life Sciences, Xiamen University, Xiamen 361005, Fujian Province, China. Tel: +86 592 2187959; Fax: +86 592 2086630; Email: qiaow{at}xmu.edu.cn
Acutely transforming retrovirus AKT8 in rodent T cell lymphoma (Akt) phosphorylates and regulates the function of many cellular proteins involved in processes such as metabolism, apoptosis and proliferation. However, the precise mechanisms by which Akt promotes cell survival and inhibits apoptosis have been characterized in part only. TR3, an orphan receptor, functions as a transcription factor that can both positively or negatively regulate gene expression. We have reported previously that the translocation of TR3 from the nucleus to the mitochondria can elicit a proapoptotic effect in gastric cancer cells. In our present study, we demonstrate that Akt phosphorylates cytoplasmic TR3 through its physical interaction with the N-terminus of TR3. When coexpressed with Akt, TR3 mitochondrial targeting was blocked and this protein adopted a diffuse expression pattern in the cytoplasm. Moreover, Akt displayed an ability to disrupt the interaction of TR3 with Bcl-2, which is thought to be a critical requirement for mitochondrial TR3 to elicit apoptosis. Consistently, insulin was also found to induce the phosphorylation of TR3 and abolish 12-O-tetradecanoylphorbol-13-acetate-induced mitochondrial localization, which was dependent upon the activation of the phophatidylinositol-3-OH-kinase–Akt signaling pathway. Taken together, our current data demonstrate a unique role for Akt in inhibiting TR3 functions that are not related to transcriptional activity but that correlate with the regulation of its mitochondrial association. This may represent a novel signal pathway by which Akt exerts its antiapoptotic effects in gastric cancer cells, i.e. by regulating the phosphorylation and redistribution of orphan receptors.
Abbreviations: AKT, acutely transforming retrovirus AKT8 in rodent T cell lymphoma; CIAP, calf intestine alkaline phosphatase; DBD, DNA-binding domain; DN-Akt, dominant-negative Akt; ERK, extracellular signal-regulated kinase; OA, okadaic acid; PBS, phosphate-buffered saline; PI3K, phophatidylinositol-3-OH-kinase; TM, transmembrane domain; TPA, 12-O-tetradecanoylphorbol-13-acetate
Received February 26, 2008; revised August 11, 2008; accepted August 16, 2008.