Abstract 638: The impact of PI3K/Akt pathway inhibition upon the c-myb oncogene

Adenoid cystic carcinoma (ACC) is a rare variant of adenocarcinoma that most commonly arises in the salivary glands. Rarely, ACC can originate at other sites, including the lung and breast. c-myb overexpression has been identified in over 70% of all ACCs, suggesting that it is a critical driver of oncogenesis for this disease. Recently, a translocation between chromosomes 6 and 9 has been described in ACC which creates a fusion gene between the transcription factors c-myb and NFIB. The C-terminal truncation of the c-myb gene in the t(6;9) translocation leads to loss of negative regulatory microRNA binding sites and hence elevated c-myb protein levels. Full length c-myb is also reportedly overexpessed in fusion negative tumors presumably via alternate mechanisms. Based upon previous reports that the phosphatidylinositol-3 phosphate kinase (PI3K)/Akt pathway is activated in ACC tumors as well as data from other tumor models that this pathway is critical for maintaining c-myb protein stability, we hypothesized that inhibiting the PI3K/Akt pathway would be a novel therapeutic strategy for targeting c-myb in ACC tumors. Given the lack of validated ACC tumor models, we tested this hypothesis by transiently transfecting hemagglutinin (HA) tagged full length c-myb or the MYB-NFIB fusion gene in the breast cancer cell line MDA-468. The impact of three PI3K/Akt pathway inhibitors was tested: the PI3K/mTOR inhibitor LY294002, the mTOR inhibitor rapamycin, and the Akt inhibitor MK-2206 (Merck)(all drugs were obtained commercially). While all three drugs downregulated full-length c-myb, Akt inhibition with MK-2206 led to the greatest degree of c-myb protein reduction. For the c-myb/NFIB fusion, again MK-2206 was the most effective for downregulating c-myb/NFIB levels. The impact of MK-2206 upon c-myb/NFIB levels was dose dependent and correlated to loss of Akt phosphorylation at serine 473. Akt inhibition with MK-2206 also resulted in increased c-myb/NFIB protein ubiquitination, supporting the hypothesis that Akt inhibition may lead to ubiquitin mediated degradation of the fusion protein. To confirm the importance of PI3K/Akt signaling for maintaining c-myb/NFIB stability, we utilized an RNA interference approach to downregulate expression of p110α, the catalytic subunit of PI3K, in cells expressing c-myb/NFIB. Transfection of the p110α siRNA downregulated the target gene, decreased Akt phosphorylation, and lowered c-myb/NFIB levels relative to cells treated with an siRNA control. Taken together, these results argue that Akt is critical for maintaining stability of both overexpressed full length c-myb and the MYB-NFIB fusion gene product. This study proposes Akt as a rational therapeutic target for ACC, and suggests that c-myb overexpression may be a potential biomarker of response to Akt inhibition. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 638. doi:10.1158/1538-7445.AM2011-638