Abstract 1039: CREB (Ser133) phosphorylation is abnormally increased in tumors showing resistance to Iressa treatment

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL EGFR is a validated therapeutic target; but unfortunately, only a small percentage of patients with EGFR-overexpressing tumors respond to therapy, and resistance develops even in responsive patients. Gefitinib (Iressa) is a small molecule tyrosine kinase inhibitor that suppresses the activation of EGFR. Understanding the molecular mechanisms and protein targets involved in the effects of Iressa requires the simultaneous identification of specific molecular markers in the complex network of signaling pathways that are secondarily modulated by Iressa in mammary cancer. In this study, female Sprague-Dawley rats (50 days old) were given methylnitrosourea (MNU) by IV injection through the jugular vein (75 mg/kg BW). The rats were palpated for mammary cancers 2 times per week. When a palpable mammary cancer of approximately 200-250 mm2 was present, the rat was administered Iressa (6 mg/kg BW/day) by gavage for 2 days. The tumor was then biopsied. The rats were treated with Iressa at the same dose for an additional 40 days and tumors were harvested. At sacrifice, the mammary cancer was rapidly removed and frozen for protein array analysis. The tumors were sorted into two groups based on sensitivity to treatment with Iressa and analyzed by a phospho-protein Proteome Profiler™ Array (R&D, Minneapolis, MN). Three tumors that regressed 27, 39, and 59% were compared to 4 tumors that continued to grow, increasing in size by 56, 106, 210, and 236%. Most notably, tumors that continued to grow in spite of Iressa treatment consistently showed a marked increased phosphorylation of CREB (Ser133) and Src (Tyr419) compared to tumors that regressed. CREB phosphorylation at Ser133 enhances its transactivation and transcriptional activities, resulting in increased expression of many downstream target genes involved in cell proliferation and cancer development. In addition, tumors that continued to grow also showed increased phosphorylation of p27 (Thr157) compared to regressed tumors. Phosphorylation of p27 at Thr157 is known to impair its nuclear localization, suggesting an acceleration of the G1/S cell cycle transition, resulting in enhanced cell proliferation. In contrast, tumors that regressed showed marked increased phosphorylation of Hck (Tyr411) compared to tumors that continued to grow. The hck gene is located at 20q11-q12, which is a region affect by interstitial deletions in some acute myeloid leukemias and myeloproliferative disorders and damage to hck might contribute to the pathogenesis of these conditions, suggesting that autophosphorylation of Hck (Tyr411) might be involved in tumor suppression. These results indicated that Iressa resistance in mammary cancer induced by MNU is closely related with activation of the signaling axis of the cytosolic tyrosine kinase, CREB, which could contribute to continuous proliferation. Supported by NCI # HHSN-261200433009C – NO1-CN-55006-72. 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 1039. doi:10.1158/1538-7445.AM2011-1039