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      Rap1/B-Raf Signaling Is Activated in Neuroendocrine Tumors of the Digestive Tract and Raf Kinase Inhibition Constitutes a Putative Therapeutic Target

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          Objective: Molecular pathogenesis of digestive neuroendocrine tumors (dNETs) is largely unknown. Recently, the serine-threonine kinase B-Raf was identified as an oncogene in endocrine cancer such as thyroid carcinoma. In endocrine cells, the small G-protein Rap1 stimulates mitogen-activated protein kinase (MAPK) signaling by activating B-Raf. We examined the expression of Rap1 and B-Raf in dNETs and their contribution to MAPK signaling in neuroendocrine cell lines. In addition, we explored the effect of suppressing B-Raf kinase by the recently developed inhibitor BAY43-9006 (Sorafinib) on growth, apoptosis and MAPK activation neuroendocrine cell lines. Methods and Results: Expression of Rap1 and B-Raf in dNETs (19 insulinomas, 15 carcinoid tumors and 10 gastrinomas) was examined by immunohistochemistry, which revealed that Rap1 and B-Raf were highly prevalent in the majority of dNETs. Overexpression of Rap1 and B-Raf activated MAPK extracellular dependent kinase (ERK) ERK-2 and ERK-dependent transcription factor Elk-1 in neuroendocrine cell lines Bon and INS-1. Suppression of B-Raf by BAY43-9006 inhibited growth and induced apoptosis in Bon and INS-1 cells. In addition, BAY43-9006 suppressed phosphorylation of MAPK ERK1/2 and its upstream kinase MEK1/2 in Bon and INS-1 cells. Conclusion: These results indicate that Rap1-B-Raf signaling may contribute to pathogenesis of dNETs and provides a molecular target for treatment of dNETs.

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          Most cited references 23

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          Meaningful relationships: the regulation of the Ras/Raf/MEK/ERK pathway by protein interactions.

           W Kolch (2000)
          The Ras/Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular-signal-regulated kinase) pathway is at the heart of signalling networks that govern proliferation, differentiation and cell survival. Although the basic regulatory steps have been elucidated, many features of this pathway are only beginning to emerge. This review focuses on the role of protein-protein interactions in the regulation of this pathway, and how they contribute to co-ordinate activation steps, subcellular redistribution, substrate phosphorylation and cross-talk with other signalling pathways.
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            BRAF mutations and RET/PTC rearrangements are alternative events in the etiopathogenesis of PTC.

            Rearrangement of RET proto-oncogene is the major event in the etiopathogenesis of papillary thyroid carcinoma (PTC). We report a high prevalence of BRAF(V599E) mutation in sporadic PTC and in PTC-derived cell lines. The BRAF(V599E) mutation was detected in 23 of 50 PTC (46%) and in three of four PTC-derived cell lines. The prevalence of the BRAF(V599E) mutation in PTC is the highest reported to date in human carcinomas, being only exceeded by melanoma. PTC with RET/PTC rearrangement as well as the TPC-1 cell line (the only one harboring RET/PTC rearrangement) did not show the BRAF(V599E) mutation. BRAF(V599E) mutation was not detected in any of 23 nodular goiters, 51 follicular adenomas and 18 follicular carcinomas. A distinct mutation in BRAF (codon K600E) was detected in a follicular adenoma. Activating mutations in RAS genes were detected in 15% of FA, 33% of FTC and 7% of PTC. BRAF(V599E) mutation did not coexist with alterations in any of the RAS genes in any of the tumors. These results suggest that BRAF(V599E) mutation is frequent in the etiopathogenesis of PTC. The BRAF(V599E) mutation appears to be an alternative event to RET/PTC rearrangement rather than to RAS mutations, which are rare in PTC. BRAF(V599E) may represent an alternative pathway to oncogenic MAPK activation in PTCs without RET/PTC activation.
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              Rap1 signalling: adhering to new models.

              Ras-like GTPases are ubiquitously expressed, evolutionarily conserved molecular switches that couple extracellular signals to various cellular responses. Rap1, the closest relative of Ras, has attracted much attention because of the possibility that it regulates Ras-mediated signalling. Rap1 is activated by extracellular signals through several regulatory proteins, and it might function in diverse processes, ranging from modulation of growth and differentiation to secretion, integrin-mediated cell adhesion and morphogenesis.

                Author and article information

                S. Karger AG
                April 2007
                05 March 2007
                : 85
                : 1
                : 45-53
                aDepartment of Internal Medicine, Division of Gastroenterology and Endocrinology, and bDepartment of Surgery, Philipps University Marburg, Marburg, Germany
                100508 Neuroendocrinology 2007;85:45–53
                © 2007 S. Karger AG, Basel

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                Page count
                Figures: 8, Tables: 1, References: 30, Pages: 9
                Clinical Neuroendocrinology and Neuroendocrine Tumors


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