Thyroid cancer is the most common endocrine malignancy, and incidences are rising. MAP kinase (MAPK) signaling has been implicated in playing a critical role in the initiation and maintenance of thyroid cancer, as evidenced by the high incidence of non-overlapping mutations of the genes encoding RET and TRK, as well as of NRAS, HRAS, KRAS and BRAF. RAS mutations are found in 40-50% of follicular carcinomas (FTC) and 20-40% of follicular adenomas (FA), as well as follicular variant of papillary cancer (FVPTC). The most common mutations associate with thyroid tumors are in HRAS and NRAS, most often in codon 61. This is in contrast to most other epithelial tumors where KRAS codon 12/13 mutations are most common. We hypothesized that Ras isoforms have different transformational potential in thyroid epithelium. To test this hypothesis, we developed mice with a thyroid-specific knock-in of oncogenic Hras combined homozygous loss of Pten (HrasG12V/PtenHom). We report that a thyroid specific knock-in of HrasG12V coupled with Pten inactivation leads to the development of FTCs and poorly differentiated thyroid cancer (PDTCs) by 47-52 weeks of age in HrasG12V/PtenHom/TPO-cre mice. In stark contrast, mice with a thyroid specific knock-in of KrasG12D and loss of Pten (KrasG12D/PtenHom/TPO-cre) develop FTCs which rapidly progress to PDTCs and anaplastic thyroid cancer with 50% lethality at 7 weeks and complete lethality by 20 weeks. These data suggest that Kras and Hras, in cooperation with PI3K signaling, likely play distinct roles in the development and progression of disease. We sought to dissect the role of Hras versus Kras activation in FTC development. We derived stable cell lines from HrasG12V/PtenHom/TPO-cre and obtained a similar line from Kras tumors to identify cellular and molecular responses to Hras versus Kras activation and their impact on tumor phenotype. Utilizing a phospho-protein expression array, we determined that Kras activation leads to increased phosphorylation of members of the p38 MAPK pathway, whereas Hras activation increases mTOR effectors. Pathway profiling indicated differences in transcript levels of MAPK effects in response to Kras versus Hras activation. KrasG12D/Ptenhom cells had increased expression of cell cycle regulators including cyclinD1, whereas extracellular matrix protein Col1a1 was unregulated in Hras-driven cells. Further, Kras-driven cells have an increased ability to form thyrospheres on low-adherence tissue culture plate. These results suggest that Kras induced tumors have a larger population of self-renewing cells and therefore may account for the increase virulence of this oncogene in thyrocytes. These studies demonstrate a clear phenotypic difference between mutant Hras and Kras in thyroid epithelium, and suggest that the increase transformative property of mutant Kras might be mediated by over-expression of proteins that increase cell proliferation and mediate transition through the cell cycle.
Citation Format: Lee Ann King, Nolan West, Aime Franco. Hras and Kras have different transformational capacity in murine models of thyroid carcinoma. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr B04. doi: 10.1158/1557-3125.RASONC14-B04
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