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Reactivation of Suppressed RhoB is a Critical Step for the Inhibition of Anaplastic Thyroid Cancer Growth

¹ Department of Cancer Biology, Mayo Clinic Comprehensive Cancer Center and ² Division of Endocrinology, Internal Medicine Department, Mayo Clinic, Jacksonville, Florida; ³ Daiichi Sankyo Co., Ltd., Edogawa-ku, Tokyo, Japan; Departments of ⁴ Laboratory Medicine/Pathology, ⁵ Surgery, and ⁶ Medicine, ⁷ Endocrine Malignancy Working Group, Mayo Clinic, Rochester, Minnesota; and ⁸ Department of Otolaryngology-Head and Neck Surgery, Eastern Virginia Medical School, Norfolk, Virginia

Requests for reprints: John A. Copland, Department of Cancer Biology, Mayo Clinic Comprehensive Cancer Center, Mayo Clinic College of Medicine, 4500 San Pablo Road, Jacksonville, FL 32224. Phone: 904-953-6120; Fax: 904-953-0277; E-mail: copland.john{at}mayo.edu.

Key Words: Peroxisome proliferator–activated receptor • RhoB • Anaplastic thyroid carcinoma • p21^WAF1/CIP1 • Histone deacetylase inhibitor

Anaplastic thyroid carcinoma (ATC) is a highly aggressive form of the disease for which new therapeutic options are desperately needed. Previously, we showed that the high-affinity peroxisome proliferator–activated receptor (PPAR) agonist, RS5444, inhibits cell proliferation of ATC cells via induction of the cyclin-dependent kinase inhibitor p21^WAF1/CIP1 (p21). We show here that up-regulation of RhoB is a critical step in PPAR-mediated activation of p21-induced cell stasis. Using multiple independently derived ATC cell lines, we found that treatment with RS5444 leads to the up-regulation of RhoB and subsequent activation of p21, and that silencing of RhoB by RNAi blocks the ability of RS5444 to induce p21 and to inhibit cell proliferation. Our results show that transcriptional regulation of RhoB by the nuclear transcription factor PPAR is responsible for the induction of p21 mRNA and protein. We further implicate RhoB as a key signaling effector for the growth inhibition of ATC, as treatment with a histone deacetylase inhibitor shown to increase RhoB expression in lung cancer cells caused the up-regulation of RhoB in ATC cells accompanied by increased expression of p21 and inhibition of cell proliferation; this effect occurred even in ATC cells that were unresponsive to RS5444 due to a lack of expression of PPAR. Our results implicate RhoB as a novel intermediate in critical signaling pathways and as an additional target for therapeutic intervention in ATC. [Cancer Res 2009;69(4):1536–44]

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