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The Possible Role of Cytokeratin 8 in Cadmium-Induced Adaptation and Carcinogenesis

Department of Anatomy, Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong SAR, People's Republic of China

Requests for reprints: Jen-Fu Chiu, Department of Anatomy, Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong SAR, People's Republic of China. Phone: 852-2299-0777; Fax: 852-2817-1006; E-mail: jfchiu{at}hkucc.hku.hk.

Chronic exposures to cadmium compounds are carcinogenic. It was hypothesized that the development of resistance to cadmium may drive carcinogenesis. This is achieved by selection of resistant cells in which the apoptotic response is significantly attenuated. The induction of cadmium resistance in rat lung epithelial cells (LEC) was used to explore the mechanisms of cadmium-induced adaptation and carcinogenesis. Our previous results showed that LECs developed resistance to apoptosis during cadmium adaptation possibly due to perturbation of the c-Jun NH₂-terminal kinase pathway. Here, we further study these cells by comparative proteomics. Interestingly, we showed that two intermediate filament proteins, cytokeratin 8 (CK8) and cytokeratin 14 (CK14), were increased significantly and stably maintained only in the adapted cells but not in cadmium-treated parental cells. It has been documented that CK8/cytokeratin 18 provided resistance to tumor necrosis factor (TNF)–induced apoptosis and CK14 may function as an inhibitor of TNF-TNF receptor 1 (TNFR1) signaling through an association with TNFR1-associated death domain protein, suggesting that up-regulation of CK8 and CK14 may be responsible for apoptotic resistance. Finally, we showed that small interfering RNA–specific knockdown of CK8 in cadmium-adapted cells attenuated the cadmium resistance, indicating the potential role of CK8 in cadmium resistance. This acquired self-resistance to apoptosis could account for cadmium-induced carcinogenesis, as this promotes neoplastic cell survival as well as subsequent clonal expansion and then progression of tumor development. Thus, increased expression of these cytokeratins represents an adaptive survival mechanism that resists cadmium-induced apoptosis and it is unprecedented that cells respond to long-term cadmium exposure by modulating keratin dynamics. [Cancer Res 2007;67(5):2107–13]