Abstract B55: Involvement of RSK2 in human skin cancer
Authors: Lee Cheol-Jung , Lee Mee Hyun , Yao Ke , Lee Hye Suk , Bode Ann M. , Dong Zigang , Cho Yong Yeon 
Published: January 2014

Abstract
Our previous study demonstrated that the total p90 ribosomal S6 kinase 2 (RSK2) protein level was significantly higher in human skin cancer tissues and cancer cells compared with normal skin tissues and premalignant cell lines. We also demonstrated that the N-terminal kinase domain of RSK2 plays an important role in transducing the RSK2 activation signal to its substrates. RSK2 requires activation of its C-terminal domain by its upstream kinases, ERKs, in order to phosphorylate and activate its substrates. For example, a recombinant RSK2 protein purified from E. Coli cannot phosphorylate the RSK2 substrate, NFAT3, but phosphorylation of RSK2 by ERKs at the linker region of RSK2 gives RSK2 the ability to phosphorylate its substrates through its N-terminal kinase domain. This suggests that ERKs/RSK2 signaling plays an important role in RSK2-mediated cell proliferation and cell transformation. Importantly, our study demonstrated that co-expression of RasG12V and RSK2 in NIH3T3 cells increased foci formation compared with RasG12V alone and si-RNA knockdown of RSK2 in RasG12V- or RasG12V/RSK2-co-expressing NIH3T3 cells completely blocked foci formation. Furthermore, we demonstrated that kaempferol, a natural compound abundantly found in dietary foods, inhibited RSK2 N-terminal kinase activity and EGF-induced neoplastic cell transformation. These indicated that the Ras/ERKs/RSK2 signaling pathway plays a key role in cell transformation. However, the role of RSK2 activation and signaling in human skin cancer has not yet been fully elucidated. To elucidate the relationship of RSK2 activity and human skin cancer, we examined the effect of knocking down RSK2 expression on EGF-induced anchorage-independent transformation in the premalignant HaCaT human skin keratinocyte cell line and on soft agar colony growth of SK-MEL-28 malignant melanoma (MM) cells. We found that the phosphorylated protein levels of RSK2 were enhanced in cancer tissues compared with normal tissues in a human skin cancer tissue array. We found that UVB stimulation induced increased in not only the total and phosphorylated protein levels of ERKs and RSK2, but also the nuclear localization and gene expression of RSK2. RSK2 knockdown inhibited proliferation and anchorage-independent transformation of HaCaT cells and soft agar colony growth of MM cells. Moreover, RSK2−/− mouse embryonic fibroblasts (MEFs) showed enhanced sub-G1 accumulation induced by UVB stimulation compared with RSK2+/+ MEFs, indicating that RSK2 might play an important role in tolerance against stress associated with UV. Importantly, activated RSK2 protein levels were highly abundant in human skin cancer tissues compared with matched skin normal tissues. Taken together, our results demonstrated that RSK2 plays a key role in neoplastic transformation of human skin cells and in skin cancer growth.
Citation Format: Cheol-Jung Lee, Mee Hyun Lee, Ke Yao, Hye Suk Lee, Ann M. Bode, Zigang Dong, Yong Yeon Cho. Involvement of RSK2 in human skin cancer. [abstract]. In: Proceedings of the Eleventh Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2012 Oct 16-19; Anaheim, CA. Philadelphia (PA): AACR; Cancer Prev Res 2012;5(11 Suppl):Abstract nr B55.