RSK2 as a key regulator in human skin cancer

Our previous report demonstrated that RSK2 plays an important role in cell proliferation and transformation induced by tumor promoters such as epidermal growth factor mediated through the N-terminal kinase domain of RSK2 in JB6 Cl41 mouse skin epidermal cells in vitro. However, no direct evidence has been reported regarding the relationship of RSK2 activity and human skin cancer. To elucidate the relationship of RSK2 activity and human skin cancer, we examined the effect of knocking down RSK2 expression on epidermal growth factor-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 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 malignant melanoma cells. Moreover, RSK2(/) mouse embryonic fibroblast (MEF) showed enhanced sub-G(1) accumulation induced by UVB stimulation compared with RSK2(/) MEFs, indicating that RSK2 might play an important role in tolerance against stress associated with ultraviolet. 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.