Relocalized p27Kip1 tumor suppressor functions as a cytoplasmic metastatic oncogene in melanoma

The p27 tumor suppressor negatively regulates G1 cell cycle progression. However, human malignancies rarely select for deletion/inactivation of p27, a hallmark of tumor suppressor genes. Instead, p27 is degraded or relocalized to the cytoplasm in aggressive malignancies, supporting the notion that p27 sequestration from its nuclear cyclin:cyclin-dependent kinase (cdk) targets is critical. However, emerging cell biology data suggest a novel cdk-independent cytoplasmic function of p27 in cell migration. Here, we find cytoplasmic p27 in 70% of invasive and metastatic melanomas. In contrast, no cytoplasmic p27 was detected in noninvasive, basement membrane-confined melanoma in situ, suggesting a late oncogenic role for cytoplasmic p27 in metastasis. Targeted cytoplasmic expression of wild-type or non-cdk-binding p27 at subphysiologic levels induced melanoma motility and resulted in numerous metastases to lymph node, lung, and peritoneum. These observations point to a prominent role of cytoplasmic p27 in metastatic disease that is independent of cyclin:cdk regulation or mere nuclear loss.