Senescence and pre-malignancy: How do tumors progress?

Cellular senescence is a tumor suppressor response that has been observed both in vitro and in vivo, and features of senescence have been documented in various human premalignant lesions, including melanoma, colon and lung adenoma, prostatic intraepithelial neoplasia, and others. The fact that a subset of these lesions eventually progress to malignant invasive tumors suggests that premalignant cells can either bypass or escape the senescent response. Much work has been done to understand the mechanisms underlying such progression, but it remains unclear whether tumors progress by evasion of senescence induction, or by disruption of senescence maintenance, or whether both mechanisms can occur in human cancer development. This review presents the current evidence for mechanisms of senescence evasion and reversion, and discusses what has been learnt about this process using in vitro and in vivo experimental systems. As we learn more about the key signaling effectors of senescence, the hope is that appropriate targets will be identified for preservation and/or re-induction of senescence in human tumors. Such knowledge may also find application in better estimation of risks of cancer progression in individual premalignant lesions, which will lead to more accurate allocation of appropriate treatment options for such patients.