The absence of p21Cip1/WAF1 alters keratinocyte growth and differentiation and promotes ras-tumor progression.

Abstract

p21Cip1/WAF1 was the first cyclin-dependent kinase (CDK) inhibitor to be identified, as a mediator of p53 in DNA damage-induced growth arrest, cell senescence, and direct CDK regulation. p21 may also play an important role in differentiation-associated growth arrest, as its expression is augmented in many terminally differentiating cells. A general involvement of p21 in growth/differentiation control and tumor suppression has been questioned, as mice lacking p21 undergo a normal development, harbor no gross alterations in any of their organs, and exhibit no increase in spontaneous tumor development. However, a significant imbalance between growth and differentiation could be unmasked under conditions where normal homeostatic mechanisms are impaired. We report here that primary keratinocytes derived from p21 knockout mice, transformed with a ras oncogene, and injected subcutaneously into nude mice exhibit a very aggressive tumorigenic behavior, which is not observed with wild-type control keratinocytes nor with keratinocytes with a disruption of the closely related p27 gene. p21 knockout keratinocytes tested under well-defined in vitro conditions show a significantly increased proliferative potential, which is also observed but to a lesser extent with p27 knockout cells. More profound differences were found in the differentiation behavior of p21 versus p27 knockout keratinocytes, with p21 (but not p27) deficiency causing a drastic down-modulation of differentiation markers linked with the late stages of the keratinocyte terminal differentiation program. Thus, our results reveal a so far undetected role of p21 in tumor suppression, demonstrate that this function is specific as it cannot be attributed to the closely related p27 molecule, and point to an essential involvement of p21 in terminal differentiation control, which may account for its role in tumor suppression.

Does Notch play a tumor suppressor role across diverse squamous cell carcinomas?

“…The inhibition of Notch signaling by genetic methods or by using pharmacological Notch inhibitors in human keratinocytes, together with activated ras, can cause aggressive SCC formation in grafted mice 26. This is similar to that previously revealed for ras‐transformed p21 knockout keratinocytes 32, suggesting that NOTCH1 and p21 may overlap to perform their tumor suppressing functions. However, the general role of p21 remains to be investigated as p21 levels were selectively decreased in a subset of human skin SCCs tested 26, whereas in human keratinocytes, activation of NOTCH1 was found to lead to modest increase in p21 levels 33.…”

“…In mouse keratinocytes, cyclin‐dependent kinase inhibitor p21 was identified as a downstream positive target of NOTCH1, where activated NOTCH1 induces p21 expression to suppress growth in a CSL‐dependent manner 29, 30. p21's role in tumor suppression was earlier investigated where p21 knockout keratinocytes exhibited significant downmodulation of differentiation markers and dramatic increase in proliferative potential 32. The inhibition of Notch signaling by genetic methods or by using pharmacological Notch inhibitors in human keratinocytes, together with activated ras, can cause aggressive SCC formation in grafted mice 26.…”

Deregulated expression of cell-cycle proteins during premalignant progression in SENCAR mouse skin

“…Thus, p21 Cip1 and p27 Kip1 protein levels may a ect the kinase activity of this complex. Thus, our results could explain the reason that keratinocytes derived from p21 Cip1 knockout mice, transformed with ras oncogene, exhibit a very aggressive behavior in nude mice (Missero et al, 1996), although opposed results were recently reported in a similar model (Weinberg et al, 1997).…”

Comparative effects of overexpression of p27Kip1 and p21Cip1/Waf1 on growth and differentiation in human colon carcinoma cells

“…As mentioned in the Introduction, p21 is often induced during di erentiation. In addition antisense to p21 decreases monocytic di erentiation in HL60 cells (Freemerman et al, 1997) and p21 de®cient keratinocytes display a decrease in di erentiation (Missero et al, 1996). Therefore, we were surprised to ®nd that HT29 derivatives that stably overexpressed increased levels of p21 displayed a decrease in SB-induced differentiation (Figures 4c and 5b).…”