Wild type p53 is targeted by CEACAM1 to facilitate melanoma cell proliferation

CEACAM1 is a multifunctional protein, which is associated with melanoma progression and poor prognosis. We have previously demonstrated that CEACAM1 serves as an immune evasion mechanism and facilitates cell cycle and proliferation of melanoma cells. Here we describe the downstream signaling leading to enhanced proliferation.

Co-immunoprecipitation showed strong interactions between CEACAM1, SHP-1 and RAP1 in melanoma cells. Point mutations in CEACAM1 phosphorylation sites not only abrogated the enhanced proliferation but also abolished the interactions with SHP-1 and RAP1.Knockdown of SHP-1 or RAP1A inhibited CEACAM1-mediated proliferation. Conversely, CEACAM1 overexpression increased the active fraction of RAP1 as demonstrated by pull-down assays, and an inhibitor of activated RAP1A hindered proliferation in CEACAM1-positive cells. Thus, CEACAM1 recruits RAP1 to its cytoplasmic tail, leading to its activation and increased proliferation.

Comparative microarray analysis showed significant alteration in p53 and RAP1 signaling pathways following CEACAM1 overexpression. Expression of p53 effectors such as p21 was suppressed by CEACAM1 in a SHP1 dependent manner, confirmed by concomitant knockdown of SHP-1. Binding of p53 to pulled-down active RAP1A was confirmed by immunoblotting. Importantly, inhibition of RAP1A enhanced p53 phosphorylation and expression of p21. Studies in various melanoma lines showed that the effects of RAP1A and CEACAM1 depend on the presence of wild type p53.

In conclusion, as opposed to other cancers, p53 is infrequently mutated in melanoma. Our results show that wild type p53 is targeted by the commonly expressed CEACAM1, which sheds new light on the role of p53 in melanoma biology.