Collaborator of ARF (CARF) regulates proliferative fate of human cells by dose-dependent regulation of DNA damage signaling.

Collaborator of ARF (CARF) has been shown to directly bind to and regulate p53, a central protein that controls tumor suppression via cellular senescence and apoptosis. However, the cellular functions of CARF and the mechanisms governing its effect on senescence, apoptosis, or proliferation are still unknown. Our previous studies have shown that (i) CARF is up-regulated during replicative and stress-induced senescence, and its exogenous overexpression caused senescence-like growth arrest of cells, and (ii) suppression of CARF induces aneuploidy, DNA damage, and mitotic catastrophe, resulting in apoptosis via the ATR/CHK1 pathway. In the present study, we dissected the cellular role of CARF by investigating the molecular pathways triggered by its overexpression in vitro and in vivo. We found that the dosage of CARF is a critical factor in determining the proliferation potential of cancer cells. Most surprisingly, although a moderate level of CARF overexpression induced senescence, a very high level of CARF resulted in increased cell proliferation. We demonstrate that the level of CARF is crucial for DNA damage and checkpoint response of cells through ATM/CHK1/CHK2, p53, and ERK pathways that in turn determine the proliferative fate of cancer cells toward growth arrest or proproliferative and malignant phenotypes. To the best of our knowledge, this is the first report that demonstrates the capability of a fundamental protein, CARF, in controlling cell proliferation in two opposite directions and hence may play a key role in tumor biology and cancer therapeutics.