Many years of oncogenic insult and hormonal flux have important long-term implications for breast cancer emergence and progression. Premature-induced senescence is proposed to be a protective response against cancer development. We hypothesize that the senescence response can be subverted in some breast carcinomas, which exhibit latent hormone-dependent proliferative capacity.

The ARF-p53 pathway has long been known to play an important role in tumour surveillance halting cell cycle progression to transiently or permanently stop aberrant cells from proliferating. It is well documented that disruption of this pathway is involved in tumourigenesis, in part, through inactivation of cyclin D1. Cyclin D1 plays an important role in breast cancer progression as a regulator of CDK4/6. In addition cyclin D1 has been reported to stimulate estrogen receptor (ER) activity independent of CDK4/6.

Our initial observations showed that reactivation of ARF-p53-p21 is indeed a potent rapid inhibitor of cell proliferation in MCF-7 breast cancer cells with a senescent phenotype, however controversially cyclin D1 expression was increased and predominantly located in the nucleus. High expression of nuclear cyclin D1 and p14ARF inversely correlated with cell proliferation. Our studies showed that p14ARF regulated cyclin D1 at the post-transcriptional level. Furthermore, we have evidence from microRNA array analysis that microRNAs known to bind to the cyclin D1 3'UTR sequence were down regulated upon induction of p14ARF, contributing to cyclin D1 stability.

We found that long-term exposure to p14ARF was not a failsafe barrier to tumour progression, senescent-like cells re-entering the cell cycle correlated with reduction of nuclear cyclin D1. Conversely, continuous p14ARF expression resulted in chaotic multinucleation and preceded neoplasia. These aberrant nuclei co-stained with cyclin D1 and Ki-67, a well-known marker of cell proliferation.

These results suggest that induction of the p14ARF-p53 pathway does not entirely remove the threat of cancer resulting from hyperproliferative oncogenic signals and cyclin D1 plays a promiscuous role in prevention and progression. Arguably, in some breast cancers premature senescent cells may be precursor, or latent cancer cells.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P1-04-03.

2010