Ganem et al. have identified that the Hippo signalling pathway is a key mediator of cell cycle arrest in tetraploid cells in vitro and in vivo.

Genetic instability is a fundamen‑tal characteristic of cancer cells, and tetraploid cells can promote tumori‑genesis. Although p53 was known to induce cell cycle arrest of tetraploid cells in vitro, how p53 is activated and whether this activation in tetraploid cells is different from p53 activation in response to DNA damage was unclear.

To elucidate this mechanism, Ganem et al. used a cytokinesis inhibitor to induce tetraploidy in the nontransformed retinal pigment epithelial (RPE‑1) cell line, which has a normal p53 response pathway. Cell cycle progression beyond G1 phase was inhibited in most tetraploid cells (~86%) and these expressed higher levels of p53 compared with diploid cells. However, no upregulation in DNA damage markers was observed, suggesting that the cell cycle arrest in tetraploid cells is distinct from that associated with DNA damage.

To further investigate this distinction, the authors carried out an RNA interference (RNAi) screen in both tetraploid cells and diploid cells with induced DNA damage. This screen identified large tumour suppressor kinase 2 (LATS2) — which encodes a key kinase in the Hippo tumour suppressor pathway — as a gene that is required for cell cycle arrest in tetraploid cells, but not cells with DNA damage. Knockdown of LATS2 with small interfering RNA (siRNA) in tetra‑ploid cells promoted their prolifera‑tion, and re‑expression of wild‑type LATS2 in these cells was sufficient to cause cell cycle arrest. Depletion of LATS2 also restored p53 levels to basal levels, indicating that LATS2 has a direct effect on p53 and that this is specific to tetraploid cells. In addition, primary mouse hepato‑cytes, which spontaneously become tetraploid in vivo, had reduced proliferation and Hippo pathway activation, suggesting that this mechanism is relevant in vivo.

One known function of LATS2 is to phosphorylate and inactivate the transcriptional co‑activator YAP1 (Yes‑associated protein 1), thereby

preventing it from translocating to the nucleus and activating

growth‑promoting genes. The authors found that YAP1 is phosphory lated and seques‑tered in the cytoplasm of tetraploid cells. In addition, co‑immunoprecipitation

experiments showed that LATS2 binds to MDM2, a

negative regulator of p53, in tetraploid, but not diploid cells.

Together, these results suggest that, in tetraploid cells, LATS2 activation promotes cell cycle arrest both by sequestering YAP1 and by inhibiting MDM2 and thus stabilizing p53.

How can cells sense tetraploidy and activate Hippo signalling? Other studies have shown that reduced activity of RHOA can induce LATS2 in the context of reduced contractility of the actin cytoskeleton. Accordingly, tetraploid cells had lower levels of active RHOA relative to diploid cells, and reduced contrac‑tile force. Using either a construct that constitutively expresses active RHOA or treatment with glycophos‑pholipids known to activate RHOA (such as lysophosphatidic acid), the authors showed that active RHOA was sufficient to inactivate Hippo signalling, reduce p53 to basal levels, and to initiate proliferation in tetra‑ploid cells but not in DNA‑damaged diploid cells. Further experiments showed that the extra centrosomes present in tetraploid cells stimulate the upregulation of RAC1, which represses RHOA activity, and that inactivation of RAC1 is sufficient to inactivate Hippo signalling. This observation suggests that centrosome number may provide a cellular sensor for tetraploidy.

This study provides insight into the distinct mechanisms underlying cell cycle arrest in tetraploid cells, which is crucial to the development of therapies for high ploidy tumours.

Isabel Lokody

S I G N A L L I N G

Hippo signalling arrests tetraploid cell growth

ORIGINAL RESEARCH PAPER Ganem, N. J. et al. Cytokinesis failure triggers hippo tumor suppressor pathway activation. Cell 158, 833–848 (2014)

LATS2 has a direct effect on p53 and … this is specific to tetraploid cells

Lara Crow / NPG

R E S E A R C H H I G H L I G H T S

NATURE REVIEWS | CANCER VOLUME 14 | OCTOBER 2014

Nature Reviews Cancer | AOP, published online 18 September 2014; doi:10.1038/nrc3824

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