Cyclin D1 is a component of the core cell cycle machinery1. Abnormally high levels of cyclin D1 are detected in many human cancer types2. To elucidate the molecular functions of cyclin D1 in human cancers, here we performed a proteomic screen for cyclin D1 protein partners in several types of human tumors. Analyses of cyclin D1-interactors revealed a network of DNA repair proteins, including RAD51, a recombinase that drives the homologous recombination process3. We found that cyclin D1 directly binds RAD51, and that cyclin D1-RAD51 interaction is induced by radiation. Like RAD51, cyclin D1 is recruited to DNA damage sites in a BRCA2-dependent fashion. Reduction of cyclin D1 levels in human cancer cells impaired recruitment of RAD51 to damaged DNA, impeded the homologous recombination-mediated DNA repair, and increased sensitivity of cells to radiation in vitro and in vivo. This effect was seen in cancer cells lacking the retinoblastoma protein, which do not require D-cyclins for proliferation4, 5. These findings reveal an unexpected function of a core cell cycle protein in DNA repair and suggest that targeting cyclin D1 may be beneficial also in retinoblastoma-negative cancers which are currently thought to be oblivious to cyclin D1 inhibition.

Loss of G1/S control is a hallmark of cancer, and is often caused by inactivation of the retinoblastoma pathway. However, mouse embryonic fibroblasts lacking the retinoblastoma genes RB1, p107, and p130 (TKO MEFs) are still subject to cell cycle control: Upon mitogen deprivation, they enter and complete S phase, but then firmly arrest in G2. We now show that G2-arrested TKO MEFs have accumulated DNA damage. Upon mitogen readdition, cells resume proliferation, although only part of the damage is repaired. As a result, mitotic cells show chromatid breaks and chromatid cohesion defects. These aberrations lead to aneuploidy in the descendent cell population. Thus, our results demonstrate that unfavorable growth conditions can cause genomic instability in cells lacking G1/S control. This mechanism may allow premalignant tumor cells to acquire additional genetic alterations that promote tumorigenesis.[Keywords: Cell cycle; DNA damage; Rb; genomic instability; sister chromatid cohesion] Supplemental material is available at http://www.genesdev.org.

The chromosomal passenger complex (CPC) is a critical regulator of chromosome segregation during mitosis by correcting nonbipolar microtubule-kinetochore interactions. By severing these interactions, the CPC is thought to create unattached kinetochores that are subsequently sensed by the spindle assembly checkpoint (SAC) to prevent premature mitotic exit. We now show that spindle checkpoint function of the CPC and its role in eliminating nonbipolar attachments can be uncoupled. Replacing the chromosomal passenger protein INCENP with a mutant allele that lacks its coiled-coil domain results in an overt defect in a SAC-mediated mitotic arrest in response to taxol treatment, indicating that this domain is critical for CPC function in spindle checkpoint control. Surprisingly, this mutant could restore alignment and cytokinesis during unperturbed cell divisions and was capable of resolving syntelic attachments. Also, Aurora-B kinase was localized and activated normally on centromeres in these cells, ruling out a role for the coiled-coil domain in general Aurora-B activation. Thus, mere microtubule destabilization of nonbipolar attachments by the CPC is insufficient to install a checkpoint-dependent mitotic arrest, and additional, microtubule destabilization-independent CPC signaling toward the spindle assembly checkpoint is required for this arrest, potentially through amplification of the unattached kinetochore-derived checkpoint signal.

The establishment and maintenance of apical-basal cell polarity is essential for the functionality of glandular epithelia. Cell polarity is often lost in advanced tumours correlating with acquisition of invasive and malignant properties. Despite extensive knowledge regarding the formation and maintenance of polarity, the mechanisms that deregulate polarity in metastasizing cells remain to be fully characterized. Here we show that AmotL2 expression correlates with loss of tissue architecture in tumours from human breast and colon cancer patients. We further show that hypoxic stress results in activation of c-Fos-dependent expression of AmotL2 leading to loss of polarity. c-Fos/hypoxia-induced p60 AmotL2 interacts with the Crb3 and Par3 polarity complexes retaining them in large vesicles and preventing them from reaching the apical membrane. The resulting loss of polarity potentiates the response to invasive cues in vitro and in vivo in mice. These data provide a molecular mechanism how hypoxic stress deregulates cell polarity during tumour progression.