MicroRNAs (miRNAs) are small, noncoding RNAs that can contribute to cancer development and progression by acting as oncogenes or tumor suppressor genes. Recent studies have also linked different sets of miRNAs to metastasis through either the promotion or suppression of this malignant process. Interestingly, epigenetic silencing of miRNAs with tumor suppressor features by CpG island hypermethylation is also emerging as a common hallmark of human tumors. Thus, we wondered whether there was a miRNA hypermethylation profile characteristic of human metastasis. We used a pharmacological and genomic approach to reveal this aberrant epigenetic silencing program by treating lymph node metastatic cancer cells with a DNA demethylating agent followed by hybridization to an expression microarray. Among the miRNAs that were reactivated upon drug treatment, miR-148a, miR-34b/c, and miR-9 were found to undergo specific hypermethylationassociated silencing in cancer cells compared with normal tissues. The reintroduction of miR-148a and miR-34b/c in cancer cells with epigenetic inactivation inhibited their motility, reduced tumor growth, and inhibited metastasis formation in xenograft models, with an associated down-regulation of the miRNA oncogenic target genes, such as C-MYC, E2F3, CDK6, and TGIF2. Most important, the involvement of miR-148a, miR-34b/c, and miR-9 hypermethylation in metastasis formation was also suggested in human primary malignancies (n ‫؍‬ 207) because it was significantly associated with the appearance of lymph node metastasis. Our findings indicate that DNA methylation-associated silencing of tumor suppressor miRNAs contributes to the development of human cancer metastasis.

Inactivation of APC is a strongly predisposing event in the development of colorectal cancer1,2, prompting us to search for vulnerabilities specific to cells that have lost APC function. Signalling through the mTOR pathway is known to be required for epithelial cell proliferation and tumour growth3-5 and the current paradigm suggests that a critical function of mTOR activity is to upregulate translational initiation through phosphorylation of 4EBP16,7. This model predicts that the mTOR inhibitor rapamycin, which does not efficiently inhibit 4EBP18, would be ineffective in limiting cancer progression in APC deficient lesions. Here we show that mTORC1 activity is absolutely required for the proliferation of APC deficient (but not wild type) enterocytes, revealing an unexpected opportunity for therapeutic intervention. Although APC deficient cells show the expected increases in protein synthesis, our studies reveals that it is translation elongation, and not initiation, which is the rate-limiting component. Mechanistically, mTORC1 mediated inhibition of eEF2 kinase is required for the proliferation of APC deficient cells. Importantly, treatment of established APC deficient adenomas with rapamycin (which can target eEF2 through the mTORC1 – S6K – eEF2K axis) causes tumour cells to undergo growth arrest and differentiation. Taken together our data suggest that inhibition of translation elongation using existing, clinically approved drugs such as the Rapalogs, would provide clear therapeutic benefit for patients at high-risk of developing colorectal cancer.

Metastases are the major cause of death from melanoma, a skin cancer which has the fastest rising incidence of any malignancy in the Western world. Molecular pathways that drive melanoblast migration in development are believed to underpin the movement and ultimately the metastasis of melanoma. Here we show that mice lacking P-Rex1, a Rac-specific Rho GTPase guanine nucleotide exchange factor (GEF), have a melanoblast migration defect during development evidenced by a white belly. Moreover, these P-Rex1−/− mice are resistant to metastasis when crossed to a murine model of melanoma. Mechanistically, this is associated with P-Rex1 driving invasion in a Rac-dependent manner. P-Rex1 is elevated in the great majority of human melanoma cell lines as well as tumor tissue. We conclude that P-Rex1 plays an important role in melanoblast migration and cancer progression to metastasis in mice and humans.

Cellular senescence is a stable proliferation arrest that suppresses tumorigenesis. Cellular senescence and associated tumor suppression depend on control of chromatin. Histone chaperone HIRA deposits variant histone H3.3 and histone H4 into chromatin in a DNA replication-independent manner. Appropriately for a DNA replication-independent chaperone, HIRA is involved in control of chromatin in nonproliferating senescent cells, although its role is poorly defined. Here, we show that nonproliferating senescent cells express and incorporate histone H3.3 and other canonical core histones into a dynamic chromatin landscape. Expression of canonical histones is linked to alternative mRNA splicing to eliminate signals that confer mRNA instability in nonproliferating cells. Deposition of newly synthesized histones H3.3 and H4 into chromatin of senescent cells depends on HIRA. HIRA and newly deposited H3.3 colocalize at promoters of expressed genes, partially redistributing between proliferating and senescent cells to parallel changes in expression. In senescent cells, but not proliferating cells, promoters of active genes are exceptionally enriched in H4K16ac, and HIRA is required for retention of H4K16ac. HIRA is also required for retention of H4K16ac in vivo and suppression of oncogeneinduced neoplasia. These results show that HIRA controls a specialized, dynamic H4K16ac-decorated chromatin landscape in senescent cells and enforces tumor suppression.

Melanoma cells break down lysophosphatidic acid from the environment, creating a chemotactic gradient that the tumor cells then follow; this provides an explanation for the rapid metastasis of melanoma.