Editors' ChoiceCancer

Keratinocytes have the metastatic touch

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Sci. Signal.  08 Sep 2015:
Vol. 8, Issue 393, pp. ec255
DOI: 10.1126/scisignal.aad3677

Melanoma metastasis involves a shift from radial (lateral) growth in the epidermis to vertical invasion into the dermis. Golan et al. found that contact with differentiated keratinocytes in the upper epidermis triggered vertical invasion by melanoma cells. Various signaling pathways, including Notch, are active in the epidermis. The authors found that the Notch receptor was present on the surface of both invasive and noninvasive melanoma cell lines, and Notch ligands and nuclear-localized Notch intracellular domain (NICD, a cleavage product of the Notch receptor that is the active mediator of this pathway) were present in the upper layer of the epidermis in human skin samples (where keratinocytes are present), but not in the normal melanocytes in the basal epidermis. Noninvasive melanoma cells became invasive in Matrigel when cocultured with differentiated keratinocytes but not with basal keratinocytes or fibroblasts, and transfection with an mCherry Notch reporter revealed that coculture with keratinocytes induced Notch signaling in melanoma cells. Treating cocultures with the γ-secretase inhibitor DAPT that blocks the production of the NICD or depleting the Notch ligand DLL1 from keratinocytes prevented invasion into Matrigel. Compared with mice injected with melanoma cells and either basal keratinocytes or fibroblasts, mice injected with melanoma cells and differentiated keratinocytes had greater numbers of lung metastases. In melanoma patient samples, contact between melanoma cells and DLL1-positive upper epidermal cells correlated with invasive grades. Sequencing of the small RNAs in melanoma cells cultured on DLL1-coated plates or cocultured with keratinocytes revealed that Notch signaling induced the expression of the miR-222 and miR-221 (miR-222/221) cluster, the expression of which was observed only in invasive melanoma samples. Expressing miR-222 in noninvasive melanoma xenografts promoted metastasis in mice. Various cell culture experiments, involving promoter histone methylation analysis, luciferase reporter assays and RNA interference, revealed that the transcription factor MITF formed a complex with RBPJK that bound the promoter of miR-222/221 and recruited the demethylase KDM5A to repress miR-222/221 expression, but that when Notch signaling is stimulated, the NICD disrupted this repressive complex by reducing the binding of MITF to the miR-222/221 promoter. In invasive melanoma samples, the expression of Notch target genes was increased, and expression of miR222/221 target genes was decreased. Knocking down MITF or overexpressing the NICD in noninvasive cells conferred invasive properties in culture, which were blocked by DAPT to inhibit Notch signaling or an miR222/221 antagomir. Overexpressing MITF reduced invasive behavior in highly invasive cells. The findings suggest that inhibiting intercellular Notch signaling may prevent melanoma metastasis.

T. Golan, A. R. Messer, A. Amitai-Lange, Z. Melamed, R. Ohana, R. E. Bell, O. Kapitansky, G. Lerman, S. Greenberger, M. Khaled, N. Amar, J. Albrengues, C. Gaggioli, P. Gonen, Y. Tabach, D. Sprinzak, R. Shalom-Feuerstein, C. Levy, Interactions of melanoma cells with distal keratinocytes trigger metastasis via Notch signaling inhibition of MITF. Mol. Cell 59, 664–676 (2015). [PubMed]