Murine Neonatal Melanocytes Exhibit a Heightened Proliferative Response to Ultraviolet Radiation and Migrate to the Epidermal Basal Layer

Melanocytes respond to UVR not only by producing melanin, but also by proliferating. This is essentially a protective response. We have studied the melanocyte proliferative response after a single UVR exposure to neonatal mice. At 3 days post-UVR in wild-type neonates we observed a marked melanocyte activation not seen in adults. Melanocytes migrated to the epidermal basal layer, their numbers peaking at 3-5 days after UVR then diminishing. They appeared to emanate from the hair follicle, migrating to the epidermis via the outer root sheath. In melanoma-prone mice with melanocyte-specific overexpression of Hras(G12V), basal layer melanocytes were increased in size and dendricity compared to UVR-treated wild-type mice. Melanocytes in mice carrying a pRb pathway cell-cycle defect (oncogenic Cdk4(R24C)) did not show an enhanced response to UVR such as those carrying Hras(G12V). The exquisite sensitivity to UVR-induced proliferation and migration that characterizes neonatal mouse melanocytes may partly explain the utility of this form of exposure for inducing melanoma in mice that carry oncogenic mutations

Metastasizing melanoma formation caused by expression of activated N-RasQ61K on an INK4a-deficient background

Friedrich Beermann, Manon Frutschi

In human cutaneous malignant melanoma, a predominance of activated mutations in the N-ras gene has been documented. To obtain a mouse model most closely mimicking the human disease, a transgenic mouse line was generated by targeting expression of dominant-active human N-ras (N-RasQ61K) to the melanocyte lineage by tyrosinase regulatory sequences (Tyr::N-RasQ61K). Transgenic mice show hyperpigmented skin and develop cutaneous metastasizing melanoma. Consistent with the tumor suppressor function of the INK4a locus that encodes p16INK4A and p19(ARF), >90% of Tyr::N-RasQ61K INK4a-/- transgenic mice develop melanoma at 6 months. Primary melanoma tumors are melanotic, multifocal, microinvade the epidermis or epithelium of hair follicles, and disseminate as metastases to lymph nodes, lung, and liver. Primary melanoma can be transplanted s.c. in nude mice, and if injected i.v. into NOD/SCID mice colonize the lung. In addition, primary melanomas and metastases contain cells expressing the stem cell marker nestin suggesting a hierarchical structure of the tumors comprised of primitive nestin-expressing precursors and differentiated cells. In conclusion, a novel mouse model with melanotic and metastasizing melanoma was obtained by recapitulating genetic lesions frequently found in human melanoma.

2005

Melanocytes and pigmentation are affected in dopachrome tautomerase knockout mice

Friedrich Beermann

The tyrosinase family comprises three members, tyrosinase (Tyr), tyrosinase-related protein 1 (Tyrp1), and dopachrome tautomerase (Dct). Null mutations and deletions at the Tyr and Tyrp1 loci are known and phenotypically affect coat color due to the absence of enzyme or intracellular mislocalization. At the Dct locus, three mutations are known that lead to pigmentation phenotype. However, these mutations are not null mutations, and we therefore set out to generate a null allele at the Dct gene locus by removing exon 1 of the mouse Dct gene. Mice deficient in Dct [Dct(tm1(Cre)Bee)] lack Dct mRNA and dopachrome tautomerase protein. They are viable and do not show any abnormalities in Dct-expressing sites such as skin, retinal pigment epithelium, or brain. However, the mice show a diluted coat color phenotype, which is due to reduced melanin content in hair. Primary melanocytes from Dct knockout mice are viable in culture and show a normal distribution of tyrosinase and tyrosinase-related protein 1. In comparison to the knockout, the slaty mutation (Dct(slt)/Dct(slt)) has less melanin and affects growth of primary melanocytes severely. In summary, we have generated a knockout of the Dct gene in mice with effects restricted to pigment production and coat color.

2004

Melanomas require HEDGEHOG-GLI signaling regulated by interactions between GLI1 and the RAS-MEK/AKT pathways

Friedrich Beermann, Carmen Mas

Melanoma is one of the most aggressive cancers, and its incidence is increasing. These tumors derive from the melanocyte lineage and remain incurable after metastasis. Here we report that SONIC HEDGEHOG (SHH)-GLI signaling is active in the matrix of human hair follicles, and that it is required for the normal proliferation of human melanocytes in culture. SHH-GLI signaling also regulates the proliferation and survival of human melanomas: the growth, recurrence, and metastasis of melanoma xenografts in mice are prevented by local or systemic interference of HH-GLI function. Moreover, we show that oncogenic RAS-induced melanomas in transgenic mice express Gli1 and require Hh-Gli signaling in vitro and in vivo. Finally, we provide evidence that endogenous RAS-MEK and AKT signaling regulate the nuclear localization and transcriptional activity of GLI1 in melanoma and other cancer cells. Our data uncover an unsuspected role of HH-GLI signaling in melanocytes and melanomas, demonstrate a role for this pathway in RAS-induced tumors, suggest a general integration of the RAS/AKT and HH-GLI pathways, and open a therapeutic approach for human melanomas.

2007