Molecular and functional characterization of inflammation-induced cell state transitions in malignant-transformed melanocytes

Abstract

Cellular phenotype plasticity is a hallmark of embryonic development in order to achieve cell type specification from precursor populations. The molecular mechanisms orchestrating phenotypic plasticity are of interest because they may also help to understand how its dysregulation contributes to disease development, in particular cancer. The neural crest is an embryonic cell population that is well known for its migratory capacity and phenotypic plasticity. Malignant melanoma, an aggressive type of skin cancer, is a neural crest-related malignancy originating from the pigment producing melanocytes in the epidermis. In the recent years, it became clear to the field of neural crest biologists that melanoma is an ideal model system to study basic principles of phenotype plasticity of the neural crest cell lineage. Melanoma cells are highly prone to phenotype switching and this is linked to the expression level of MITF (Microphthalmia-associated transcription factor), the master transcription factor of the melanocyte cell lineage that induces the expression of pigmentation-related genes among other effects. On this ba- sis, melanoma cell state transitions have been classified as a binary switch between a differentiated MITF^high proliferative phenotype and a dedifferentiated MITF^low invasive phenotype. However, intermediate melanoma cell states, as well as the detailed molecular mechanisms concerting the transitions remain elusive.<br /> In this study we identified a novel <em>nascent invasive</em> cell state in melanoma. This pheno- type exhibits early inflammatory and early invasive signaling programs although keeping the differentiation signature. Expression of the cell surface 5'ectonucleotidase CD73 was revealed as a marker for <em>nascent invasiveness</em> and further increased upon full dedifferentiation. Interestingly, CD73 is not only known as a mesenchymal stem cell marker, but also involved in tumor immunosuppression and therefore emerges as a target for cancer immunotherapy. Computational and <em>in vitro</em> studies confirmed a critical cooperation of mitogenic growth factors and pro-inflammatory cytokines inducing CD73 expression. This was dependent on the MEK-ERK signaling cascade and coordinated by the transcription factor c-Jun, which critically bound to an intronic enhancer within the CD73 genomic locus. The findings reported in this thesis provide novel mechanistic insight into neural crest plasticity, melanoma cell state transitions and the regulation of a CD73-dependent immunosuppressive tumor microenvironment.