The protein kinase IKK epsilon shapes the melanoma tumor microenvironment by modulating angiogenesis and the T-cell immunity

Inhibitor of nuclear factor kappa-B kinase epsilon (IKKε) contributes to tumorigenesis and metastasis in various cancers. In melanoma, it is overexpressed and constitutively active. Inhibition of IKKε by either knockdown or pharmaceutical compounds reduce tumor progression by suppressing key molecular pathways within melanoma tumor cells. However, within the tumor microenvironment (TME), the role of this protein remains to be fully elucidated.

Multiplex immunohistochemistry was performed on human primary melanomas to assess IKKε expression in tumor and immune cell compartments. In the mouse model, B16BL6 melanoma cells were injected subcutaneously into the flanks of wildtype and IKKε-deficient mice, and tumor growth was monitored over time. Flow-cytometric analyses characterized intratumoral immune cell populations in both genotypes, followed by in vivo CD8⁺ T-cell depletion to determine their contribution to tumor control. To elucidate molecular mechanisms underlying the differential tumor growth, we conducted targeted and untargeted proteomic analyses, which indicated regulation of angiogenic pathways which were further examined by assessing specific protein changes in tumor tissue and performing in vitro angiogenesis assays.

The reduced tumor growth observed in IKKε knock-out (KO) mice was concomitant with elevated levels of cytotoxic CD8⁺ and γδ-T-cells, and a diminished presence of neutrophils. Furthermore, our data revealed that isolated CD8⁺ T-cells from IKKε KO mice exhibited a higher activity in comparison to their wildtype counterparts. In addition, the depletion of CD8⁺ cells reversed the antitumor efficacy of IKKε depletion. Proteomic analysis of tumor lysates revealed a reduction of several chemokines in IKKε KO mice compared to wildtype controls, that might contribute to the reduced tumor growth and suggested that an inhibition of angiogenesis may support the antitumor activity in IKKε KO mice. This hypothesis could be confirmed by further protein analyses and angiogenesis assays.

Our data imply that IKKε expression in melanoma is not only important in the tumor cells but also in the tumor microenvironment by regulation of the immune response and angiogenesis. Consequently, inhibition of IKKε may offer a novel therapeutic approach for melanoma, complementing existing therapies that target pathways within tumor cells and antitumor immune responses in the tumor microenvironment.