A light shines on melanoma metastagenesis

Despite exciting, recent major advances in the development and clinical application of immune checkpoint and signalling pathway targeted therapies for patients with advanced stage disease; melanoma remains a major public health problem. Not only is it one of the commonest malignancies in young adults in many Western countries (Thompson et al., 2009); in contrast to most other cancers, it possesses an innate capacity to metastasis when at a very early clinical stage. In fact, many metastasising melanomas are less than 1 mm thick at the time when the primary tumour is first recognized. For these reasons, much scientific research has focused on the pathogenic mechanisms underlying melanoma metastasis. Metastases develop following a complex series of interactions between the tumour and the host microenvironment. The classical invasion and metastasis paradigm are typically portrayed as commencing with adhesion to and invasion through the basement membrane around the primary tumour, followed by passage through the extracellular matrix, invasion of vessels, lymphatic and/or haematogenous dissemination and ending in extravasation and colonization and growth of the metastatic deposit at a new site separated from the primary tumour. Over the past decade or so, an increasing body of research, much of which has been championed by Barnhill and Lugassy and underpinned by careful and meticulous pathological observations, has recognized that melanoma may also metastasise by utilizing some of the migratory systems employed by neural crest cells, the precursors of melanocytes (reviewed by Lugassy et al., 2013). During embryogenesis, neural crest cells are in part guided along their migratory route from the dorsal most region of the neural tube by a network of nascent vessels (Lugassy et al., 2013). Likewise, angiotropism in melanoma, defined as tropism of cells for small and medium-sized vessels (Figure 1), has been identified as a potential route for both local recurrence and distant spread (Van Es et al., 2008; Wilmott et al., 2012). This process, whereby melanoma cells track along the outside of vessels in a pericytic location, termed extravascular migratory metastasis (EVMM)(Lugassy et al., 2013), has also been associated with the acquisition of embryonic properties promoting melanoma plasticity and invasion (Lugassy et al., 2013). In a recent study published in the journal Nature, Bald et al. identified a mechanism capable of both inducing the migration of melanoma cells into the perivascular compartment and promoting metastasis. They showed that a neutrophil-dependent inflammatory response to repeated ultraviolet (UV)-induced keratinocyte damage results in enhancement of the migratory and static capacity of melanoma (Figure 2). A complex relationship exists between UV radiation and melanoma. The results of the study by Bald et al. encourage distinction between the effects of UV radiation as a major aetiological factor for primary melanoma development on the one hand and its effects as a promotor of invasion and metastasis in established tumours on the other. They discovered that in chemically induced murine melanomas, repeated UV exposure did not affect the growth capacity of the primary tumour at the irradiated cutaneous site. Instead, the observed effect was an increase in the number of lung metastases compared with the non-UV-irradiated melanoma group. The UV-irradiated primary murine melanomas also showed increased co-localization of tumour cells with the abluminal surface of dermal blood vessels, that is, angiotropism. The number of lung metastases was positively correlated with the degree of both neutrophilic inflammation and perivascular invasion. These factors, together with others, including primary tumour ulceration, were also recognized as poor prognostic factors in the authors' concurrent analysis of 178 unselected sentinel lymph node-staged patients with primary cutaneous melanoma. The results of Bald et al. may also provide a biological basis for the adverse prognostic effect of primary melanoma ulceration, via its potent stimulation of an acute inflammatory response. Bald et al.'s study also employed a combination of murine knock-outs of four key effectors of the Toll-like receptor signalling pathway to uncover the mechanism coupling UV irradiation with neutrophil infiltration. The UV-induced recruitment of neutrophils was identified as being highly dependent on the function of the myeloid differentiation primary-response gene 88 (MyD88)-dependent pathway mediated by activation of Toll-like receptor 4 (TLR4), but not the TLR3-TRIF MyD88-independent pathway. In this system, activation of TLR4 is triggered by binding to the high-mobility group box 1 (HMGB1). This HMGB1/TLR4-MyD88 signalling axis was also shown to be a requisite for the UV irradiation-induced increase in both the metastatic potential and angiotropic invasion of skin transplanted melanoma cell lines HCmel12 and HCmel31. The authors then turned their attention to the interaction between melanoma and endothelium under ex vivo inflammatory conditions. Using transwell assays, Bald et al. showed that either neutrophil-conditioned media, or tissue necrosis factor (TNF) alone, could induce the migration of HCmel12 cells towards endothelium. In particular, TNF enhanced migration distance and velocity of melanoma cells along endothelium when compared to spread along keratinocytes or extracellular matrix extracts. These findings were also supported by two other observations: (i) in aortic ring explants, TNF in neutrophil-conditioned media promoted endothelial sprouting and melanoma co-migration; (ii) HCmel12 cells introduced into mouse ear explants showed marked angiotropism, but only in the presence of UV-induced skin inflammation. The human corollary of this was the observation that patient-derived BRAF and RAS mutant cell lines showed the same enhanced migratory properties in the presence of TNF. Taken together, these experiments support the notion that neutrophil-mediated inflammation promotes melanoma angiotropism. Finally, the authors confirmed the presence of a TNF-response gene expression signature in melanoma cell lines, involving regulators of tumour-endothelial cell interactions and angiogenesis. The results of Bald et al.'s study may appear at odds with the clinical studies reporting more favourable outcomes in melanomas associated with cumulative sun exposure and evidence of a lower mortality rate for melanomas diagnosed in summer. However, a similar seasonal effect is observed in patients with other tumours, including colorectal, prostate, lung and breast cancer implicating UV-induced systemic effects, possibly through the anti-tumour properties of enhanced vitamin D production. Interestingly, in their disease model, Bald et al. prove TNF is responsible for enabling the pro-invasive and pro-metastatic effects of UV-induced inflammation. This stands in contrast to recent findings identifying the use of tissue necrosis factor inhibitors, in the context of immunomodulation in patients with rheumatoid arthritis, as being at increased risk of developing invasive melanoma. Still, the identification of Toll-like receptor signalling as being a key mediator of angiotropism and metastasis opens up a potential avenue of research into the use of TLR4 antagonists as a therapeutic strategy for melanoma. “provide further evidence of the central role UV radiation plays not only in the initiation but also the promotion of melanomagenesis”