A new function for a classic photoreceptor: melanopsin as an oncogene

In our paper published in Communication Biology, we show that melanopsin acts as an oncogene in melanoma. In the next lines, we contextualize our findings to the literature as well as its relevance for cancer treatment.
Published in Cancer
A new function for a classic photoreceptor: melanopsin as an oncogene

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Light detection is an important ability among different species ranging from bacteria to humans [1]. For each animal, light means a different set of information to the organism. Focusing on mammals, light detection takes place in the eyes via the presence of different photoreceptors. As classically known, rods and cones participate in the process of image formation. However, in early 2000’s, a novel type of photopigment, named melanopsin (OPN4), was discovered in Xenopus laevis melanophores [2] and subsequently in mouse and human retina ganglion cells [3]. This discovery was very important for the progress of the chronobiology field as OPN4 was found to participate as a light sensor that signals environmental light information to the central clock (suprachiasmatic nucleus, [4, 5]). 

Although the skin is an intuitive organ to be considered photosensitive due to its exposure to environmental light, only in the early 2010’s, studies started investigating the photosensitivity of the skin. As one would expect, the expression of several opsins, including OPN4, was reported (reviewed in [6]). Today, it is clear to the scientific community that the skin is an organ equipped with photosensitive molecules similar to the ones found in the retina, thus making the skin an important light-detecting organ. Current knowledge supports the fact that skin cells such as melanocytes, keratinocytes, and dermal fibroblasts express different sets of light-sensing molecules (opsins), which participate in important physiological processes [6]. Recent studies, however, have suggested that opsins can also display light- and thermo-independent roles [7, 8].

Within this context, our laboratory has been investigating the physiological processes regulated by the photosensitive system in the skin melanocytes. We have provided evidence that OPN4 acts an ultraviolet A (UVA) radiation sensor and participates in pigmentation and apoptosis mechanisms [9]. We also demonstrated that OPN4 can, intriguingly, detect temperature, thus acting as a thermosensor [10], similarly to what was shown in sperm cells [11, 12] and Drosophila larvae [13].

Not surprisingly, we identified different responses to white light and UVA radiation in melanoma cancer cells when compared to melanocytes [14, 15]. The differential responses were in part associated with the deregulation of the clock gene machinery in a process known as chronodisruption [14, 16]. Such differences led us to investigate in-depth the role of opsins and clock genes in melanoma cancer, a journey that led to a very interesting set of discoveries.

Taking into consideration our previous studies and the current literature, we questioned whether OPN4 might be involved in the carcinogenic process of melanoma.

In the study published in Communication Biology, we demonstrated that indeed OPN4 acts as an oncogene in melanoma. Using CRISPR editing technique, we created melanoma cells expressing a non-functional (KO) OPN4. With these cells, we observed impaired proliferation and slower cell cycle progression when compared to functional OPN4WT cells. Our data were further validated in an in vivo model of subcutaneous injection of melanoma cells. Tumor growth was also significantly smaller in mice inoculated with OPN4KO cells. We explored the composition of the tumor microenvironment (TME) and found that several key immune system cells were more present in the TME of OPN4KOmelanoma. It is still elusive why TME of OPN4KO cancer contains more immune system components, which indeed corroborates the reduced tumor growth in in vivo.

We found that OPN4KO cells also express higher gene and protein levels of the clock gene BMAL1. In fact, we previously identified that BMAL1 in human melanoma is a positive prognostic marker as patients expressing higher levels of BMAL1 show longer overall survival. This effect was associated with a higher immune response shown by highly expressing BMAL1 tumors [17]. It is curious and elusive the association between BMAL1, OPN4, and the reduced tumor growth. Nonetheless, our subsequent step was to in-depth investigate the differences between the proteomes of OPN4WT and OPN4KO cells. To this end, we employed label-free proteomics, which confirmed our previous experimental findings, and provided new and exciting pathways to be investigated.

Among these pathways, we stress here that the absence of OPN4 resulted in a significant reduction of melanocyte-inducing transcription factor (MITF) gene and protein expression. Indeed, suppression of this important pathway can explain the reduced proliferative capacities of OPN4KO cells. We also detected a reduction in guanylyl cyclase activity in the absence of OPN4, which has been directly associated with increased proliferative capacity. Conversely, in the presence of OPN4, we identified increased GTPase activity and a higher immune-suppressive TME, which corroborates the elevated aggressiveness of OPN4WT tumors. Our study now shows compelling evidence that OPN4 can be considered as an oncogene in melanoma, which is a novel function to the growing repertoire of opsins actions.

Taken altogether, our study has added a new layer of complexity to this fascinating class of proteins, which were initially classified as light-, then as thermo-sensor, and more recently as light- and thermo-independent sensors. This argues for a more complex level of regulation than previously anticipated and opens research fronts to different and potentially therapeutical usage of opsins in human cancer. It is very likely that our study has just scratched one more layer (of many) regarding the complex realm of the opsin field. 


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  15. de Assis, L.V.M., M.N. Moraes, and A.M.L. Castrucci, Heat shock antagonizes UVA-induced responses in murine melanocytes and melanoma cells: an unexpected interaction. Photochem Photobiol Sci, 2017. 16(5): p. 633-648.
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  17. de Assis, L.V.M., et al., Circadian clock gene BMAL1 positively correlates with antitumor immunity and patient survival in metastatic melanoma. Front Oncol, 2018. 8: p. 185.


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