Loss of Melanopsin (OPN4) Leads to a Faster Cell Cycle Progression and Growth in Murine Melanocytes

Abstract

Skin melanocytes harbor a complex photosensitive system comprised of opsins, which were shown, in recent years, to display light- and thermo-independent functions. Based on this premise, we investigated whether melanopsin, OPN4, displays such a role in normal melanocytes. In this study, we found that murine Opn4KO melanocytes displayed a faster proliferation rate compared to Opn4WT melanocytes. Cell cycle population analysis demonstrated that OPN4KO melanocytes exhibited a faster cell cycle progression with reduced G0–G1, and highly increased S and slightly increased G2/M cell populations compared to the Opn4WT counterparts. Expression of specific cell cycle-related genes in Opn4KO melanocytes exhibited alterations that corroborate a faster cell cycle progression. We also found significant modification in gene and protein expression levels of important regulators of melanocyte physiology. PER1 protein level was higher while BMAL1 and REV-ERBα decreased in Opn4KO melanocytes compared to Opn4WT cells. Interestingly, the gene expression of microphthalmia-associated transcription factor (MITF) was upregulated in Opn4KO melanocytes, which is in line with a higher proliferative capability. Taken altogether, we demonstrated that OPN4 regulates cell proliferation, cell cycle, and affects the expression of several important factors of the melanocyte physiology; thus, arguing for a putative tumor suppression role in melanocytes.

Melanopsin (Opn4) is an oncogene in cutaneous melanoma

“…3 e–h ). One would expect an increase in Per1 bioluminescence in response to forskolin, as previously shown for murine melanocytes 27 . However, the lack of Per1 bioluminescence induction may be related to an already upregulated cAMP signaling, which is a common feature known in melanoma cells 45 .…”

“…Based on our previous study 27 , 6 genes were selected for qPCR validation. The genes ataxia-telangiectasia-mutated ( Atm ) and ataxia telangiectasia and Rad3-related ( Atr ) encode proteins that act on DNA damage response and are responsible for maintaining genome integrity 35 .…”

“…Since B16-F10 cells show a reduced synchronization capacity to different methods 18 , 19 , 43 , we focused on the evaluation of the acute clock response in this assay. Opn4 WT and Opn4 KO cells were challenged with dexamethasone and forskolin, both known modulators of the molecular clock 44 , using a Per1 :Luc bioluminescence reporter assay, as previously described 25 , 27 . As expected, dexamethasone increased cellular metabolism and Per1 bioluminescence in Opn4 WT melanocytes compared to control Opn4 WT cells.…”

“…Contributing to the new role of opsins, our group has recently demonstrated that OPN4 removal in malignant melanocytes resulted in insensitivity to UVA-induced effects such as melanin content increase and apoptosis 25 . In normal melanocytes, OPN4 removal resulted in the loss of UVA-induced reduced cellular growth and pigmentation 25 and a higher proliferation and faster cell cycle progression, which was associated with higher Mitf expression 27 .…”

“…We have previously investigated the role of melanopsin (OPN4) in murine melanocytes and demonstrated that this protein acts as UVA-sensor for pigmentary and apoptosis-dependent processes 24 , 25 as well as a thermal sensor 26 . More recently, we demonstrated that OPN4 may also display light- and thermo-independent roles as Opn4 knockout murine melanocytes show faster proliferation and cell cycle progression 27 …”