Regulators of the UVA-Evoked Signaling Pathway in Human Epidermal Melanocytes

Abstract

Melanin is the natural pigment that gives color and protection to our skin. Human epidermal melanocytes (HEMs) are the skin cells responsible for the synthesis and storage of melanin. There are two types of melanin: black-brown eumelanin (EM), which is photoprotective and a reactive oxygen species (ROS) scavenger, and yellow-red pheomelanin (PM), which is photolabile and pro-oxidant. HEMs are constantly exposed to solar ultraviolet radiation (UVR), an environmental carcinogen responsible for the increasing incidence of skin cancers. UVR is composed of 95% long-wavelength UVA rays, and 5% short-wavelength UVB rays. UVA and UVB exposure triggers distinct signaling cascades, both of which have been shown to increase melanin production in HEMs. The well-characterized UVB-induced delayed pigmentation is considered an innate defense mechanism against UVR, while melanin induced by the less understood UVA pathway remains uncharacterized. Our laboratory previously identified a retinal-dependent UVA pathway mediated by the activation of a G protein, which in turn leads to calcium (Ca2+) mobilization and subsequent melanin production within minutes to hours (early melanin synthesis) in HEMs. My dissertation aims to further characterize the UVA pathway in HEMs by understanding how Ca2+ travels from the cytosol to the mitochondria in the UVA pathway and identifying whether cellular melanin affects the UVA-induced signaling. Furthermore, I describe a novel synergistic interaction between the UVB-activated pathway and UVA-activated pathway. Although UVA exposure is a major source of ROS in HEMs, the origin and function of the ROS elicited by physiological doses of UVA remain elusive. As part of my dissertation work, I contributed to the discovery that physiological doses of UVA elicit ROS that cross-talk with Ca2+ to mediate sustained Ca2+ uptake by the mitochondria via mitochondrial calcium uniporter (MCU), which subsequently leads to mitochondrial ROS production and melanin production. Moreover, the relationship between melanin and ROS within HEMs is controversial, owing to the pro-oxidant state of melanin synthesis and the opposing roles of EM and PM in ROS balance and UVA filtration. I show that in immortalized HEMs, low EM and low PM concentrations amplify the UVA-induced ROS responses and produce greater UVA-evoked double strand breaks (DSBs) compared with cells with high EM and high PM. Most studies attribute the presence of the pro-oxidant PM for the increased susceptibility for melanoma in red-haired individuals (RHI). Here, we highlight the contribution of low EM and low PM in melanocytes from RHI in the increased levels of ROS and DSBs in response to UVA. Furthermore, I present preliminary data that indicate a bidirectional synergistic interaction between the UVA and UVB pathways; these data suggest that the skin produces the maximal melanin response to mixed UVR. Collectively, these findings improve our understanding of the UVA pathway and aid the development of better skin protection approaches.