Tyrosinase catalyzes the first and rate-limiting reaction during the synthesis of melanin, the pigment that protects skin against ultraviolet radiation-induced damage, thus reducing the risk of developing cutaneous cancers. Disruption of tyrosinase folding and trafficking results in several forms of oculocutaneous albinism. In addition, tyrosinase polymorphisms are associated with increased risk of developing vitiligo and melanoma, and the protein may also play a role in Parkinson's disease, macular degeneration, and congenital glaucoma. Tyrosinase is also a determinant of skin pigmentation. Enzyme activity varies as much as ten-fold in lightly versus darkly pigmented skin; however, mRNA expression levels are remarkably similar. Tyrosinase regulation is therefore post-translational. Our objective is to elucidate the mechanisms that regulate melanin synthesis and characterize the pathogenesis of tyrosinase-related disorders. In this study, we aim to define the role of the Skp, Cullin, F-box (SCF) containing complexes, particularly the FBXO11 component, in tyrosinase regulation. SCF is a multi-protein E3 ubiquitin ligase complex that is a crucial component of the ubiquitin proteasome system (UPS). SCF complexes catalyze the ubiquitylation of proteins, thereby targeting them for proteasomal degradation. The F Box family protein FBXO11 has recently been implicated in the degradation of tyrosinase, while downregulation of FBXO11 is associated with the depigmentation disorder vitiligo. Previous studies as well as our own have demonstrated that melanocytes with reduced FBXO11 expression display increased levels of tyrosinase. Conversely, melanocytes overexpressing FBXO11 exhibit decreased levels of tyrosinase.
In Specific Aim 1, we will investigate the mechanisms involved in FBXO11-mediated tyrosinase targeting. A number of external stimuli, such as exposure to vitiligo-inducing phenols or fatty acids, promote a reduction in tyrosinase levels.
In Specific Aim 2. 1, we will explore whether these stimuli promote FBXO11-mediated degradation of tyrosinase. In addition, p53, which promotes tyrosinase expression in response to ultraviolet light exposure, is itself a target of FBXO11.
In Specific Aim 2. 2, we will determine whether p53 is also targeted by FBXO11 in the melanocyte and whether degradation impacts tyrosinase levels. Given the critical role of p53 in cell cycle regulation and cell survival, FBXO1 may also play a role in determining melanocyte viability. We will thus also explore the effects of FXBO11 expression on melanocyte viability following exposure to stress induced by ultraviolet light or vitiligo-inducing phenols in Specific Aim 2.3. FBXO11 may thus play a key role in the regulation of normal pigmentation and in the pathogenesis of pigmentation disorders such as vitiligo. Understanding the role of this protein in the melanocyte will be crucial for delineating he etiology of pigmentation disorders and developing effective therapies.
Tyrosinase is an enzyme essential for: (1) production of melanin, the pigment that protects skin and eyes from ultraviolet radiation, (2) for correct optic tract routing during development and (3) has been implicated in the pathogenesis of albinism, vitiligo and skin cancers. While expression of tyrosinase is modulated by a number of factors, tyrosinase protein levels are primarily controlled at the post-translational level. In this study w will investigate the mechanisms involved in FBXO11-mediated modulation of skin pigmentation.
