Human melanocortin 1 receptor-mediated ubiquitination of nonvisual arrestins. Role of Mahogunin Ring Finger 1 E3 ligase

MGRN1 depletion promotes intercellular adhesion in melanoma by upregulation of E-cadherin and inhibition of CDC42

Mahogunin Ring Finger 1 is an E3-ubiquitin ligase encoded by the color gene MGRN1. Our previous in vitro and in vivo studies demonstrated that Mgrn1 deletion in mouse melanoma cells induced cell differentiation and adhesion, and decreased cell motility and invasion on collagen I, and lung colonization in an in vivo model. Here, we investigated the role of MGRN1 on human melanoma cell morphology, adhesion and expression of genes/proteins involved in an EMT-like transition. We demonstrated that wild-type BRAF human melanoma cells adopted a clustering-like morphology on collagen I, with permanent MGRN1 abrogation resulting in bigger cell clusters. Enhanced intercellular adhesion was mostly mediated by induction of E-cadherin and higher co-localization with β-catenin. Transcriptional upregulation of E-cadherin likely occurred through downregulation of the ZEB1 repressor. Finally, pulldown assays showed reduced activation of CDC42 in the absence of MGRN1, which was reverted after E-cadherin silencing. Overall, these findings highlight a new MGRN1-dependent pathway regulating melanoma cell shape, motility, and invasion potential.

Structure-function relationship and physiological role of apelin and its G protein coupled receptor

Apelin receptor (APJR) is a class A peptide (apelin) binding G protein-coupled receptor (GPCR) that plays a significant role in regulating blood pressure, cardiac output, and maintenance of fluid homeostasis. It is activated by a wide range of endogenous peptide isoforms of apelin and elabela. The apelin peptide isoforms contain distinct structural features that aid in ligand recognition and activation of the receptor. Site-directed mutagenesis and structure-based studies have revealed the involvement of extracellular and transmembrane regions of the receptor in binding to the peptide isoforms. The structural features of APJR activation of the receptor as well as mediating G-protein and β-arrestin-mediated signaling are delineated by multiple mutagenesis studies. There is increasing evidence that the structural requirements of APJR to activate G-proteins and β-arrestins are different, leading to biased signaling. APJR also responds to mechanical stimuli in a ligand-independent manner. A multitude of studies has focused on developing both peptide and non-peptide agonists and antagonists specific to APJR. Apelin/elabela-activated APJR orchestrates major signaling pathways such as extracellular signal-regulated kinase (ERKs), protein kinase B (PKB/Akt), and p70S. This review focuses on the structural and functional characteristics of apelin, elabela, APJR, and their interactions involved in the binding and activation of the downstream signaling cascade. We also focus on the diverse signaling profile of APJR and its ligands and their involvement in various physiological systems.

MGRN1 as a Phenotypic Determinant of Human Melanoma Cells and a Potential Biomarker

Mahogunin Ring Finger 1 (MGRN1), a ubiquitin ligase expressed in melanocytes, interacts with the α melanocyte-stimulating hormone receptor, a well-known melanoma susceptibility gene. Previous studies showed that MGRN1 modulates the phenotype of mouse melanocytes and melanoma cells, with effects on pigmentation, shape, and motility. Moreover, MGRN1 knockdown augmented the burden of DNA breaks in mouse cells, indicating that loss of MGRN1 promoted genomic instability. However, data concerning the roles of MGRN1 in human melanoma cells remain scarce. We analyzed MGRN1 knockdown in human melanoma cells. Transient MGRN1 depletion with siRNA or permanent knockdown in human melanoma cells by CRISPR/Cas9 caused an apparently MITF-independent switch to a more dendritic phenotype. Lack of MGRN1 also increased the fraction of human cells in the S phase of the cell cycle and the burden of DNA breaks but did not significantly impair proliferation. Moreover, in silico analysis of publicly available melanoma datasets and estimation of MGRN1 in a cohort of clinical specimens provided preliminary evidence that MGRN1 expression is higher in human melanomas than in normal skin or nevi and pointed to an inverse correlation of MGRN1 expression in human melanoma with patient survival, thus suggesting potential use of MGRN1 as a melanoma biomarker.

Mahogunin Ring Finger 1 regulates pigmentation by controlling the pH of melanosomes in melanocytes and melanoma cells

Mahogunin Ring Finger 1 (MGRN1) is an E3-ubiquitin ligase absent in dark-furred mahoganoid mice. We investigated the mechanisms of hyperpigmentation in Mgrn1-null melan-md1 melanocytes, Mgrn1-KO cells obtained by CRISPR-Cas9-mediated knockdown of Mgrn1 in melan-a6 melanocytes, and melan-a6 cells depleted of MGRN1 by siRNA treatment. Mgrn1-deficient melanocytes showed higher melanin content associated with increased melanosome abundance and higher fraction of melanosomes in highly melanized maturation stages III-IV. Expression, post-translational processing and enzymatic activity of the rate-limiting melanogenic enzyme tyrosinase measured in cell-free extracts were comparable in control and MGRN1-depleted cells. However, tyrosinase activity measured in situ in live cells and expression of genes associated with regulation of pH increased upon MGRN1 repression. Using pH-sensitive fluorescent probes, we found that downregulation of MGRN1 expression in melanocytes and melanoma cells increased the pH of acidic organelles, including melanosomes, strongly suggesting a previously unknown role of MGRN1 in the regulation of melanosomal pH. Among the pH regulatory genes upregulated by Mgrn1 knockdown, we identified those encoding several subunits of the vacuolar adenosine triphosphatase V-ATPase (mostly Atp6v0d2) and a calcium channel of the transient receptor potential channel family, Mucolipin 3 (Mcoln3). Manipulation of expression of the Mcoln3 gene showed that overexpression of Mcoln3 played a significant role in neutralization of the pH of acidic organelles and activation of tyrosinase in MGRN1-depleted cells. Therefore, lack of MGRN1 led to cell-autonomous stimulation of pigment production in melanocytes mostly by increasing tyrosinase specific activity through neutralization of the melanosomal pH in a MCOLN3-dependent manner.

The GTPase-activating protein-related domain of neurofibromin interacts with MC1R and regulates pigmentation-mediated signaling in human melanocytes

The melanocortin 1 receptor (MC1R) is a G-protein coupled receptor (GPCR) which plays a major role in controlling melanogenesis. A large body of evidence indicates that GPCRs are part of large protein complexes that are critical for their signal transduction properties. Among proteins which may affect MC1R signaling, neurofibromin (Nf1), a GTPase activating protein (GAP) for Ras, is of special interest as it regulates adenylyl cyclase activity and ERK signaling, two pathways involved in MC1R signaling. Moreover, mutations in this gene encoding Nf1 are responsible for neurofibromatosis type I, a disease inducing hyperpigmented flat skin lesions. Using co-immunoprecipitation and Bioluminescence Resonance Energy Transfer experiments we demonstrated a physical interaction of Nf1 with MC1R. In particular, the GAP domain of Nf1 directly and constitutively interacts with MC1R in melanocytes. Pharmacologic and genetic approaches revealed that the GAP activity of Nf1 is important to regulate intracellular signaling pathways involved in melanogenesis and, consequently, melanogenic enzyme expression and melanin production. These finding shed new light on the understanding and cure of skin pigmentation disorders.

Mahogunin Ring Finger 1 Is Required for Genomic Stability and Modulates the Malignant Phenotype of Melanoma Cells

Simple Summary

Melanoma, the most aggressive skin cancer, accounts for the majority of deaths due to this disease. Therefore, identification of genes/proteins involved in melanoma genesis and/or progression is urgent. Mutations abrogating expression of Mahogunin Ring Finger 1 (MGRN1) in mice cause complex phenotypes with hyperpigmentation, and known MGRN1 interactors are important regulators of cell shape and movement. This suggests that MGRN1 may modulate the malignant phenotype of melanoma cells. Analysis of MGRN1-KO mouse melanocytes and melanoma cells showed that lack of MGRN1 leads to cell cycle defects and to a more differentiated, less aggressive phenotype, with increased adhesion to various matrices, decreased motility and high genomic instability. The higher aggressivity of MGRN1-expressing melanoma cells was confirmed in an in vivo mouse melanoma model and is consistent with higher survival of human melanoma patients expressing low levels of MGRN1. Therefore, MGRN1 appears an important determinant of the malignant phenotype of melanoma.

Abstract

The mouse mahoganoid mutation abrogating Mahogunin Ring Finger-1 (MGRN1) E3 ubiquitin ligase expression causes hyperpigmentation, congenital heart defects and neurodegeneration. To study the pathophysiology of MGRN1 loss, we compared Mgrn1-knockout melanocytes with genetically matched controls and melan-md1 (mahoganoid) melanocytes. MGRN1 knockout induced a more differentiated and adherent phenotype, decreased motility, increased the percentage of cells in the S phase of the cell cycle and promoted genomic instability, as shown by stronger γH2AX labelling, increased burden of DNA breaks and higher abundance of aneuploid cells. Lack of MGRN1 expression decreased the ability of melanocytes to cope with DNA breaks generated by oxidizing agents or hydroxyurea-induced replicative stress, suggesting a contribution of genomic instability to the mahoganoid phenotype. MGRN1 knockout in B16-F10 melanoma cells also augmented pigmentation, increased cell adhesion to collagen, impaired 2D and 3D motility and caused genomic instability. Tumors formed by Mgrn1-KO B16-F10 cells had lower mitotic indices, fewer Ki67-positive cells and showed a trend towards smaller size. In short-term lung colonization assays Mgrn1-KO cells showed impaired colonization potential. Moreover, lower expression of MGRN1 is significantly associated with better survival of human melanoma patients. Therefore, MGRN1 might be an important phenotypic determinant of melanoma cells.

Elucidation of Melanogenesis Cascade for Identifying Pathophysiology and Therapeutic Approach of Pigmentary Disorders and Melanoma

Melanogenesis is the biological and biochemical process of melanin and melanosome biosynthesis. Melanin is formed by enzymic reactions of tyrosinase family proteins that convert tyrosine to form brown-black eumelanin and yellow-red pheomelanin within melanosomal compartments in melanocytes, following the cascades of events interacting with a series of autocrine and paracrine signals. Fully melanized melanosomes are delivered to keratinocytes of the skin and hair. The symbiotic relation of a melanocyte and an associated pool of keratinocytes is called epidermal melanin unit (EMU). Microphthalmia-associated transcription factor (MITF) plays a vital role in melanocyte development and differentiation. MITF regulates expression of numerous pigmentation genes for promoting melanocyte differentiation, as well as fundamental genes for maintaining cell homeostasis. Diseases involving alterations of EMU show various forms of pigmentation phenotypes. This review introduces four major topics of melanogenesis cascade that include (1) melanocyte development and differentiation, (2) melanogenesis and intracellular trafficking for melanosome biosynthesis, (3) melanin pigmentation and pigment-type switching, and (4) development of a novel therapeutic approach for malignant melanoma by elucidation of melanogenesis cascade.

Functional characterization of a C-terminal splice variant of the human melanocortin 1 receptor

The melanocortin 1 receptor (MC1R) is a major determinant of skin pigmentation and sensitivity to ultraviolet radiation. When stimulated by its natural agonists, it promotes the switch from synthesis of poorly photoprotective and lightly colored pheomelanins to production of photoprotective and darker eumelanins. In addition to an unusually high number of single nucleotide polymorphisms, the MC1R is expressed as 3 protein-coding splice variants. Two transcripts display different 5' untranslated sequences but yield the same open reading frame corresponding to the canonical 317 aminoacids protein (termed MC1R). An alternative transcript named MC1R-203 encodes for a 382 amino acids protein of poorly characterized functional properties containing an additional 65 aminoacids C-terminal extension. Given the known roles of the MC1R C-terminal extension in forward trafficking, coupling to intracellular effectors and desensitization, the different structure of this domain in MC1R and MC1R-203 may lead to significant functional alteration(s). We have assessed the functional properties of MC1R-203, as compared with the canonical MC1R form. We show that unstimulated HBL human melanoma cells express the MC1R-203 spliceoform, although at much lower levels than canonical MC1R. When expressed in heterologous HEK293 cells, the presence of the 65 aminoacid-long cytosolic extension immediately after Cys316 in MC1R-203 did not impair the intracellular stability of the protein, but it interfered with functional coupling to the cAMP cascade and with the ubiquitylation of ARRB2 associated with MC1R desensitization. Conversely, MC1R-203 retained full capacity to activate ERK1/2 signaling. Accordingly, MC1R-203 displays biased signaling when expressed in HEK293 cells.