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.

Application of a combined sulphorhodamine B and melanin assay to the evaluation of Chinese medicines and their constituent compounds for hyperpigmentation treatment

AIM OF THE STUDY

To investigate the efficacy of traditional Chinese medicine (TCM) in the treatment of hyperpigmentation problems, extracts of herbs selected based on traditional Chinese medical literature were screened.

MATERIALS AND METHODS

Forty extracts were extracted from 10 selected herbs using hexane, dichloromethane, methanol and water. They were then screened using melan-a cells, an immortalized non-tumorigenic mouse melanocyte cell line. Sulphorhodamine B (SRB) assay and measurement of melanin production were performed to examine the effects of the extracts as well as some natural compounds from these herbs on melanogenesis in the melan-a cells.

RESULTS

The hexane and dichloromethane extracts of Angelica sinensis exhibited strong hypopigmentary effects.

CONCLUSIONS

Natural compounds occurring in this herb were also investigated. Among them 4-ethylresorcinol, 4-ethylphenol and 1-tetradecanol demonstrated positive effects in attenuating melanin synthesis in the cultured cells.

Abstract 5076: Dissection of RAS downstream pathways in melanomagenesis

Cutaneous malignant melanoma is considered one of the most deadly human cancers, based upon both its penchant for metastatic spread and its typical resistance to currently available therapy. Long known to harbor oncogenic NRAS mutations, melanomas were more recently reported to be frequent bearers of activating mutations in BRAF, one of the effectors situated downstream of wildtype NRAS. RAS gene products (HRas, KRas and NRas) are 21kd G-proteins that serve as molecular switches converting cell-surface kinase activation events to nuclear events, thus influencing cell behavior. The major downstream effectors of Ras are the Rafs (ARaf, BRaf and CRaf), phosphatidylinositol 3-kinase (PI3K) and the Ral guanine exchange factors (RalGEF). NRAS and BRAF mutations are rarely found in the same melanoma, suggesting that they may possess important overlapping oncogenic activities. Here we compare and contrast the oncogenic roles of the three major NRas downstream effectors, Raf, PI3K and RalGEF, using genetically engineered Arf-deficient immortalized mouse melanocytes as a model system. Activation of each of the three major downstream pathways of NRas has overt phenotypic effects on immortalized mouse melanocytes, many distinct. Our data indicate a prominent role for BRaf and PI3K in melanocyte senescence and invasiveness, respectively. More surprisingly, we found that constitutively activated RalGEF stimulated melanocyte invasiveness and anchorage-independent growth almost as impressively as NRasQ61K. The suppressive effects of expression of a dominant negative Ral corroborated the role of the RalGEF-Ral pathway in anchorage-independent growth. Although no single downstream pathway could recapitulate all of the consequences of oncogenic NRas expression, melanocytes expressing constitutively activated mutants of all three NRas downstream factors (RLF-CAAX plus p110α-CAAX plus BRafV600E) achieved soft agar colony forming efficiency that was fully comparable to those expressing NRasQ61K alone. In conclusion, our data suggested a distinct function for all three NRas downstream pathways - Raf, PI3K and Ral in melanocyte senescence, invasiveness and transformation, respectively. Importantly, our finding that constitutive RalGEF activation confers malignant phenotypes in melanocytes, particularly anchorage-independent growth, indicates that this often overlooked pathway should be more carefully evaluated as a possible therapeutic target.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5076.

UV-induced Expression of Key Component of the Tanning Process, the POMC and MC1R Genes, Is Dependent on the p-38-activated Upstream Stimulating Factor-1 (USF-1)

Protection against UV-mediated DNA damage and the onset of oncogenesis is afforded by the tanning response in which UV irradiation triggers melanocytes to increase production of melanin that is then transferred to keratinocytes. A key component of the tanning process is the UV-mediated induction of the pro-opiomelanocortin (POMC) and MC1R genes encoding the alpha-melanocyte-stimulating hormone and its receptor, respectively, which play a crucial role in pigmentation by regulating the intracellular levels of cAMP. How these genes are regulated in response to UV irradiation is not known. Here we have shown that UV-induced activation of the POMC and MC1R promoters is mediated by p38 stress-activated kinase signaling to the transcription factor, upstream stimulating factor-1 (USF-1). Importantly, melanocytes derived from USF-1 -/- mice exhibit a defective UV response and fail to activate POMC and MC1R expression in response to UV irradiation. The results define USF-1 as a critical UV-responsive activator of genes implicated in protection from solar radiation.

In early development of the rat mRNA for the major myelin protein P(0) is expressed in nonsensory areas of the embryonic inner ear, notochord, enteric nervous system, and olfactory ensheathing cells

The myelin protein P(0) has a major structural role in Schwann cell myelin, and the expression of P(0) protein and mRNA in the Schwann cell lineage has been extensively documented. We show here, using in situ hybridization, that the P(0) gene is also activated in a number of other tissues during embryonic development. P(0) mRNA is first detectable in 10-day-old embryos (E10) and is at this time seen only in cells in the cephalic neural crest and in the otic placode/pit. P(0) expression continues in the otic vesicle and at E12 P(0) expression in this structure largely overlaps with expression of another myelin gene, proteolipid protein. In the developing ear at E14, P(0) expression is complementary to expression of serrate and c-ret mRNAs, which later are expressed in sensory areas of the inner ear, while expression of bone morphogenetic protein (BMP)-4 and P(0), though largely complementary, shows small areas of overlap. P(0) mRNA and protein are detectable in the notochord from E10 to at least E13. In addition to P(0) expression in a subpopulation of trunk crest cells at E11/E12 and in Schwann cell precursors thereafter, P(0) mRNA is also present transiently in a subpopulation of cells migrating in the enteric neural crest pathway, but is down-regulated in these cells at E14 and thereafter. P(0) is also detected in the placode-derived olfactory ensheathing cells from E13 and is maintained in the adult. No signal is seen in cells in the melanocyte migration pathway or in TUJ1 positive neuronal cells in tissue sections. The activation of the P(0) gene in specific tissues outside the nervous system was unexpected. It remains to be determined whether this is functionally significant, or whether it is an evolutionary relic, perhaps reflecting ancestral use of P(0) as an adhesion molecule.

Investigation of the effect of Angelica sinensis root extract on the proliferation of melanocytes in culture

Aqueous extracts of Angelica sinensis root, a herb commonly used in the treatment of vitiligo in Traditional Chinese Medicine, were tested for their activity on mouse melanocyte proliferation in culture. At concentrations of 0.5-2500 micrograms/ml, these extracts were not able to stimulate melanocyte cell division. On the contrary, they exerted a general cytotoxicity to the cells at higher concentrations. Cytotoxicity was reduced by prior treatment of the extract with polyvinylpolypyrrolidone, which was shown by thin layer chromatography to reduce the coumarin content.