GPNMB is expressed in human epidermal keratinocytes but disappears in the vitiligo lesional skin

Abstract

GPNMB is involved in multiple cellular functions including cell adhesion, stress protection and stem cell maintenance. In skin, melanocyte-GPNMB is suggested to mediate pigmentation through melanosome formation, but details of keratinocyte-GPNMB have yet to be well understood. We confirmed the expression of GPNMB in normal human epidermal keratinocytes (NHEKs) by reducing the expression using siRNA. A higher calcium concentration of over 1.25 mM decreased the GPNMB expression. Histological staining showed that GPNMB was expressed in the basal layer of normal skins but completely absent in vitiligo skins. the normal expression of GpnMB in nevus depigmentosus skin suggested that lack of GPNMB is characteristic of vitiligo lesional skins. IFN-γ and IL-17A, two cytokines with possible causal roles in vitiligo development, inhibited GPNMB expression in vitro. Approximately 4-8% of the total GPNMB expressed on NHEKs were released possibly by ADAM 10 as a soluble form, but the process of release was not affected by the cytokines. The suppressive effect of IFN-γ on GpnMB was partially via IFN-γ/JAK2/STAT1 signaling axis. Decreased GPNMB expression in keratinocytes may affect melanocyte maintenance or survival against oxidative stress although further studies are needed. These findings indicate a new target for vitiligo treatment, focusing on the novel role of IFN-γ and IL-17 in downregulating keratinocyte-GPNMB. GPNMB (Glycoprotein nonmetastatic melanoma protein B)-also known as osteoactivin, dendritic cell-heparin integrin ligand (DC-HIL), or hematopoietic growth factor inducible neurokinin-1 type-is a type I transmembrane glycoprotein. GPNMB has 2 transcript variants encoding 560 and 572 amino acid isoforms in human and shares a 25% amino acid sequence homology with PMEL-17, a melanocyte-specific melanosomal protein 1. The extracellular part of GPNMB mainly contains an RGD motif that binds to integrin in the process of maintaining cell-cell adhesion, and an Ig-like polycystic kidney disease (PKD) domain involved in protein-protein and protein-carbohydrate interactions. The cytoplasmic tail contains an immunoreceptor tyrosine-based activation motif (ITAM) that takes part in the intracellular signaling via Src and Syk cytoplasmic kinases, and a di-leucine motif required for its endosomal⁄melanosomal sorting signal 2. GPNMB is widely expressed in various tissues such as the skin, brain, breasts, muscle, and bone 3-7. The known functions of GPNMB include cellular adhesion through integrin 1 , regulation of the degeneration/regeneration of the extracellular matrix in skeletal muscles 6 , the mineralization of bone 2 , the differentiation of osteoclasts 8 and osteoblasts 9 , the impairment of T-cell activation 10 , the regulation of inflammatory responses in macrophages 11 , the suppression of motor neuron degeneration in amyotrophic lateral sclerosis 12 , and the invasion and metastasis of several cancers 13-17. GPNMB has also been widely demonstrated to increase the endoplasmic reticulum (ER) stress response by inducing th...

Rhododendrol‐induced leukoderma update II: Pathophysiology, mechanisms, risk evaluation, and possible mechanism‐based treatments in comparison with vitiligo

“… 37 Notably, it was confirmed that IFN‐γ and IL‐17A, which are thought to be involved in the development of vitiligo, suppress keratinocyte GPNMB expression. 37 It has also been found that keratinocyte GPNMB was absent in basal layer lesions in RDL, and that the extracellular domain of GPNMB is released as soluble GPNMB, which protects melanocytes against cytotoxicity (unpubl. data, Professor Katayama, Department of Pigmentation Research and Therapeutics, Osaka City University, Osaka, Japan), suggesting that keratinocyte GPNMB may be involved in vitiligo and RDL pathology.…”

“…The glycoprotein non‐metastatic melanoma protein B (GPNMB), a membrane protein known to be expressed in melanocytes and involved in processes such as melanosome formation, stress resistance, and cell adhesion, is expressed in cultured human keratinocytes and in the basal layer of healthy epidermis, whereas it is absent in basal layer lesions in vitiligo 37 . Notably, it was confirmed that IFN‐γ and IL‐17A, which are thought to be involved in the development of vitiligo, suppress keratinocyte GPNMB expression 37 . It has also been found that keratinocyte GPNMB was absent in basal layer lesions in RDL, and that the extracellular domain of GPNMB is released as soluble GPNMB, which protects melanocytes against cytotoxicity (unpubl.…”

Skin Pigmentation Abnormalities and Their Possible Relationship with Skin Aging

“…GPNMB has also been downregulated in vitiligo keratinocytes, causing the loss of melanocytes [ 158 ]. The finding indicates the regulatory role of GPNMB in skin hypomelanosis by reducing melanocyte survival in addition to reducing melanogenesis.…”

Functional Domains and Evolutionary History of the PMEL and GPNMB Family Proteins

“…GPNMB and PMEL are related genes that encode for proteins with a common functional domain architecture: a signal peptide, core amyloid fragment, Kringle-like domain and a single transmembrane domain [ 1 ]. Consistent with this common architecture, both have also been implicated in the same diverse biological roles via their association with phenotypic presentation and disease, i.e., cancer [ 2 , 3 , 4 , 5 , 6 ], glaucoma [ 7 , 8 , 9 ], neurodegenerative disease [ 10 , 11 , 12 , 13 ], melanin-based pigmentation [ 14 , 15 , 16 , 17 ] and amyloid formation/amyloidosis [ 18 , 19 , 20 , 21 , 22 ]. It is therefore surprising that these related proteins with a strikingly similar suite of functional domains have yet to be identified as interacting in, or being partially redundant in, any biological process or pathway.…”

“…Like PMEL, the GPNMB protein undergoes glycosylation and proteolytic cleavage to produce the mature peptide [ 52 , 53 , 54 , 55 ]. GPNMB also localizes to melanosomes [ 54 , 56 , 57 ], is transcriptionally regulated by the Melanocyte-Inducing Transcription Factor (MITF) [ 58 , 59 , 60 , 61 ] and, when mutated, can lead to hypopigmented lesions and pigmentary glaucoma in mice ( Figure 2 ) [ 7 , 8 , 18 , 19 ]. Surprisingly, there has been no functional evidence of the amyloid-forming roles of GPNMB, perhaps owing to the post-transcriptional modification of its PKD domain [ 62 ], although disease presentation suggests a relationship between GPNMB and the amyloid.…”