Transmembrane glycoprotein NMB is a protein that in humans is encoded by the GPNMBgene. Two transcript variants encoding 560 and 572 amino acid isoforms have been characterized for this gene in humans. The mouse and rat orthologues of GPNMB are known as DC-HIL and Osteoactivin (OA), respectively.
GPNMB has been reported to be expressed in various cell types, including: melanocytes, osteoclasts, osteoblasts, dendritic cells, and it is overexpressed in various cancer types. In melanocytic cells and osteoclasts the GPNMB gene is transcriptionally regulated by Microphthalmia-associated transcription factor.
In osteoblast progenitor cells, Osteoactivin works as a positive regulator of osteoblast differentiation during later stages of matrix maturation and mineralization  that is mediated at least in part by BMP-2 in a SMAD1 dependent manner to promote osteoblast differentiation. In addition, using a rat fracture model, Osteoactivin (OA) enhances the repairing process in bone fracture, demonstrated by its high expression during chondrogenesis (soft callus) and osteogenesis (hard callus) compared to the intact femurs  that is why Osteoactivin (OA) could be a novel therapeutic agent used to treat generalized osteoporosis or localized osteopenia during fracture repair by stimulating bone growth and regeneration. Similarly, Osteoactivin expression increases during osteoclast differentiation and it is functionally implicated in this process, possibly by promoting the fusion of osteoclast progenitor cells.
Clinical and functional significance in cancer
GPNMB was originally identified as a gene that was expressed in poorly metastatic human melanoma cell lines and xenografts and not expressed in highly metastatic cell lines. However, several recent studies have identified high GPNMB expression in aggressive melanoma,glioma, and breast cancer specimens.
Based on Immunohistochemical analysis, two studies have shown that GPNMB is commonly expressed in breast tumors. In the first study, GPNMB was detected in 71% (10/14) of breast tumors. In the second study, 64% of human breast tumors express GPNMB in the tumor stroma and an additional 10% of tumors express GPNMB in the tumor epithelium. In this study it was reported that GPNMB expression in the tumor epithelium was an independent prognostic indicator of breast cancer recurrence. Moreover, epithelial GPNMB expression was most abundant in triple negative breast cancers and it was found to be a prognostic marker for shorter metastasis-free survival times within this breast cancer subtype. Finally, GPNMB expression in breast cancer cells is capable of promoting cell migration, invasion, and metastasis both in vitro and in vivo.
^Weterman MA, Ajubi N, van Dinter IM, Degen WG, van Muijen GN, Ruitter DJ, Bloemers HP (January 1995). "nmb, a novel gene, is expressed in low-metastatic human melanoma cell lines and xenografts". International Journal of Cancer. 60 (1): 73–81. PMID7814155. doi:10.1002/ijc.2910600111.
^Hoek KS, Schlegel NC, Eichhoff OM, Widmer DS, Praetorius C, Einarsson SO, Valgeirsdottir S, Bergsteinsdottir K, Schepsky A, Dummer R, Steingrimsson E (December 2008). "Novel MITF targets identified using a two-step DNA microarray strategy". Pigment Cell & Melanoma Research. 21 (6): 665–76. PMID19067971. doi:10.1111/j.1755-148X.2008.00505.x.
^Abdelmagid SM, Barbe MF, Rico MC, Salihoglu S, Arango-Hisijara I, Selim AH, Anderson MG, Owen TA, Popoff SN, Safadi FF (1 August 2008). "Osteoactivin, an anabolic factor that regulates osteoblast differentiation and function". Exp Cell Res. 314 (13): 2334–51. PMID18555216. doi:10.1016/j.yexcr.2008.02.006.
^Abdelmagid SM, Barbe MF, Arango-Hisijara I, Owen TA, Popoff SN, Safadi FF (2007). "Osteoactivin acts as downstream mediator of BMP-2 effects on osteoblast function". J Cell Physiol. 210 (1): 26–37. PMID17034042. doi:10.1002/jcp.20841.
^Abdelmagid SM, Barbe MF, Hadjiargyrou M, Owen TA, Razmpour R, Rehman S, Popoff SN, Safadi FF (1 Oct 2010). "Temporal and spatial expression of osteoactivin during fracture repair". J Cell Biochem. 111 (2): 295–309. PMID20506259. doi:10.1002/jcb.22702.
^Abdelmagid, Samir (2010). Role of Osteoactivin in Bone Formation and Fracture Repair. USA: LAP Lambert Academic Publishing. p. 144. ISBN978-3-8383-5436-1.
^Sheng MH, Wergedal JE, Mohan S, Lau KH (October 2008). "Osteoactivin is a novel osteoclastic protein and plays a key role in osteoclast differentiation and activity". FEBS Lett. 582 (10): 1451–8. PMID18381073. doi:10.1016/j.febslet.2008.03.030.
^Tse KF, Jeffers M, Pollack VA, McCabe DA, Shadish ML, Khramtsov NV, Hackett CS, Shenoy SG, Kuang B, Boldog FL, MacDougall JR, Rastelli L, Herrmann J, Gallo M, Gazit-Bornstein G, Senter PD, Meyer DL, Lichenstein HS, LaRochelle WJ (February 2006). "CR011, a fully human monoclonal antibody-auristatin E conjugate, for the treatment of melanoma". Clinical Cancer Research. 12 (4): 1373–82. PMID16489096. doi:10.1158/1078-0432.CCR-05-2018.
^Kuan CT, Wakiya K, Dowell JM, Herndon JE, Reardon DA, Graner MW, Riggins GJ, Wikstrand CJ, Bigner DD (April 2006). "Glycoprotein nonmetastatic melanoma protein B, a potential molecular therapeutic target in patients with glioblastoma multiforme". Clinical Cancer Research. 12 (7 Pt 1): 1970–82. PMID16609006. doi:10.1158/1078-0432.CCR-05-2797.
^ abRose AA, Pepin F, Russo C, Abou Khalil JE, Hallett M, Siegel PM (October 2007). "Osteoactivin promotes breast cancer metastasis to bone". Molecular Cancer Research. 5 (10): 1001–14. PMID17951401. doi:10.1158/1541-7786.MCR-07-0119.
^ abRose AA, Grosset AA, Dong Z, Russo C, Macdonald PA, Bertos NR, St-Pierre Y, Simantov R, Hallett M, Park M, Gaboury L, Siegel PM (April 2010). "Glycoprotein Nonmetastatic B Is an Independent Prognostic Indicator of Recurrence and a Novel Therapeutic Target in Breast Cancer". Clinical Cancer Research. 16 (7): 2147–2156. PMID20215530. doi:10.1158/1078-0432.CCR-09-1611.
Owen TA, Smock SL, Prakash S, Pinder L, Brees D, Krull D, Castleberry TA, Clancy YC, Marks SC, Safadi FF, Popoff SN (2003). "Identification and characterization of the genes encoding human and mouse osteoactivin". Critical Reviews in Eukaryotic Gene Expression. 13 (2–4): 205–20. PMID14696968.
Haralanova-Ilieva B, Ramadori G, Armbrust T (April 2005). "Expression of osteoactivin in rat and human liver and isolated rat liver cells". Journal of Hepatology. 42 (4): 565–72. PMID15763343. doi:10.1016/j.jhep.2004.12.021.
Rual JF, Venkatesan K, Hao T, Hirozane-Kishikawa T, Dricot A, Li N, Berriz GF, Gibbons FD, Dreze M, Ayivi-Guedehoussou N, Klitgord N, Simon C, Boxem M, Milstein S, Rosenberg J, Goldberg DS, Zhang LV, Wong SL, Franklin G, Li S, Albala JS, Lim J, Fraughton C, Llamosas E, Cevik S, Bex C, Lamesch P, Sikorski RS, Vandenhaute J, Zoghbi HY, Smolyar A, Bosak S, Sequerra R, Doucette-Stamm L, Cusick ME, Hill DE, Roth FP, Vidal M (October 2005). "Towards a proteome-scale map of the human protein-protein interaction network". Nature. 437 (7062): 1173–8. Bibcode:2005Natur.437.1173R. PMID16189514. doi:10.1038/nature04209.
Oh JH, Yang JO, Hahn Y, Kim MR, Byun SS, Jeon YJ, Kim JM, Song KS, Noh SM, Kim S, Yoo HS, Kim YS, Kim NS (December 2005). "Transcriptome analysis of human gastric cancer". Mammalian Genome. 16 (12): 942–54. PMID16341674. doi:10.1007/s00335-005-0075-2.
Kuan CT, Wakiya K, Dowell JM, Herndon JE, Reardon DA, Graner MW, Riggins GJ, Wikstrand CJ, Bigner DD (April 2006). "Glycoprotein nonmetastatic melanoma protein B, a potential molecular therapeutic target in patients with glioblastoma multiforme". Clinical Cancer Research. 12 (7 Pt 1): 1970–82. PMID16609006. doi:10.1158/1078-0432.CCR-05-2797.
Chi A, Valencia JC, Hu ZZ, Watabe H, Yamaguchi H, Mangini NJ, Huang H, Canfield VA, Cheng KC, Yang F, Abe R, Yamagishi S, Shabanowitz J, Hearing VJ, Wu C, Appella E, Hunt DF (November 2006). "Proteomic and bioinformatic characterization of the biogenesis and function of melanosomes". Journal of Proteome Research. 5 (11): 3135–44. PMID17081065. doi:10.1021/pr060363j.
Rose AA, Pepin F, Russo C, Abou Khalil JE, Hallett M, Siegel PM (October 2007). "Osteoactivin promotes breast cancer metastasis to bone". Molecular Cancer Research. 5 (10): 1001–14. PMID17951401. doi:10.1158/1541-7786.MCR-07-0119.