A Phosphatidylinositol 3-Kinase–Pax3 Axis Regulates Brn-2 Expression in Melanoma

Abstract

bDeregulation of transcription arising from mutations in key signaling pathways is a hallmark of cancer. In melanoma, the most aggressive and lethal form of skin cancer, the Brn-2 transcription factor (POU3F2) regulates proliferation and invasiveness and lies downstream from mitogen-activated protein kinase (MAPK) and Wnt/␤-catenin, two melanoma-associated signaling pathways. In vivo Brn-2 represses expression of the microphthalmia-associated transcription factor, MITF, to drive cells to a more stem cell-like and invasive phenotype. Given the key role of Brn-2 in regulating melanoma biology, understanding the signaling pathways that drive Brn-2 expression is an important issue. Here, we show that inhibition of phosphatidylinositol 3-kinase (PI3K) signaling reduces invasiveness of melanoma cells in culture and strongly inhibits Brn-2 expression. Pax3, a transcription factor regulating melanocyte lineage-specific genes, directly binds and regulates the Brn-2 promoter, and Pax3 expression is also decreased upon PI3K inhibition. Collectively, our results highlight a crucial role for PI3K in regulating Brn-2 and Pax3 expression, reveal a mechanism by which PI3K can regulate invasiveness, and imply that PI3K signaling is a key determinant of melanoma subpopulation diversity. Together with our previous work, the results presented here now place Brn-2 downstream of three melanoma-associated signaling pathways.

The master role of microphthalmia-associated transcription factor in melanocyte and melanoma biology

“…14,16 Meanwhile, PAX3 seems to repress MITF expression through POU3F2 (see below) in some melanomas. 17,18 α-Melanocyte-stimulating hormone (α-MSH) increases pigmentation through increasing tyrosinase (Tyr) expression in mouse B16-F1 melanoma cells, 19 as well as numerous additional pigmentation related genes (see below). Based on the results that α-MSH activates the cAMP-CREB signaling pathway and MITF activates TYR transcription, Bertolotto et al 20 and Price et al 21 hypothesized and found that cAMP-CREB signaling activates MITF-M transcription in a cis-acting fashion.…”

PAX3 and FOXD3 Promote CXCR4 Expression in Melanoma

“…PAX3 drives melanoma migration and invasiveness, and this is due at least in part by the activation of the BRN2 gene (18).…”

“…PAX3 inhibition leads to a decrease in melanoma cell proliferation and survival (14 -17). PAX3 has also been implicated in melanoma migration, invasion, and metastasis (18). The mechanism by which PAX3 acts as an oncogene is poorly understood, although it most likely parallels its role during development by regulating growth and differentiation.…”

“…PAX3 likely exerts its oncogenic potential through the regulation of downstream effector genes. A limited number of PAX3 direct targets have been discovered in melanoma including the MET tyrosine kinase receptor as well as transcription factors TBX2 and BRN2 (15,18,19). The full scope of PAX3 activity in melanoma cannot be explained by the regulation of these factors alone, therefore, PAX3 must also regulate other downstream effectors.…”

Haplosufficiency of PAX3 for melanoma development in Tyr

“…In addition, silencing Pax3 using RNA interference was shown to inhibit proliferation, and induce terminal differentiation and apoptosis following activation of caspase-3 and p53 in melanoma cells [4][5][6]. More recently, Pax3 silencing was reported to inhibit the invasiveness of melanoma cells in culture [7] as well as the growth of melanoma cells with acquired resistance to vemurafenib [8]. PAX3 is also expressed in muscle progenitor cells before myogenic transcription factors, such as MyoD, Myogenin, and Myf5, and gradually decreases during muscle differentiation.…”