Plexin C1, A Receptor for Semaphorin 7A, Inactivates Cofilin and Is a Potential Tumor Suppressor for Melanoma Progression

Abstract

Melanocytes are progenitor cells for melanoma, which arises through step-wise progression from dysplastic to invasive, to metastatic tumor. Our previous data showed that semaphorin 7A (Sema7A), a protein involved in axon guidance, stimulates melanocyte adhesion and dendricity through opposing actions of beta1-integrin and Plexin C1 receptors. We now show that Plexin C1 is diminished or absent in human melanoma cell lines; analysis of tissue microarrays of nevi, melanoma, and metastatic melanoma showed a decrease in Plexin C1 expression in metastatic melanoma, and an inverse correlation of Plexin C1 expression with depth of invasion. We examined the signaling intermediates of Sema7A and downstream targets of Plexin C1 in human melanocytes. Sema7A activated mitogen-activated protein kinase and inactivated cofilin, an actin-binding protein involved in cell migration. When Plexin C1 expression was silenced, Sema7A failed to phosphorylate cofilin, indicating that cofilin is downstream of Plexin C1. Further, Lim kinase II, a protein that phosphorylates cofilin, is upregulated by Sema7A in a Plexin C1-dependent manner. These data identify Plexin C1 as a potential tumor suppressor protein in melanoma progression, and suggest that loss of Plexin C1 expression may promote melanoma invasion and metastasis through loss of inhibitory signaling on cofilin activation.

Semaphorin 7A Promotes Chemokine-Driven Dendritic Cell Migration

“…Thus, Sema7A on DCs acts by controlling the balance between adhesion/ deadhesion, similar to Sema7A function on tumor cells and melanocytes (15,17,38). A prominent role for the actin-binding protein cofilin was shown in these studies.…”

“…A prominent role for the actin-binding protein cofilin was shown in these studies. Cofilin regulates cell migration downstream of plexin C1, integrins, and CCR7 (15,39), making it a possible candidate for controlling Sema7A-dependent DC migration. Experiments to unravel a role for cofilin in our studies were hampered by the low number of DCs available for biochemical experiments, as well as by the unconventional mechanisms regulating cofilin activity (40).…”

“…The two known receptors for Sema7A are plexin C1 and b 1 integrin (a 1 and a v heterodimers). Binding of Sema7A to plexin C1 in melanocytes leads to inhibition of the actin-binding protein cofilin and reduced cell spreading (15). In contrast, Sema7A binding to integrins in neurons or melanocytes activates FAK and MAPK pathways, leading to remodeling of the actin cytoskeleton and increased cell spreading (16,17).…”

Plexin B1 inhibits MET through direct association and regulates Shp2 expression in melanocytes

“…Recombinant Sema4D and Sema7A were expressed as Fc-tagged proteins as previously described (Scott et al, 2008;Scott et al, 2009). Protein purity, quantity and identity were assessed by silver staining of gels and western blotting.…”

“…Neonatal foreskins were obtained according to the University of Rochester Research Subjects Review Board guidelines and were the source of cultured human melanocytes as previously described (Scott et al, 2009). Cells were cultured in melanocyte growth media (MGM) consisting of Opti-MEM (Gibco-BRL) containing: 5% FBS (fetal bovine serum; Atlanta Biologicals; Lawrenceville, GA), 10 24 M iso-butyl-methylxanthine (IBMX), Anti-Anti (Gibco-BRL), 2.5 nM Cholera Toxin, 0.1 mM dbcAMP, 25 ng/ml phorbol ester.…”