Nuclear Factor-κB Activation by the CXC Chemokine Melanoma Growth-stimulatory Activity/Growth-regulated Protein Involves the MEKK1/p38 Mitogen-activated Protein Kinase Pathway

Melanoma growth stimulatory activity/growth-regulated protein (MGSA/GRO), a CXC chemokine, plays an important role in inflammation, wound healing, growth regulation, angiogenesis, and tumorigenesis. Constitutive expression of MGSA/GROα in melanoma tumors is associated with constitutive nuclear factor (NF)-κB activity. We show here that either exogenous addition or continuous expression of MGSA/GROα in immortalized melanocytes enhances NF-κB activation, as well as mitogen-activated protein (MAP) kinase kinase kinase (MEKK) 1, MAP kinase kinase (MEK) 3/6, and p38 MAP kinase activation. Expression of dominant negative M-Ras (S27N), dominant negative MEKK1 (K432M), or specific chemical inhibitors for p38 MAP kinase (SB202190 and SB203580) block MGSA/GROα-induced NF-κB transactivation, demonstrating that Ras, MEKK1, and p38 are involved in the signal pathways of MGSA/GROα activation of NF-κB. Expression of dominant active Ras or dominant active MEKK1 alone can also stimulate NF-κB activation. The expression of dominant negative MEKK1 inhibits the Ras-induced NF-κB activation, suggesting that MEKK1 is a downstream target of Ras. Moreover, MGSA/GROα induction of NF-κB is independent of the MEK1/ERK cascade, because MGSA/GROα failed to increase ERK and ELK activation, and specific chemical inhibitors for MEK1 (PD98059) had no effect on MGSA/GROα-enhanced NF-κB activation. These data demonstrate that NF-κB activation is required for MGSA/GROα-induced melanocyte transformation through a Ras/MEKK1/p38 cascade in melanocytes.