The basal transcription of the CXC chemokine, melanocyte growth stimulatory activity (MGSA)/growth-regulated protein (GRO)-α, is upregulated in Hs294T melanoma cells compared with the normal retinal pigment epithelial (RPE) cells. Previous studies characterized a cytokine-inducible, functional nuclear factor (NF) -κB consensus element in the immediate 5' regulatory region of the MGSA/GRO-α gene at -78 bp. Although the cytokine-inducible mechanisms for transcription of this gene are fairly well delineated, the mechanisms involved in its basal up-regulation of transcription in Hs294T melanoma cells are poorly understood. Recently, we demonstrated an increased rate of IκB-α degradation in Hs294T cells, which leads to an increased nuclear localization of NF-κB (R. L. Shattuck-Brandt and A. Richmond. Cancer Res., 57: 3032-3039, 1997). Here we demonstrate that Hs294T melanoma cells have elevated basal IκB kinase (IKK) activity relative to RPE cells, causing an increased constitutive IκB-α phosphorylation and degradation. We also show here that the resultant elevated nuclear NF-κB (p50/p65) in these cells is responsible for the increased basal transcription of MGSA/GRO-α. Pretreatment of Hs294T or RPE cells with proteasome inhibitors MG115 or MG132 captures the slower migrating, constitutively phosphorylated form of IκB-α in Hs294T melanoma cells, but not in RPE cells. In addition, a phospho- specific antibody that specifically recognizes the inhibitory form of IκB that is phosphorylated at Ser-32 reacted with IκB-α in Hs294T cell, but not in unstimulated RPE cells. Although the basal level of protein expression of IKK-α or IKK-β are the same in both Hs294T and RPE cells, immunoprecipitation with IKK-α antibody combined with activity assay reveal a constitutively active IKK complex in Hs294T melanoma cells. Cotransfection of a 350-bp MGSA/GRO-α promoter-luciferase reporter construct with either the dominant negative IKK-α or the repressors of NF-κB, the IκB-α wild type or mutants lacking the inducible phosphorylation sites, demonstrates that the increased basal MGSA/GRO-α transcription in the Hs294T cells is due to the enhanced nuclear activation of NF-κB.
|Original language||English (US)|
|Number of pages||6|
|State||Published - Mar 15 1999|
ASJC Scopus subject areas
- Cancer Research