Abstract
The mechanisms by which c‐__erb__B‐dependent signaling contribute to the invasive potential of HNSCC remain to be fully elucidated. We have previously shown that c‐__erb__B autocrine and/or paracrine stimulation upregulates MMP‐9 but has no effect on the related gelatinase, MMP‐2. BTC, a major c‐__erb__B ligand, has the ability to efficiently activate all c‐__erb__B receptors and to bind directly to EGFR and c‐__erb__B‐4. BTC is commonly expressed in HNSCC cells and exerts the most potent effects in terms of MMP induction relative to other c‐__erb__B ligands so far tested. In the present study, we explored the contribution of major downstream events triggered by BTC/c‐__erb__B receptor signaling to the regulation of MMP‐9 and in vitro invasiveness of HNSCC cells. In human HNSCC cell lines, SIHN‐006 and Detroit‐562, BTC treatment resulted in rapid tyrosine phosphorylation of all c‐__erb__B receptors whereas both endogenous MMP‐9 and BTC‐stimulated MMP‐9 were predominantly mediated via EGFR. BTC induced ERK1/2, JNK/SAPK and Akt phosphorylation with differing kinetics but not p38 kinase. The BTC‐dependent activation of JNK and PI3K/Akt pathways occurred predominantly via EGFR, whereas activation of the MEK‐1/ERK pathway occurred via all 4 c‐__erb__B receptors, although again predominantly via EGFR. Selective inhibition of ERK/MAPK (by PD98059 or U0126) and PI3K (by LY294002 or wortmannin) led to marked reduction of both basal and BTC‐induced MMP‐9 activity and invasive ability of HNSCC cells. In contrast, inhibition of p38 kinase with SB203580 produced no such effects. A specific inhibitor of NF‐κB, BAY 11‐7085, also blocked the stimulatory effect of BTC. No remarkable inhibition of MMP‐9 and invasion was observed on targeting other cellular activities, such as PKA, PKC and PLC‐γ. Taken together, our data show that BTC induces MMP‐9 production and invasion primarily through activation of EGFR, MAPK and PI3K/Akt in HNSCC cells. © 2004 Wiley‐Liss, Inc.