Tumor cell invasion and metastasis require precise coordination of adherence to extracellular matrix (ECM) and controlled degradation of its components. Invasive cells secrete proteolytic enzymes known as matrix metalloproteinases (MMPs) which degrade specific basement membrane molecules. Expression of these enzymes is regulated by multiple signaling mechanisms, including ECM attachment itself. Previously, we have shown that retinoic acid (RA) inhibits tumor cell invasion of ECM by regulating MMP expression. We determined that RA mediated inhibition of MMP-9 expression was dependent on ECM attachment in RA sensitive but not RA resistant human carcinoma lines. In the present study we examined the mechanisms of this regulation. Both RA and attachment to type IV collagen inhibited the activity of one of the terminal effectors of the mitogen activated protein kinase (MAPK) pathway, namely ERK1. RA also induced ets-1 expression, a transcription factor known to be phosphorylated by ERK1, in RA sensitive but not RA resistant lines. Increased expression of hypophosphorylated ets-1 was sufficient to inhibit MMP-9 expression in these lines. This inhibition was mediated via two ets-1 binding sites in the MMP-9 promoter. The results of these experiments indicate that induction of hypophosphorylated ets-1 as the result of RA and ECM mediated decreases in ERK1 activity represents a novel mechanism by which RA regulates MMP-9 gene expression.

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