This investigation has confirmed previous studies of uveal melanoma that suggested that these cells are responsive to HGF.
21 Although some movement in response to HGF was thought to be due to random motility, the observation that all tumors studied had a significant response to HGF, which was relatively high in comparison with the response to other factors tested, suggests specific involvement of HGF in uveal melanoma metastasis
(Fig. 3) . It is also of particular interest that cultures classified as noninvasive and weakly invasive, showed the greatest responses to HGF and that such increased responses were not seen with the other stimulatory factors. There are several possible explanations for this effect. First, in this situation, it is possible that, these invasive melanomas (SOM-196B, -267, -277, -279, -280, -282, -288, -289, and -290) are already stimulated by autocrine HGF, additional stimulation by inclusion of this growth factor in the assay would only produce a limited response in comparison to weakly invasive melanomas (SOM-157d, -281, and -286) where autocrine stimulation may be less apparent. Evidence suggests that invasive uveal melanoma cells are capable of expressing HGF, with micrometastases in the liver having been found to stain positively for HGF.
21 Second, the difference may in part arise from the involvement of the HGF receptor, c-Met. All uveal melanoma cultures tested were found to express c-Met (data not shown), and blocking of c-Met in SOM-196B with a functional blocking antibody completely abrogated the invasive response to HGF
(Figs. 5c 5d 5e) , thus confirming that HGF directly stimulates even invasive uveal melanoma cells. However, blocking c-Met in the absence of stimulation by HGF also decreased invasion of SOM-196B, suggesting that either innate production of HGF by the tumor itself was blocked, or c-Met itself contributes to the invasive response. Certainly, much evidence exists suggesting the role of c-Met in the invasion of other tumors,
33 34 and there is growing evidence implicating both c-Met and HGF in a number of autocrine and paracrine responses in a variety of cell types, promoting, among other responses, junctional breakdown, directed migration and invasion, and cell survival. The possibility therefore exists that other pathways, perhaps used by the more invasive melanomas, are also responsible for stimulating c-Met in uveal melanomas, and recently it has been reported that autocrine TGFα, binding to its receptor (EGFR), causes phosphorylation and activation of c-Met, in the absence of HGF.
35 Therefore, by blocking c-Met, alternative pathways such as the TGFα/EGFR pathway may also be abrogated and the subsequent response prevented, potentially explaining the decrease in invasion noted after c-Met was blocked, in the absence of HGF.