Upregulation of mRNA for the melanocortin-1 receptor but not for melanogenic proteins in macrophage x melanoma fusion hybrids exhibiting increased melanogenic and metastatic potential

Fusion of mouse peritoneal macrophages or human blood monocytes with weakly metastatic mouse Cloudman S91 melanoma cells resulted in hybrids with enha nced metastatic potential (Rachkovsky et al., 1998. Clin. Exp. Metastasis, 16: 299-312). With few exceptions, such hybrids also showed increased basal - and MSH-induced pigmentation, at least in part through increased N-glycos ylation of melanogenic proteins (Sodi et al., 1998. Pigment Cell Res., 11: 299-399). Here we report analyses regarding expression of the melanocyte-st imulating hormone (MSH) receptor (melanocortin-1 receptor, MC1-R) and the m elanogenic proteins, tyrosinase (E.C. 1.14.18.1), tyrosinase-related protei n 1 (TRP-1), and the tyrosinase-related protein 2 (TRP-2, F.C. 5.3.2.3), by a panel of cell lines consisting of parental Cloudman S91 melanoma cells, macrophages from DBA/2J mice, artificially derived macrophage x melanoma hy brids of high and low metastatic potential, and a naturally occurring highl y metastatic hybrid between a Cloudman 891 tumor cell and a DBA/2J tumor-in filtrating cell. We show that incubation of cells with MSH/isobutylmethylxa nthine (IBMX) resulted in strong melanogenic and morphologic responses in h igh metastatic hybrids compared to parental cells and the low metastatic hy brid, and that high metastatic hybrids exhibit increased mRNA expression fo r MC1-R accompanied by increased I-125-alpha MSH binding. Although tyrosina se activity and the protein level for tyrosinase and TRP-S, but not for TRP -1, were increased in the high metastatic hybrids versus the other cells, n o significant changes in mRNA either for tyrosinase or for TRPs were observ ed in them. Furthermore, unlike tyrosinase, the abundance and gel mobility pattern of TRP-S did not correlate with changes in acitvity in all hybrids and parental melanoma cells. The results suggest that although the activity MC1-R and tyrosinase correlate with enhanced basal as well as MSH-induced melanogenesis in metastatic/melanotic hybrids, their expression is differen tially regulated, i.e., regulation of MC1-R while at transcriptional level, the TRPs are primarily regulated via post-transcriptional mechanisms in hi gh metastatic hybrids.