Agonists of the retinoic acid- and retinoid X-receptors inhibit hepatocyte growth factor secretion and expression in U87 human astrocytoma cells.

Retinoids participate in the onset of differentiation, apoptosis and the inhibition of growth in a wide variety of normal and cancerous cells. Several recent reports have shown that hepatocyte growth factor (HGF), and its receptor, c-Met, are expressed at abnormally high levels in various human malignant gliomas and exert a strong proliferative action in an autocrine fashion. These results, consequently, imply that HGF and its receptor may represent a major contributor to the progression of such malignancies. Since astrocytomas are the most frequently occurring glioma, we have shown here that U87 cells - a well-established, human astrocytoma cell line - express both HGF and c-Met, thereby providing a suitable astrocytic tumor model for studying the potential role of HGF, functioning in an autocrine mode, in astrocytic tumorigenesis. Furthermore, we demonstrated the expression of the retinoic acid receptor (RAR) isoforms, RARalpha, -beta and -gamma, as well as the retinoid x-receptor (RxR) isoforms, RxRalpha and -beta, by RT-PCR and western blot analysis in these cells. Since ligands of the RARs and RxRs are known to exert growth inhibitory effects on various tumor cells which include some astrocytomas, we speculated that such effect of retinoids might be mediated via inhibition of HGF secretion in human astrocytoma cells. Indeed, we have shown that the RAR agonists, all-trans retinoic acid (ATRA) and (E)-4-[2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1-propenyl] benzoic acid (TTNPB), inhibited HGF secretion with half maximal inhibition occurring at 3.0 microM and 15 nM, respectively, as did the RxR agonists, 9-cis- and 13-cis retinoic acid (9cRA and 13cRA, respectively), which exerted half-maximal inhibitory effects at 40 and 25 nM, respectively. These actions of the RAR and RxR agonists appear to be exerted at the transcriptional level as assessed by Northern blot analysis. Taken together, our results show for the first time that retinoids, acting via the RAR and RxRs, significantly inhibit both the secretion and expression of HGF, thereby interrupting a potentially highly tumorigenic autocrine loop in astrocytoma cells.