Studies on the effect of ketoconazole on the fusion of L6 myoblasts

Ketoconazole and phorbol myristate acetate regulate osteoclast precursor fusion in primary murine marrow culture

Osteoclast formation requires both precursor proliferation and then fusion into a multinuclear cell. These processes can be separated in primary murine marrow culture where osteoclastogenesis is stimulated by 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). Here we investigate the regulation of precursor fusion. Ketoconazole, an agent known to inhibit cell fusion, added during the fusion period (days 5-6), dose-dependently inhibited formation of tartrate-resistant acid phosphatase+ (TRAP+) multinucleated cells (TRAP+MNCs), maximally at 62 +/- 4% (n = 10). TRAP+MNCs in cultures exposed to 48 h of ketoconazole (1 microM) during fusion had fewer nuclei compared with control (11.7 +/- 0.6 vs. 15.1 +/- 0.9). This inhibitory effect was completely reversed 24 h after removal of ketoconazole from culture. Phorbol myristate acetate (PMA) stimulated TRAP+MNC formation when given during the last 12 h of culture (2.3 +/- 0.2 fold compared with control). This increased formation was unaffected by the addition of hydroxyurea and accompanied by an increase in nuclei per TRAP+MNC (15.5 +/- 0.9 vs. 13.1 +/- 0.6). Finally, staurosporine decreased TRAP+MNC formation in the presence or absence of PMA, implying that protein kinase C is involved in fusogenic processes. Regulation of fusion appears to be another mechanism by which bone remodeling can be modulated in vivo.

Studies on the effect of mevinolin (lovastatin) and mevastatin (compactin) on the fusion of L6 myoblasts

The effect of mevastatin and mevinolin on the fusion of L6 myoblasts was studied. Both compounds were present inhibitors of myoblast fusion at concentrations as low as 0.25 microM, but fusion was restored when the inhibitors were removed. Both compounds resulted in decreased binding of conA and WGA to cell surface oligosaccharides showing they were causing a reduction in N-linked cell surface glycoproteins. There was a reduction in creatine phosphokinase activities in the presence of both compounds showing that they were affecting biochemical differentiation. The presence of both compounds inhibited the incorporation of labeled mannose from GDP-mannose into lipid-sugar and N-linked glycoprotein, but the inhibition was reversed by addition of exogenous dolichol phosphate to the incorporation mixture. The main conclusion from these studies is that mevinolin and mevastatin are inhibiting myoblast fusion by affecting the synthesis of fusogenic cell surface N-linked glycoproteins probably by affecting the synthesis of dolichol phosphate containing oligosaccharides that are required as intermediates in N-linked glycoprotein biosynthesis.