Concanamycin B

The macrolide antibiotic concanamycin B is a highly selective inhibitor (IC50 = 5 nM) of the H(+)-ATPases of the vacuolar system. We have examined the effects of concanamycin B on the constitutive secretory pathway of the human hepatoma cell line, HepG2. In cells exposed to 10 nM concanamycin B, transport from the endoplasmic reticulum to the Golgi occurs at normal rates, as determined by pulse-chase analysis of endoglycosidase H-sensitive product in conjunction with subcellular fractionation experiments. However, intra-Golgi trafficking or Golgi to plasma membrane delivery is significantly impaired. A delay in the onset of secretion of the major secretory proteins, albumin, alpha 1-antitrypsin and transferrin is observed. Processing of N-linked glycans by sialyltransferases is inhibited, resulting in secreted glycoproteins which are modified less extensively. In view of the acidic pH of the trans-Golgi and the trans-Golgi network, these studies suggest that acidification by vacuolar ATPases is critical to achieving timely secretion and correct N-linked glycan modifications of proteins which follow the constitutive secretory pathway.

The antibiotic concanamycin B was found to inhibit oxidized-low-density-lipoprotein(LDL)-induced accumulation of lipid droplets in the macrophage J774 at a concentration of 5-10 nM. Concanamycin B inhibited cholesteryl-ester synthesis from [14C]oleate by 50% at 14 nM without affecting the synthesis of triacylglycerol and polar lipids. Degradation of internalized oxidized 125I-LDL was inhibited by about 80% in cells treated with 25 nM concanamycin B, while cell-surface binding of oxidized 125I-LDL at 4 degrees C, uptake of surface-bound oxidized 125I-LDL and microsomal acyl-CoA:cholesterol acyltransferase activity were not significantly affected by the antibiotic at 25 nM. When J774 cells were treated with 25 nM concanamycin B at 37 degrees C for 60 min, there was a reduction of about 50% in the activity of cell-surface receptors. This reduction appeared to be due to partial trapping of the receptors within the cells. Concanamycin B significantly inhibited ATP-dependent acidification of endosomes and lysosomes of the J774 cells at a concentration of 4 nM. Since acidic condition of these organelles is required for receptor recycling and hydrolysis of lipoproteins, the results demonstrate that concanamycin-B inhibition of oxidized-LDL-induced accumulation of lipid droplets and cholesteryl esters in macrophages J774 is associated with reduced ATP-dependent acidification of these organelles.

Effects of concanamycin B, a specific inhibitor of the vacuolar type H(+)-ATPase (V-ATPase), on the stimulation of bone resorption induced by parathyroid hormone (PTH) were examined in vitro. Concanamycin B was found to inhibit PTH-stimulated osteoclastic pit formation and to suppress the acidification of vacuolar organelles by V-ATPase in the osteoclasts. PTH-stimulated 45Ca release from prelabelled chick embryonic calvariae was also inhibited by concanamycin B in a dose-dependent manner. These results suggest that osteoclastic acidification of lacunae by V-ATPase plays an essential role in mineral dissolution and degradation of the organic matrix during bone resorption.