Calcitonin Stimulates Lysosomal Enzyme Release and Uptake in LLC-PK1 Cells

Abstract

Abstract. Renal tubular targeted hormones increase urinary excretion of a lysosomal enzyme, N-acetyl-β-D-glucosaminidase (NAG). To elucidate the mechanism of this event, the calcitonin effect on NAG handling by LLC-PK1 cells was examined. Calcitonin (1 nM to 1 μM), phorbol myristate (10 nM to 1 μM), and ionomycin (1 to 10 μM) promoted NAG release without any increase in lactate dehydrogenase release or any reduction of mitochondrial dehydrogenase activity. Treatment with 100 nM calphostin C or 50 μM KN-93 partially reversed the calcitonin effect on NAG release. Calcitonin promoted secretion of fluorescence ceramide, a reporter of protein transport from Golgi apparatus to cell surface. Calcitonin-stimulated NAG release was partially inhibited by 10 μg/ml brefeldin A, a blocker of protein transport through the Golgi apparatus. Calcitonin accelerated cellular uptake of exogenous NAG, which was inhibited by low temperature, 0.1 mM monodansyl cadaverine (receptor-mediated endocytosis inhibitor), and 10 mM mannose-6-phosphate. Furthermore, calcitonin promoted progression of intracellular membranes stained by a fluorescence membrane marker, styryl pyridinium dye, from cell periphery to perinuclear regions (commonly referred to as recycling vesicles) and increased dye release from preloaded cells. Fluorescence release from the cells preloaded with FITC-labeled NAG or albumin was also stimulated by calcitonin. These calcitonin effects on endocytotic and re-exocytotic pathways were inhibited by 100 nM cytochalasin D, 100 nM nocodazole, 0.1 to 1 μM bafilomycin A1, or 0.1 mM monodansyl cadaverine. Increased urinary NAG excretion has been considered to reflect renal tubular damage. However, it was demonstrated here that stimulation of secretory and recycling pathways may be an alternative mechanism for calcitonin-induced enzymuria, which will become a new indicator of renal tubular response to this hormone.