We studied the mechanism of transepithelial zinc (Zn) transport using monolayers of Caco-2 cells grown on permeable filter supports. Zn-65 transport could be fitted to a modified Michaelis-Menten equation, which includes a nonsaturable [linear diffusion constant of nonsaturable component (K(d)) = 0.08% . cm-2 . 90 min-1] and a saturable component [upper well Zn concentration at half J(max) (K(t)) = 226 muM and maximal rate of saturable Zn transport (J(max)) = 1.06 nmol . cm-2 . 90 min-1]. Caco-2 cells contained metal-inducible metallothionein (MT) protein and mRNA as well as mRNA for cysteine-rich intestinal protein (CRIP). Cells pretreated with 10 nM 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] for 3 days transported more Zn (159%) than controls (0.48 +/- 0.02 nmol . cm-2 . 90 min-1) when each was incubated with 100 muM Zn for 90 min. This effect was significant after 24 h of 1alpha,25-OH)2D3 pretreatment and continued to increase up to 72 h, with concomitant increases in MT mRNA levels being observed (4-fold by 24 h, 10-fold by 72 h). MT protein levels were only modestly elevated by 72 h 1alpha,25(OH)2D3 treatment (from 0.32 +/- 0.04 to 0.45 +/- 0.03 nmol MT/mg protein). CRIP mRNA levels were reduced by 1alpha,25(OH)2D3 treatment. The lysosome-disrupting agent quinacrine (0.5 mM) inhibited basal Zn transport by 68%, suggesting the possible presence of a lysosome-mediated component for transepithelial Zn transport in Caco-2 cells. 1alpha,25(OH)2D3-stimulated Zn transport was not affected by quinacrine, suggesting that 1alpha,25(OH)2D3-induced Zn transport is distinct from the putative lysosome-mediated Zn transport pathway.
