Hepatic zinc in metallothionein-null mice following zinc challenge: in vivo and in vitro studies

Abstract

Hepatic zinc uptake and accumulation were compared in freshly isolated and cultured hepatocytes prepared from control (MT+/+) and metallothionein (MT)-null (MT-/-) mice. In freshly isolated hepatocytes, rapid (10-15 min) exchange of 65Zn was proportional to the Zn concentration in the medium and occurred to the same extent in hepatocytes from MT+/+ and MT-/- mice. In 24 h culture experiments with MT+/+ and MT-/- hepatocytes it was shown that approx. 40% of newly acquired cell-associated Zn was attached to the cell surface and not internalized. In MT+/+ and MT-/- hepatocyte cultures, internalized Zn (intZn) increased in proportion to extracellular Zn. Zn accumulation in MT-/- hepatocytes was only 60% that of MT+/+ cells. Addition of 1 microM dexamethasone (Dex) and recombinant mouse interleukin-6 (IL-6; 100 units/ml) increased MT accumulation by 8.6-fold in MT+/+ hepatocytes (at 50 microM Zn) and there was an associated parallel increase in intZn. Dex and IL-6 did not increase intZn in the MT-/- hepatocytes. At 16 h after an intraperitoneal injection of 5 micrograms/g Zn, plasma and urine Zn concentrations were 69 +/- 10 microM and 86 +/- 25 microM respectively in MT-/- mice (n = 10) and 27 +/- 1 microM and 23 +/- 4 microM respectively in MT+/+ controls (n = 9) (P < 0.001, plasma; P < 0.05, urine). Hepatic cytosolic Zn concentrations doubled in MT+/+ mice and increased by a significant 15% in MT-/- mice. There was no increase in hepatic Zn (dry wt.) concentrations or in total hepatic Zn, demonstrating that the increase in cytosolic Zn in MT-/- mice was due to hepatic water loss rather than net Zn uptake. It appears that even at extreme plasma concentrations of Zn, little if any accumulates within the liver when there is no MT available for its sequestration. That this is not fully demonstrated in vitro is probably due to nature of cell culture, where organ architecture is lost and the external protein binding milieu is less complex.