Cell cycle regulation of metallothionein in human colonic cancer cells

Elevated levels of metallothionein (MT) found in rapidly growing tissues such as neonatal liver and various types of human tumors have suggested a role for MT in cell proliferation. To further explore this possibility we investigated the concentration of MT in human colonic cancer (HT-29) cells at different stages of proliferation by means of immunocytochemistry and competitive binding. MT is increased in subconfluent proliferating cells relative to growth-inhibited confluent cells, much as it is in growing tissues. Cycling cells synchronized with compactin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, revealed an oscillation of cytoplasmic MT that reached a maximum in successive late G1 phases and at the G1/S transition. Individual phase of the cell cycle were assessed by [3H]thymidine incorporation and by immunofluorescence employing an antibody that detects a nuclear antigen associated with proliferation. An enzyme-linked immunosorbent assay was used to quantify the relative amounts of MT in homogenate supernatants of HT-29 cells. A 2- to 3-fold increase in MT in actively proliferating cells and the regulation of the protein during the mitotic cell cycle point to a physiological role for MT in cellular proliferation and suggest that it may also serve as a proliferation marker.