INDUCTION AND CHARACTERIZATION OF METALLOTHIONEIN IN CHICKEN EPIPHYSEAL GROWTH-PLATE CARTILAGE CHONDROCYTES

Following exposure to cadmium or zinc, chickens were sacrificed and th e liver, kidney, and bone epiphyseal growth plates harvested. When cyt osolic extracts of the growth plate cartilage were fractionated by gel filtration chromatography a protein with high metal-binding capacity and low ultraviolet (UV) absorbance eluted in the same position as riv er metallothionein (MT) and a MT standard. Cd or Zn treatment resulted in a 25-fold or 5-fold induction in growth plate MT, respectively. In liver the greatest level of MT induction was seen with short-term Cd exposures. In contrast, MT levels in the growth plate increased as the duration of Cd exposure increased. Induction of MT in growth plate ch ondrocyte cell cultures was observed for media Cd concentrations of gr eater than or equal to 0.1 mu M and Zn concentrations of greater than or equal to 100 mu M. Basal and inducible levels of MT declined throug h the culture period and were lowest in the terminally differentiated mineralized late stages of the culture. Alkaline phosphatase activity was also lowest in the late-stage cu Itu res, while total cellular pro tein increased throughout the culture period. Treatment of chondrocyte s with Zn prior to Cd exposure resulted in a protective induction of M T. Pre-treatment of chondrocytes with dexamethasone resulted in suppre ssed synthesis of MT upon Cd exposure and greater Cd toxicity. Both Cd and Zn resulted in significantly increased levels of MT mRNA in chond rocyte cell cultures. Dexamethasone treatment resulted in an approxima te 2- to 3-fold increase in MT mRNA. This is contrary to the finding t hat MT protein levels were decreased by dexamethasone. The findings su ggest that an increased rate of MT degradation in dexamethasone-treate d and late-stage chondrocyte cultures may be associated with the termi nally differentiated phenotype. (C) 1998 Wiley-Liss, Inc.