Lead may affect glucocorticoid signal transduction in cultured hepatoma cells through inhibition of protein kinase C

Abstract

Specific cellular sites of lead action have not been completely defined. To elucidate the effects of lead exposure on glucocorticoid-mediated signal transduction in hepatic hormonal target tissues, the induction of tyrosine aminotransferase (TAT) specific activity in the H4-IIE-C3 hepatoma cell culture model system was employed. It had been found that lead acetate (3–10 μM) exposure of HTC cells significantly reduced TAT specific activity in a concentration- and time-dependent manner. Two possible molecular targets of the lead-induced effect were investigated: interference with calcium-mediated cellular processes and calcium- and phospholipid-dependent protein kinase C (PKC) activity and isoform-type interactions. Lead acetate treatment (5 μM) reduced TAT specific activity below sodium acetate treated controls by 31%. One-half of the TAT specific activity was recovered by co-treatment with 5 μM lead acetate and 10 mM calcium chloride. As the concentration of lead acetate was increased to 10 μM, interference in calcium-mediated events also increased. Potentiation of glucocorticoid induction by phorbol myristate acetate (PMA) (300 nM) in control cells was 34%, but was abolished by exposure of cells to 10 μM lead acetate (48 h). Treatment with the kinase inhibitor genistein decreased TAT specific activity by 55% and 45% in control and lead acetate exposed cells, respectively. Following treatment with dexamethasone (100 nM), significant increases in both cytosolic and particulate PKC were noted in control cells but not lead acetate exposed cells. Western blot results indicated that lead exposure may increase PKC β and decrease PKC α translocation from cytosolic to particulate fractions, respectively. Taken together, these results suggest that glucocorticoid signal transduction pathways in HTC cells involve calcium-mediated cellular events and PKC isoforms. Exposure of cells to lead results in interference with calcium-mediated events and aberrant modulation of PKC activities. Within hormonal target cells, these may be toxic molecular sites of action of the heavy metal lead and may contribute to the overall toxicity of lead exposure.