[Modifications of protein kinase C activity and phosphorylation of lipocortin I in cultures of pig thyroid cells].

When cultured in the absence of thyreostimulin (TSH), thyroid cells lose some of their differentiated functions such as iodide transport and its incorporation into thyroglobulin. In the presence of TSH (0.1 mU/ml), these differentiated functions are preserved ("TSH cells"). The addition of tetradecanoyl phorbol 13 acetate (TPA) inhibits some differentiated functions of the cells and provokes important modifications of bio-signalling pathways. The protein kinase C (pKC) activity, unchanged in "control" and "TSH cells", was dramatically modified in TPA treated cells. After translocation, the pKC activity was down-regulated and the phosphorylation of its endogenous substrates (35-38 kDa) disappeared. Among these substrates, we identified the lipocortin I (LC I) (35 kDa), a phospholipase A2 inhibitory protein related to the Ca2+ binding protein family. By monodimensional electrophoresis (PAGE-SDS) and western-blot, we evidenced the presence of LCI in cytosols and particulate extracts. By 2 dimensional electrophoresis (PAGE-SDS and IEF) and western-blot we identified a phosphorylated and unphosphorylated LCI protein. The phosphorylation of LCI by pKC decreased its isoelectric point from 6.9-6.6. The modifications of pKC activity and LCI phosphorylation and the changes in the bio-signalling pathways can partly account for the loss of differentiation observed in control or TPA treated cells.