Initiation of check cell division by trypsin action at the cell surface

Binding and internalization of 125I thrombin in chick embryo fibroblasts: possible role in mitogenesis

Human alpha thrombin acts as a mitogen for cultures of resting chick embryo fibroblasts (CEF) in serum free medium. The use of 125I-labeled thrombin shows that thrombin specifically binds to CEF and that after a lag of approximately 30 to 60 minutes it can not be removed by subsequent exposure to trypsin. The entry of 125I thrombin into the trypsin-insensitive domain is not inhibited to any great extent by excess unlabelled thrombin. The cell-associated thrombin retains its native molecular weight and its catalytic activity toward synthetic amide substrates. It appears to be located in the crude nuclear fraction of homogenized CEF cells. The association of thrombin with CEF is specific, since the non-mitogenic serine protease chymotrypsin is internalized to a much lesser extent than thrombin. The data are discussed in terms of a possible intracellular site for thrombin's mitogenic action.

Transmembrane action of thrombin initiates chick cell division

Thrombin immobilized on polystyrene beads initiates DNA synthesis and cell division in quiescent cultures of chick embryo (CE) cells in serum-free medium. These thrombin beads also produce morphological changes in CE cells similar to those produced by soluble thrombin. The amount of acid-precipitable material released from 125I-thrombin-beads into the culture medium was 60-fold less than the amount of soluble thrombin required to produce an equivalent increase in cell number. Moreover, EM autoradiography of CE cells 10 h after 125I-thrombin-bead addition showed that there was no direct release and accumulation of radioactive material in the cytoplasm of these cells. These results demonstrate that thrombin action at the cell surface is sufficient to initiate proliferation of CE cells.