Question
In vitro studies on isolated human fibroblasts indicate that exposure of these cells to platelet-derived growth factor (PDGF) leads to an increase in the rate of cell division and the secretion of epidermal growth factor (EGF).
Use the following pieces of experimental data to sketch a flow diagram of the cell signalling pathways that control these processes.
Stimulation of fibroblasts with PDGF causes tyrosine phosphorylation of the PDGF receptor, the adaptor protein Shc and MAP kinase. MAP kinase also becomes phosphorylated on a threonine residue that is required for kinase activation.
The general tyrosine kinase inhibitor Tyrphostin A23, inhibits both PDGF-stimulated increase in cell division and EGF secretion.
The MAP kinase inhibitor PD98059 inhibits PDGF-stimulated cell division but PDGF-stimulated EGF secretion is unaffected.
Only EGF secretion is inhibited by the PI 3-kinase inhibitors, LY294002 and wortmannin.
PDGF treatment also causes phosphorylation and inactivation of glycogen synthase kinase 3 (GSK3). This effect is inhibited by LY294002 and wortmannin.
A fibroblast cell line that lacks the adaptor protein Shc fails to divide in response to PDGF. In these cells MAP kinase fails to become phosphorylated and activated by PDGF. EGF secretion in response to PDGF in these cells is normal.
A mutant form of Ras that fails to bind to GTP is expressed in a different fibroblast cell line. These cells fail to divide in response to PDGF, although EGF secretion is normal. In these cells the PDGF receptor becomes tyrosine phosphorylated, as does Shc, but MAP kinase fails to become phosphorylated and activated.
