Mechanism in the sequential control of cell morphology and S phase entry by epidermal growth factor involves distinct MEK/ERK activations

Cell shape plays a role in cell growth, differentiation, and death. Herein, we used the hepatocyte, a normal, highly differentiated cell characterized by a long G1 phase, to understand the mechanisms that link cell shape to g rowth. First, evidence was provided that the mitogen-activated protein kina se kinase (MEK)/extracellular signal-regulated kinase (ERK) cascade is a ke y transduction pathway controlling the hepatocyte morphology. MEK2/ERK2 act ivation in early Gl phase did not lead to cell proliferation but induced ce ll shape spreading and demonstration was provided that this MAPK-dependent spreading was required for reaching G1/S transition and DNA replication. Mo reover, epidermal growth factor (EGF) was found to control this morphogenic signal in addition to its mitogenic effect. Thus, blockade of cell spreadi ng by cytochalasin D or PD98059 treatment resulted in inhibition of EGF-dep endent DNA replication. Our data led us to assess the first third of G1, is exclusively devoted to the growth factor-dependent morphogenic events, whe reas the mitogenic signal occured at only approximately mid-Gl phase. Moreo ver, these two growth factor-related sequential signaling events involved s uccessively activation of MEK2-ERK2 and then MEK1/2-ERK1/2 isoforms. In add ition, we demonstrated that inhibition of extracellular matrix receptor, su ch as integrin beta1 subunit, leads to cell arrest in G1, whereas EGF was f ound to up-regulated integrin beta1 and fibronectin in a MEK-ERK- dependent manner. This process in relation to cytoskeletal reorganization could indu ce hepatocyte spreading, making them permissive for DNA replication. Our re sults provide new insight into the mechanisms by which a growth factor can temporally control dual morphogenic and mitogenic signals during the G1 pha se.