Mitogen-activated protein kinases (MAPK) and protein kinase B (PKB or Akt) are major signal transduction molecules regulating cell proliferation, differentiation, and apoptosis. We examined how cultured rat aortic vascular smooth muscle cells (VSMC) at different cell densities respond to selected stimuli and how this is reflected in the two distinct (MAPK and Akt) and yet cross-talking signaling pathways. VSMC were cultured to 100% confluence, reaching contact inhibition, and to 60-70% confluence, as sparse, proliferating cells. They were treated with menadione (an intracellular generator of O(-2)) and/or platelet-derived growth factor homodimer BB (PDGF). In sparse cells, menadione or PDGF alone activated ERK, and together the effect was synergistic, whereas in confluent cells menadione's and PDGF's activations of ERK were, at most, additive. Activation of the upstream ERK kinase (MEK-1) paralleled ERK activation except in sparse cells in which the synergistic effects of menadione and PDGF on ERK could not be fully accounted for by MEK-1 activation. Another member of the MAPK family, p38, did not show significant changes. Akt activation by PDGF alone was present under both cell culture conditions; Akt activation is blocked by menadione. Co-incubation with the reducing agent dithiothreitol or calcium chelators (EDTA/EGTA) inhibited partially or completely menadione's effects on MEK/ERK and Akt pathways, as well as menadione's effects on PDGF-induced ERK and Akt activations. These data suggest that in VSMC, the state of cell confluence determines how distinct pathways of MAPK activation cross talk. In addition while PDGF may function as a survival factor by inducing Akt activation, menadione could promote apoptosis by inhibiting PDGF-induced Akt activation independent of cell density. The effects of menadione, but not those of PDGF, are more dependent on the cellular redox status and extracellular calcium.
Copyright 2001 Academic Press.