Cell Density-Dependent Inhibition of Epidermal Growth Factor Receptor Signaling by p38α Mitogen-Activated Protein Kinase via Sprouty2 Downregulation

ABSTRACT Contact inhibition is a fundamental process in multicellular organisms aimed at inhibiting proliferation at high cellular densities through poorly characterized intracellular signals, despite availability of growth factors. We have previously identified the protein kinase p38α as a novel regulator of contact inhibition, as p38α is activated upon cell-cell contacts and p38α-deficient cells are impaired in both confluence-induced proliferation arrest and p27Kip1 accumulation. Here, we establish that p27Kip1 plays a key role downstream of p38α to arrest proliferation at high cellular densities. Surprisingly, p38α does not directly regulate p27Kip1 expression levels but leads indirectly to confluent upregulation of p27Kip1 and cell cycle arrest via the inhibition of mitogenic signals originating from the epidermal growth factor receptor (EGFR). Hence, confluent activation of p38α uncouples cell proliferation from mitogenic stimulation by inducing EGFR degradation through downregulation of the EGFR-stabilizing protein Sprouty2 (Spry2). Accordingly, confluent p38α-deficient cells fail to downregulate Spry2, providing them in turn with sustained EGFR signaling that facilitates cell overgrowth and oncogenic transformation. Our results provide novel mechanistic insight into the role of p38α as a sensor of cell density, which induces confluent cell cycle arrest via the Spry2-EGFR-p27Kip1 network.