Inhibition of Integrin-mediated Crosstalk with Epidermal Growth Factor Receptor/Erk or Src Signaling Pathways in Autophagic Prostate Epithelial Cells Induces Caspase-independent Death

Abstract

In vivo in the prostate gland, basal epithelial cells adhere to laminin 5 (LM5) via ␣3␤1 and ␣6␤4 integrins. When placed in culture primary prostate basal epithelial cells secrete and adhere to their own LM5-rich matrix. Adhesion to LM5 is required for cell survival that is dependent on integrin-mediated, ligand-independent activation of the epidermal growth factor receptor (EGFR) and the cytoplasmic tyrosine kinase Src, but not PI-3K. Integrin-mediated adhesion via ␣3␤1, but not ␣6␤4 integrin, supports cell survival through EGFR by signaling downstream to Erk. PC3 cells, which do not activate EGFR or Erk on LM5-rich matrices, are not dependent on this pathway for survival. PC3 cells are dependent on PI-3K for survival and undergo caspase-dependent death when PI-3K is inhibited. The death induced by inhibition of EGFR or Src in normal primary prostate cells is not mediated through or dependent on caspase activation, but depends on the induction of reactive oxygen species. In addition the presence of an autophagic pathway, maintained by adhesion to matrix through ␣3␤1 and ␣6␤4, prevents the induction of caspases when EGFR or Src is inhibited. Suppression of autophagy is sufficient to induce caspase activation and apoptosis in LM5-adherent primary prostate epithelial cells. INTRODUCTIONIn vivo, the precise regulation of epithelial cell homeostasis involves interactions between cells and their microenvironment. Cells receive signals from both the extracellular matrix in the basement membrane and soluble factors secreted by the stroma that precisely control the timing of cell division, growth arrest, differentiation, and survival. Integrins on the cell surface that interact with laminin 5 (LM5) in the extracellular matrix, such as ␣3␤1 and ␣6␤4, are critically involved in mediating survival. Genetic loss of LM5, or its receptors ␣3, ␣6, or ␤4 integrins, in vivo results in cell detachment and induction of caspase-mediated apoptosis, even in the presence of soluble factors (Ryan et al., 1999;DiPersio et al., 2000). This detachment-induced form of apoptosis has been termed anoikis (Frisch and Screaton, 2001). In vitro anoikis can be rescued by expression of an activated form of FAK, Rac, or Akt (Frisch et al., 1996;Rytomaa et al., 2000;Coniglio et al., 2001), suggesting that integrin-mediated signaling through these molecules is required to maintain cell survival. However, studies in which specific signaling pathways are inhibited while integrins are still engaged suggest alternative pathways, such as Ras/Erk or Jnk, are required for integrin-mediated survival (Almeida et al., 2000;Manohar et al., 2004). Whether signaling from multiple pathways is involved in mediating integrin-dependent survival and whether different pathways are unique to specific cell types have not been extensively investigated.In addition to classical caspase-mediated apoptosis, such as that observed during anoikis, several other mechanisms of cell death have been described (Melino et al., 2005). Other forms of cell death include caspase-independe...

Activation of EGFR on monocytes is required for human cytomegalovirus entry and mediates cellular motility

“…More genes were regulated by the intracellular signaling of EGFR when compared with genes regulated by the extracellular signaling of EGFR ( Fig. S6; R ϭ 0.78 vs. 0.93), which is likely attributable to the cross-talk phosphorylation of the cytoplasmic domain of EGFR by integrins activated during HCMV entry (22,23,39). We suggest that HCMV initiates a multireceptor activation process during viral entry that depends on the expression of cell-specific HCMV receptors on different cell types.…”

TEAD1 and c-Cbl are novel prostate basal cell markers that correlate with poor clinical outcome in prostate cancer

“…Interestingly, however, knockdown of c-Cbl in PC3 cells decreased proliferation. Importantly, c-Cbl also functions as an adaptor protein in signal transduction, for example downstream of integrins in which c-Cbl has been shown to interact with c-Src (Edick et al, 2007). Further investigation is needed to establish the role of c-Cbl in regulating prostate cancer cell proliferation.…”

Extracellular matrix regulation of autophagy

“…In contrast, decreased autophagosome formation is observed in starved prostate epithelial cells (PECs) upon inhibiting integrin function. Addition of function blocking antibodies directed against α3 integrin result in decreased autophagosome formation in starved PECs suggesting ECM attachment is required to maintain autophagy [20]. However, in this study, autophagy levels are assessed by measuring GFP-LC3 puncta, which does not account for autophagosome maturation and turnover in the lysosome.…”