title: The UV (Ribotoxic) stress response of human keratinocytes involves the unexpected uncoupling of the Ras-extracellular signal-regulated kinase signaling cascade from the activated epidermal growth factor receptor.
abstract: In mammals, UVB radiation is of biological relevance primarily for the cells of the epidermis. We report here the existence of a UVB response that is specific for proliferating human epidermal keratinocytes. Unlike other cell types that also display a UVB response, keratinocytes respond to UVB irradiation with a transient but potent downregulation of the Ras-extracellular signal-regulated kinase (ERK) signaling cascade. The downregulation of ERK precedes a profound decrease in the steady-state levels of cyclin D1, a mediator of the proliferative action of ERK. Keratinocytes exhibit high constitutive activity of the Ras-ERK signaling cascade even in culture medium lacking supplemental growth factors. The increased activity of Ras and phosphorylation of ERK in these cells are maintained by the autocrine production of secreted molecules that activate the epidermal growth factor receptor (EGFR). Irradiation of keratinocytes increases the phosphorylation of EGFR on tyrosine residues Y845, Y992, Y1045, Y1068, Y1086, Y1148, and Y1173 above the basal levels and leads to the increased recruitment of the adaptor proteins Grb2 and ShcA and of a p55 form of the regulatory subunit of the phosphatidylinositide 3-kinase to the UVB-activated EGFR. Paradoxically, however, UVB causes, at the same time, the inactivation of Ras and a subsequent dephosphorylation of ERK. By contrast, the signaling pathway leading from the activated EGFR to the phosphorylation of PKB/Akt1 is potentiated by UVB. The UVB response of keratinocytes appeared to be a manifestation of the more general ribotoxic stress response inasmuch as the transduction of the UVB-generated inhibitory signal to Ras and ERK required the presence of active ribosomes at the time of irradiation.
authors.affiliation: Department of Cell and Developmental Biology, Oregon Health Sciences University, Portland, OR 97201, USA.
authors.forename: Mihail S
authors.initials: MS
authors.lastname: Iordanov
authors.forename: Remy J
authors.initials: RJ
authors.lastname: Choi
authors.forename: Olga P
authors.initials: OP
authors.lastname: Ryabinina
authors.forename: Thanh-Hoai
authors.initials: TH
authors.lastname: Dinh
authors.forename: Robert K
authors.initials: RK
authors.lastname: Bright
authors.forename: Bruce E
authors.initials: BE
authors.lastname: Magun
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D033942:Adaptor Proteins, Vesicular Transport; D019898:Autocrine Communication / Q000528:radiation effects; D002470:Cell Survival / Q000502:physiology / Q000528:radiation effects; D002478:Cells, Cultured; D019938:Cyclin D1 / Q000378:metabolism; D015536:Down-Regulation / Q000528:radiation effects; D004789:Enzyme Activation / Q000528:radiation effects; D066246:ErbB Receptors / Q000378:metabolism*; D051380:GRB2 Adaptor Protein; D006367:HeLa Cells; D006801:Humans; D015603:Keratinocytes / Q000166:cytology / Q000378:metabolism / Q000528:radiation effects*; D008024:Ligands; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation / Q000528:radiation effects; D017346:Protein Serine-Threonine Kinases; D021122:Protein Subunits; D011506:Proteins / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D012270:Ribosomes / Q000378:metabolism*; D055765:Shc Signaling Adaptor Proteins; D015398:Signal Transduction / Q000528:radiation effects*; D000071425:Src Homology 2 Domain-Containing, Transforming Protein 1; D014466:Ultraviolet Rays; D018631:ras Proteins / Q000378:metabolism*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D033942:Adaptor Proteins, Vesicular Transport; D019938:Cyclin D1; D066246:ErbB Receptors; D051380:GRB2 Adaptor Protein; D008024:Ligands; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D021122:Protein Subunits; D011506:Proteins; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D055765:Shc Signaling Adaptor Proteins; D000071425:Src Homology 2 Domain-Containing, Transforming Protein 1; D018631:ras Proteins; C495948:GRB2 protein, human; C528024:SHC1 protein, human; C494918:AKT1 protein, human