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.

Suppression of UVB-induced phosphorylation of mitogen-activated protein kinases and nuclear factor kappa B by green tea polyphenol in SKH-1 hairless mice

“…JNK pathways are also involved in the phosphorylation and activation of several transcription factors, including c-Jun (Derijard et al, 1994), ATF-2 (Read et al, 1997), Elk-1 (Cavigelli et al, 1995), and p53 (Milne et al, 1995). Studies have demonstrated that cell exposure to UV irradiation causes activation of the MAPK pathway (Chen et al, 2002;Chouinard et al, 2002;Iordanov et al, 2002;She et al, 2002). JNK1/2 and p38 kinases are potently activated by various forms of stress, such as UV, heat shock, and inflammation Figure 7 Inhibitory effect of green tea polyphenol on UVBinduced activation of IKKa, and phosphorylation and degradation of IkBa in SKH-1 hairless mice.…”

“…of 12 mice (*Po0.001 vs UVB). ns, nonspecific Suppression of UVB effects in mouse skin by GTP F Afaq et al (Derijard et al, 1994;Chen et al, 2002;Chouinard et al, 2002;Iordanov et al, 2002). Experimental studies have shown that ERK1/2 and p38 are involved in the transcriptional activation of NF-kB (Adderley and Fitzgerald, 1999;Carter et al, 1999).…”

Homeodomain-interacting protein kinase-2 mediates CtBP phosphorylation and degradation in UV-triggered apoptosis

“…The spectral characterization of the UV-B source and the mode of the irradiation of cells with UV-B are described in ref. 13. Proteasome inhibitor MG-132 was from Calbiochem.…”

Fas Ligand-induced Proinflammatory Transcriptional Responses in Reconstructed Human Epidermis

“…Inflammation in the Skin-We have previously reported that the basal levels of ERK activity in HEKn or HEKn-E6/E7 in tissue culture were determined by the autocrine secretion of ligands that bind to the EGFR (20). Interfering with either the extracellular ligand-binding portion of EGFR by means of the LA-1 antibody or the intracellular kinase domain of EGFR reduced the basal phosphorylation of ERK1 and ERK2 to undetectable levels (20).…”

“…were established and propagated as described (20,21). For RHE, we employed the method of Poumay et al (22), with modifications as described in Ref.…”