Dephosphorylation of receptor tyrosine kinases as target of regulation by radiation, oxidants or alkylating agents.

Abstract

Several non‐physiologic agents such as radiation, oxidants and alkylating agents induce ligand‐independent activation of numerous receptor tyrosine kinases (RTKs) and of protein tyrosine kinases at the inner side of the plasma membrane (e.g. Dévary et al., 1992; Sachsenmaier et al., 1994; Schieven et al., 1994; Coffer et al., 1995). Here we show additional evidence for the activation of epidermal growth factor receptor (EGFR), and we show activation of v‐ErbB, ErbB2 and platelet‐derived growth factor receptor. As a common principle of action the inducing agents such as UVC, UVB, UVA, hydrogen peroxide and iodoacetamide inhibit receptor tyrosine dephosphorylation in a thiol‐sensitive and, with the exception of the SH‐alkylating agent, reversible manner. EGFR dephosphorylation can also be modulated by these non‐physiologic agents in isolated plasma membranes in the presence of Triton X‐100. Further, substrate (EGFR) and phosphatase have been separated: a membrane preparation of cells that have been treated with epidermal growth factor (EGF) and whose dephosphorylating enzymes have been permanently destroyed by iodoacetamide can be mixed with a membrane preparation from untreated cells which re‐establishes EGFR dephosphorylation. This dephosphorylation can be modulated in vitro by UV and thiol agents. We conclude that RTKs exhibit significant spontaneous protein kinase activity; several adverse agents target (an) essential SH‐group(s) carried by (a) membrane‐bound protein tyrosine phosphatase(s).

Phenolic antioxidants trolox and caffeic acid modulate the oxidized LDL‐induced EGF‐receptor activation

“…This is also consistent with the concept that oxidative stress (possibly H 2 O 2 ) acts as intracellular messenger (Rhee, 1999). Hypothetically, this sustained EGFR activation may result from the inhibition of phosphotyrosine phosphatases (PTPases) which are known to be inhibited by oxidants such as H 2 O 2 (Knebel et al, 1996) or sulphydryl reagents (Monteiro & Stern, 1996). Direct determination of PTPases activity and evaluation of the rate of EGFR dephosphorylation in cells preincubated with oxLDL, suggest that, after 5 h-incubation with oxLDL, PTPase activity and EGFR dephosphorylation are inhibited by around 50%.…”

“…A hypothetical mechanism involved in the late phase of oxLDL-induced EGFR activation may result from peroxidemediated inhibition of protein tyrosine phosphatases (PTPases) implicated in EGFR dephosphorylation (Knebel et al, 1996). The validity of this hypothesis was examined by evaluating, under conditions of the late phase of oxLDLinduced EGFR activation: (i) the activity of PTPases and (ii) the rate of dephosphorylation of EGFR.…”

Solar ultraviolet light activates extracellular signal-regulated kinases and the ternary complex factor in human normal keratinocytes

“…Thus in human keratinocytes, U.V.-A and U.V.-B irradiation are able to activate both MAP and SAP kinase pathways. In two recent publications, an activation of ERK1, ERK2 and JNK in response to U.V.-B irradiation was observed in Hela and A 431 cells (Knebel et al, 1996;Rosette and Karin, 1996). Our results appear to have more physiological relevance since we have used normal human keratinocytes which are directly exposed in vivo to U.V.…”

Age-dependent decline in EGF-induced signaling is independent of intracellular thiols

“…Therefore, it seems likely that an age-dependent difference in a phosphatase may be responsible for the decreased proliferation of hepatocytes from old animals. Interestingly, Knebel et al, (35) have proposed an intriguing hypothesis about the regulation of UV-induced ROS-induced tyrosine phosphorylation by phosphatases. They suggested that reversible SH-group oxidation at the active site of one or several protein tyrosine phosphatases by UV-induced ROS or thiol agents will inhibit the dephosphorylation of the EGF receptor.…”