We have isolated and characterized a new human cDNA, coding for a protein kinase, related to the protein kinase C (PKC) gene family. Although this protein kinase shares some homologous sequences and structural features with the four members of the PKC family initially isolated (alpha, beta I, beta II, and gamma), it shows more homology with the recently described PKC-related subfamily, encoded by the cDNAs delta, epsilon, and zeta. The transcript for this gene product, termed PKC-L, is most abundant in lung tissue, less expressed in heart and skin tissue, and exhibited very low expression in brain tissue. Thus, its tissue distribution is different from that described for other mammalian members of the PKC gene family, their expression being enriched in brain tissues. PKC-L is also expressed in several human cell lines, including the human epidermoid carcinoma line A431. The ability of phorbol esters to bind to and stimulate the kinase activity of PKC-L was revealed by introducing the cDNA into COS cells.

Cultured NIH‐3T3 cells devoid of endogenous EGF‐receptors were transfected with cDNA constructs encoding normal human EGF‐receptor and with a construct encoding an insertional mutant of the EGF‐receptor containing four additional amino acids in the kinase domain after residue 708. Unlike the wild‐type receptor expressed in these cells which exhibits EGF‐stimulatable protein tyrosine kinase activity, the mutant receptor lacks protein tyrosine kinase activity both in vitro and in vivo. Despite this deficiency the mutant receptor is properly processed, it binds EGF and it exhibits both high and low affinity binding sites. Moreover, it undergoes efficient EGF‐mediated endocytosis. However, EGF fails to stimulate DNA synthesis and is unable to stimulate the phosphorylation of S6 ribosomal protein in cells expressing this receptor mutant. Hence, it is proposed that the protein tyrosine kinase activity of EGF‐receptor is essential for the initiation of S6 phosphorylation and for DNA synthesis induced by EGF. However, EGF‐receptor processing, the expression of high and low affinity surface receptors and receptor internalization, require neither kinase activity nor receptor autophosphorylation. Interestingly, phorbol ester (TPA) fails to abolish the high affinity state and is also unable to stimulate the phosphorylation of this receptor mutant. This result is consistent with the notion that kinase‐C phosphorylation of EGF‐receptor is essential for the loss of high affinity EGF‐receptors caused by TPA.

Here we describe the Achilles' Heel Method (AHM), a new function-based approach for identification of inhibitors of signaling pathways, optimized for human cells. The principle of AHM is the identification of 'sensitizing' cDNAs based on their decreased abundance following selection. As a proof of principle, we have employed AHM for the identification of Fas/CD95/APO-1 pathway inhibitors. HeLa cells were transfected with an antisense cDNA expression library in an episomal vector followed by selection with a suboptimal dose of the apoptotic inducer. Antisense inactivation of Fas inhibitors rendered the cells more sensitive to apoptosis resulting in their preferential death and consequent loss of their sensitizing episomes that were identified by subtraction. We show that the resulting products were enriched for sensitizing cDNAs as seven out of eight candidates tested were confirmed as inhibitors of Fas-induced killing either by transfection or by pharmacological inhibition. Furthermore, we demonstrate by multiple approaches that one candidate, NF-E2 related factor 2 (Nrf2), is an inhibitor of Fas-induced apoptosis. Inactivation of Nrf2 by antisense or by a membrane permeable dominantnegative polypeptide sensitized cells while overexpression of Nrf2 protected cells from Fas-induced apoptosis. In addition, dicumarol, an inhibitor of the phase II detoxifying enzyme NQO1, a downstream target of Nrf2, sensitized cells. Nrf2 induces the production of Glutathione (GSH) and we demonstrated that N-acetyl L-cysteine (NAC), a precursor to GSH, protected cells from Fas-mediated killing. Taken together, AHM is a powerful approach for the identification of inhibitors of a signaling pathway with a low rate of false positives that opens new avenues for function profiling of human genes and discovery of new drug targets.

The tumor promoter phorbol ester (TPA) modulates the binding affinity and the mitogenic capacity of the epidermal growth factor (EGF) receptor. Moreover, TPA-induced kinase C phosphorylation occurs mainly on Thr-654 of the EGF receptor, suggesting that the phosphorylation state of this residue regulates ligand-binding affinity and kinase activity of the EGF receptor. To examine the role of this residue, we prepared a Tyr-654 EGF receptor cDNA construct by in vitro site-directed mutagenesis. Like the wild-type receptor, the mutant receptor exhibited typical high-and low-affinity binding sites when expressed on the surface of NIH 3T3 cells.Moreover, TPA regulated the affinity of both wild-type and mutant receptors and stimulated receptor phosphorylation of serine and threonine residues other than Thr-654. The addition of TPA to NIH 3T3 cells expressing a wild-type human EGF receptor blocked the mitogenic capacity of EGF. However, this inhibition did not occur in cells expressing the Tyr-654 EGF receptor mutant. In the latter cells, EGF was able to stimulate DNA synthesis even in the presence of inhibitory concentrations of TPA. While phosphorylation of sites other than Thr-654 may regulate ligand-binding affinity, the phosphorylation of Thr-654 by kinase C appears to provide a negative control mechanism for EGF-induced mitogenesis in mouse NIH 3T3 fibroblasts.The mitogenic polypeptide epidermal growth factor (EGF) mediates its biological responses by activating an integral membrane glycoprotein denoted as the EGF receptor (reviewed in references 5 and 26; J. Schlessinger, Biochemistry, in press). Scatchard analysis of binding of 125I-EGF to intact cells reveals two affinity classes of EGF toward the receptor (16). The addition of the phorbol ester (TPA) to cells abolishes the high-affinity binding sites (4,16,18,27) and reduces the protein-tyrosine kinase activity of the receptor molecule (6, 9, 15). Moreover, TPA-induced kinase C phosphorylation of the EGF receptor occurs mainly on Thr-654 (7, 13), suggesting that the phosphorylation of this residue regulates the ligand-binding affinity and the kinase activity of the EGF receptor. The phosphorylation state of Thr-654 may also play a role in endocytosis as an Ala-654 mutant of the EGF receptor fails to internalize in response to TPA, while the native EGF receptor undergoes TPA-induced internalization (19). We have shown that mutations in the cytoplasmic domain of the EGF receptor can cause the loss of high-affinity EGF-binding sites (20,23). Hence, the so-called effect of TPA on EGF receptor internalization (2, 19, 26; Schlessinger, in press) most probably represents TPA-induced loss of high-affinity receptors rather than receptor internalization (26; Schlessinger, in press). The physiological role of the high-and low-affinity receptors is not known (26). Interestingly, growth factors other than EGF such as platelet-derived growth factor and bombesin, which bind to their own distinct membrane receptors, are also able to abolish the high-affinity state of the EGF r...

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