Incomplete epidermal differentiation of A431 epidermoid carcinoma cells

Summary A431 malignant keratinocytes, although derived from a muco-cutaneous carcinoma of the vulva, fail to achieve terminal epidermal differentiation in culture as shown by their inability to form cornified envelopes. Even after culture in a serum-free medium (MCDB 153) containing no retinoic acid and a high (10⁻³ M) calcium concentration (conditions known to facilitate epidermal differentiation), the cells do not become competent as shown by the fact that subsequent treatment with a calcium ionophore is unable to provoke the formation of cornified envelopes. Nevertheless, A431 cells are able to synthesize the envelope precursor involucrin. The block in formation of cornified envelopes is thus not due to a lack in involucrin. The results described here suggest that the absence of cross-linking of this molecule is due to a lowered epidermal membrane-bound transglutaminase activity in A431 cells, enhances involucrin accumulation in these cells, although in normal human keratinocytes it stimulates growth and reduces involucrin synthesis. These results suggest that involucrin synthesis is triggered by the arrest of growth. EDITOR'S STATEMENT The A431 cell line has been used extensively in the study of EGF receptors and effects, and recently has been employed in studies of surface membrane receptors for other factors, as well as studies of extracellular matrix synthesis and deposition and tumor promoter activities. The expanding use of A431 cells calls for a more thorough understanding of the cell type it represents and the degree to which it represents a general in vitro model of normal or neoplastic epidermal cells. This article addresses some of these questions.     

Augmented desensitization to epidermal growth factor (EGF) immediate actions: a novel mechanism for altered EGF growth response in mutant A431 cells.

Epidermal growth factor (EGF) may either stimulate or inhibit cell growth. To elucidate the mechanism of these varied effects, we compared EGF action in parental A431 cells in which cell growth is inhibited, and clone 15, a mutant of these cells resistant to EGF growth inhibition. In both lines, EGF receptor was present in similar concentrations and underwent tyrosine phosphorylation to the same extent. Likewise, in both lines, acute exposure to EGF stimulated an increase in free cytoplasmic [Ca2+], as well as a similar increase in phosphorylation of lipocortin 1, a major substrate for the EGF receptor kinase whose phosphorylation is calcium-dependent. On the other hand, pretreatment of clone 15 cells with EGF for 72 h abolished EGF-induced phosphorylation of lipocortin 1 and led to a loss of the increase in cytoplasmic free [Ca2+], whereas no such desensitization was seen in the parental A431 cells. These data indicate a link between EGF-induced increase in cytoplasmic calcium, lipocortin phosphorylation, and cell growth and suggest that differences in mechanisms of desensitization to these immediate actions of EGF may lead to altered growth response to this hormone.     

Relation of epidermal growth factor receptor concentration to growth of human epidermoid carcinoma A431 cells

The relation between the concentration of epidermal growth factor (EGF) receptor/kinase and effects of EGF on cell proliferation has been studied using variant A431 cells and antagonist anti-EGF receptor monoclonal antibodies. Clonal A431 cell variants selected for escape from the EGF-mediated growth inhibition of parental A431 cells all have reduced concentrations of EGF receptor/kinase; Harvey sarcoma virus-transformed A431 cells, which have escaped from EGF-mediated growth inhibition, also have reduced EGF receptors. Three clonal variants which have reacquired EGF-mediated growth inhibition have 2- to 4-fold more EGF receptor than their respective parent variant. A biphasic response with stimulation at low and inhibition at high concentrations of EGF was especially evident in revertants of clone 29. Three separate antagonist monoclonal anti-EGF receptor antibodies block the growth inhibitory effects of EGF and uncover EGF-mediated growth stimulation. These studies indicate that in A431 cell variants a continuum of ligand-activated EGF receptors determines proliferative responses from low concentrations of active receptors under basal conditions to intermediate concentrations causing growth stimulation to high concentrations, causing inhibition of cell proliferation.     

PKCdelta-dependent deubiquitination and stabilization of Gadd45 in A431 cells overexposed to EGF.

Epidermal growth factor (EGF) receptor-overexpressing p53-deficient A431 cells response to toxic dose of EGF by G1 arrest and apoptosis was studied. We previously reported an increased expression of growth arrest and DNA-damage-inducible gene, Gadd45, in EGF-overexposed A431 cells. The mechanism for this induction was increased half-lives of mRNA and protein. In this study, using phorbol ester (a PKC activator) and specific inhibitors of PKC isoforms, we showed that protein kinase C-delta (PKCdelta) was involved in the increase of Gadd45 protein stability. We further demonstrated that Gadd45 is ubiquitinated and is regulated by proteolysis. While EGF induced ubiquitination of total cellular proteins, there was a decrease in Gadd45 ubiquitination, which could be inhibited by Rottlerin, a PKCdelta-specific inhibitor. These results suggest that an increase in Gadd45 stability may involve PKCdelta-dependent ubiquitin-proteasome pathway.     

Stimulation and inhibition of growth by EGF in different A431 cell clones is accompanied by the rapid induction of c-fos and c-myc proto-oncogenes

Stimulation of quiescent fibroblasts to growth by polypeptide growth factors is accompanied by the rapid induction of c-fos and c-myc proto-oncogenes. In contrast to fibroblasts, A431 cells respond to epidermal growth factor (EGF) with a decreased growth rate. Here we report that, in spite of its growth inhibitory effect, EGF rapidly induces transient expression of c-fos mRNA, followed by the synthesis of nuclear c-fos protein. In addition, EGF treatment resulted in elevated levels of c-myc expression. Practically identical results were obtained with variant A431 clones that are resistant to the inhibitory effect of EGF on cell proliferation. These observations suggest that in A431 cells c-fos and c-myc induction is a primary consequence of growth factor-receptor interaction. Indeed, efficient induction of both genes was also observed with cyanide bromide-cleaved EGF, which has previously been shown to be non-mitogenic but able to trigger early events induced by EGF. We observed strong induction of c-fos and to a lesser extent of c-myc also by TPA, and by the calcium ionophore A23187, indicating an important role for kinase C in proto-oncogene activation by growth factors.     

Role of epidermal growth factor-stimulated protein kinase in control of proliferation of A431 cells

Epidermal growth factor (EGF), which stimulates tyrosine-specific protein kinase activity both in vivo and in vitro, inhibits proliferation of A431 human epidermoid carcinoma cells. After mutagenesis clonal cell lines that were resistant to the growth inhibitory effects of EGF were selected. All six variants examined contained decreased EGF-stimulated protein kinase. The number of EGF receptors in variant cells decreased in parallel with EGF-stimulated protein kinase activity so that the specific activity of EGF-stimulated protein kinase per EGF receptor remained constant in variant cell lines with up to tenfold reductions in both activities. This result suggests that both EGF binding and kinase activities reside in the same or closely coupled molecules. The effect of EGF on growth of two resistant variants was examined in detail. Clone 29 contains approximately 50% and clone 4 contains approximately 20% of the EGF-stimulated protein kinase activity of the parental A431 cell line. In serum-supplemented medium, EGF stimulated proliferation of clone 29 but did not affect growth of clone 4. In a 1:1 mixture of DME and F-12 medium without serum, EGF caused both clone 29 and clone 4 to grow as well as in 10% serum. These variants, which were selected for resistance to the growth inhibitory effects of EGF, thus exhibit a strong mitogenic response to EGF. This result suggests that resistance to the growth inhibitory effect of EGF may involve both a decrease in EGF-stimulated protein kinase and an alteration in the response pathway.     

EGF induces cell cycle arrest of A431 human epidermoid carcinoma cells

The human carcinoma cell line A431 is unusual in that physiologic concentrations of epidermal growth factor (EGF) inhibit proliferation. In the presence of 5-10 nM EGF proliferation of A431 cells is abruptly and markedly decreased compared to the untreated control cultures, with little loss of cell viability over a 4-day period. This study was initiated to examine how EGF affects the progression of A431 cells through the cell cycle. Flow cytometric analysis of DNA in EGF-treated cells reveals a marked change in the cell cycle distribution. The percentage of cells in late S/G2 increases and early S phase is nearly depleted. Since addition of the mitotic inhibitor vinblastine causes accumulation of cells in mitosis and prevents reentry of cells into G1, it is possible to distinguish between slow progression through G1 and G2 and blocks in those phases. When control cells, not treated with EGF, are exposed to vinblastine, the cells accumulate mitotic figures, as expected, and show progression into S, thus diminishing the number of cells in G1. In contrast, no mitotic figures are found among the EGF-treated cells in the presence or absence of vinblastine, and progression from G1 into S is not observed, as the number of cells in G1 remains constant. These results suggest that there are two EGF-induced blocks in cell cycle transversal; one is in late S and/or G2, blocking entry into mitosis, and the other is in G1, blocking entry into S phase. After 24 hours of EGF treatment, DNA synthesis is reduced to less than 10% compared to untreated controls as measured by the incorporation of [3H]thymidine or BrdU. In contrast, protein synthesis is inhibited by about twofold. Although inhibition of protein synthesis is less extensive, it occurs 6 hours prior to an equivalent inhibition of DNA synthesis. The rapid decrease in protein synthesis may result in the subsequent cell cycle arrest which occurs several hours later.     

Prolonged induction of p21Cip1/WAF1/CDK2/PCNA complex by epidermal growth factor receptor activation mediates ligand-induced A431 cell growth inhibition

Proliferation of some cultured human tumor cell lines bearing high numbers of epidermal growth factor (EGF) receptors is paradoxically inhibited by EGF in nanomolar concentrations. In the present study, we have investigated the biochemical mechanism of growth inhibition in A431 human squamous carcinoma cells exposed to exogenous EGF. In parallel, we studied a selected subpopulation, A431-F, which is resistant to EGF-mediated growth inhibition. We observed a marked reduction in cyclin-dependent kinase-2 (CDK2) activity when A431 and A431-F cells were cultured with 20 nM EGF for 4 h. After further continuous exposure of A431 cells to EGF, the CDK2 activity remained at a low level and was accompanied by persistent G1 arrest. In contrast, the early reduced CDK2 activity and G1 accumulation in A431-F cells was only transient. We found that, at early time points (4-8 h), EGF induces p21Cip1/WAF1 mRNA and protein expression in both EGF-sensitive A431 cells and EGF-resistant A431-F cells. But only in A431 cells, was p21Cip1/WAF1 expression sustained at a significantly increased level for up to 5 d after addition of EGF. Induction of p21Cip1/WAF1 by EGF could be inhibited by a specific EGF receptor tyrosine kinase inhibitor, tyrphostin AG1478, suggesting that p21Cip1/WAF1 induction was a consequence of receptor tyrosine kinase activation by EGF. We also demonstrated that the increased p21Cip1/WAF1 was associated with both CDK2 and proliferating cell nuclear antigen (PCNA). Taken together, our results demonstrate that p21Cip1/WAF1 is an important mediator of EGF-induced G1 arrest and growth inhibition in A431 cells.     

Human keratinocyte growth factor activity on proliferation and differentiation of human keratinocytes: differentiation response distinguishes KGF from EGF family.

Human keratinocyte growth factor (KGF) is an epithelial cell specific mitogen which is secreted by normal stromal fibroblasts. In the present studies, we demonstrate that KGF is as potent as EGF in stimulating proliferation of primary or secondary human keratinocytes in tissue culture. Exposure of KGF- or EGF-stimulated keratinocytes to 1.0 mM calcium, an inducer of differentiation, led to cessation of cell growth. However, immunologic analysis of early and late markers of terminal differentiation, K1 and filaggrin, respectively, revealed striking differences in keratinocytes propagated in the presence of these growth factors. With KGF, the differentiation response was associated with expression of both markers whereas their appearance was retarded or blocked by EGF. TGF alpha, which also interacts with the EGF receptor, gave a similar response to that observed with EGF. These findings functionally distinguish KGF from the EGF family and support the role of KGF in the normal proliferation and differentiation of human epithelial cells.     

Epidermal growth factor induces terminal differentiation in human epidermoid carcinoma cells

Previous studies have reported that the proliferation of A431 cells, a human squamous cell carcinoma cell line, was stimulated by picomolar epidermal growth factor (EGF) but inhibited by nanomolar EGF. This biphasic dose-response phenomenon is not observed in normal human epithelial cells where nanomolar EGF is usually mitogenic. We have examined the effects of inhibitory and stimulatory concentrations of EGF on the growth and differentiation of A431 cells. In the presence of 100 pM EGF, A431 cells showed a mild increase in growth rate (129% of control) compared to cells grown in the absence of EGF. At 10 nM EGF, growth inhibition to 63% of control was observed. EGF at 10 nM stimulates a twofold increase both in cornified envelope formation and in epidermal transglutaminase activity, suggesting that high concentrations of EGF induce terminal differentiation in A431 cells. Mitogenic concentrations of EGF (100 pM) had no significant effect on these differentiation markers. Chronic exposure of A431 cells to 20 or 50 nM EGF resulted in EGF-resistant A431 variants that are neither growth arrested nor induced to terminally differentiate by 10 nM EGF. Removal of EGF from the growth medium of the EGF-resistant cells resulted in the reversion of these cells back to the wild-type A431 biphasic response pattern within 2 weeks. Our results suggest that A431 cells have the capacity to non-mutatively alter their response pattern to EGF in vitro to maintain themselves in a state of optimum proliferation and away from terminal differentiation. © 1993 Wiley-Liss, Inc.     

EGF induces differentiation of an IL-3-dependent cell line expressing the EGF receptor.

Interleukin 3 (IL-3) is a T cell-derived lymphokine that supports the growth and development of hematopoietic cells. Tyrosine phosphorylation has been suggested to play an important role in IL-3-dependent cell proliferation. To test whether a growth factor receptor carrying a tyrosine kinase can be functional in IL-3 dependent cells, we used a retroviral vector to introduce the human EGF receptor into a murine IL-3-dependent pre-mast cell line, IC2. The EGF receptors expressed on the infected clones bind EGF with both high and low affinities. EGF stimulates the infected cells for a short term growth response. In the presence of IL-3 and EGF, infected clones differentiate into more mature mast cells characterized by increases in intracellular granulation and histamine content. This differentiation is reversible when EGF is removed. EGF induces tyrosine phosphorylation of several cellular proteins and the expression of oncogenes c-fos and c-myc, in a manner analogous to IL-3 stimulation. These results indicate that the EGF receptor is functional in the pre-mast IC2 cells; EGF can support short-term proliferation and activates the signals that induce cell differentiation. Thus, EGF receptor-expressing IC2 cells provide a unique cellular system for in vitro study of mast cell differentiation.     

Rap1, a small GTP-binding protein is upregulated during arrest of proliferation in human keratinocytes

Rap1 is a small GTP-binding protein (SMG) that exists in two 95% homologous isoforms, rap1A and rap1B. The functions of the rap1 proteins are not well understood. In this report we examined expression and function of rap1 in primary (HOKs) and immortalized (IHOKs) human oral keratinocytes under different growth conditions. In HOKs, rap1 increased with passage number, suggesting a role in differentiation and arrest of proliferation. Similarly, when inhibition of proliferation and differentiation were induced in HOKs by 1.2 mM CaCl2, both rap1 and involucrin increased with increasing concentrations of CaCl2. However, when similar experiments were done with IHOKs, which continue to proliferate in the presence of 1.2 mM CaCl2, the increase in involucrin expression was similar to HOKs but there was no substantial increase in rap1, suggesting that increased expression of rap1 is linked to inhibition of proliferation rather than differentiation of keratinocytes. Upon transfection of immortalized keratinocytes with rapGAP, which inactivates both isoforms of endogenous active rap1, enhanced proliferation was observed. Thus, we conclude that rap1 inhibits proliferation in keratinocytes.     

Mesenchymal stem cells differentiate into keratinocytes and express epidermal kallikreins: Towards an in vitro model of human epidermis

Epidermal differentiation is a complex process in which keratinocytes go through morphological and biochemical changes in approximately 15 to 30 days. Abnormal keratinocyte differentiation is involved in the pathophysiology of several skin diseases. In this scenario, mesenchymal stem cells (MSCs) emerge as a promising approach to study skin biology in both normal and pathological conditions. Herein, we have studied the differentiation of MSC from umbilical cord into keratinocytes. MSC were cultured in Dulbecco's modified Eagle's medium (DMEM) (proliferation medium) and, after characterization, differentiation was induced by culturing cells in a defined keratinocyte serum-free medium (KSFM) supplemented with epidermal growth factor (EGF) and calcium chloride ions. Cells cultivated in DMEM were used as control. Cultures were evaluated from day 1 to 23, based on the cell morphology, the expression of p63, involucrin and cytokeratins (KRTs) KRT5, KRT10 and KRT14, by quantitative polymerase chain reaction, Western blot analysis or immunofluorescence, and by the detection of epidermal kallikreins activity. In cells grown in keratinocyte serum-free medium with EGF and 1.8 mM calcium, KRT5 and KRT14 expression was shown at the first day, followed by the expression of p63 at the seventh day. KRT10 expression was detected from day seventh while involucrin was observed after this period. Data showed higher kallikrein (KLK) activity in KSFM-cultured cells from day 11th in comparison to control. These data indicate that MSC differentiated into keratinocytes similarly to that occurs in the human epidermis. KLK activity detection appears to be a good methodology for the monitoring the differentiation of MSC into the keratinocyte lineage, providing useful tools for the better understanding of the skin biology.     

The Characteristics and Function of S100A7 Induction in Squamous Cell Carcinoma: Heterogeneity,Promotion of Cell Proliferation and Suppression of Differentiation

S100A7 is highly expressed in squamous cell carcinomas (SCC) and is related to the terminal differentiation of keratinocytes. However, its characteristic and function in SCC is not very known. In this present study, we used immunohistochemistry to examine the expression of S100A7 in 452 SCC specimens, including the lung, esophagus, oral cavity, skin, cervix, bladder, and three SCC cell lines. We found that S100A7-positive staining showed significant heterogeneity in six types of SCC specimen and three SCC cell lines. Further examination found that S100A7-positive cells and its expression at mRNA and protein levels could be induced in HCC94, FaDu, and A-431 cells both in vitro and in vivo using immunohistochemistry, real-time PCR, and Western blotting. Notably, the upregulation of squamous differentiation markers, including keratin-4, keratin-13, TG-1, and involucrin, also accompanied S100A7 induction, and a similar staining pattern of S100A7 and keratin-13 was found in HCC94 cells both in vitro and in vivo. Further study revealed that the overexpression of S100A7 significantly increased proliferation and inhibited squamous differentiation in A-431 cells both in vitro and in vivo. Conversely, silencing S100A7 inhibited cell growth and survival and increased the expression of keratin-4, keratin-13, TG-1, and involucrin in HCC94 cells. Therefore, these results demonstrate that S100A7 displays heterogeneous and inducible characteristic in SCC and also provide novel evidence that S100A7 acts as a dual regulator in promoting proliferation and suppressing squamous differentiation of SCC.     

cAMP-mediated modulation of signal transduction of epidermal growth factor (EGF) receptor systems in human epidermoid carcinoma A431 cells. Depression of EGF-dependent diacylglycerol production and EGF receptor phosphorylation.

While a cAMP-dependent protein kinase (protein kinase A) has been suggested to phosphorylate epidermal growth factor (EGF) receptor in vitro, both intrinsic and EGF- or potent phorbol tumor promoter-induced phosphorylation of EGF receptor were found to be depressed in human epidermoid carcinoma A431 cells by prior incubation of the cells with various protein kinase A activators (e.g. cholera toxin, forskolin, cAMP analogues, or a combination of prostaglandin E1 and 3-isobutyl-1-methylxanthine). Protein kinase A activators did not change significantly either the number of EGF receptors or their affinity for EGF. The tryptic phosphopeptide map of EGF receptors from cells treated with cholera toxin alone or cholera toxin followed by EGF revealed unique peptides whose serine phosphorylation was preferentially depressed. However, the catalytic subunit of protein kinase A phosphorylated no threonine and little serine in the EGF receptors in the plasma membranes of isolated A431 cells in vitro, while serine residues in an unidentified 170-kDa membrane protein(s) other than EGF receptor were heavily phosphorylated. Pretreatment of the cells with forskolin blocked 1,2-diacylglycerol induction by EGF; growth inhibition by nanomolar levels of EGF could be partially restored by the presence of forskolin. These results indicate that an increase in intracellular cAMP modulates the EGF receptor signal transduction system by reducing EGF-induced production of diacylglycerol without direct phosphorylation of EGF receptors by protein kinase A in A431 cells.     

Treatment of A431 cells with epidermal growth factor (EGF) induces desensitization of EGF-stimulated phosphatidylinositol turnover

Epidermal growth factor (EGF) stimulates the turnover of phosphoinositides in A431 cells. In cells that were pretreated with EGF for 30 min at 37 degrees C and then washed to remove surface-bound hormone, a 70-100% decrease in the EGF-stimulated production of inositol monophosphate, inositol bisphosphate, and inositol triphosphate was noted when the cells were exposed to the agonist a second time. Since only a 15% decrease in receptor number was observed in these pretreated cells, the loss of responsiveness to EGF for the production of inositol phosphates could not be attributed to a down-regulation of the EGF receptors. These data suggest that pretreatment of A431 cells with high concentrations of EGF leads to a desensitization of the EGF receptor. This desensitization of the receptor by EGF is apparent within 10-15 min of the addition of EGF and is maximal by 30 min. The desensitization appears to be homologous in nature since pretreatment of cells with EGF did not diminish their responsiveness to bradykinin; and conversely, pretreatment with bradykinin did not diminish the subsequent responsiveness of the cells to EGF. Desensitization to EGF was observed in cells in which protein kinase C had been down-regulated by prolonged treatment with 12-O-tetradecanoylphorbol-13-acetate, implying that EGF receptor desensitization is independent of protein kinase C. The desensitizing effects of EGF on growth factor-induced phosphatidylinositol turnover could be prevented by pretreatment of the cells with the calmodulin antagonist trifluoperazine, suggesting that calmodulin may be involved in the regulation of EGF receptor sensitivity.     

Association of EGF and LDL receptors with the cytoskeleton of cultured keratinocytes

In this paper we demonstrate that isolated cytoskeletons of normal keratinocytes cultured under differentiation inducing conditions exhibit a high level of epidermal growth factor (EGF) binding. This binding is approximately 300% higher than the binding of intact cells. In contrast, various squamous carcinoma cell lines or normal keratinocytes cultured under differentiation retarding conditions exhibit EGF binding to isolated cytoskeletons which is around 10-20% of the binding to intact cells. Incubation of normal keratinocytes in the presence of arotinoid ethyl sulfone resulted in a marked decrease of the ability of the cells to differentiate, and a decrease of EGF binding to isolated cytoskeletons. These results suggest a close relationship between the differentiation capacity of the cells and the presence of cytoskeleton-associated EGF receptors. Similar results were obtained for low density lipoprotein (LDL) binding.     

Low electromagnetic field (50 Hz) induces differentiation on primary human oral keratinocytes (HOK)

This work concerns the effect of low frequency electromagnetic fields (ELF) on biochemical properties of human oral keratinocytes (HOK). Cells exposed to a 2 mT, 50 Hz, magnetic field, showed by scanning electron microscopy (SEM) modification in shape and morphology; these modifications were also associated with different actin distribution, revealed by phalloidin fluorescence analysis. Moreover, exposed cells had a smaller clonogenic capacity, and decreased cellular growth. Indirect immunofluorescence with fluorescent antibodies against involucrin and beta-catenin, both differentiation and adhesion markers, revealed an increase in involucrin and beta-catenin expression. The advance in differentiation was confirmed by a decrease of expression of epidermal growth factor (EGF) receptor in exposed cells, supporting the idea that exposure to electromagnetic field carries keratinocytes to higher differentiation level. These observations support the hypothesis that 50 Hz electromagnetic fields may modify cell morphology and interfere in differentiation and cellular adhesion of normal keratinocytes.     

Epidermal growth factor potentiates cyclic AMP accumulation in A-431 cells.

Epidermal growth factor (EGF) treatment of A-431 cells potentiates up to 5-fold the intracellular cyclic AMP (cAMP) accumulation induced by isoproterenol, cholera toxin, forskolin, or 3-isobutyl-1-methylxanthine (IBMX). EGF potentiates cAMP accumulation in several epithelial cell lines which overexpress the EGF receptor including A-431 cells, HSC-1 cells, and MDA-468 cells, and in the A-431-29S clone which expresses a normal complement of EGF receptors. Although EGF potentiates cAMP accumulation, EGF by itself does not measurably alter the basal level of cAMP. EGF rapidly enhances cAMP accumulation (within 1 to 3 min) in A-431 cells treated with these cAMP-elevating agents. EGF potentiation of cAMP accumulation does not reflect enhancement of beta-adrenergic receptor activation and is not a consequence of intracellular cAMP elevation or the concomitant activation of cAMP-dependent protein kinase. Since EGF potentiates accumulation of both intracellular and extracellular cAMP in isoproterenol-treated A-431 cells, EGF does not potentiate intracellular cAMP accumulation by inhibition of cAMP export. EGF potentiation of cAMP accumulation is pertussis toxin-insensitive and does not result from EGF inhibition of cAMP degradation in A-431 cells. These results demonstrate that EGF transmembrane signaling includes an interaction with a component of the adenylate cyclase system and that this interaction stimulates cAMP synthesis resulting in enhancement of cAMP accumulation.     

EGF receptor phosphorylation is affected by ionizing radiation

Eukaryotic cells respond to ionizing radiation with cell cycle arrest, activation of DNA repair mechanisms, and lethality. However, little is known about the molecular mechanisms that constitute these responses. Here we report that ionizing radiation enhances epidermal growth factor (EGF) receptor tyrosine phosphorylation in intact cells as well as in isolated membranes of A431 cells. Phosphoamino acid analysis revealed that ionizing radiation preferentially enhances tyrosine phosphorylation, while EGF enhances the phosphorylation of all three phosphoamino acids (serine, threonine and tyrosine) of the EGF receptor. In addition, radiation reduces the turnover rate of the EGF receptor, while EGF increases the rate of the receptor turnover and down-regulation. Moreover, the confined radiation-induced phosphorylation of tyrosine residues is inhibited by genistein, indicating that this phosphorylation of EGF receptor is due to protein tyrosine kinase activation. These studies provide novel insights into the capacity of radiation to modulate EGF receptor phosphorylation and function. The radiation-induced elevation in the EGF receptor tyrosine phosphorylation and the receptor's slower rate of turnover are discussed in terms of their possible role in cell growth and apoptosis modulation.