Epidermal growth factor stimulates the synthesis of its own receptor in a human breast cancer cell line

Targeting Epidermal Growth Factor Receptor for Cancer Treatment: Abolishing Both Kinase-Dependent and Kinase-Independent Functions of the Receptor

Epidermal growth factor receptor (EGFR), a receptor tyrosine kinase, is activated by ligand binding, overexpression, or mutation. It is well known for its tyrosine kinase-dependent oncogenic activities in a variety of human cancers. A large number of EGFR inhibitors have been developed for cancer treatment, including monoclonal antibodies, tyrosine kinase inhibitors, and a vaccine. The EGFR inhibitors are aimed at inhibiting the activation or the activity of EGFR tyrosine kinase. However, these agents have shown efficacy in only a few types of cancers. Drug resistance, both intrinsic and acquired, is common even in cancers where the inhibitors have shown efficacy. The drug resistance mechanism is complex and not fully known. The key vulnerability of cancer cells that are resistant to EGFR inhibitors has not been identified. Nevertheless, it has been increasingly recognized in recent years that EGFR also possesses kinase-independent oncogenic functions and that these noncanonical functions may play a crucial role in cancer resistance to EGFR inhibitors. In this review, both kinase-dependent and -independent activities of EGFR are discussed. Also discussed are the mechanisms of actions and therapeutic activities of clinically used EGFR inhibitors and sustained EGFR overexpression and EGFR interaction with other receptor tyrosine kinases to counter the EGFR inhibitors. Moreover, this review discusses emerging experimental therapeutics that have shown potential for overcoming the limitation of the current EGFR inhibitors in preclinical studies. The findings underscore the importance and feasibility of targeting both kinase-dependent and -independent functions of EGFR to enhance therapeutic efficacy and minimize drug resistance. SIGNIFICANCE STATEMENT: EGFR is a major oncogenic driver and therapeutic target, but cancer resistance to current EGFR inhibitors remains a significant unmet clinical problem. This article reviews the cancer biology of EGFR as well as the mechanisms of actions and the therapeutic efficacies of current and emerging EGFR inhibitors. The findings could potentially lead to development of more effective treatments for EGFR-positive cancers.

Expression of EGFR isoform D is regulated by HER receptor activators in breast cancer cells

Human epidermal growth factor receptor isoform D (EGFR; isoform D) is a soluble protein from a 3 kb alternate mRNA transcript that arises from the human EGFR gene. Several studies have identified this circulating isoform of EGFR as a potential diagnostic biomarker for the detection of early stage of cancers. While the expression of the full-length EGFR (isoform A) is regulated by its cognate ligand, EGF, as well as by phorbol myristate acetate (PMA), no studies have examined the factors regulating the expression of EGFR isoform D. In this study, using breast cancer cell lines, we show that the HER receptor ligands, EGF and neuregulin (NRG-1β), as well as the phorbol ester, PMA, can increase the expression of EGFR isoform D, as well as isoform A. Our results, based on measurement of mRNA levels, suggest that EGF induced expression of both isoform A and isoform D occur through a mitogen activated protein kinase (MAPK)-dependent mechanism, and also suggest that protein kinase C is involved in PMA-induced regulation of both isoforms. We also demonstrate that NRG-1β increases isoform A and isoform D expression via the MAPK-dependent pathway, but this regulation occurs independently of phosphatidylinositol 3-kinase/Akt activation. These results suggest that regulation of EGFR isoform A and isoform D expression occur using similar mechanisms. Despite commonalities in the transcriptional regulation of these two EGFR isoforms, the half-lives of these two transcripts is quite different. Moreover, EGFR isoform D, unlike isoform A, is not post-transcriptionally modulated by EGFR activators in the breast cancer cell line MDA-MB-468.

mRNA expression of specific HER ligands and their association with clinical outcome in patients with metastatic breast cancer treated with trastuzumab

Prognostic and predictive biomarkers are being studied for the diagnosis and treatment of breast cancer. The present study retrospectively assessed the mRNA expression of HER family receptor ligands and of other potential prognostic biomarkers and their association with time to progression (TTP), survival and clinicopathological characteristics in patients with metastatic breast cancer (MBC) treated with trastuzumab. A total of 145 tumour tissue samples were analysed. mRNA expression analysis of the transcripts of interest was performed and the association of these markers with selected clinicopathological parameters was examined. HER2 status was centrally re-evaluated. Only 67.6% of patients were truly HER2-positive according to the central HER2 re-evaluation. Heparin binding epidermal growth factor (EGF)-like growth factor, transforming growth factor β1 (TGFB1) and thyroid hormone receptor α (THRA) mRNA expression was higher in HER2-positive patients (P=0.026, P<0.001 and P<0.001). Insulin-like growth factor binding protein 4 was correlated with retinoic acid receptor α, TGFB1 and THRA (rho=0.45, rho=0.60 and rho=0.45). In HER2-positive patients, high neuregulin 1 and high betacellulin were unfavourable factors for TTP [hazard ratio (HR) = 1.78, P=0.040 and HR=2.00, P=0.043, respectively]. In patients with de novo MBC, high EGF expression was associated with a non-significant prolongation of TTP (HR=0.52, P=0.080) and significantly longer survival (HR=0.40, P=0.020). The present study examined clinical and biological implications of specific genes and it was concluded that their expression has an impact on the outcome of trastuzumab-treated patients with MBC.

RNF144A sustains EGFR signaling to promote EGF-dependent cell proliferation

RNF144A is a single-pass transmembrane RBR E3 ligase that interacts with and degrades cytoplasmic DNA-PKcs, which is an epidermal growth factor receptor (EGFR)-interacting partner. Interestingly, RNF144A expression is positively correlated with EGFR mRNA and protein levels in several types of cancer. However, the relationship between RNF144A and EGFR is poorly understood. This study reports an unexpected role for RNF144A in the regulation of EGF/EGFR signaling and EGF-dependent cell proliferation. EGFR ligands, but not DNA-damaging agents, induce a DNA-PKcs-independent interaction between RNF144A and EGFR. RNF144A promotes EGFR ubiquitination, maintains EGFR protein, and prolongs EGF/EGFR signaling during EGF stimulation. Moreover, depletion of RNF144A by multiple independent approaches results in a decrease in EGFR expression and EGF/EGFR signaling. RNF144A knockout cells also fail to mount an immediate response to EGF for activation of G₁/S progression genes. Consequently, depletion of RNF144A reduces EGF-dependent cell proliferation. These defects may be at least in part due to a role for RNF144A in regulating EGFR transport in the intracellular vesicles during EGF treatment.

EGFR feedback-inhibition by Ran-binding protein 6 is disrupted in cancer

Transport of macromolecules through the nuclear pore by importins and exportins plays a critical role in the spatial regulation of protein activity. How cancer cells co-opt this process to promote tumorigenesis remains unclear. The epidermal growth factor receptor (EGFR) plays a critical role in normal development and in human cancer. Here we describe a mechanism of EGFR regulation through the importin β family member RAN-binding protein 6 (RanBP6), a protein of hitherto unknown functions. We show that RanBP6 silencing impairs nuclear translocation of signal transducer and activator of transcription 3 (STAT3), reduces STAT3 binding to the EGFR promoter, results in transcriptional derepression of EGFR, and increased EGFR pathway output. Focal deletions of the RanBP6 locus on chromosome 9p were found in a subset of glioblastoma (GBM) and silencing of RanBP6 promoted glioma growth in vivo. Our results provide an example of EGFR deregulation in cancer through silencing of components of the nuclear import pathway.

The role of myosin II in glioma invasion: A mathematical model

Gliomas are malignant tumors that are commonly observed in primary brain cancer. Glioma cells migrate through a dense network of normal cells in microenvironment and spread long distances within brain. In this paper we present a two-dimensional multiscale model in which a glioma cell is surrounded by normal cells and its migration is controlled by cell-mechanical components in the microenvironment via the regulation of myosin II in response to chemoattractants. Our simulation results show that the myosin II plays a key role in the deformation of the cell nucleus as the glioma cell passes through the narrow intercellular space smaller than its nuclear diameter. We also demonstrate that the coordination of biochemical and mechanical components within the cell enables a glioma cell to take the mode of amoeboid migration. This study sheds lights on the understanding of glioma infiltration through the narrow intercellular spaces and may provide a potential approach for the development of anti-invasion strategies via the injection of chemoattractants for localization.

Metastatic triple-negative breast cancer is dependent on SphKs/S1P signaling for growth and survival

About 40,000 American women die from metastatic breast cancer each year despite advancements in treatment. Approximately, 15% of breast cancers are triple-negative for estrogen receptor, progesterone receptor, and HER2. Triple-negative cancer is characterized by more aggressive, harder to treat with conventional approaches and having a greater possibility of recurrence. Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid signaling mediator has emerged as a key regulatory molecule in breast cancer progression. Therefore, we investigated whether cytosolic sphingosine kinase type 1 (SphK1) and nuclear sphingosine kinase type 2 (SphK2), the enzymes that make S1P are critical for growth and PI3K/AKT, ERK-MAP kinase mediated survival signaling of lung metastatic variant LM2-4 breast cancer cells, generated from the parental triple-negative MDA-MB-231 human breast cancer cell line. Similar with previous report, SphKs/S1P signaling is critical for the growth and survival of estrogen receptor positive MCF-7 human breast cancer cells, was used as our study control. MDA-MB-231 did not show a significant effect of SphKs/S1P signaling on AKT, ERK, and p38 pathways. In contrast, LM2-4 cells that gained lung metastatic phenotype from primary MDA-MB-231 cells show a significant effect of SphKs/S1P signaling requirement on cell growth, survival, and cell motility. PF-543, a selective potent inhibitor of SphK1, attenuated epidermal growth factor (EGF)-mediated cell growth and survival signaling through inhibition of AKT, ERK, and p38 MAP kinase pathways mainly in LM2-4 cells but not in parental MDA-MB-231 human breast cancer cells. Moreover, K-145, a selective inhibitor of SphK2, markedly attenuated EGF-mediated cell growth and survival of LM2-4 cells. We believe this study highlights the importance of SphKs/S1P signaling in metastatic triple-negative breast cancers and targeted therapies.

Strategies of eradicating glioma cells: a multi-scale mathematical model with MiR-451-AMPK-mTOR control

The cellular dispersion and therapeutic control of glioblastoma, the most aggressive type of primary brain cancer, depends critically on the migration patterns after surgery and intracellular responses of the individual cancer cells in response to external biochemical and biomechanical cues in the microenvironment. Recent studies have shown that a particular microRNA, miR-451, regulates downstream molecules including AMPK and mTOR to determine the balance between rapid proliferation and invasion in response to metabolic stress in the harsh tumor microenvironment. Surgical removal of main tumor is inevitably followed by recurrence of the tumor due to inaccessibility of dispersed tumor cells in normal brain tissue. In order to address this multi-scale nature of glioblastoma proliferation and invasion and its response to conventional treatment, we propose a hybrid model of glioblastoma that analyses spatio-temporal dynamics at the cellular level, linking individual tumor cells with the macroscopic behaviour of cell organization and the microenvironment, and with the intracellular dynamics of miR-451-AMPK-mTOR signaling within a tumour cell. The model identifies a key mechanism underlying the molecular switches between proliferative phase and migratory phase in response to metabolic stress and biophysical interaction between cells in response to fluctuating glucose levels in the presence of blood vessels (BVs). The model predicts that cell migration, therefore efficacy of the treatment, not only depends on oxygen and glucose availability but also on the relative balance between random motility and strength of chemoattractants. Effective control of growing cells near BV sites in addition to relocalization of invisible migratory cells back to the resection site was suggested as a way of eradicating these migratory cells.

Modeling the role of TGF-β in regulation of the Th17 phenotype in the LPS-driven immune system

Airway exposure levels of lipopolysaccharide (LPS) are known to determine type I versus type II helper T cell induced experimental asthma. While low doses of LPS derive Th2 inflammatory responses, high (and/or intermediate) LPS levels induce Th1- or Th17-dominant responses. The present paper develops a mathematical model of the phenotypic switches among three Th phenotypes (Th1, Th2, and Th17) in response to various LPS levels. In the present work, we simplify the complex network of the interactions between cells and regulatory molecules. The model describes the nonlinear cross-talks between the IL-4/Th2 activities and a key regulatory molecule, transforming growth factor β (TGF-β), in response to high, intermediate, and low levels of LPS. The model characterizes development of three phenotypes (Th1, Th2, and Th17) and predicts the onset of a new phenotype, Th17, under the tight control of TGF-β. Analysis of the model illustrates the mono-, bi-, and oneway-switches in the key regulatory parameter sets in the absence or presence of time delays. The model also predicts coexistence of those phenotypes and Th1- or Th2-dominant immune responses in a spatial domain under various biochemical and bio-mechanical conditions in the microenvironment.

A hybrid model of tumor-stromal interactions in breast cancer

Ductal carcinoma in situ (DCIS) is an early stage noninvasive breast cancer that originates in the epithelial lining of the milk ducts, but it can evolve into comedo DCIS and ultimately, into the most common type of breast cancer, invasive ductal carcinoma. Understanding the progression and how to effectively intervene in it presents a major scientific challenge. The extracellular matrix (ECM) surrounding a duct contains several types of cells and several types of growth factors that are known to individually affect tumor growth, but at present the complex biochemical and mechanical interactions of these stromal cells and growth factors with tumor cells is poorly understood. Here we develop a mathematical model that incorporates the cross-talk between stromal and tumor cells, which can predict how perturbations of the local biochemical and mechanical state influence tumor evolution. We focus on the EGF and TGF-β signaling pathways and show how up- or down-regulation of components in these pathways affects cell growth and proliferation. We then study a hybrid model for the interaction of cells with the tumor microenvironment (TME), in which epithelial cells (ECs) are modeled individually while the ECM is treated as a continuum, and show how these interactions affect the early development of tumors. Finally, we incorporate breakdown of the epithelium into the model and predict the early stages of tumor invasion into the stroma. Our results shed light on the interactions between growth factors, mechanical properties of the ECM, and feedback signaling loops between stromal and tumor cells, and suggest how epigenetic changes in transformed cells affect tumor progression.

The role of the microenvironment in tumor growth and invasion

Mathematical modeling and computational analysis are essential for understanding the dynamics of the complex gene networks that control normal development and homeostasis, and can help to understand how circumvention of that control leads to abnormal outcomes such as cancer. Our objectives here are to discuss the different mechanisms by which the local biochemical and mechanical microenvironment, which is comprised of various signaling molecules, cell types and the extracellular matrix (ECM), affects the progression of potentially-cancerous cells, and to present new results on two aspects of these effects. We first deal with the major processes involved in the progression from a normal cell to a cancerous cell at a level accessible to a general scientific readership, and we then outline a number of mathematical and computational issues that arise in cancer modeling. In Section 2 we present results from a model that deals with the effects of the mechanical properties of the environment on tumor growth, and in Section 3 we report results from a model of the signaling pathways and the tumor microenvironment (TME), and how their interactions affect the development of breast cancer. The results emphasize anew the complexities of the interactions within the TME and their effect on tumor growth, and show that tumor progression is not solely determined by the presence of a clone of mutated immortal cells, but rather that it can be 'community-controlled'.

Transformed epithelial cells and fibroblasts/myofibroblasts interaction in breast tumor: a mathematical model and experiments

It is well known that tumor and its microenvironment, or stroma, interact with each other and that this interaction plays a critical role in tumor initiation, growth, and metastasis. This interaction consists of complex relations between tumor cells, stromal cells such as fibroblasts, epithelial cells and immunocytes, the vascular system, the extracellular matrix, and cytokines secreted by the cells. Understanding these relationships may lead to new therapeutic approaches to cancer. In the present paper, we consider tumor-stroma crosstalk in a simple in vitro situation which involves interaction between tumor epithelial cells from breast cancer and a microenvironment consisting of just fibroblasts. The two populations of cells are separated by a semi-permeable membrane that allows only cytokines to cross over. We develop a mathematical model that includes two critical growth factors: TGF-beta, produced by the tumor cells, and EGF, secreted by the fibroblasts. The TGF-beta modifies the microenvironment by transforming fibroblasts into myofibroblasts. Myofibroblasts secrete higher concentrations of EGF than fibroblasts, thereby, increasing the proliferation of tumor cells. Thus already in this simple setup one sees a mutual interaction between tumor cells and their microenvironment. We conducted experiments which show good agreement with the model's simulations, hence confirming the model's ability to predict aspects of tumor cell behavior in response to signaling from fibroblasts.

ErbB/HER ligands in human breast cancer, and relationships with their receptors, the bio-pathological features and prognosis

BACKGROUND

The aim of this study is to provide an expression profile of ErbB/HER ligands in breast cancer. We analysed the relationships with their receptors, the bio-pathological features and prognosis.

PATIENTS AND METHODS

Epidermal growth factor (EGF), transforming growth factor-alpha (TGFalpha), amphiregulin (AREG), betacellulin (BTC), heparin-binding EGF-like growth factor (HB-EGF), epiregulin (EREG) and neuregulins1-4 (NRG1-4) were quantified in 363 tumours by real-time reverse transcription-polymerase chain reaction using TaqMan probes.

RESULTS

Ligands were detected in 80%-96% of the cases, except NRG3 (42%) and EREG (45.5%). At least one ligand was expressed in 304 cases (cut-off: upper quartile). Almost all combinations of receptor and ligand co-expressions were observed, but TGFalpha is preferentially expressed in tumours co-expressing EGFR/HER3, NRG3 in those co-expressing EGFR/HER4, AREG and EREG in those co-expressing HER2/HER4. EGF and AREG were associated with estradiol receptors, small tumour size, low histoprognostic grading, high HER4 levels. TGFalpha, HB-EGF and NRG2 were negatively related to these parameters. In Cox univariate analyses, EGF was a prognostic factor.

CONCLUSION

Our study demonstrates that (i) ErbB/HER ligands, including BTC and EREG, are expressed in most breast cancers; and (ii) TGFalpha, HB-EGF and NRG2 high expressions are related to the biological aggressiveness of the tumours.

Role of the Sec61 translocon in EGF receptor trafficking to the nucleus and gene expression

The epidermal growth factor (EGF)-dependent trafficking of the intact EGF receptor to the nucleus and its requirement for growth factor induction of cyclin D and other genes has been reported. Unresolved is the mechanism by which this or other transmembrane proteins are excised from a lipid bilayer before nuclear translocalization. We report that, after the addition of EGF, the cell surface EGF receptor is trafficked to the endoplasmic reticulum (ER) where it associates with Sec61beta, a component of the Sec61 translocon, and is retrotranslocated from the ER to the cytoplasm. Abrogation of Sec61beta expression prevents EGF-dependent localization of EGF receptors to the nucleus and expression of cyclin D. This indicates that EGF receptors are trafficked from the ER to the nucleus by a novel pathway that involves the Sec61 translocon.

Mechanism of action of potato carboxypeptidase inhibitor (PCI) as an EGF blocker

The epidermal growth factor receptor (EGFR) signal transduction pathway plays a prominent role in the development of carcinomas, and is an interesting target for antitumoral therapy. We have previously described how potato carboxypeptidase inhibitor (PCI), a 39-amino acid protease inhibitor with a T-Knot motif, binds to EGFR receptor and inhibits the activation of receptor protein tyrosine kinase. In this paper it is shown that PCI interferes with EGFR activation through inhibition of receptor dimerization and receptor transphosphorylation induced by epidermal growth factor (EGF) and by transforming growth factor alpha (TGF-alpha). Moreover, PCI blocks the formation and activation of ErbB1/ErbB-2 heterodimers that have a prominent role in carcinoma development. As a result of these effects, PCI interferes in the EGFR signal transduction pathway by reversing the effects of EGF on the growth of two tumoral cell lines, A431 and MDA-MB-453, and promotes EGFR down-regulation. These results show that PCI acts as an EGF/TGF-alpha antagonist, which suggests its therapeutic potential in the treatment of carcinomas.

Identification of a novel AU-Rich element in the 3' untranslated region of epidermal growth factor receptor mRNA that is the target for regulated RNA-binding proteins

The epidermal growth factor receptor (EGF-R) plays an important role in the growth and progression of estrogen receptor-negative human breast cancers. EGF binds with high affinity to the EGF-R and activates a variety of second messenger pathways that affect cellular proliferation. However, the underlying mechanisms involved in the regulation of EGF-R expression in breast cancer cells are yet to be described. Here we show that the EGF-induced upregulation of EGF-R mRNA in two human breast cancer cell lines that overexpress EGF-R (MDA-MB-468 and BT-20) is accompanied by stabilization (>2-fold) of EGF-R mRNA. Transient transfections using a luciferase reporter identified a novel EGF-regulated approximately 260-nucleotide (nt) cis-acting element in the 3' untranslated region (3'-UTR) of EGF-R mRNA. This cis element contains two distinct AU-rich sequences (~75 nt), EGF-R1A with two AUUUA pentamers and EGF-R2A with two AUUUUUA extended pentamers. Each independently regulated the mRNA stability of the heterologous reporter. Analysis of mutants of the EGF-R2A AU-rich sequence demonstrated a role for the 3' extended pentamer in regulating basal turnover. RNA gel shift analysis identified cytoplasmic proteins (~55 to 80 kDa) from breast cancer cells that bound specifically to the EGF-R1A and EGF-R2A cis-acting elements and whose binding activity was rapidly downregulated by EGF and phorbol esters. RNA gel shift analysis of EGF-R2A mutants identified a role for the 3' extended AU pentamer, but not the 5' extended pentamer, in binding proteins. These EGF-R mRNA-binding proteins were present in multiple human breast and prostate cancer cell lines. In summary, these data demonstrate a central role for mRNA stabilization in the control of EGF-R gene expression in breast cancer cells. EGF-R mRNA contains a novel complex AU-rich 260-nt cis-acting destabilizing element in the 3'-UTR that is bound by specific and EGF-regulated trans-acting factors. Furthermore, the 3' extended AU pentamer of EGF-R2A plays a central role in regulating EGF-R mRNA stability and the binding of specific RNA-binding proteins. These findings suggest that regulated RNA-protein interactions involving this novel cis-acting element will be a major determinant of EGF-R mRNA stability.

Effects of epidermal growth factor (EGF) on the development of EGF-receptor (EGF-R) binding in fetal rabbit lung organ culture

Epidermal growth factor (EGF) causes gender- and development-specific changes in fetal lung surfactant synthesis. We hypothesized that the effects of EGF on development of surfactant synthesis are related to effects on EGF receptor (EGF-R) expression. We prepared sex-specific fetal rabbit lung organ cultures on gestational days 21 and 24 (term = 31 days) in Waymouth's medium + 10% charcoal-stripped fetal calf serum as control or with added EGF (10 ng/mL). After 3, 5, and 7 days of culture, we measured specific EGF-R binding in fetal lung plasma membrane preparations. Analysis of variance (ANOVA) revealed significant effects of fetal gender (P = 0.0003), time in culture (P = 0.01), and EGF treatment (P = 0. 0003) on EGF specific binding. In control cultures from days 21 and 24 (both male and female), EGF specific binding tended to decrease with time in culture. Specific binding in EGF-treated female 21-day cultures was significantly higher than in controls, both after 5 days (184% of control, P = 0.007) and after 7 days (151% of control, P = 0.01; Bonferroni multiple comparisons) of treatment, whereas males exhibited no response to EGF treatment. As opposed to these effects in 21-day cultures, EGF had little effect on 24-day cultures. We conclude that EGF affects the expression of the EGF-R on EGF specific binding in the fetal lung. The development of surfactant synthesis in the fetal lung may be controlled by upregulation of the EGF-R.

Conditional expression of the ErbB2 oncogene elicits reversible hyperplasia in stratified epithelia and up-regulation of TGFalpha expression in transgenic mice

The ErbB2 receptor tyrosine kinase (RTK) is expressed in basal cells of squamous epithelia and the outer root sheath of hair follicles. We previously showed that constitutive expression of activated ErbB2 directed to these sites in the skin by the keratin 14 (K14) promoter produces prominent hair follicle abnormalities and striking skin hyperplasia in transgenic mice. However, perinatal lethality precluded the establishment of a transgenic line for analysis of ErbB2 function in adult animals. To investigate the significance of ErbB2 signaling in epithelial tissues during and post development, we developed a K14-rtTA/TetRE-ErbB2 'Tet-On' bitransgenic mouse system. These mice were normal until the ErbB2 transgene was induced by exposure to doxycycline (Dox). Prenatal induction resulted in perinatal death. Postnatally, ErbB2 transgene expression was observed at 4 h after the initiation of Dox, and reached a plateau at 24 h. Skin hyperplasia followed after 2 days and these changes reverted to normal upon Dox withdrawal. In adults, as in the neonates, prolonged ErbB2 induction caused prominent skin and hair follicle hyperplasias. Severe hyperplasias in the cornea, eye lids, tongue and esophagus were also observed. ErbB2 transgene induction was accompanied by increased expression of TGFalpha, a ligand of epidermal growth factor receptor (EGFR), and to a lesser extent, EGFR, further enhancing RTK signal transduction. We conclude that ErbB2 plays important roles in both development and maintenance of hair follicles and diverse squamous epithelia and that this ligand-inducible and tissue-specific 'Tet-On' transgenic mouse system provides a means to study transgenes with perinatal toxicity.

Small bowel adaptation is dependent on site of massive enterectomy

BACKGROUND

Changes in amino acid transport after massive enterectomy occur in a nutrient-dependent fashion and may affect long-term outcome. Epidermal growth factor (EGF) can enhance nutrient transport and a defective epidermal growth factor receptor (EGF-R) has been noted to attenuate adaptation. Most animal studies, however, have examined only a single site of resection. This does not mimic the clinical situation where disease dictates the site of resection leading to proximal, middle, or distal enterectomies. We hypothesize that the site of massive enterectomy will alter nutrient transport and EGF-R levels in the residual gut.

MATERIALS AND METHODS

New Zealand White rabbits were randomized to control, midgut division, or 70% resection (proximal, midgut, or distal). After 1 week, sodium-dependent transport of glucose, glutamine, alanine, and leucine into brush border membrane vesicles was quantitated. EGF-R protein was determined by Western blot analysis.

RESULTS

At baseline, amino acid transport was greater in ileum than jejunum. Surgery alone elevated glutamine and leucine jejunal transport by 130 and 97%, respectively, over controls (P < 0.05). Midgut resection increased jejunal glutamine transport 61% over control (P < 0.05). In contrast, distal resection increased jejunal alanine transport by 150% over controls with no change in glutamine (P < 0.05). After midgut resection, EGF-R was significantly greater (124%) in ileum then in jejunum in whole mucosa homogenates. Proximal resection significantly lowered ileal EGF-R compared to that seen in midgut resected residual ileum.

CONCLUSIONS

Site of massive resection is important in determining postoperative changes in nutrient transport and EGF-R.

Complex post-transcriptional regulation of EGF-receptor expression by EGF and TGF-alpha in human prostate cancer cells

The epidermal growth factor receptor (EGFR) plays an important role in the development and progression of prostate cancer and its overexpression is associated with decreased survival. With progression, prostate cancer cells switch from epidermal growth factor (EGF) to transforming growth factor alpha (TGF-alpha) synthesis, which contributes to autocrine growth and unrestrained proliferation. To define the molecular mechanisms involved in the regulation of EGFR expression by EGF and TGF-alpha we studied three human prostate cancer cell lines, androgen-responsive (LNCaP) and -unresponsive (DU145 and PC3). Here we show that TGF-alpha stabilized EGFR mRNA two- to threefold in all three cell lines, whilst EGF stabilized EGFR mRNA approximately twofold in LNCaP and DU145 cells, but not in PC3 cells. Both ligands increased EGFR transcription in LNCaP and DU145 cells, with less effect in PC3 cells. In all three cell lines EGF reduced total EGFR protein levels more than TGF-alpha, but this was associated with a greater increase in de novo protein synthesis with EGF compared to TGF-alpha. Only EGF, however, shortened EGFR protein stability (half-life decreased from 5 h to 120 min), resulting in rapid disappearance of newly synthesized EGFR protein. Both ligands increased total LNCaP and DU145 cell numbers. These studies demonstrate that the EGF- and TGF-alpha-induced upregulation of EGFR mRNA and protein in human prostate cancer cell lines is complex and occurs at multiple, transcriptional and post-transcriptional levels. Taken together, these data provide novel insight into the molecular mechanisms by which TGF-alpha would preferentially maintain an autocrine loop in human prostate cancer cells. Furthermore, this work suggests that in human prostate cancer cells ligand-specific differential intracellular trafficking of the EGFR plays a major role in regulating its expression.

How far can a juxtacrine signal travel?

Juxtacrine signalling is the process of cell communication in which ligand and receptors are both anchored in the cell membrane. We develop three mathematical models for this process, involving different mathematical representations of the dynamics of membrane-bound ligand and free and bound receptors, within an epithelial sheet. We consider the dynamics of this system following a localized disturbance, such as would be provided by a source of ligand or by the generation of a free edge via wounding. We study the ability of the juxtacrine mechanism to transmit a signal away from this disturbance, and show analytically that the spatial half-life of the signal can in fact be arbitrarily large. This result is quite general, since we use a generic reaction kinetic scheme; the key assumption is that ligand and receptor production are both upregulated by binding. Moreover, the result applies to all three of our model formulations. We conclude by discussing applications of the result to the particular case of the transforming growth factor alpha binding to epidermal growth factor receptor in epidermal wound healing.

Regulation of EGF-receptor expression by EGF and TGF alpha in epidermoid cancer cells is cell type-specific

The epidermal growth factor receptor (EGF-R) and its major ligands EGF and transforming growth factor alpha (TGF alpha) play an important role in the development of multiple human tumors. However, little is known of the comparative effects of each ligand on the regulation of EGF-R expression. To investigate this issue we used two similar human epidermoid cancer cell lines that overexpress EGF-Rs (KB and A431). In KB cells, EGF and TGF alpha increased EGF-R mRNA and protein levels by 2-3 fold over 8 h, associated with a greater than 4-fold stabilization of EGF-R mRNA half-life. EGF and TGF alpha also increased transcription of EGF-R mRNA 2-3-fold in KB cells. In contrast, EGF and TGF alpha only minimally increased EGF-R mRNA and protein in A431 cells, without changing EGF-R mRNA half-life. Basal EGF-R mRNA half-life was 2 fold greater in A431 cells than in KB cells (6-7 h versus 2-3 h), whilst the half-life of a mutant 2.6 kb EGF-R mRNA present in A431 cells, which lacks the 3-untranslated region (3'-UTR), was 2 fold greater than the full-length EGF-R mRNA. RNA gel-shift studies demonstrated that KB and A431 cells contain cytoplasmic proteins that bind specifically to an AU-rich sequence from the 3'-UTR of EGF-R mRNA. Taken together, these results demonstrate that in KB cells EGF and TGF alpha upregulate EGF-R expression at both transcriptional and post-transcriptional levels. The identification of AU-rich EGF-R mRNA-specific RNA-binding proteins from epidermoid cancer cells that overexpress EGF-Rs suggests that regulated RNA-protein interactions involving this region may play a central role in modulating EGF-R mRNA stability.

Hyperexpression of epidermal growth factor receptors in granulosa cells from women with polycystic ovary syndrome

OBJECTIVE

To compare the localization and quantitation of epidermal growth factor (EGF) receptor in granulosa cells from women with normal and polycystic ovaries.

DESIGN

Controlled, comparative study.

SETTING

Academic research laboratory.

PATIENT(S)

Forty-two women with normal or polycystic ovaries who attended our facility for the recovery of their immature or mature oocytes or for therapeutic purposes.

INTERVENTION(S)

Patients underwent intravaginal ultrasound-guided oocyte retrieval or laparoscopic follicular aspiration with or without prior stimulation.

MAIN OUTCOME MEASURE(S)

Quantitation of EGF receptor in granulosa cells.

RESULT(S)

Granulosa cells from polycystic ovaries expressed significantly higher levels of EGF receptor than granulosa cells from normal ovaries. In contrast with patients who were treated with clomiphene citrate, those who were treated with gonadotropins showed low levels of the receptor. However, the levels of the receptor in granulosa cells were not correlated with circulating levels of LH, FSH, progesterone, or E2. Immunolabeling of EGF receptor was confined to the cell membrane of granulosa cells. This receptor was fully functional, mediating the ligand-induced inhibition of E2 production in culture.

CONCLUSION(S)

These results provide further evidence supporting a possible role of EGF/transforming growth factor-alpha in the aberration of ovarian function in polycystic ovary syndrome.

Epidermal growth factor upregulates the expression of its own intestinal receptor after small bowel resection

BACKGROUND/PURPOSE

Epidermal growth factor (EGF) binds to its enterocyte receptor and enhances intestinal adaptation after massive small bowel resection (SBR). To ascertain the mechanism for enhanced adaptation by EGF, we sought to determine the effect of EGF administration on in vivo expression of the intestinal EGF receptor after SBR.

METHODS

Male ICR mice underwent a 50% proximal SBR and then were assigned randomly to EGF (150 microg/kg/d) or saline by twice daily intraperitoneal injection. After 3 days, the ileum was harvested and total protein and DNA content were measured. Northern hybridization and a ribonuclease protection assay were used to detect qualitative and quantitative expression of EGF receptor mRNA. The remaining ileum was pooled for each group and Western blotting used to determine expression of EGF receptor protein.

RESULTS

EGF augmented adaptation after SBR as monitored by significant increases in ileal protein (2.7+/-0.08 saline versus 3.9+/-0.17 mg/cm EGF; P<.001) and DNA (55.8+/-1.6 saline versus 104+/-8.4 microg/cm EGF; P<.001) content. Northern blotting results showed a marked (>fivefold) increase in ileal EGF receptor mRNA, which was confirmed with the ribonuclease protection assay. Administration of EGF after SBR induced a similar expression of EGF receptor protein.

CONCLUSIONS

EGF enhanced intestinal adaptation after SBR. This augmented response is associated with increased ileal expression of EGF receptor mRNA and protein. Increased EGF receptor expression and subsequent enhanced ligand/ receptor activity may be one important mechanism for the beneficial effect of EGF administration during intestinal adaptation.

Plasma membrane phosphotyrosine, Her2-NEU, and epidermal growth factor receptor in human breast cancer. A comparative study

Experimental therapeutic regimens for breast cancer include strategies to block the activity of specific oncogenes. Because oncogenesis is a multistep process, specific oncogenes may drive tumor production at one stage yet not function in another. Since the effectiveness of therapy targeted against oncogenes depends on their function in the tumor, correlation of oncogene function to specific stages of tumor development has therapeutic implications. Among the oncogenes known to be important in breast cancer production are two cell surface growth factor receptors, epidermal growth factor receptor (EGFR) and Her2-NEU (NEU). These proteins are receptor tyrosine kinases that autophosphorylate specific tyrosine residues on activation. The oncogenic potential of these receptors depends on this autophosphorylation. We examined 86 primary formalin-fixed, paraffin-embedded breast tumors for overexpression of EGFR and NEU and correlated our findings with the presence of cell surface phosphotyrosine as an indicator of tyrosine kinase activity at the plasma membrane. Our data indicate that only 34% of tumors that overexpress EGFR or NEU show plasma membrane phosphotyrosine, indicating that in the majority of these tumors, the overexpressed oncogene may not be active at this stage.

In vivo and in vitro effects of epidermal growth factor on its receptor gene expression in rat dental follicle cells

Epidermal growth factor receptor (EGFR) is known to be localized early postnatally in the follicle of rat mandibular molars and to decline later. Here, EGFR mRNA present early postnatally in the follicle was revealed by in situ hybridization and reverse transcription-polymerase chain reaction. Injections of epidermal growth factor (EGF) enhanced the expression of EGFR mRNA but not EGF mRNA through day 5 postnatally in the follicle. By day 7 postnatally, the amount of EGFR mRNA was dramatically reduced and EGF injection at that time or later did not enhance its expression. Cultured dental follicle cells were also shown to contain both EGFR mRNA and EGF mRNA, but only EGFR mRNA expression was enhanced after incubation of the cells with EGF. The EGFR mRNA was apparently translated, because the cultured cells could be immunostained for EGFR. This enhancement of EGFR mRNA in the dental follicle cells by EGF may be a necessary prelude to the mitogenic effect of EGF on the cultured cells because incubating the dental follicle cells in EGF over 6 days resulted in almost a doubling in their number as compared to controls. These results indicate that the tissue necessary for eruption to occur, the dental follicle, contains EGFR mRNA in the early postnatal days. In turn, this mRNA is enhanced by EGF, a molecule known to accelerate eruption if injected early postnatally. Whether or not the mitogenic effect of the EGF on the cultured dental follicle cells is a mechanistic factor that contributes to the subsequent tooth eruption is unknown.

Dual regulation of the epidermal growth factor family of growth factors in breast cancer by sex steroids and protein kinase C

There has been increased interest in the last few years in seeking a better understanding of the local regulation of polypeptide growth factors by systemic hormones, such as sex steroids and by polypeptide hormones. Growth factors and systemic hormones play pivotal roles in hormone-regulated cancers such as breast cancer. In this review, we discuss the regulation of members of the epidermal growth factor (EGF) family by sex steroids and by regulators of the polypeptide hormone signal transduction enzyme termed protein kinase C (PKC). Regulation of the EGF family of genes will be discussed as a model system to evaluate interactions between these two important types of regulatory pathways in breast cancer.

Heterodimerization and functional interaction between EGF receptor family members: a new signaling paradigm with implications for breast cancer research

The EGF receptor (EGFR) and HER2 are members of a growth factor receptor family. Overexpression of either protein in advanced breast cancer correlates with poor prognosis. EGF stimulates growth by binding to EGFR, activating the receptor's intracellular tyrosine kinase. The initial consequence is phosphorylation of specific tyrosine-containing sequences in the receptor's carboxyl terminus. These phosphotyrosines serves as high affinity recognition sites for proteins that, in turn, transmit the growth signal inside the cell. Mechanistic studies suggest that EGF binds to a single EGFR, triggering dimerization with another like receptor molecule. This dimerization is thought to initiate the tyrosine kinase activation. The EGF receptor family was recently expanded with the sequencing of HER3 and HER4. Each of the four family members was postulated to regulate a unique growth or differentiation signaling repertoire when activated by a receptor-specific ligand. However, new data from numerous laboratories suggest that EGFR family members may play a complex and ultimately more flexible role in signaling by forming heterodimers between family members, e.g. EGFR:HER2 or HER4:HER2. These heterodimers may form even when only one member of the pair binds its ligand. This review summarizes current work on heterodimerization and attempts to predict the consequences for downstream signaling. In brief, when compared to ligand-dependent receptor homodimers comprised of two proteins with the same internalization sequence and phosphorylated tyrosine residues, heterodimers are likely to: i) expand substrate selection and downstream signaling pathway activation; ii) promote interaction between sets of substrates in the mixed receptor complexes that would not ordinarily be physically juxtaposed; iii) alter the duration of receptor signaling by changing rates of receptor internalization, ligand loss, kinase inactivation, recycling, etc.; and iv) alter rates of receptor and substrate dephosphorylation. In addition to understanding interactions of heterodimers with the internalization machinery, identification of receptor-specific substrates and binding proteins for each EGFR family member will be necessary to explicate the role of heterodimers in growth and differentiation.

Disseminated eruptive nevocellular nevi

A 13-year-old boy suddenly developed about 2,000 dark brown to black colored papules on his face and neck and about 500 lesions on his trunk and upper extremities during a six month period. Histopathologic features were compatible with junctional nevus. The results of alpha melanocyte stimulating hormone (MSH), proliferating cell nuclear antigen (PCNA), and epidermal growth factor receptor (EGF/R) studies are presented. To the best of our knowledge, this report represents an outbreak of the highest number of nevocellular nevi in a short period without any malignant nature or evident triggering factor.

Expression of epidermal growth factor receptor (EGFR) in non-affected and tumorous mammary tissue of female dogs

Epidermal growth factor (EGFR), oestrogen (ER), and progestin (PR) receptor concentrations were determined by radioligand binding assay in non-affected mammary tissues (n = 13) and benign (n = 11) and primary/locally recurrent malignant proliferative mammary lesions (n = 45) and metastases (n = 19) in 65 female dogs. The number of specimens expressing EGFR was not significantly different among these tissues, but EGFR concentration was lower in metastases (P = 0.02) than in benign or primary/locally recurrent malignant lesions not mixed with non-affected mammary tissue. The presence of non-affected mammary tissue in primary cancer specimens was noticed as a factor that may influence results of receptor measurements. No relation was found between the expression of EGFR and that of ER or PR in non-affected or in tumorous mammary tissues. It was concluded that in the dog mammary gland EGFR expression is not associated with conditions of steroid receptor absence of biological agressiveness of neoplastic growth.

EGF receptor expression, regulation, and function in breast cancer

Epidermal growth factor receptor (EGFR) overexpression correlates with both loss of estrogen receptor (ER) and poor prognosis in breast cancer. Interestingly, in normal breast EGFR appears to be expressed more frequently than in malignant tissue, and there may be a different relationship between ER and EGFR. A variety of cellular regulators, such as EGF, TGF alpha, phorbol esters, and steroid hormones, are capable of altering the level of EGFR expression in breast cells. However, much work remains to be done on the mechanistic details of EGFR regulation in this disease. The significance of EGFR as an oncogene in breast cancer is compounded by its potential interactions with other oncogenes such as c-erbB-2 and c-myc. Additionally, several recent studies have placed EGFR prominently in the signal transduction pathway, demonstrating that the EGFR-ligand system may play important roles throughout the course of malignant progression in breast cancer.

Mechanisms of EGF receptor regulation in breast cancer cells

Overexpression of the EGF receptor in breast cancer correlates with poor prognosis and failure on endocrine therapy for both ER-/EGFR+ and ER+/EGFR+ tumors, suggesting a role for EGFR in the progression to hormone independence. The identification of specific DNAse I hypersensitive site patterns for the EGFR gene in ER+ vs. ER- cells implicates regions of the EGFR first intron in up-regulation of EGFR, while estrogen regulation studies indicate the involvement of a repressor(s) in the maintenance of low levels of EGFR. Based on these findings, a multi-step model is proposed for the progression of breast cancer from a hormone-dependent, ER+/EGFR-phenotype to an aggressive, hormone-independent, ER-/EGFR+ stage.

Evidence for up-regulation of epidermal growth-factor receptors on rat periodontal ligament fibroblastic cells associated with stabilization of phenotype in vitro

This study sought to understand the role of epidermal growth factor receptor (EGF-R) in periodontal ligament (PDL) fibroblasts. Rat PDL fibroblastic cells and ROS 17/2.8 cells (highly differentiated osteoblastic osteosarcoma cells) were cultured and treated with transforming growth factor-alpha (TGF-alpha), EGF, dexamethasone (Dex) or a combination of EGF and Dex. Alkaline phosphatase (ALP) activity, an early differentiation marker for mineralized tissue-forming cells, was measured using p-nitrophenylphosphate as a substrate. For Scatchard analysis of [125I]-EGF binding, cells were incubated in Dulbecco's modified Eagle's medium containing 0.2% bovine serum albumin and 0-64 ng/ml of [125I]-EGF for 4 h at 4 degrees C. Also, the synthesis of EGF-R protein and the expression of mRNA for EGF-R were measured by immunoprecipitation and Northern blot analysis, respectively. Untreated PDL fibroblastic cells showed a gradual increase in spontaneous ALP activity from 32.4 U/10(6) cells at 2 days to 49.6 U/10(6) cells at 7 days of culture. ALP activity was further increased to 70.8 U/10(6) cells at 7 days after treatment with Dex, whereas EGF treatment reduced it to 19.4 U/10(6) cells. Culture of PDL fibroblastic cells in the presence of a combination of Dex and EGF decreased the Dex-induced ALP activity from 70.8 U to 41.8 U/10(6) cells at 7 days. A similar inhibitory effect on ALP activity was found after treatment with TGF-alpha. In contrast, ROS cells maintained a high ALP activity (1748 U/10(6) cells) throughout culture, unaffected by EGF. Scatchard analysis demonstrated that PDL fibroblastic cells have both high- and low-affinity forms of EGF-R, while ROS cells did not have any detectable EGF-R. Treatment of PDL cells with Dex for 2 days decreased the synthesis of EGF-R protein, the expression of EGF-R mRNA and the number of EGF-R. In contrast, EGF treatment increased the expression of EGF-R mRNA. These data suggest that PDL fibroblastic cells express numerous EGF-R, but the number decreases during their differentiation into mineralized tissue-forming cells under the influence of Dex. Thus, EGF-R may function in the stabilization of phenotype in PDL fibroblastic cells.

Atypical receptor-mediated signal transduction events in the EGF-dependent growth-inhibited cell line, MDA-468

It is now generally considered that early signalling from tyrosine kinases that induce mitogenesis is initiated through the formation of heteromeric complexes consisting of the autophosphorylated tyrosine kinase and a number of tyrosylphosphorylated proteins, including phospholipase C-gamma (PLC-gamma) and GTPase activating protein (GAP). However, since much of this work has been performed on proliferative, chimeric cell lines expressing heterologous receptor molecules, we examined the nature of the epidermal growth factor receptor (EGFR) signalling complex formation in the human breast cancer cell line, MDA-468. This cell line has an amplified, native EGFR gene, correspondingly overexpresses the EGFR, and its growth in culture is inversely related to the EGF concentration. Our results indicate that in MDA-468 cells, both the EGFR and PLC-gamma are phosphorylated on tyrosine residues and can be co-immunoprecipitated. This occurs at both high and low EGF concentrations regardless of the proliferative endpoint. The molecular association is correlated with a significant increase in total inositol phosphates formed in response to the growth factor treatment. In contrast, however, there is no evidence that GAP is either phosphorylated on tyrosine residues or forms a complex with the activated EGFR in EGF-treated MDA-468 cells. These observations suggest that as a model for growth factor action, the formation of heteromeric protein signalling complexes may demonstrate considerable diversity depending upon both cell type and physiology.

Northern and in situ hybridization analyses of the effects of somatotropin on bovine mammary gene expression

We investigated the potential roles of insulin-like growth factor-I and epidermal growth factor, both multifunctional regulators of mammary physiology, in somatotropin-stimulated lactation of dairy cattle. One insulin-like growth factor-I mRNA (7.4 kb) was detected in lactating mammary tissue; however, it became undetectable after somatotropin administration. Four type I insulin-like growth factor receptor mRNA (11.3, 6.2, 4.9, and 3.3 kb) were detected in mammary tissue and primarily expressed in alveolar epithelial cells. All of these receptor mRNA markedly decreased in mammary tissue from somatotropin-treated animals, although the cellular distribution pattern of receptor gene expression did not change. One epidermal growth factor receptor mRNA (10.5 kb) was present in mammary tissue and predominantly expressed in alveolar epithelial cells. Epidermal growth factor receptor mRNA decreased after somatotropin administration. The results from this study are consistent with the idea that the stimulatory effect of somatotropin during lactation is in part due to its ability to regulate the expression of specific genes or the stability of their mRNA in mammary tissue. The data also suggest roles for insulin-like growth factor-I and epidermal growth factor, as well as their homologous receptors, in controlling cellular changes in the mammary gland during somatotropin-stimulated lactation.

Increased epidermal growth factor receptors in melanocytic lesions

BACKGROUND

Epidermal growth factor receptors (EGF/R) have been reported to be absent in melanomas or, in contrast, to be markers for potential malignancy in melanocytic lesions.

OBJECTIVE

Our purpose was to evaluate the literature discrepancies regarding the presence of EGF/R in melanocytic lesions and to determine whether EGF/R presence correlates with the potential for malignancy of melanocytic lesions.

METHODS

An EGF/R-specific polyclonal antibody was used to study melanomas, dysplastic nevi, congenital nevi, and nevocellular nevi.

RESULTS

All melanocytic cell types (nevus and melanoma cells) in the lesions studied had immunoreactive EGF/R. EGF/R immunoreactivity was also observed throughout the epidermal basal to granular cell layers overlying the melanocytic lesions, although dermal fibroblasts were negative.

CONCLUSION

The pattern of increased immunoreactive EGF/R in both benign and malignant nevocellular lesions suggests that although EGF/R are not a specific marker of potential malignancy in melanocytic lesions, they may mediate or coordinate growth of keratinocytes and nevus cells.

Variation in EGF-induced EGF receptor downregulation in human hepatoma-derived cell lines expressing different amounts of EGF receptor

The effect of epidermal growth factor (EGF) receptor overexpression on ligand-induced EGF receptor downregulation was examined using a hepatoma-derived cell line, PLC/PRF/5, which expresses normal amounts of the EGF receptor, and a subline, NPLC/PRF/5, which expresses 10-fold more receptors at its cell surface. PLC/PRF/5 cells efficiently downregulated surface receptor levels upon exposure to saturating and subsaturating concentrations of EGF; the rate of receptor downregulation corresponded to that of ligand-receptor internalization. Upon internalization, EGF receptors were degraded and receptor biosynthesis remained at basal levels. EGF surface receptor remained downregulated for as long as cells were exposed to EGF. By contrast, surface EGF receptor abundance in NPLC/PRF/5 cells decreased by only 5-15% after 1-4 h incubation with subsaturating doses of EGF and actually increased by 67% within 20 h. Exposure of these cells to saturating concentrations of EGF induced modest decreases in surface receptor abundance during the initial 12 h incubation, followed by a progressive decline to 30% of initial values by 24 h. Relative ligand-receptor internalization rates in NPLC/PRF/5 cells were lower than those in PLC/PRF/5, although their surface receptor population was even higher than that predicted by the decreased internalization rates. EGF receptor degradation in NPLC/PRF/5 cells was also inhibited; exposure to saturating levels of EGF for more than 16 h was necessary before significant degradation occurred. Receptor protein and mRNA biosynthesis in NPLC/PRF/5 were stimulated by 8 h exposure to EGF but when saturating concentrations of EGF were present for 16 h, receptor biosynthesis was inhibited. EGF receptor overexpression circumvents the downregulatory effect of EGF by decreasing the rate of receptor internalization, inhibiting degradation of the internalized receptor pool, and stimulating EGF receptor biosynthesis. Conversely, receptor downregulation becomes pronounced at late times when receptor degradation is high and biosynthesis is inhibited.

Epidermal growth factor receptor internalization and biosynthesis in the diabetic rat

The number of surface EGF receptors as well as their internalization rate and biosynthesis were analyzed in hepatocytes freshly isolated from control, streptozotocin-diabetic, and insulin-treated diabetic rats. All three parameters were decreased in diabetic animals and values were corrected by insulin treatment. Moreover, the inhibition of synthesis was specific for the EGF receptor since the other biosynthetically labeled proteins were not affected. These data demonstrate that the reduced number of hepatocyte surface EGF receptors results from an inhibition of EGF-receptor synthesis which is not compensated by a reduced internalization rate.

Use of transgenic mice in the study of proto-oncogene functions

The introduction of genes into the germ line of mammals has been utilized to study the regulation of gene expression, the role of certain genes during development and to establish animal models of human diseases. The present report explores the application of transgenic mice methodology to the study of the normal and transforming activity of some proto-oncogenes in the living animal. The major findings of these studies and their contribution to our understanding of the role of these proto-oncogenes in development and transformation will be evaluated.