A number of different epidermal growth factor (EGF)-related peptides such as EGF, transforming growth factor α (TGFα), amphiregulin (AR), heregulin (HRG), and cripto-1 (CR-1), are coexpressed to varying degrees in both normal and malignant mammary epithelial cells. However, in general the frequency and level of expression of TGFα, AR, and CR-1 are higher in malignant breast epithelial cells than in normal mammary epithelium. In addition, several of these peptides such as TGFα and AR can function as autocrine and/or juxtacrine growth factors in mammary epithelial cells, and their expression is stringently regulated by mammotrophic hormones such as estrogens, activated proto-oncogenes that have been implicated in the pathogenesis of breast cancer, and other growth factors. The redundancy of expression that is observed for a number of these structurally related peptides in both normal and malignant mammary epithelial cells suggests that some of these peptides may be involved in regulating other aspects of cellular behavior such as differentiation in addition to proliferation.

Worldwide, breast cancer is the second leading type of cancer in women. In Germany, nearly 40000 new cases of breast cancer and 15000 deaths are reported each year. Determining the etiological and pathogenetic factors that contribute to breast cancer will provide important insights into the design of novel preventive, diagnostic and therapeutic approaches. One extremely productive avenue is the study of growth factors and their receptors in breast cancer. Different families of growth factors are involved in the regulation of growth, differentiation, transformation and apoptosis as well as angiogenesis, migration and metastasis. The EGF-superfamily is one of the largest known family of growth factors. Members of this family include epidermal growth factor (EGF), transforming growth factor alpha (TGF alpha), amphiregulin, betacellulin, epiregulin, and heparin-binding EGF, as well as the Cripto- and heregulin-subfamilies. Activation of specific intracellular signaling pathways by EGF-related growth factors occurs by activation of the type I EGF receptor-related erb B-tyrosine kinase family. The EGF-receptor family consists of erb B (EGF-receptor, EGF-R), erb B-2 (HER1/neu), erb B-3 (HER3/neu) and erb B-4 (HER4/neu). It is well established that the overexpression of the EGF-R or erb B-2 is correlated with a more aggressive tumor behavior and is one of the more unfavorable prognostic markers in breast cancer. The erb B-receptors are important targets for novel therapeutic approaches as evidenced by the fact that Herceptin(TM), a humanized monoclonal antibody generated against erb B-2, has been shown to be therapeutically effective in advanced human breast cancer. In the forseeable future, the modern arsenal of breast cancer treatment will be supplemented by effective, biological-oriented therapeutic modalities.

Cripto-1 (CR-1), a recently discovered protein of the epidermal growth factor (EGF) family, was found to interact with a high affinity, saturable binding site(s) on HC-11 mouse mammary epithelial cells and on several different human breast cancer cell lines. This receptor exhibits specificity for CR-1, since other EGF-related peptides including EGF, transforming growth factor α, heparin-binding EGF-like growth factor, amphiregulin, epiregulin, betacellulin, or heregulin β1 that bind to either the EGF receptor or to other type 1 receptor tyrosine kinases such as erb B-3 or erb B-4 fail to compete for binding. Conversely, CR-1 was found not to directly bind to or to activate the tyrosine kinases associated with the EGFR, erb B-2, erb B-3, or erb B-4 either alone or in various pairwise combinations which have been ectopically expressed in Ba/F3 mouse pro-B lymphocyte cells. However, exogenous CR-1 could induce an increase in the tyrosine phosphorylation of 185- and 120-kDa proteins and a rapid (within 3-5 min) increase in the tyrosine phosphorylation of the SH2-containing adaptor proteins p66, p52, and p46 Shc in mouse mammary HC-11 epithelial cells and in human MDA-MB-453 and SKBr-3 breast cancer cells. CR-1 was also found to promote an increase in the association of the adaptor Grb2-guanine nucleotide exchange factor-mouse son of sevenless (mSOS) signaling complex with tyrosine-phosphorylated Shc in HC-11 cells. Finally, CR-1 was able to increase p42erk-2 mitogen-activated protein kinase (MAPK) activity in HC-11 cells within 5-10 min of treatment. These data demonstrate that CR-1 can function through a receptor which activates intracellular components in the ras/raf/MEK/MAPK pathway.

Amphiregulin (AR), a member of the epidermal growth factor (EGF) family, was found to be as potent as EGF in stimulating the anchorage-dependent growth (ADG) of immortalized, nontransformed human mammary epithelial MCF-10A cells. MCF-10A cells transformed by either an activated human c-Ha-ras protooncogene (MCF-10A ras) or by overexpression of a nonactivated rat c-neu gene (MCF-10A neu) exhibited a 35% reduction in the response to AR in ADG when compared to MCF-10A cells, but AR was still as potent as EGF in these transformants. Exogenous AR exhibited only 15-20% of the activity of EGF in stimulating the anchorage-independent growth, a response that is normally dependent upon exogenous EGF, of the oncogene-transformed MCF-10A cells. MCF-10A cells express low levels of a 1.4-kb AR mRNA transcript, while MCF-10A ras and MCF-10A neu cells display a 15- to 30-fold increase in the levels of AR mRNA and endogenous AR protein as determined by Western blot analysis. Exogenous EGF was found to induced both the AR mRNA and protein in the MCF-10A parental and transformed cells. A 20-mer phosphorothioate antisense deoxyoligonucleotide complementary to the 5' sequence of AR mRNA was able to significantly reduce the levels of endogenous AR protein and to inhibit the EGF-stimulated ADG and anchorage-independent growth of MCF-10A ras and MCF-10A neu cells. These data suggest that AR may function as an EGF-dependent autocrine growth factor in mammary epithelial cells that have been transformed by either a point-mutated c-Ha-ras or c-neu.

The expression of three epidermal growth factor (EGF)-related peptides, transforming growth factor alpha (TGF-alpha), amphiregulin (AR) and cripto-1 (CR-1), was examined by immunocytochemistry (ICC) in 68 primary infiltrating ductal (IDCs) and infiltrating lobular breast carcinomas (ILCs), and in 23 adjacent non-involved human mammary tissue samples. Within the 68 IDC and ILC specimens, 54 (79%) expressed immunoreactive TGF-alpha, 52 (77%) expressed AR and 56 (82%) expressed CR-1. Cytoplasmic staining was observed with all of the antibodies, and this staining could be eliminated by preabsorption of the antibodies with the appropriate peptide immunogen. Cytoplasmic staining with all of the antibodies was confined to the carcinoma cells, since no specific immunoreactivity could be detected in the surrounding stromal or endothelial cells. In addition to cytoplasmic reactivity, the AR antibody also exhibited nuclear staining in a number of the carcinoma specimens. No significant correlations were found between the percentage of carcinoma cells that were positive for TGF-alpha, AR or CR-1 and oestrogen receptor status, axillary lymph node involvement, histological grade, tumour size, proliferative index, loss of heterozygosity on chromosome 17p or overall patient survival. However, a highly significant inverse correlation was observed between the average percentage of carcinoma cells that expressed AR in individual tumours and the presence of a point-mutated p53 gene. Likewise, a significantly higher percentage of tumour cells in the ILC group expressed AR as compared with the average percentage of tumour cells that expressed AR in the IDC group. Of the 23 adjacent, non-involved breast tissue samples, CR-1 could be detected by ICC in only three (13%), while TGF-alpha was found in six (26%) and AR in ten (43%) of the non-involved breast tissues. These data demonstrate that breast carcinomas express multiple EGF-related peptides and show that the differential expression of CR-1 in malignant breast epithelial cells may serve as a potential tumour marker for breast cancer.

The human and mouse cripto-1 (CR-1) genes can code for proteins related in structure to epidermal growth factor (EGF). A specific 36-kDa immunoreactive protein was detected by Western blot analysis in human cell lines that express CR-1 mRNA but not in cell lines that fail to express this transcript. Immunoprecipitation of GEO colon carcinoma or mouse embryonal carcinoma cells detected 27-29-kDa and 24-kDa proteins, respectively. Cell lysates and conditioned medium that were prepared from several CHO clones and were expressing either a recombinant human or mouse CR-1 cDNA contained immunospecific 27-29-kDa and 24-kDa proteins, respectively. Monensin or tunicamycin treatment resulted in a shift of the 27-29-kDa human CR-1 protein to 24 kDa and 20 kDa, respectively. The 20-kDa protein was also observed after digestion of the 27-29-kDa human CR-1 protein with N-glycosidase F. Using two CR-1 synthetic refolded peptides that correspond to the EGF-like domain of the human CR-1 sequence or conditioned medium obtained from human CR-1 expressing CHO cells, growth stimulatory activity could be detected on non-transformed human mammary epithelial cells and on two human breast cancer cell lines. EGF receptor-blocking antibody did not inhibit the growth stimulatory action of the CR-1 protein. Likewise, the CR-1 refolded peptides or conditioned medium from the human CR-1-expressing CHO cells failed to inhibit the binding of 125I-EGF in an EGF-radioreceptor assay. These data demonstrate that the CR-1 is a glycoprotein that can function as a growth factor through an EGF receptor-independent pathway.

CRIPTO is an epidermal growth factor-related gene expressed in a majority of human colorectal tumors. To assess the role of CRIPTO in the growth control of human colon cancer, we have treated human colon carcinoma GEO and CBS cells, that possess high levels of CRIPTO, and WIDR colon cancer cells, that are negative for CRIPTO expression, with two antisense phosphorothioate oligodeoxynucleotides complementary to the 5' end of the human CRIPTO mRNA. Both antisense oligodeoxynucleotides significantly reduced endogenous CRIPTO protein levels and inhibited GEO and CBS cell growth in monolayer and in semisolid medium, whereas they did not affect WIDR cell growth. In addition, GEO, CBS and WIDR cells were infected with a recombinant retroviral vector containing the hygromycin-resistance gene and a 900 bp EcoRI-EcoRI coding fragment of the human CRIPTO cDNA oriented in the 3' to 5' direction. GEO and CBS CRIPTO antisense infectants exhibited a 60 to 70% reduction in CRIPTO protein expression, in monolayer growth and in soft agar cloning efficiency as compared to parental noninfected cells. In contrast, infection of WIDR cells with the CRIPTO antisense retrovirus did not alter their growth. Finally, GEO CRIPTO antisense infectants formed tumors in nude mice that were significantly smaller and had a larger latency period as compared to noninfected GEO cells.

Cripto-1 (CR-1), amphiregulin (AR), and heregulin alpha (HRGalpha) are three recently discovered epidermal growth factor (EGF)-related peptides. The expression of these proteins was determined in MCF-7, ZR-75-1, T-47D, SK-BR-3, MDA-MB-231, MDA-MB-468, and Hs-578T human breast cancer cell lines using reverse transcriptase-polymerase chain reaction (RT-PCR), Northern blotting, and immunocytochemistry (ICC). The expression of CR-1 mRNA was detected by RT-PCR in all of the breast cancer cell lines. AR mRNA was detected by Northern blot analysis in MCF-7, ZR-75-1, T-47D, MDA-MB-231, and MDA-MB-468 cells while HRGalpha mRNA was expressed in only MDA-MB-231 and Hs-578T cells. All estrogen receptor-positive cell lines were found to express AR mRNA, and estrogen was able to induce AR mRNA expression in estrogen-depleted MCF-7 cells. CR-1 and AR proteins could be immunocytochemically detected in the breast cancer cell lines that were expressing CR-1 and AR mRNA using monospecific rabbit polyclonal antibodies. The anti-CR-1 antibody was also used to examine 26 human primary breast carcinomas by ICC for CR-1 expression. Seventy-five percent of the carcinomas were found to express the CR-1 protein while the adjacent non-involved breast epithelium was negative. These data demonstrate that CR-1, AR, and HRGalpha are coexpressed in human breast cancer cells and suggest that these three EGF-related peptides might perform a role in the autocrine growth regulation of human breast carcinoma cells.