Heregulin-beta is especially potent in activating phosphatidylinositol 3-kinase in nontransformed human mammary epithelial cells

The ERBB3 receptor in cancer and cancer gene therapy

ERBB3, a member of the epidermal growth factor receptor (EGFR) family, is unique in that its tyrosine kinase domain is functionally defective. It is activated by neuregulins, by other ERBB and nonERBB receptors as well as by other kinases, and by novel mechanisms. Downstream it interacts prominently with the phosphoinositol 3-kinase/AKT survival/mitogenic pathway, but also with GRB, SHC, SRC, ABL, rasGAP, SYK and the transcription regulator EBP1. There are likely important but poorly understood roles for nuclear localization and for secreted isoforms. Studies of ERBB3 expression in primary cancers and of its mechanistic contributions in cultured cells have implicated it, with varying degrees of certainty, with causation or sustenance of cancers of the breast, ovary, prostate, certain brain cells, retina, melanocytes, colon, pancreas, stomach, oral cavity and lung. Recent results link high ERBB3 activity with escape from therapy targeting other ERBBs in lung and breast cancers. Thus a wide and centrally important role for ERBB3 in cancer is becoming increasingly apparent. Several approaches for targeting ERBB3 in cancers have been tested or proposed. Small inhibitory RNA (siRNA) to ERBB3 or AKT is showing promise as a therapeutic approach to treatment of lung adenocarcinoma.

Species differences in renal Src activity direct EGF receptor regulation in life or death response to EGF

In rodent models of obstructive nephropathy, exogenous epidermal growth factor (EGF) attenuates tubule cell death in rats and exacerbates cell death in mice. To identify species differences in EGF receptor (EGFR) regulation and signaling, cell lysates were prepared from rat, mouse, and human proximal tubule cells (PTC) and compared by immunoblot analysis for expression and phosphorylation of Src and EGFR. Frozen kidney tissue was also analyzed. Results indicate mouse PTC have constitutive Src- and EGFR-kinase activities not detected in rat or human PTC. Immunoblots of rat, mouse, and human kidney homogenates confirmed this finding in vivo. Src-specific inhibitor PP2 and EGFR kinase inhibitor AG1478 decreased EGF-induced apoptosis in mouse PTC by 74% (P < 0.001) and 70% (P < 0.001), respectively. Expression of a constitutive Src mutant cDNA in rat PTC rendered cells susceptible to EGF-induced death. EGF decreased stretch-induced apoptosis by 66% (P < 0.001) relative to vehicle control in human PTC, similar to rat PTC response. We conclude that elevated Src activity in mouse tubular cells alters downstream EGFR signaling and increases susceptibility to EGF-induced cell death. The unexpected finding that a therapeutic agent (EGF) in rats is detrimental in mice underscores the importance of determining which animal best represents the response of human kidneys to a given agent.

System properties of ErbB receptor signaling for the understanding of cancer progression

An intracellular signal transduction network constitutes an assembled machinery to control the dynamics of kinase-phosphatase cascade and gene expression. Spatio-temporal analyses of the cellular process can explain the biochemical role of the receptor tyrosine kinases in cancer development from a system point of view.

Quantitative transcriptional control of ErbB receptor signaling undergoes graded to biphasic response for cell differentiation

ErbB receptor ligands, epidermal growth factor (EGF) and heregulin (HRG), induce dose-dependent transient and sustained intracellular signaling, proliferation, and differentiation of MCF-7 breast cancer cells, respectively. In an effort to delineate the ligand-specific cell determination mechanism, we investigated time course gene expressions induced by EGF and HRG that induce distinct cellular phenotypes in MCF-7 cells. To analyze independently the effects of ligand dosage and time for gene expression, we developed a statistical method for estimating the two effects. Our results indicated that signal transduction pathways convey quantitative properties of the dose-dependent activation of ErbB receptor to early transcription. The results also implied that moderate changes in the expression levels of a number of genes, not the predominant regulation of a few specific genes, might cooperatively work at the early stage of the transcription for determining cell fate. However, the EGF- and HRG-induced distinct signal durations resulted in the ligand-oriented biphasic induction of proteins after 20 min. The selected gene list and HRG-induced prolonged signaling suggested that transcriptional feedback to the intracellular signaling results in a graded to biphasic response in the cell determination process and that each ErbB receptor is inextricably responsible for the control of amplitude and duration of cellular biochemical reactions.

The BAD protein integrates survival signaling by EGFR/MAPK and PI3K/Akt kinase pathways in PTEN-deficient tumor cells

Tumor cells with mutated PTEN proliferate in an EGFR-independent manner. Induction of PTEN sensitizes cells to EGFR inhibition, and the combination causes synergistic apoptosis. Synergy is due to inhibition of two parallel pathways that phosphorylate the proapoptotic protein BAD at distinct sites. Serine 112 phosphorylation is EGFR/MEK/MAPK dependent, whereas serine 136 phosphorylation is PI3K/Akt dependent. Either phosphorylation is sufficient to sequester BAD to 14-3-3. BAD is released and apoptosis is induced only if both serines are dephosphorylated in response to inhibition of both pathways. Reduction of BAD expression by RNA interference prevents apoptosis in response to pathway inhibition. Thus, BAD integrates the antiapoptotic effects of both pathways. Combined inhibition of EGFR and PI3K signaling may be a useful therapeutic strategy.

Deranged Kv channel regulation in fibroblasts from mice lacking the serum and glucocorticoid inducible kinase SGK1

Coexpression of the serum and glucocorticoid inducible kinase 1 (SGK1) up-regulates Kv channel activity in Xenopus oocytes and human embryonic kidney cells. To investigate the physiological impact of SGK1 dependent Kv channel regulation, we recorded whole-cell currents in lung fibroblasts from SGK1 knockout mice (sgk1-/-) and wild-type littermates (sgk1+/+). Serum-grown mouse lung fibroblasts (MLF) from both genotypes exhibited voltage-gated outwardly rectifying K(+)-currents with time-dependent activation (tau(act) approximately 3 msec), slow inactivation (tau(inact) approximately 700 msec), use-dependent inactivation, and (partial) inhibition by K(+) channel blockers TEA, 4-AP, and margatoxin. In serum grown MLF peak Kv current density at +100 mV was significantly lower in sgk1-/- (14 +/- 2 pA/pF, n = 13) than in sgk1+/+ (31 +/- 4 pA/pF, n = 16). PCR amplification of different Kv1 and Kv3 subunits from mouse fibroblasts demonstrated the expression of Kv1.1-1.7, Kv3.1, and Kv3.3 mRNA in both sgk1+/+ and sgk1-/- cells. Upon serum deprivation Kv currents almost disappeared in sgk1+/+ (4 +/- 1 pA/pF, n = 11) but not in sgk1-/- (10 +/- 1 pA/pF, n = 6) MLF. Accordingly, following serum deprivation Kv current density was significantly lower in sgk1+/+ than in sgk1-/-. Stimulation of serum-depleted cells with dexamethasone (dex) (1 microM, 1 day), IGF-1 (6.7 microM, 4-6 h) or both, significantly activated Kv currents in sgk1+/+ but not in sgk1-/- MLF. In the presence of both, dex and IGF-1, the Kv current density was significantly larger in sgk1+/+ (27 +/- 3 pA/pF, n = 12) than in sgk1-/- (13 +/- 3 pA/pF, n = 10) cells. Similar to MLF, Kv currents were significantly higher in sgk1+/+ mouse tail fibroblasts (MTF). In sgk1+/+ but not sgk1-/- MTF the Kv currents were inhibited upon serum deprivation and reincreased after stimulation of serum deprived MTF with dex (1 microM, 1 day) and afterwards with IGF-1 (6.7 microM, 4-6 h). According to Fura-2-fluorescence capacitative Ca(2+) entry was lower in sgk1-/- MTF compared to sgk1+/+ MTF. Upon serum deprivation capacitative Ca(2+) entry decreased significantly in sgk1+/+ but not in sgk1-/- MTF. Stimulation of depleted cells with dex (1 microM, 1 day) and afterwards with IGF-1 (6.7 microM, 4-6 h) reincreased capacitative Ca(2+) entry in sgk1+/+ MTF, whereas in sgk1-/- cells it remained unchanged. In conclusion, lack of SGK1 does not abrogate Kv channel activity but abolishes regulation of those channels by serum, glucocorticoids and IGF-1, an effect influencing capacitative Ca(2+) entry.

Increased Constitutive Activity of PKB/Akt in Tamoxifen Resistant Breast Cancer MCF-7 Cells

The tamoxifen-resistant (TAM-R) MCF-7 breast cancer cell line has been used as a model to identify the signalling pathways that enable resistant cancer cells to grow independently of steroid hormones. In TAM-R cells, peptide growth factor signalling pathways appear to be important in modified cell behaviour, growth and survival. The PI3 kinase signalling components Akt1 and Akt2 are expressed at similar levels by both parental wild-type MCF-7 and TAM-R cells, but Akt1 phosphorylation is significantly increased in TAM-R cells grown under basal conditions. High levels of basal Akt, GSK3 alpha / beta and p70S6 kinase phosphorylation are all inhibited by the PI3 kinase inhibitor, LY 294002. The ligands for the EGFR/erbB1 receptor, EGF (epidermal growth factor) and TGF alpha (transforming growth factor- alpha ) demonstrate an increased ability to activate Akt in TAM-R compared with parental MCF-7 cells and it is proposed that the preferred autocrine or paracrine activation of Akt occurs via the erbB heterodimer EGFR/erbB2 in TAM-R cells. Akt phosphorylation is reduced by gefitinib ("Iressa"/ZD1839). The results suggest that the PI3 kinase pathway plays a role in proliferation of TAM-R cells and is important in the increased EGF induced membrane ruffling detected in the resistant cells. Increased Akt1 activation may contribute to the aggressive phenotype of tamoxifen resistant ER (oestrogen receptor) positive breast cancers.

Regulation of the voltage gated K+ channel Kv1.3 by the ubiquitin ligase Nedd4-2 and the serum and glucocorticoid inducible kinase SGK1

The stimulation of cell proliferation by insulin like growth factor IGF-1 has previously been shown to depend on activation of voltage gated K(+) channels. The signaling involved in activation of voltage gated K(+) channel Kv1.3 includes the phosphatidylinositol-3 (PI3) protein kinase, 3-phosphoinositide dependent protein kinase PDK1 and the serum and glucocorticoid inducible kinase SGK1. However, nothing is known about mechanisms mediating the stimulation of Kv1.3 by SGK1. Most recently, SGK1 has been shown to phosphorylate and thus inactivate the ubiquitin ligase Nedd4-2. The present study has been performed to explore whether the regulation of Kv1.3 involves Nedd4-2. To this end Kv1.3 has been expressed in Xenopus oocytes with or without coexpression of Nedd4-2 and/or constitutively active (S422D)SGK1. In oocytes expressing Kv1.3 but not in water injected oocytes, depolarization from a holding potential of -80 mV to +20 mV triggers rapidly inactivating currents typical for Kv1.3. Coexpression of Nedd4-2 decreases, coexpression of (S422D)SGK1 enhances the currents significantly. The effects of either Nedd4-2 or of SGK1 are abrogated by destruction of the respective catalytic subunits ((C938S)Nedd4-2 or (K127N)SGK1). Further experiments revealed that wild type SGK1 and SGK3 and to a lesser extent SGK2 are similarly effective in stimulating Kv1.3 in both, presence and absence of Nedd4-2. It is concluded that Kv1.3 is downregulated by Nedd4-2 and stimulates by SGK1, SGK2, and SGK3. The data thus disclose a novel mechanism of Kv1.3 channel regulation.

Blockade of epidermal growth factor- or heregulin-dependent ErbB2 activation with the anti-ErbB2 monoclonal antibody 2C4 has divergent downstream signaling and growth effects

Due to heterodimerization and a variety of stimulating ligands, the ErbB receptor system is both diverse and flexible, which proves particularly advantageous to the aberrant signaling of cancer cells. However, specific mechanisms of how a particular receptor contributes to generating the flexibility that leads to aberrant growth regulation have not been well described. We compared the utilization of ErbB2 in response to epidermal growth factor (EGF) and heregulin stimulation in colon carcinoma cells. Anti-ErbB2 monoclonal antibody 2C4 blocked heregulin-stimulated phosphorylation of ErbB2 and ErbB3; activation of mitogen-activated protein kinase (MAPK), phosphatidylinositol 3'-kinase (PI3K), and Akt; proliferation; and anchorage-independent growth. 2C4 blocked EGF-mediated phosphorylation of ErbB2 and inhibited PI3K/Akt and anchorage-independent growth but did not affect ErbB1 or MAPK. Immunoprecipitations showed that ErbB3 and Grb2-associated binder (Gab) 1 were phosphorylated and associated with PI3K activity after heregulin treatment and that Gab1 and Gab2, but not ErbB3, were phosphorylated and associated with PI3K activity after EGF treatment. These data show that monoclonal antibody 2C4 inhibited all aspects of heregulin signaling as well as anchorage-independent and monolayer growth. Furthermore, we identify ErbB2 as a critical component of EGF signaling to the Gab1/Gab2-PI3K-Akt pathway and anchorage-independent growth, but EGF stimulation of MAPK and monolayer growth can occur efficiently without the contribution of ErbB2.

Herstatin inhibits heregulin-mediated breast cancer cell growth and overcomes tamoxifen resistance in breast cancer cells that overexpress HER-2

Ligands of the ErbB family of receptors and estrogens control the proliferation of breast cancer cells. Overexpression of human EGF receptor HER-2 (erbB2) leads to amplified heregulin (HRG) signaling, promoting more aggressive breast cancer that is nonresponsive to estrogen and the antiestrogenic drug tamoxifen. Herstatin (Hst), a secreted HER-2 gene product, binds to the HER-2 receptor ectodomain blocking receptor activation. The aim of this study was to investigate the impact of this HER-2 inhibitor on HRG-induced signaling, proliferation, and sensitivity to tamoxifen in breast cancer cells with and without HER-2 overexpression. The expression of Hst in MCF7 cells eliminated HRG signaling through both mitogen-activated protein kinase and Akt pathways and prevented HRG-mediated proliferation. The loss in signaling corresponded to downregulation of the HRG receptors, HER-3 and HER-4, whereas HER-2 overexpression strongly stimulated the levels of both HRG receptors. Although Hst blocked HRG signaling in both parental and HER-2 transfected cells, it enhanced sensitivity to tamoxifen only in the MCF7 cells that overexpressed HER-2. To evaluate further the efficacy of Hst as an anticancer agent, His-tagged Hst was expressed in transfected insect cells, purified, and added to the breast cancer cells. As in the transfected cells, purified Hst inhibited HER-3 levels and suppressed HRG-induced proliferation of MCF7 and BT474 breast cancer cells. In contrast, the HER-2 monoclonal antibody, herceptin, downregulated HER-2, but not HER-3. These results suggest the potential use of Hst against HRG-mediated growth of breast cancers with high and low levels of HER-2 and against tamoxifen resistance in HER-2 overexpressing breast cancer.

Novel agents for the prevention of breast cancer: targeting transcription factors and signal transduction pathways

Transformation of breast cells occurs through loss or mutation of tumor suppressor genes, or activation or amplification of oncogenes, leading to deregulation of signal transduction pathways, abnormal amplification of growth signals, and aberrant expression of genes that ultimately transform the cells into invasive cancer. The goal of cancer preventive therapy, or "chemoprevention," is to eliminate premalignant cells or to block the progression of normal cells into cancer. Multiple alterations in signal pathways and transcription factors are observed in mammary gland tumorigenesis. In particular, estrogen receptor (ER) deregulation plays a critical role in breast cancer development and progress, and targeting ER with selective ER modulators (SERMs) has achieved significant reduction of breast cancer incidence in women at high risk for breast cancer. However, not all breast cancer is prevented by SERMs, because 30-40% of the tumors are ER-negative. Other receptors for retinoids, vitamin D analogs and peroxisome proliferator-activiator, along with transcription factors such as AP-1, NF-kappaB, and STATs (signal transducers and activators of transcription) affect breast tumorigenesis. This is also true for the signal transduction pathways, for example cyclooxygenase 2 (Cox-2), HER2/neu, mitogen-activated protein kinase (MAPK), and PI3K/Akt. Therefore, proteins in pathways that are altered during the process of mammary tumorigenesis may be promising targets of future chemopreventive drugs. Many newly-developed synthetic or natural compounds/agents are now under testing in preclinical studies and clinical trials. Receptor selective retinoids, receptor tyrosine kinase inhibitors (TKIs), SERMs, Cox-2 inhibitors, and others are some of the promising novel agents for the prevention of breast cancer. The chemopreventive activity of these agents and other novel signal transduction inhibitors are discussed in this chapter.

Induction or suppression of expression of cytochrome C oxidase subunit II by heregulin beta 1 in human mammary epithelial cells is dependent on the levels of ErbB2 expression

The ErbB family of receptor kinases is composed of four members: epidermal growth factor receptor (EGFR/ErbB1), ErbB2/neu, ErbB3, and ErbB4. Amplification of the ErbB2/neu is found in about 30% of breast cancer patients and is associated with a poor prognosis. Heregulin (HRG) activates the ErbB2 via induction of heterodimerization with ErbB3 and ErbB4 receptors. With suppression subtractive hybridization, we demonstrated that the expression of cytochrome c oxidase subunit II (COXII) is HRG-responsive. Two nontransformed human mammary epithelial cell lines, the HB2 and the HB2(ErbB2) (the HB2 engineered to overexpress ErbB2), displayed an opposite response to HRG-mediated regulation. HRG upregulated mRNA expression of COXII in the HB2 cells, but suppressed COXII expression in the HB2(ErbB2) cells. A human breast cancer cell line (T47D), which expresses ErbB2 at a level similar to that of the HB2 cells, also responded to HRG by increasing COXII mRNA levels. Therefore, HRG regulation of COXII expression depends on the levels of ErbB2 expression. Furthermore, the expression of COXII was inversely correlated to the levels of ErbB2, i.e., the cells overexpressing ErbB2 exhibited lower COXII levels. HRG-evoked signal transduction differed between the cells with normal ErbB expression and the cells overexpressing ErbB2. The activation of both ERK and PI3-K was essential for HRG regulation of COXII, i.e., blockage of either pathway eliminated HRG-mediated alteration. This is the first report demonstrating that the expression of mitochondria-encoded COXII is HRG-responsive. The levels of ErbB2 expression are decisive for the diverse biological activities of HRG.

Expression of membrane-associated mucins MUC1 and MUC4 in major human salivary glands

Mucins are high molecular weight glycoproteins secreted by salivary glands and epithelial cells lining the digestive, respiratory, and reproductive tracts. These glycoproteins, encoded in at least 13 distinct human genes, can be subdivided into gel-forming and membrane-associated forms. The gel-forming mucin MUC5B is secreted by mucous acinar cells in major and minor salivary glands, but little is known about the expression pattern of membrane-associated mucins. In this study, RT-PCR and Northern blotting demonstrated the presence of transcripts for MUC1 and MUC4 in both parotid and submandibular glands, and in situ hybridization localized these transcripts to epithelial cells lining striated and excretory ducts and in some serous acinar cells. The same cellular distribution was observed by immunohistochemistry. Soluble forms of both mucins were detected in parotid secretion after immunoprecipitation with mucin-specific antibodies. These studies have shown that membrane-associated mucins are produced in both parotid and submandibular glands and that they are expressed in different cell types than gel-forming mucins. Although the function of these mucins in the oral cavity remains to be elucidated, it is possible that they both contribute to the epithelial protective mucin layer and act as receptors initiating one or more intracellular signal transduction pathways.

AKT2 is frequently upregulated in HER-2/neu-positive breast cancers and may contribute to tumor aggressiveness by enhancing cell survival

Amplification or overexpression of the HER-2/neu gene in breast cancers is associated with aggressive behavior and resistance to therapeutic regimens. The molecular mechanisms that contribute to therapeutic resistance/survival of HER-2/neu-overexpressing tumor cells have not been well defined. To determine if phosphatidylinositol 3-kinase/AKT signaling contributes to cell survival in HER-2/neu-positive breast cancers, we performed immunohistochemical analyses to evaluate expression of HER-2/neu and AKT in a series of 52 breast carcinomas. Elevated expression of HER-2/neu was found to correlate with overexpression of AKT2 protein and activation of AKT kinase. HER-2/neu-overexpressing breast cancer cell lines were resistant to apoptosis induced by UV treatment and hypoxia, which was suppressed in the presence of the phosphatidylinositol 3-kinase inhibitors LY294002 and wortmannin, indicating a link between AKT activation and stress resistance in HER-2/neu-overexpressing cells. These observations suggest that AKT signaling augments resistance to stress-induced apoptosis in breast cancer cells overexpressing HER-2/neu.

Effects of in vivo heregulin beta1 treatment in wild-type and ErbB gene-targeted mice depend on receptor levels and pregnancy

Mice heterozygous (+/-) for either heregulin (HRG), ErbB2, or ErbB3 were created by gene targeting, resulting in the loss of one functional gene copy and an associated decrease in targeted protein. We examined the in vivo activity of recombinant HRG peptide, rHRG beta1 (amino acids 177 to 241), in the three heterozygous mouse lines and in wild-type (WT) mice, both pregnant and nonpregnant. Nonpregnant WT and HRG(+/-) mice of both sexes were sensitive to rHRG beta1 treatment as evidenced by a high mortality rate associated with abdominal enlargement and parietal cell loss. However, pregnant WT mice and ErbB2 and ErbB3 heterozygous mice treated with rHRG beta1 were less affected, with significantly lower mortality rates and a less severe abdominal phenotype. Histological analysis revealed extensive breast ductal hyperplasia in females of all genotypes after rHRG beta1 treatment. Hyperplasia of other epithelial tissues such as the pancreas and intestine and the growth of cardiac nerve bundles were also observed, independent of sex.