Antagonism of EGFR and HER3 enhances the response to inhibitors of the PI3K-Akt pathway in triple-negative breast cancer

A critical role for HER3 in HER2-amplified and non-amplified breast cancers: function of a kinase-dead RTK

ERBB3/HER3 is the most intriguing RTK by virtue of its ability to transduce multiple cytosolic signals for the proliferation and growth of tumor cells in spite of being a "kinase dead" receptor that binds to its true ligand, heregulin. Although other members of the HER3 family like EGFR and HER2 have long been recognized to be associated with breast tumorigenesis and studied because of their predictive and prognostic value, the significance of HER3 as an irrefutable component of HER family signalosome is a relatively new development. The recent understanding of signals originating from the oncogenic partnership of HER3 with HER2 in the context of HER2 amplification/overexpression showed the critical clinical value for the treatment of HER2+BC. The downstream signaling cascade (included but not limited to the PI3K signaling) associated with signals originating from HER2:HER3 dimers play a vital role in the tumorigenesis, drug-resistance and tumor progression of HER2+BC. The upregulation of HER3 activity provides an alternate "escape route" via which tumor cells bypass either the inhibition of the HER family RTKs or the inhibition of the downstream PI3K-AKT-mTOR signaling pathway. By understanding the signaling that provides this "escape route" for these tumor cells treated with a targeted therapy (HER2 inhibitors or inhibitors of downstream PI3K-AKT-mTOR signaling pathway), we are just beginning to appreciate the prognostic value of HER3 in breast cancer. In this review, we will discuss the relevance of HER3 signaling in the context of, (1) downstream oncogenic signals and (2) therapeutic options in HER2 amplified BC.

Quantification of HER family receptors in breast cancer

The clinical success of trastuzumab in breast cancer taught us that appropriate tumor evaluation is mandatory for the correct identification of patients eligible for targeted therapies. Although HER2 protein expression by immunohistochemistry (IHC) and gene amplification by fluorescence in situ hybridization (FISH) assays are routinely used to select patients to receive trastuzumab, both assays only partially predict response to the drug. In the case of epidermal growth factor receptor (EGFR), the link between the presence of the receptor or its amplification and response to anti-EGFR therapies could not be demonstrated. Even less is known for HER3 and HER4, mainly due to lack of robust and validated assays detecting these proteins. It is becoming evident that, besides FISH and IHC, we need better assays to quantify HER receptors and categorize the patients for individualized treatments. Here, we present the current available methodologies to measure HER family receptors and discuss the clinical implications of target quantification.

Melatonin-mediated inhibition of Purkinje neuron P-type Ca²⁺ channels in vitro induces neuronal hyperexcitability through the phosphatidylinositol 3-kinase-dependent protein kinase C delta pathway

Although melatonin receptors are widely expressed in the mammalian central nervous system and peripheral tissues, there are limited data regarding the functions of melatonin in cerebellar Purkinje cells. Here, we identified a novel functional role of melatonin in modulating P-type Ca(2+) channels and action-potential firing in rat Purkinje neurons. Melatonin at 0.1 μm reversibly decreased peak currents (I(Ba)) by 32.9%. This effect was melatonin receptor 1 (MT(R1)) dependent and was associated with a hyperpolarizing shift in the voltage dependence of inactivation. Pertussis toxin pretreatment, intracellular application of QEHA peptide, and a selective antibody raised against the Gβ subunit prevented the inhibitory effects of melatonin. Pretreatment with phosphatidylinositol 3-kinase (PI3K) inhibitors abolished the melatonin-induced decrease in I(Ba). Surprisingly, melatonin responses were not regulated by Akt, a common downstream target of PI3K. Melatonin treatment significantly increased protein kinase C (PKC) activity 2.1-fold. Antagonists of PKC, but not of protein kinase A, abolished the melatonin-induced decrease in I(Ba). Melatonin application increased the membrane abundance of PKCδ, and PKCδ inhibition (either pharmacologically or genetically) abolished the melatonin-induced IBa response. Functionally, melatonin increased spontaneous action-potential firing by 53.0%; knockdown of MT(R1) and blockade of P-type channels abolished this effect. Thus, our results suggest that melatonin inhibits P-type channels through MT(R1) activation, which is coupled sequentially to the βγ subunits of G(i/o)-protein and to downstream PI3K-dependent PKCδ signaling. This likely contributes to its physiological functions, including spontaneous firing of cerebellar Purkinje neurons.

Urotensin-II receptor stimulation of cardiac L-type Ca2+ channels requires the βγ subunits of Gi/o-protein and phosphatidylinositol 3-kinase-dependent protein kinase C β1 isoform

Recent studies have demonstrated that urotensin-II (U-II) plays important roles in cardiovascular actions including cardiac positive inotropic effects and increasing cardiac output. However, the mechanisms underlying these effects of U-II in cardiomyocytes still remain unknown. We show by electrophysiological studies that U-II dose-dependently potentiates L-type Ca(2+) currents (ICa,L) in adult rat ventricular myocytes. This effect was U-II receptor (U-IIR)-dependent and was associated with a depolarizing shift in the voltage dependence of inactivation. Intracellular application of guanosine-5'-O-(2-thiodiphosphate) and pertussis toxin pretreatment both abolished the stimulatory effects of U-II. Dialysis of cells with the QEHA peptide, but not scrambled peptide SKEE, blocked the U-II-induced response. The phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin as well as the class I PI3K antagonist CH132799 blocked the U-II-induced ICa,L response. Protein kinase C antagonists calphostin C and chelerythrine chloride as well as dialysis of cells with 1,2bis(2aminophenoxy)ethaneN,N,N',N'-tetraacetic acid abolished the U-II-induced responses, whereas PKCα inhibition or PKA blockade had no effect. Exposure of ventricular myocytes to U-II markedly increased membrane PKCβ1 expression, whereas inhibition of PKCβ1 pharmacologically or by shRNA targeting abolished the U-II-induced ICa,L response. Functionally, we observed a significant increase in the amplitude of sarcomere shortening induced by U-II; blockade of U-IIR as well as PKCβ inhibition abolished this effect, whereas Bay K8644 mimicked the U-II response. Taken together, our results indicate that U-II potentiates ICa,L through the βγ subunits of Gi/o-protein and downstream activation of the class I PI3K-dependent PKCβ1 isoform. This occurred via the activation of U-IIR and contributes to the positive inotropic effect on cardiomyocytes.

Trial watch: Tumor-targeting monoclonal antibodies for oncological indications

An expanding panel of monoclonal antibodies (mAbs) that specifically target malignant cells or intercept trophic factors delivered by the tumor stroma is now available for cancer therapy. These mAbs can exert direct antiproliferative/cytotoxic effects as they inhibit pro-survival signal transduction cascades or activate lethal receptors at the plasma membrane of cancer cells, they can opsonize neoplastic cells to initiate a tumor-targeting immune response, or they can be harnessed to specifically deliver toxins or radionuclides to transformed cells. As an indication of the success of this immunotherapeutic paradigm, international regulatory agencies approve new tumor-targeting mAbs for use in cancer patients every year. Moreover, the list of indications for previously licensed molecules is frequently expanded to other neoplastic disorders as the results of large, randomized clinical trials become available. Here, we discuss recent advances in the preclinical and clinical development of tumor-targeting mAbs for oncological indications.

Targeting HER3 by interfering with its Sec61-mediated cotranslational insertion into the endoplasmic reticulum

There is increasing evidence implicating HER3 in several types of cancer. But the development of targeted therapies to inactivate HER3 function has been a challenging endeavor. Its kinase domain functions in allostery not catalysis, and the classical ATP-analog class of tyrosine kinase inhibitors fail to inactivate it. Here we describe a novel approach that eliminates HER3 expression. The small-molecule cotransin CT8 binds the Sec61 translocon and prevents the signal peptide of the nascent HER3 protein from initiating its cotranslational translocation, resulting in the degradation of HER3 but not the other HER proteins. CT8 treatment suppresses the induction of HER3 that accompanies lapatinib treatment of HER2-amplified cancers and synergistically enhances the apoptotic effects of lapatinib. The target selectivities of cotransins are highly dependent on their structure and the signal sequence of targeted proteins and can be narrowed through structure-function studies. Targeting Sec61-dependent processing identifies a novel strategy to eliminate HER3 function.

HER Targeting in HER2-Negative Breast Cancers: Looking for the HER3 Positive

Targeting HER2 for the treatment of HER2-positive breast cancers is now a validated treatment paradigm. However, evidence suggests that this family of receptors may have important roles outside of the realm of HER2 amplification. There is considerable interest in the development of biomarkers to identify such breast cancers.

PI3K in cancer: divergent roles of isoforms, modes of activation and therapeutic targeting

Phosphatidylinositol 3-kinases (PI3Ks) are crucial coordinators of intracellular signalling in response to extracellular stimuli. Hyperactivation of PI3K signalling cascades is one of the most common events in human cancers. In this Review, we discuss recent advances in our knowledge of the roles of specific PI3K isoforms in normal and oncogenic signalling, the different ways in which PI3K can be upregulated, and the current state and future potential of targeting this pathway in the clinic.

Therapeutic targeting of tumor suppressor genes

Carcinogenesis is a multistep process attributable to both gain-of-function mutations in oncogenes and loss-of-function mutations in tumor suppressor genes. Currently, most molecular targeted therapies are inhibitors of oncogenes, because inactivated tumor suppressor genes have proven harder to "drug." Nevertheless, in cancers, tumor suppressor genes undergo alteration more frequently than do oncogenes. In recent years, several promising strategies directed at tumor suppressor genes, or the pathways controlled by these genes, have emerged. Here, we describe advances in a number of different methodologies aimed at therapeutically targeting tumors driven by inactivated tumor suppressor genes.

Regulation of ERBB3/HER3 signaling in cancer

ERBB3/HER3 is emerging as a molecular target for various cancers. HER3 is overexpressed and activated in a number of cancer types under the conditions of acquired resistance to other HER family therapeutic interventions such as tyrosine kinase inhibitors and antibody therapies. Regulation of the HER3 expression and signaling involves numerous HER3 interacting proteins. These proteins include PI3K, Shc, and E3 ubiquitin ligases NEDD4 and Nrdp1. Furthermore, recent identification of a number of HER3 oncogenic mutations in colon and gastric cancers elucidate the role of HER3 in cancer development. Despite the strong evidence regarding the role of HER3 in cancer, the current understanding of the regulation of HER3 expression and activation requires additional research. Moreover, the lack of biomarkers for HER3-driven cancer poses a big challenge for the clinical development of HER3 targeting antibodies. Therefore, a better understanding of HER3 regulation should improve the strategies to therapeutically target HER3 for cancer therapy.

Overcoming acquired resistance to cetuximab by dual targeting HER family receptors with antibody-based therapy

Background

Cetuximab, an anti-EGFR monoclonal antibody, is used to treat several cancers. However, many patients who initially respond to cetuximab acquire resistance. To examine mechanisms of acquired resistance, we developed a series of cetuximab-resistant (Ctx^R) clones derived from the cetuximab sensitive (Ctx^S) non-small cell lung cancer (NSCLC) cell line H226. Previous studies characterizing this model revealed that: 1) EGFR was robustly overexpressed in Ctx^R clones due to decreased EGFR ubiquitination and degradation and 2) Ctx^R clones expressed increased HER2 and HER3 activation resulting in constitutive activation of the PI3K/AKT signaling axis. These findings suggest that dual targeting HER family receptors would be highly beneficial in the Ctx^R setting.

Results

Since HER3 has been implicated in resistance to EGFR inhibitors, the efficacy of dually targeting both EGFR and HER3 in Ctx^R models was evaluated. First, EGFR and HER3 expression were knocked down with siRNAs. Compared to the Ctx^S parental cell line (HP), all Ctx^R clones exhibited robust decreases in cell proliferation upon dual knockdown. Analysis of Ctx^R clones indicated that neuregulin-1 was highly overexpressed compared to HP cells. Incubation of HP cells with neuregulin-1 rendered them resistant to cetuximab. Next, dual treatment of Ctx^R clones with cetuximab and the HER3 neutralizing monoclonal antibody (mAb) U3-1287 led to potent anti-proliferative effects. Blockade of EGFR with cetuximab resulted in inactivation of MAPK, while blockade of HER3 with U3-1287 resulted in the inactivation of AKT. Treatment with both mAbs resulted in knockdown of both signaling pathways simultaneously. HER2 was also strongly inactivated upon dual mAb therapy, suggesting that this treatment regimen can diminish signaling from three HER family receptors. De novo Ctx^R H226 mouse xenografts were established to determine if dual therapy could overcome acquired resistance to cetuximab in vivo. Tumors that had acquired resistance to cetuximab were significantly growth delayed upon dual treatment of U3-1287 and cetuximab compared to those continued on cetuximab only. Combinatorial-treated xenograft tumors expressed decreased Ki67 and increased cleaved caspase-3 levels compared to tumors treated with either monotherapy.

Conclusions

These studies demonstrate that dually targeting HER family receptors with antibody-based therapies can overcome acquired resistance to cetuximab.

The ErbB4 CYT2 variant protects EGFR from ligand-induced degradation to enhance cancer cell motility

The epidermal growth factor receptor (EGFR) is a member of the ErbB family that can promote the migration and proliferation of breast cancer cells. Therapies that target EGFR can promote the dimerization of EGFR with other ErbB receptors, which is associated with the development of drug resistance. Understanding how interactions among ErbB receptors alter EGFR biology could provide avenues for improving cancer therapy. We found that EGFR interacted directly with the CYT1 and CYT2 variants of ErbB4 and the membrane-anchored intracellular domain (mICD). The CYT2 variant, but not the CYT1 variant, protected EGFR from ligand-induced degradation by competing with EGFR for binding to a complex containing the E3 ubiquitin ligase c-Cbl and the adaptor Grb2. Cultured breast cancer cells overexpressing both EGFR and ErbB4 CYT2 mICD exhibited increased migration. With molecular modeling, we identified residues involved in stabilizing the EGFR dimer. Mutation of these residues in the dimer interface destabilized the complex in cells and abrogated growth factor-stimulated cell migration. An exon array analysis of 155 breast tumors revealed that the relative mRNA abundance of the ErbB4 CYT2 variant was increased in ER+ HER2- breast cancer patients, suggesting that our findings could be clinically relevant. We propose a mechanism whereby competition for binding to c-Cbl in an ErbB signaling heterodimer promotes migration in response to a growth factor gradient.

Therapeutic antibodies in breast cancer

The discovery of HER2 and development of trastuzumab pioneered the field of targeted therapy in breast cancer. Hoping to emulate the same clinical success, pharmaceutical companies have developed several antibodies against newly identified membrane-bound targets. Unfortunately, none of these agents has yet matched the thousands of lives saved by trastuzumab. In this article we review the most advanced therapeutic antibodies in breast cancer. While acknowledging their unquestionable benefit, we emphasize the need to better understand their biology and mechanisms of action in order to optimize their use in defined patient populations.

Rationale-based therapeutic combinations with PI3K inhibitors in cancer treatment

The PI3K/AKT/mTOR signaling is important for cell proliferation, survival, and metabolism. Hyperactivation of this pathway is one of the most common signaling abnormalities observed in cancer and a substantial effort has recently been made to develop molecules targeting this signaling cascade. However, it is becoming evident that PI3K inhibitors used as single agents do not elicit dramatic or durable responses. Given the numerous mechanisms mediating intrinsic and acquired resistance to these agents, hypothesis-based combinatorial strategies are probably needed to fully exploit their antitumor activity. In the first part of this review, we briefly dissect the PI3K/AKT/mTOR axis and list the most advanced compounds targeting different nodes of this cascade. The second part focuses on what we believe to be the most promising rationale-based therapeutic combinations with PI3K/AKT/mTOR inhibitors in solid tumors, with special emphasis on breast cancer.

Science Signaling Podcast: 25 March 2014

A combination therapy that targets the PI3K-Akt pathway and signaling through the growth factor receptors EGFR and HER3 may be effective against triple-negative breast cancer.