Protein kinase C mediated internalization of ErbB2 is independent of clathrin, ubiquitination and Hsp90 dissociation

Molecular chaperones HSP40, HSP70, STIP1, and HSP90 are involved in stabilization of Cx43

To investigate the involvement of stress induced phosphoprotein 1 (STIP1), heat shock protein (HSP) 70, and HSP90 in ubiquitination of connexin 43 (Cx43) in rat H9c2 cardiomyocytes. Co-immunoprecipitation was used to detect protein-protein interactions and Cx43 ubiquitination. Immunofluorescence was used for protein co-localization. The protein binding, Cx43 protein expression, and Cx43 ubiquitination were reanalyzed in H9c2 cells with modified STIP1 and/or HSP90 expression. STIP1 bound to HSP70 and HSP90, and Cx43 bound to HSP40, HSP70, and HSP90 in normal H9c2 cardiomyocytes. Overexpression of STIP1 promoted the transition of Cx43-HSP70 to Cx43-HSP90 and inhibited Cx43 ubiquitination; knockdown of STIP1 resulted in the opposite effects. Inhibition of HSP90 counteracted the inhibitory effect of STIP1 overexpression on Cx43 ubiquitination. STIP1 suppresses Cx43 ubiquitination in H9c2 cardiomyocytes by promoting the transition of Cx43-HSP70 to Cx43-HSP90.

Irreversible tyrosine kinase inhibitors induce the endocytosis and downregulation of ErbB2

Erb-b2 receptor tyrosine kinase 2 (ErbB2) is an oncogene that frequently overexpressed in a subset of cancers. Anti-ErbB2 therapies have been developed to treat these types of cancers. However, less is known about how anti-ErbB2 drugs affect the trafficking and degradation of ErbB2. We demonstrate that the reversible and irreversible tyrosine kinase inhibitors (TKIs) differentially modulate the subcellular trafficking and downregulation of ErbB2. Only the irreversible TKIs can induce the loss of ErbB2 expression, which is not dependent on proteasome or lysosome. The irreversible TKIs promote ErbB2 endocytosis from plasma membrane and enhance the ErbB2 accumulation at endosomes. The endocytosis of ErbB2 is mediated by a dynamin-dependent but clathrin-independent mechanism. Blocking of ErbB2 endocytosis can impair the TKI-induced ErbB2 downregulation.

Expanding the Disorder-Function Paradigm in the C-Terminal Tails of Erbbs

ErbBs are receptor tyrosine kinases involved not only in development, but also in a wide variety of diseases, particularly cancer. Their extracellular, transmembrane, juxtamembrane, and kinase folded domains were described extensively over the past 20 years, structurally and functionally. However, their whole C-terminal tails (CTs) following the kinase domain were only described at atomic resolution in the last 4 years. They were shown to be intrinsically disordered. The CTs are known to be tyrosine-phosphorylated when the activated homo- or hetero-dimers of ErbBs are formed. Their phosphorylation triggers interaction with phosphotyrosine binding (PTB) or Src Homology 2 (SH2) domains and activates several signaling pathways controling cellular motility, proliferation, adhesion, and apoptosis. Beyond this passive role of phosphorylated domain and site display for partners, recent structural and function studies unveiled active roles in regulation of phosphorylation and interaction: the CT regulates activity of the kinase domain; different phosphorylation states have different compaction levels, potentially modulating the succession of phosphorylation events; and prolines have an important role in structure, dynamics, and possibly regulatory interactions. Here, we review both the canonical role of the disordered CT domains of ErbBs as phosphotyrosine display domains and the recent findings that expand the known range of their regulation functions linked to specific structural and dynamic features.

A Tetravalent Biparatopic Antibody Causes Strong HER2 Internalization and Inhibits Cellular Proliferation

The overexpression of tyrosine kinase HER2 in numerous cancers, connected with fierce signaling and uncontrolled proliferation, makes it a suitable target for immunotherapy. The acquisition of resistance to currently used compounds and the multiplicity of signaling pathways involved prompted research into the discovery of novel binders as well as treatment options with multiple targeting and multispecific agents. Here we constructed an anti-HER2 tetravalent and biparatopic symmetrical IgG-like molecule by combining the Fab of pertuzumab with a HER2-specific Fcab (Fc fragment with antigen binding), which recognizes an epitope overlapping with trastuzumab. In the strongly HER2-positive cell line SK-BR-3, the molecule induced a rapid and efficient reduction in surface HER2 levels. A potent anti-proliferative effect, specific for the HER2-positive cell line, was observed in vitro, following the induction of apoptosis, and this could not be achieved with treatment with the mixture of pertuzumab and the parental Fcab. The inhibitory cytotoxic effect of our antibody as a single agent makes it a promising contribution to the armory of anti-cancer molecules directed against HER2-addicted cells.

KV1.5-KVβ1.3 Recycling Is PKC-Dependent

K_V1.5 channel function is modified by different regulatory subunits. K_Vβ1.3 subunits assemble with K_V1.5 channels and induce a fast and incomplete inactivation. Inhibition of PKC abolishes the K_Vβ1.3-induced fast inactivation, decreases the amplitude of the current K_V1.5–K_Vβ1.3 and modifies their pharmacology likely due to changes in the traffic of K_V1.5–K_Vβ1.3 channels in a PKC-dependent manner. In order to analyze this hypothesis, HEK293 cells were transfected with K_V1.5–K_Vβ1.3 channels, and currents were recorded by whole-cell configuration of the patch-clamp technique. The presence of K_V1.5 in the membrane was analyzed by biotinylation techniques, live cell imaging and confocal microscopy approaches. PKC inhibition resulted in a decrease of 33 ± 7% of channels in the cell surface due to reduced recycling to the plasma membrane, as was confirmed by confocal microscopy. Live cell imaging indicated that PKC inhibition almost abolished the recycling of the K_V1.5–K_Vβ1.3 channels, generating an accumulation of channels into the cytoplasm. All these results suggest that the trafficking regulation of K_V1.5–K_Vβ1.3 channels is dependent on phosphorylation by PKC and, therefore, they could represent a clinically relevant issue, mainly in those diseases that exhibit modifications in PKC activity.

Hsp90 inhibition and co-incubation with pertuzumab induce internalization and degradation of trastuzumab: Implications for use of T-DM1

The receptor tyrosine kinase HER2 is associated with a number of human malignancies and is an important therapeutic target. The antibody-drug conjugate trastuzumab emtansine (T-DM1; Kadcyla^® ) is recommended as a first-line treatment for patients with HER2-positive metastatic breast cancer. T-DM1 combines the antibody-induced effects of the anti-HER2 antibody trastuzumab (Herceptin^® ) with the cytotoxic effect of the tubulin inhibitor mertansine (DM1). For DM1 to have effect, the T-DM1-HER2 complex has to be internalized and the trastuzumab part of T-DM1 has to be degraded. HER2 is, however, considered endocytosis-resistant. As a result of this, trastuzumab is only internalized to a highly limited extent, and if internalized, it is rapidly recycled. The exact reasons for the endocytosis resistance of HER2 are not clear, but it is stabilized by heat-shock protein 90 (Hsp90) and Hsp90 inhibitors induce internalization and degradation of HER2. HER2 can also be internalized upon activation of protein kinase C, and contrary to trastuzumab alone, the combination of two or more anti-HER2 antibodies can induce efficient internalization and degradation of HER2. With intention to find ways to improve the action of T-DM1, we investigated how different ways of inducing HER2 internalization leads to degradation of trastuzumab. The results show that although both Hsp90 inhibition and activation of protein kinase C induce internalization of trastuzumab, only Hsp90 inhibition induces degradation. Furthermore, we find that antibody internalization and degradation are increased when trastuzumab is combined with the clinically approved anti-HER2 antibody pertuzumab (Perjeta^® ).

High-Throughput Quantification of Surface Protein Internalization and Degradation

Cell surface proteins are key regulators of fundamental cellular processes and, therefore, often at the root of human diseases. Thus, a large number of targeted drugs which are approved or under development act upon cell surface proteins. Although down-regulation of surface proteins by many natural ligands is well-established, the ability of drug candidates to cause internalization or degradation of the target is only recently moving into focus. This property is important both for the pharmacokinetics and pharmacodynamics of the drug but may also constitute a potential resistance mechanism. The enormous numbers of drug candidates targeting cell surface molecules, comprising small molecules, antibodies, or alternative protein scaffolds, necessitate methods for the investigation of internalization and degradation in high throughput. Here, we present a generic high-throughput assay protocol, which allows the simultaneous and independent quantification of internalization and degradation of surface proteins on a single-cell level. Because we fuse a HaloTag to the cell surface protein of interest and exploit the differential cell permeability of two fluorescent HaloTag ligands, no labeling of the molecules to be screened is required. In contrast to previously described approaches, our homogeneous assay is performed with adherent live cells in a 96-well format. Through channel rescaling, we are furthermore able to obtain true relative abundances of surface and internal protein. We demonstrate the applicability of our procedure to three major drug targets, EGFR, HER2, and EpCAM, examining a selection of well-investigated but also novel small molecule ligands and protein affinity reagents.

Protein kinase C regulates ErbB3 turnover

ErbB3, which belongs to the epidermal growth factor receptor (EGFR) or ErbB family of receptor tyrosine kinases, is involved in progression of several human cancers and a tight regulation of its expression is crucial. An important mechanism for regulation of ErbB proteins is endocytosis and we recently showed that ErbB3, contrary to other ErbB proteins, like EGFR and ErbB2, is constitutively internalized and degraded. Several studies show that protein kinase C (PKC) can regulate the activation, localization and stability of EGFR and ErbB2. Activation of PKC causes their down-regulation from the plasma membrane, but instead of being degraded the receptors accumulate in an endosomal recycling compartment. Since little is known about possible connections between ErbB3 and PKC, we have in the present study investigated effects PKC activity has on ErbB3 stability and intracellular trafficking. While PKC inhibition tends to increase ErbB3 degradation, activation of PKC causes ErbB3 stabilization. The stabilization was not due to inhibited internalization, on the contrary we find that expression of ErbB3 at the plasma membrane is reduced upon PMA-induced PKC activation. However, while endocytosed ErbB3 under normal conditions and upon PKC inhibition is found in early endosomal antigen 1 (EEA1) positive early endosomes and lysosomal-associated membrane protein 1 (LAMP1) positive late endosomes/lysosomes, indicating that it follows the classic degradative pathway, ErbB3 localizes to EEA1 and LAMP1 negative compartments upon PMA-induced activation of PKC. Altogether this shows that PKC regulates the stability of ErbB3, and knockdown experiments show that PKCδ is essential in this process. A likely explanation is that PKC regulates endosomal sorting of ErbB3 and that activated PKC sorts ErbB3 away from the degradative pathway.