The ERBB4/HER4 receptor tyrosine kinase regulates gene expression by functioning as a STAT5A nuclear chaperone

Nuclear Translocation of Cell-Surface Receptors: Lessons from Fibroblast Growth Factor

The nuclear localization of a number of growth factors, cytokine ligands and their receptors has been reported in various cell lines and tissues. These include members of the fibroblast growth factor (FGF), epidermal growth factor and growth hormone families. Accordingly, a number of nuclear functions have begun to emerge for these protein families. The demonstration of functional interactions of these proteins with the nuclear import machinery has further supported their functions as nuclear signal transducers. Here, we review the membrane- trafficking machinery and pathways demonstrated to regulate this cell surface to nucleus-trafficking event and highlight the many remaining unanswered questions. We focus on the FGF family, which is providing many of the clues as to the process of this unusual phenomenon.

Clinical applications for targeted therapy in bladder cancer

The tremendous amount of data accumulated through genomics, proteomics, and metabolomic technologies has not led to a definitive understanding of the mechanisms underlying cancer. The challenge remains as to how to integrate all of the relevant knowledge and data in a systematic manner so that researchers can gain the knowledge needed to devise the best therapeutic and diagnostic strategies. Human transitional cell carcinoma of the bladder is genetically heterogeneous, and it is surrounded by a complex tissue microenvironment involving vasculature, stromal cells, and connective tissue. One of the most challenging problems facing cancer researchers is the lack of correlation between in vitro cell lines and animal tumor models and human in vivo tumors. A few promising approaches are being devised that will help address this issue in the coming years. One such approach is the measurements of molecular levels of receptors, ligands, pathways components, and so on, directly in human tumors through in vivo imaging, or through proteomic profiling, as it has been proposed as standard protocol for cancer diagnostics and therapeutics.

Nuclear interaction of EGFR and STAT3 in the activation of the iNOS/NO pathway

Epidermal growth factor receptor (EGFR) exists in the nucleus of highly proliferative cells where it functions as a transcription factor. Although EGFR has transactivational activity, it lacks a DNA binding domain and, therefore, may require a DNA binding transcription cofactor for its transcriptional function. Here, we report that EGFR physically interacts with signal transducers and activators of transcription 3 (STAT3) in the nucleus, leading to transcriptional activation of inducible nitric oxide synthase (iNOS). In breast carcinomas, nuclear EGFR positively correlates with iNOS. This study describes a mode of transcriptional control involving cooperated efforts of STAT3 and nuclear EGFR. Our work suggests that the deregulated iNOS/NO pathway may partly contribute to the malignant biology of tumor cells with high levels of nuclear EGFR and STAT3.

Growth Hormone Receptor Is a Target for Presenilin-dependent γ-Secretase Cleavage

Growth hormone receptor (GHR) is a cytokine receptor superfamily member that binds growth hormone (GH) via its extracellular domain and signals via interaction of its cytoplasmic domain with JAK2 and other signaling molecules. GHR is a target for inducible metalloprotease-mediated cleavage in its perimembranous extracellular domain, a process that liberates the extracellular domain as the soluble GH-binding protein and leaves behind a cell-associated GHR remnant protein containing the transmembrane and cytoplasmic domains. GHR metalloproteolysis can be catalyzed by tumor necrosis factor-alpha-converting enzyme (ADAM-17) and is associated with down-modulation of GH signaling. We now study the fate of the GHR remnant protein. By anti-GHR cytoplasmic domain immunoblotting, we observed that the remnant induced in response to phorbol ester or platelet-derived growth factor has a reliable pattern of appearance and disappearance in both mouse preadipocytes endogenously expressing GHR and transfected fibroblasts expressing rabbit GHR. Lactacystin, a specific proteasome inhibitor, did not appreciably change the time course of remnant appearance or clearance but allowed detection of the GHR stub, a receptor fragment slightly smaller than the remnant but containing the C terminus of the remnant (receptor cytoplasmic domain). In contrast, MG132, another (less specific) proteasome inhibitor, strongly inhibited remnant clearance and prevented stub appearance. Inhibitors of gamma-secretase, an aspartyl protease, also prevented the appearance of the stub, even in the presence of lactacystin, and concomitantly inhibited remnant clearance in the same fashion as MG132. In addition, mouse embryonic fibroblasts derived from presenilin 1 and 2 (PS1/2) knockouts recapitulated the gamma-secretase inhibitor studies, as compared with their littermate controls (PS1/2 wild type). Confocal microscopy indicated that the GHR cytoplasmic domain became localized to the nucleus in a fashion dependent on PS1/2 activity. These data indicate that the GHR is subject to sequential proteolysis by metalloprotease and gamma-secretase activities and may suggest GH-independent roles for the GHR.

ERBB4/HER4 potentiates STAT5A transcriptional activity by regulating novel STAT5A serine phosphorylation events

The epidermal growth factor receptor family member ERBB4 is required for mammary gland development and lactation. ERBB4 activities in the breast are mediated through the signal transducer and activator of transcription (STAT) family member STAT5A, and ERBB4 directly activates STAT5A, in part, through phosphorylation of STAT5A at the regulatory Tyr-694. Here we show that STAT5A regulation by ERBB4 is also mediated through STAT5A serine phosphorylation. Using a reverse-phase high performance liquid chromatography tandem mass spectrometry analysis of proteolytically digested STAT5A coexpressed with ERBB4, we identified STAT5A serine phosphorylations at the previously described Ser-779 and at the novel Ser-127/Ser-128. Immunohistochemistry of wild-type and ERBB4-null mammary glands at late pregnancy showed that ERBB4 expression was required for STAT5A phosphorylation at Ser-779. Independent serine-to-alanine residue substitutions in full-length STAT5A revealed that although STAT5A Ser-779 phosphorylation was dispensable for phosphorylation of STAT5A at Tyr-694 and subsequent DNA binding, Ser-779 was required to stabilize an interaction with ERBB4 and mediate ERBB4-induced STAT5A stimulation of gene expression. STAT5A Ser-127/Ser-128, on the other hand, was required for ERBB4-induced phosphorylation of Tyr-694, whereas Ser-779 and as yet unidentified tyrosine residues were phosphorylated in the absence of Ser-127/Ser-128. In addition, STAT5A S127A/S128A remained associated with ERBB4 but failed to bind DNA or activate transcription in response to ERBB4 coexpression. Our studies demonstrate that phosphorylation of STAT5A at Ser-127/Ser-128 and Ser-779 are obligatory events regulating ERBB4-mediated activation of STAT5A.

Presenilin-dependent gamma-secretase processing regulates multiple ERBB4/HER4 activities

Transmembrane receptors typically transmit cellular signals following growth factor stimulation by coupling to and activating downstream signaling cascades. Reports of proteolytic processing of cell surface receptors to release an intracellular domain (ICD) has raised the possibility of novel signaling mechanisms directly mediated by the receptor ICD. The receptor tyrosine kinase ERBB4/HER4 (referred to here as ERBB4) undergoes sequential processing by tumor necrosis factor-alpha converting enzyme and presenilin-dependent gamma-secretase to release the ERBB4 ICD (4ICD). Our recent data suggests that regulation of gene expression by the ERBB4 nuclear protein and the proapoptotic activity of ERBB4 involves the gamma-secretase release of 4ICD. To determine the role gamma-secretase processing plays in ERBB4 signaling, we generated an ERBB4 allele with the transmembrane residue substitution V673I (ERBB4-V673I). We demonstrate that ERBB4-V673I fails to undergo processing by gamma-secretase but retains normal cell surface signaling activity. In contrast to wild-type ERBB4, however, ERBB4-V673I was excluded from the nuclei of transfected cells and failed to activate STAT5A stimulation of the beta-casein promoter. These results support the contention that gamma-secretase processing of ERBB4 is necessary to release a functional 4ICD nuclear protein which directly regulates gene expression. We also demonstrate that 4ICD failed to accumulate within mitochondria of ERBB4-V673I transfected cells and the potent proapoptotic activity of ERBB4 was completely abolished in cells expressing ERBB4-V673I. Our results provide the first formal demonstration that proteolytic processing of ERBB4 is a critical event regulating multiple receptor signaling activities.