Metalloproteinases stimulate ErbB-dependent ERK signaling in human skin organ culture

Circular RNA hsa_circ_0084443 Is Upregulated in Diabetic Foot Ulcer and Modulates Keratinocyte Migration and Proliferation

Objective: Insufficient knowledge about the molecular pathology of diabetic foot ulcer (DFU) impedes the development of effective wound treatment. Circular RNAs (circRNAs) are a novel class of RNA recently discovered to be widely expressed and have important biological functions; however, their role in skin wound healing remains largely unexplored. In this study, we investigated the role of circRNAs in DFU. Approach: CircRNA expression was profiled in normal wounds (NWs) and DFUs by microarray analysis, and hsa_circ_0084443 was identified as differentially expressed. The circularity and subcellular localization of hsa_circ_0084443 were characterized by northern blotting, real-time PCR, and fluorescence in situ hybridization. Cell migration, cell growth, and the transcriptome of human primary keratinocytes were analyzed after overexpression or RNA interference of hsa_circ_0084443. Results: hsa_circ_0084443 is downregulated in NWs compared with intact skin, and its level is higher in DFUs than NWs. We confirmed its circularity and presence in the cytoplasm of human epidermal keratinocytes. We showed that hsa_circ_0084443 reduced motility while enhancing the growth of keratinocytes. Furthermore, we identified a gene network with the potential to mediate the biological effect of hsa_circ_0084443. Innovation: CircRNAs have a functional role and a potential clinical significance in skin wound healing. Conclusions: We identified hsa_circ_0084443, a circRNA downregulated during NW healing, as a negative regulator of keratinocyte migration. Higher levels of hsa_circ_0084443 were detected in DFU samples, suggesting that it plays a role in pathology. These findings pave the way to understanding the functional role of circRNAs in human skin wound healing.

Epidermal growth factor (EGF) receptor-ligand based molecular staging predicts prognosis in head and neck squamous cell carcinoma partly due to deregulated EGF- induced amphiregulin expression

BACKGROUND

Increased expression of epidermal growth factor receptor (EGFR) and its ligands is associated with poor prognosis and chemoresistance in many carcinoma types, but its role in head and neck squamous cell carcinoma (HNSCC) is unclear. Our aim was to clarify whether mRNA expression of EGFR-ligands was linked to prognosis and cisplatin resistance, and if so, which ligand was most important and how was the expression regulated.

METHODS

To examine the prognostic effect of EGFR-ligand expression, we analyzed tumorous mRNA expression in 399 HNSCC patients. The intracellular signaling pathways controlling epidermal growth factor (EGF)-induced amphiregulin (AREG) expression were examined in three oral squamous cell carcinoma (OSCC) cell lines. Effect of AREG on cisplatin resistance was examined by viability assays in four-, and by association in 11 OSCC cell lines.

RESULTS

The patients were divided into five groups according to the median mRNA expression levels of four EGFR ligands, i.e. AREG, EGF, heparin-binding EGF-like growth factor (HBEGF) and beta-cellulin (BTC). The number of increased-expressed EGFR-ligands were progressively correlated to five-year survival, even in advanced TNM-stage IV patients, where five-year mortality increased from 26 % if tumor expressed none to one EGFR-ligand, to 45 % in three to four ligand expressing tumors. Thus, staging the tumor according to these EGFR-ligand mRNA expression pattern completely out performed TNM staging in predicting prognosis. Multivariate analysis identified AREG as the dominating predictor, and AREG was overexpressed in OSCC compared to tumors from other sites. Both EGF and HBEGF stimulation induced strong AREG increase in OSCC cell lines, which was partially mediated by the extracellular signal-regulated kinase 1/2 pathway, and negatively regulated by p38, c-Jun N-terminal kinase, and phosphoinositide-3 kinase. Although increased AREG mRNA expression predicted unfavorable prognosis in platinum treated HNSCC patients, AREG did not mediate cisplatin resistance in the OSCC cell lines.

CONCLUSIONS

Increased tumorous mRNA expression of four EGFR ligands was progressively associated with poor prognosis in HNSCC. Thus, EGFR-ligands mRNA expression pattern may be a new prognostic biomarker. The tightly regulated EGF-induced AREG mRNA expression was partly lost in the OSCC cell lines and restoring its regulation may be a new target in cancer treatment.

TRIAL REGISTRATION

Not applicable as the clinical data of the 498 HNSCC patients and their mRNA expression profiles were collected from the open TCGA database: http://cancergenome.nih.gov/cancersselected/headandneck .

Metalloproteinase-mediated, context-dependent function of amphiregulin and HB-EGF in human keratinocytes and skin

Human keratinocytes (KCs) express multiple EGF receptor (EGFR) ligands; however, their functions in specific cellular contexts remain largely undefined. To address this issue, first we measured mRNA and protein levels for multiple EGFR ligands in KCs and skin. Amphiregulin (AREG) was by far the most abundant EGFR ligand in cultured KCs, with >19 times more mRNA and >7.5 times more shed protein than any other family member. EGFR ligand expression in normal skin was low (<8 per thousand of RPLP0/36B4); however, HB-EGF and AREG mRNAs were strongly induced in human skin organ culture. KC migration in scratch wound assays was highly metalloproteinase (MP)- and EGFR dependent, and was markedly inhibited by EGFR ligand antibodies. However, lentivirus-mediated expression of soluble HB-EGF, but not soluble AREG, strongly enhanced KC migration, even in the presence of MP inhibitors. Lysophosphatidic acid (LPA)-induced ERK phosphorylation was also strongly EGFR and MP dependent and markedly inhibited by neutralization of HB-EGF. In contrast, autocrine KC proliferation and ERK phosphorylation were selectively blocked by neutralization of AREG. These data show that distinct EGFR ligands stimulate KC behavior in different cellular contexts, and in an MP-dependent fashion.

Characterization of the putative tryptophan synthase beta-subunit from Mycobacterium tuberculosis

The increasing emergence of drug-resistant tuberculosis (TB) poses a serious threat to the control of this disease. It is in urgent need to develop new TB drugs. Tryptophan biosynthetic pathway plays an important role in the growth and replication of Mycobacterium tuberculosis (Mtb). The beta-subunit of tryptophan synthase (TrpB) catalyzes the last step of the tryptophan biosynthetic pathway, and it might be a potential target for TB drug design. In this study, we overexpressed, purified, and characterized the putative TrpB-encoding gene Rv1612 in Mtb H37Rv. Results showed that Mtb His-TrpB optimal enzymatic activity is at pH 7.8 with 0.15 M Na(+) or 0.18 M Mg(2+) at 37 degrees C. Structure analysis indicated that Mtb TrpB exhibited a typical beta/alpha barrel structure. The amino acid residues believed to interact with the enzyme cofactor pyridoxal-5'-phosphate were predicted by homology modeling and structure alignment. The role of these residues in catalytic activity of the Mtb His-TrpB was confirmed by site-directed mutagenesis. These results provided reassuring structural information for drug design based on TrpB.

Differential ErbB1 signaling in squamous cell versus basal cell carcinoma of the skin

In this study, we examined ErbB1 signaling in human basal and squamous cell carcinomas (BCC and SCC) of the skin in vivo. We used enzyme-linked immunosorbent assay, laser capture microdissection-coupled real-time reverse transcriptase-polymerase chain reaction, and immunohistochemistry to assess expression and activation levels of ErbB1 protein, ligands, and potential downstream effectors, in BCC and SCC tumors, stroma, and adjacent epidermis. Although total ErbB1 protein and mRNA were similar in cancerous and normal skin, we found that ErbB1 activation (phospho-Tyr(1068)) was greater in bulk SCC versus BCC or normal skin. In addition, three ErbB1 ligand transcripts (amphiregulin, heparin-binding epidermal growth factor-like growth factor, and transforming growth factor-alpha) were up-regulated in tumor cells of SCC but not BCC. Expression of these ligands was also increased in asymptomatic epidermis adjacent to both SCC and BCC, relative to normal skin. Interestingly, betacellulin transcript levels were inversely regulated compared with the other ligands. Consistently, downstream ErbB1 effectors (Erk1/2 and Akt) were activated in tumor cells of SCC but not of BCC and in adjacent epidermis of both BCC and SCC. These results demonstrate that ErbB1 signaling is hyperactive in tumor cells of SCC but not of BCC and in nearby asymptomatic epidermis of both tumor types. Our results suggest that targeting ErbB1 signaling might be of benefit in the treatment of SCC.

β-Adrenergic Receptor Antagonists Accelerate Skin Wound Healing

The skin is our primary defense against noxious environmental agents. Upon injury, keratinocytes migrate directionally into the wound bed to initiate re-epithelialization, essential for wound repair and restoration of barrier integrity. Keratinocytes express a high level of beta2-adrenergic receptors (beta2-ARs) that appear to play a role in cutaneous homeostasis as aberrations in either keratinocyte beta2-AR function or density are associated with various skin diseases. Here we report the novel finding that beta-AR antagonists promote wound re-epithelialization in a "chronic" human skin wound-healing model. beta-AR antagonists increase ERK phosphorylation, the rate of keratinocyte migration, electric field-directed migration, and ultimately accelerate human skin wound re-epithelialization. We demonstrate that keratinocytes express two key enzymes required for catecholamine (beta-AR agonist) synthesis, tyrosine hydroxylase and phenylethanolamine-N-methyl transferase, both localized within keratinocyte cytoplasmic vesicles. Finally, we confirm the synthesis of epinephrine by measuring the endogenously synthesized catecholamine in keratinocyte extracts. Previously, we have demonstrated that beta-AR agonists delay wound re-epithelialization. Here we report that the mechanism for the beta-AR antagonist-mediated augmentation of wound repair is due to beta2-AR blockade, preventing the binding of endogenously synthesized epinephrine. Our work describes an endogenous beta-AR mediator network in the skin that can temporally regulate skin wound repair. Further investigation of this network will improve our understanding of both the skin repair process and the multiple modes of action of one of the most frequently prescribed class of drugs, hopefully resulting in a new treatment for chronic wounds.

Ligand-independent activation of the EGFR by lipid raft disruption

Normal and immortalized keratinocytes demonstrate large aggregates of lipid rafts, detectable by membrane staining with fluorescently tagged cholera toxin (CTx). As lipid rafts are known to regulate the function of many surface receptors, we wished to investigate their impact on the EGFR in HaCaT cells. When rafts were disrupted by cholesterol sequestration with methyl-beta-cyclodextrin (MbetaCD) or filipin III, EGFR rearranged into approximately micrometer large clusters outside the CTx(bright) raft aggregates. These clusters contained high concentrations of activated, tyrosine-phosphorylated EGFR exhibiting greatly reduced mobility in the fluorescence recovery after photobleaching experiments. EGFR activation led to the stimulation of extracellular signal-regulated kinase 2, the phosphorylated form of which translocated to the nucleus and stimulated growth of the MbetaCD-treated cells. Experiments with the specific antagonistic antibody proved that the activation of EGFR by lipid raft disruption occurred without the participation of the ligand. We hypothesize that cholesterol depletion leads to the release of EGFR from the damaged rafts into the small confined areas of the membrane, where the receptor molecules are likely to be spontaneously activated owing to a very high density and/or separation from the inhibitory factors remaining in the surrounding portions of the membrane.

Platelet gels and releasates

PURPOSE OF REVIEW

This review addresses potential roles for platelets and their derivatives (gels, releasates, and lysates) as therapeutic agents for regenerative medicine. Recognizing that activated platelets release chemotactic and growth factors, investigators have attempted to enhance tissue regeneration by applying platelets and various derivatives directly into sites of surgical interventions or injuries. This review analyzes the physiologic basis for this approach to tissue healing and examines the knowledge that has been derived from recent and relevant reports of in-vitro and in-vivo studies.

RECENT FINDINGS

In-vitro studies have established that platelets and their derivatives accelerate proliferation of an array of cells involved in soft and bony tissue regeneration. These effects have been evaluated, also, in vivo in humans and in animals. The outcomes of in-vivo studies are considerable less homogeneous than the outcomes of in-vitro investigations. The resultant discrepancies reflect not only differences of technical protocols, but also the greater complexity of healing vital tissues compared with circumscribed in-vitro studies.

SUMMARY

The preponderance of evidence indicates that platelets and their derivatives have the potential for a substantial therapeutic role in tissue regeneration. The results of recent research indicate that platelet-derived growth factors act in synergy with plasma-derived factors to activate a complex network of autocrine functions that modulate healing. Platelet-derivative products are promising therapeutics that offer new opportunities for research and applications of tissue engineering.

Tissue regeneration and in loco administration of platelet derivatives: clinical outcome, heterogeneous products, and heterogeneity of the effector mechanisms

BACKGROUND

In loco administration of platelet (PLT) derivatives is a relatively new auxiliary treatment for tissue regeneration to be hastened. Enthusiastic reports are faced by more critical ones. The more obvious rationale for the in vivo administration of PLT derivatives resides in their growth factor content.

STUDY DESIGN AND METHODS

The relevant literature was systematically reviewed. Close scrutiny of the technical details was carried out to find out the procedural differences accounting for conflicting results.

RESULTS

An impressively vast heterogeneity of conduct was found in both in vitro and in vivo studies. Major outcome-affecting variables were recognized such as those associated with PLT preparation; growth factor measurement; proliferation test; dose, timing, and administration of the PLT derivatives; study design; and primary endpoints.

CONCLUSIONS

So many variables were found making standardization or confrontation of the in vitro and the in vivo studies barely conceivable or manageable. The mechanisms of action are very complex. The attribution of tissue regeneration capacity of PLT derivatives solely to the PLT-derived growth factors is simplistic. The results obtained through in vitro experiments are indicative for general mechanisms. Their simplistic hold to the complex in vivo environment may be misleading.

Epidermal growth factor receptor signaling is partially responsible for the increased matrix metalloproteinase-1 expression in ocular epithelial cells after UVB radiation

Pterygia are inflammatory, invasive, and proliferative lesions of the human ocular surface in which the matrix metalloproteinase (MMP) collagenase-1 (MMP-1) is highly expressed. Pterygia development may involve MMP-1 activity against interstitial fibrillar collagen, an abundant extracellular matrix component of the cornea, and its induction by ultraviolet light (UVB). We examined the pathways responsible for enhanced expression of MMP-1 in pterygium epithelial cells after UVB exposure and/or treatment with chemical inhibitors of mitogen-activated protein kinases or epidermal growth factor receptor. The induction of MMP-1 by UVB was comparable to that mediated by heparin-binding epidermal growth factor-like growth factor and epidermal growth factor. The epidermal growth factor receptor inhibitor PD153035 partially blocked the UVB-mediated induction of MMP-1 and totally abrogated its production after stimulation with either heparin-binding epidermal growth factor-like growth factor or epidermal growth factor. UVB exposure enhanced the phosphorylated form of ERK1/2 in a time-dependent manner whereas the ERK1/2 inhibitor PD98059 decreased this induction by at least fivefold. Transcripts for c-jun and c-fos were detected as early as 2 hours after UVB exposure and were suppressed by PD98059. The identification of a specific intracellular signaling pathway responsible for the enhanced production of a key enzyme that denatures intact fibrillar collagen has important implications for understanding the pathophysiology and future therapy for pterygia.

Src family kinase inhibitors block amphiregulin-mediated autocrine ErbB signaling in normal human keratinocytes

c-Src potentiates proliferation, survival, and invasiveness in response to epidermal growth factor (EGF) in human mammary carcinoma cells. Tyrosine (Tyr) 845 of ErbB1 is phosphorylated by Src and has been implicated in control of malignant behavior. Although several lines of evidence also suggest important interactions of ErbB and Src family kinase signaling in normal epithelial cells, little is known about the mechanism of this interaction. Studying normal human keratinocytes (NHKs), here we demonstrate strong expression of the Src family kinases Src, Yes, and Fyn; Src family kinase-dependent stimulation of Tyr 845 by EGF; and potent inhibition of NHK proliferation and migration by two Src family kinase inhibitors PP1 and PD173952. EGF-stimulated extracellular signal-regulated kinase (ERK) phosphorylation occurred at much lower concentrations of EGF than required to phosphorylate Tyr 845. Moreover, the effect of Src family kinase inhibitors on EGF-stimulated ERK phosphorylation was transient, prompting a search for other targets of Src family kinase action. By enzyme-linked immunosorbent assay analysis, we found that three different Src family kinase inhibitors [6-(2,6-dichlorophenyl)-8-methyl-2-(4-morpholin-4-ylphenylamino)-8H-pyrido[2,3-d]pyrimidin-7-one (PD173952), 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1), and 2-oxo-3-(4,5,6,7-tetrahydro-1H-indol-2-ylmethylene)-2,3-dihydro-1H-indole-5-sulfonic acid dimethylamide (SU6656)] markedly inhibited elaboration of soluble amphiregulin by NHKs. The ErbB inhibitor PD158780 and the mitogen-activated protein kinase kinase inhibitor U0126 also markedly inhibited NHK proliferation, migration, and amphiregulin production. Together, these observations demonstrate that one or more Src family kinases act upstream as well as downstream of ErbB1 to promote amphiregulin-dependent autocrine stimulation of NHKs and suggest that autocrine NHK proliferation is more dependent upon ERK activation than upon Tyr 845 phosphorylation.

Expression of activated MEK1 in differentiating epidermal cells is sufficient to generate hyperproliferative and inflammatory skin lesions

Epidermal activation of Erk MAPK is observed in human psoriatic lesions and in a mouse model of psoriasis in which beta1 integrins are expressed in the suprabasal epidermal layers. Constitutive activation of the upstream kinase MEK1 causes hyperproliferation and perturbed differentiation of human keratinocytes in culture. It is not known, however, whether Erk activation in differentiating keratinocytes is sufficient to trigger hyperproliferation of basal keratinocytes and a skin inflammatory infiltrate. To investigate this, we expressed constitutively active MEK1 in the suprabasal epidermal layers of transgenic mice. Proliferation in the epidermal basal layer was stimulated and epidermal terminal differentiation was perturbed. Some older mice also developed papillomas. There was a large increase in T lymphocytes, dendritic cells, and neutrophils in the skin. The effects of suprabasal MEK1 on basal keratinocytes and leukocytes, cells that were transgene negative, suggested that MEK1 activity might stimulate cytokine release. Transgenic keratinocytes expressed elevated IL-1alpha and crossing the mice with mice overexpressing the IL-1 receptor in the epidermal basal layer led to exacerbated hyperproliferation and inflammation. These data suggest that activation of MEK1 downstream of beta1 integrins plays an important role in epidermal hyperproliferation and skin inflammation.

Autocrine extracellular signal-regulated kinase (ERK) activation in normal human keratinocytes: metalloproteinase-mediated release of amphiregulin triggers signaling from ErbB1 to ERK

ErbB signaling through extracellular signal-regulated kinase (ERK) has been implicated in regulating the expression of ErbB ligands in hyperproliferative skin disorders and wound healing. Here, we characterize the process of autocrine ERK activation in cultured normal human keratinocytes (NHKs) subjected to growth factor (GF) deprivation. Basal ERK phosphorylation was lower after 48 h than after 24 h of GF deprivation, and lowest at 30-60 min after an additional medium change. ERK phosphorylation was markedly increased by low concentrations of epidermal growth factor (EGF) (0.2-1 ng/ml) that provoked only a limited increase in ErbB1 tyrosine phosphorylation and internalization. Basal ErbB tyrosine phosphorylation and ERK phosphorylation were inhibited by two different ErbB receptor tyrosine kinase inhibitors, by the ErbB1-specific neutralizing monoclonal antibody 225 IgG, by two different metalloproteinase inhibitors, and by neutralizing antibodies against amphiregulin (AR). In contrast, these responses were unaffected by neutralizing antibodies against other ErbB1 ligands or the ErbB2 inhibitors geldanamycin and AG825. The time course of autocrine ERK phosphorylation correlated with the appearance of soluble AR, and two different metalloproteinase inhibitors blocked AR release. These results define an amphiregulin- and ErbB1-dependent mechanism by which autocrine ERK activation is maintained in NHKs, even when ErbB1 autophosphorylation and internalization are limited.

The inhibition of MAPK pathway is correlated with down-regulation of MMP-9 secretion induced by TNF-alpha in human keratinocytes

MMP-9 (92 kDa) is the major gelatinase able to degrade collagen IV, secreted by keratinocytes that are actively involved in wound-healing or tumorigenesis. Since the invasive phenotype of cancers is dependent on MMP-9 expression, it appeared of interest to precisely characterize which signal transduction pathways activated by TNF-alpha are involved in MMP-9 up-regulation induced by TNF-alpha. In HaCaT cells, activation of MMP-9 occurs at the transcriptional level. Inhibition of the MAPK pathway using specific inhibitors of the Ras, Raf, MEK1/2, and Erk1/2 cascade was correlated with a marked inhibition of MMP-9 activity, as determined by gene and protein expression. MAPK pathway activation via TNF-alpha was confirmed by marked AP-1 activation detected in EMSA. Under our experimental conditions, p38 MAPK and SAPK/JNK pathways were not activated. Gene and protein expression of other MMPs that regulate MMP-9, such as MMP-1 and MMP-13, were also up-regulated by TNF-alpha and inhibited by UO126, providing evidence that the MAPK pathway plays a fundamental role in the regulation of MMP-9 secretion by keratinocytes. As TNF-alpha is known to be a main activator of NF-kappaB pathway, the effects of campthothecin and caffeic acid were investigated, such as, TNF-alpha campthothecin up-regulated MMP-9 activity but caffeic acid only weakly inhibited MMP-9 activation induced by TNF-alpha. However, NF-kappaB is activated as shown from immunostaining data, a nuclear staining and higher Western blotting expression of p50 and p65 NF-kappaB subunits were detected after TNF-alpha treatment. A higher specific signal was also detected in EMSA for TNF-alpha-treated cells.

Keratinocyte outgrowth from human skin explant cultures is dependent upon p38 signaling

Organ culture of skin is known to recapitulate several early events in the process of wound healing. Here we investigate the function of p38 kinase signaling as a regulator of keratinocyte behavior in human skin organ culture. We first show that skin organ culture recapitulates the transition from migration to proliferation that is known to characterize the reepithelialization process. We next show that inhibition of p38 markedly impairs the formation of keratinocyte outgrowth in human skin explant cultures, as well as the migration of keratinocytes in an in vitro wound assay. In contrast, the marked induction of mRNA encoding the ErbB ligand heparin-binding epidermal growth factor-like growth factor, known to occur after skin wounding, was not blocked by inhibition of p38. As assessed by immunoblotting, phosphorylation of p38 was limited and was not increased between 0 and 7 days of organ culture. Our results show the sensitivity of reepithelialization to inhibition by p38 and suggest that p38 acts primarily during the migration phase of this process. These data also indicate that autocrine heparin-binding epidermal growth factor expression is not regulated by p38.

Ectodomain shedding of nectin-1alpha by SF/HGF and TPA in MDCK cells

Nectin is a Ca(2+)-independent immunoglobulin-like cell-cell adhesion molecule implicated in the organization of the junctional complex comprised of E-cadherin-based adherens junctions and claudin-based tight junctions in epithelial cells. Scatter factor (SF)/hepatocyte growth factor (HGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA), a tumor-promoting phorbol ester, induce cell spreading, followed by cell-cell dissociation and cell scattering, in Madin-Darby canine kidney (MDCK) cells. We found here that SF/HGF and TPA induced proteolytic cleavage of nectin-1alpha in the ectodomain, resulting in generation of the 80-kDa extracellular fragment and the 33-kDa fragment composed of the transmembrane and cytoplasmic domains, in MDCK cells. This shedding of nectin-1alpha was inhibited by metalloprotease inhibitors. These results indicate that SF/HGF and TPA induce the ectodomain shedding of nectin-1alpha presumably by a metalloprotease, and have raised the possibility that this shedding is involved in the SF/HGF- and TPA-induced cell-cell dissociation.

Differential cytoskeletal association of ErbB1 and ErbB2 during keratinocyte differentiation

ErbB1 and ErbB2 display differential subcellular localization in human skin and cultured keratinocytes. To determine whether ErbB1 and ErbB2 also differ in cytoskeletal binding properties, normal human keratinocytes grown under conditions favoring a basal or differentiated phenotype were repeatedly extracted in a non-ionic detergent buffer. In basaloid keratinocytes, cytoskeletal association of ErbB1 and ErbB2 was limited. ErbB1 ( approximately 5%) was tightly associated with the cytoskeleton, compared to <1% of ErbB2 (p=0.004). After EGF stimulation, activated ErbB1 and ERK associated with the cytoskeleton to a greater extent than did total ErbB1 and total ERK. Association of ErbB2 increased markedly in differentiated keratinocytes, whereas association of ErbB1 was similar in basaloid and differentiated cells. Cytoskeletal association of ErbB2 correlated with plasma membrane localization. These results suggest that ErbB1 and ErbB2 employ different mechanisms of plasma membrane targeting during keratinocyte differentiation, and that cytoskeletal association may facilitate the coupling of activated ErbB1 and ERK.