Effect of diverse tumor promoters on the expression of gap-junctional proteins connexin (Cx)26, Cx31.1, and Cx43 in SENCAR mouse epidermis

Induction of Apoptosis by PQ1, a Gap Junction Enhancer that Upregulates Connexin 43 and Activates the MAPK Signaling Pathway in Mammary Carcinoma Cells

The mechanism of gap junction enhancer (PQ1) induced cytotoxicity is thought to be attributed to the change in connexin 43 (Cx43) expression; therefore, the effects of Cx43 modulation in cell survival were investigated in mammary carcinoma cells (FMC2u) derived from a malignant neoplasm of a female FVB/N-Tg(MMTV-PyVT)634Mul/J (PyVT) transgenic mouse. PQ1 was determined to have an IC₅₀ of 6.5 µM in FMC2u cells, while inducing an upregulation in Cx43 expression. The effects of Cx43 modulation in FMC2u cell survival was determined through transfection experiments with Cx43 cDNA, which induced an elevated level of protein expression similar to that seen with PQ1 exposure, or siRNA to silence Cx43 protein expression. Overexpression or silencing of Cx43 led to a reduction or an increase in cell viability, respectively. The mitogen-activated protein kinase (MAPK) family has been implicated in the regulation of cell survival and cell death; therefore, the gap junctional intercellular communication (GJIC)-independent function of PQ1 and Cx43 in the Raf/Mitogen-activated protein kinase/ERK kinase/extracellular-signal-regulated kinase (Raf-MEK-ERK) cascade of cellular survival and p38 MAPK-dependent pathway of apoptosis were explored. PQ1 treatment activated p44/42 MAPK, while the overexpression of Cx43 resulted in a reduced expression. This suggests that PQ1 affects the Raf-MEK-ERK cascade independent of Cx43 upregulation. Both overexpression of Cx43 and PQ1 treatment stimulated an increase in the phosphorylated form of p38-MAPK, reduced levels of the anti-apoptotic protein Bcl-2, and increased the cleavage of pro-caspase-3. Silencing of Cx43 protein expression led to a reduction in the phosphorylation of p38-MAPK and an increase in Bcl-2 expression. The mechanism behind PQ1-induced cytotoxicity in FMC2u mammary carcinoma cells is thought to be attributed to the change in Cx43 expression. Furthermore, PQ1-induced apoptosis through the upregulation of Cx43 may depend on p38 MAPK, highlighting that the effect of PQ1 on gap junctions as well as cellular survival via a MAPK-dependent pathway.

A preliminary operational classification system for nonmutagenic modes of action for carcinogenesis

This article proposes a system of categories for nonmutagenic modes of action for carcinogenesis. The classification is of modes of action rather than individual carcinogens, because the same compound can affect carcinogenesis in more than one way. Basically, we categorize modes of action as: (1) co-initiation (facilitating the original mutagenic changes in stem and progenitor cells that start the cancer process) (e.g. induction of activating enzymes for other carcinogens); (2) promotion (enhancing the relative growth vs differentiation/death of initiated clones (e.g. inhibition of growth-suppressing cell-cell communication); (3) progression (enhancing the growth, malignancy, or spread of already developed tumors) (e.g. suppression of immune surveillance, hormonally mediated growth stimulation for tumors with appropriate receptors by estrogens); and (4) multiphase (e.g., "epigenetic" silencing of tumor suppressor genes). A priori, agents that act at relatively early stages in the process are expected to manifest greater relative susceptibility in early life, whereas agents that act via later stage modes will tend to show greater susceptibility for exposures later in life.

Reduced expression of multiple gap junction proteins is a feature of cervical dysplasia

Cervical dysplasia is a premalignant lesion associated with human papillomavirus (HPV) infection which, over time, can turn cancerous. Previous studies have indicated that loss of gap junctions may be a feature of cervical cancer and premalignant dysplasia. Loss of the gap junction protein connexin43 has been demonstrated in dysplastic cervix, but other connexins have not been investigated. In contrast we previously showed that HPV-associated cutaneous warts – and other hyperproliferative skin conditions – display a dramatic upregulation of certain connexins, in particular connexin26. By performing immunofluorescence staining after antigen retrieval of paraffin-embedded cervical tissue samples, this study reports for the first time that connexin26 and connexin30, in addition to connexin43, are expressed in differentiating cells of normal human cervical epithelia. Moreover, in dysplastic ectocervix, all connexins studied display a dramatic loss of expression compared to adjacent normal epithelia. The role of connexins in keratinocyte differentiation and carcinogenesis is discussed.

Temporal regulation of connexin phosphorylation in embryonic and adult tissues

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells in tissues and are important in development, tissue/cellular homeostasis, and carcinogenesis. Genome databases indicate that there are at least 20 connexins in the mouse and human. Connexin phosphorylation has been implicated in connexin assembly into gap junctions, gap junction turnover, and cell signaling events that occur in response to tumor promoters and oncogenes. Connexin43 (Cx43), the most widely expressed and abundant gap junction protein, can be phosphorylated at several different serine and tyrosine residues. Here, we focus on the dynamic regulation of Cx43 phosphorylation in tissue and how these regulatory events are affected during development, wound healing, and carcinogenesis. The activation of several kinases, including protein kinase A, protein kinase C, p34cdc2/cyclin B kinase, casein kinase 1, mitogen-activated protein kinase, and pp60src kinase, can lead to the phosphorylation of different residues in the C-terminal region of Cx43. The use of antibodies specific for phosphorylation at defined residues has allowed the examination of specific phosphorylation events both in tissue culture and in vivo. These new antibody tools and those under development will allow us to correlate specific phosphorylation events with changes in connexin function.

Production of chemokine CXCL1/KC by okadaic acid through the nuclear factor-kappaB pathway

The murine chemokine CXCL1/KC is known as a chemoattractant for neutrophil infiltration and as a promoter of tumor growth. To determine its relevance in tumorigenesis, we first asked whether okadaic acid (OKA), a natural tumor promoter and a potent protein phosphatase 1 and 2A inhibitor, stimulates KC expression and if it does, through what pathway, in a promotable mouse epidermal-like JB6 cell line commonly used for studying molecules related to tumor promotion. We found that OKA stimulated the de novo synthesis of KC mRNA and protein in a dose- and time-dependent manner. To determine the mechanism by which OKA stimulated the expression of KC at the transcriptional level, transient transfection assays using serially deleted sections of KC promoter fused to luciferase reporter gene were performed. These studies showed that transactivation of KC promoter by OKA specifically involved the region between -104 and -59 containing the two nuclear factor-kappaB (NF-kappaB) response elements (kappaB1 and kappaB2). Further analyses using the mutated NF-kappaB response elements kappaB1 and kappaB2 indicated that both regions were required for optimum transactivation of KC by OKA with the former NF-kappaB response element playing a more significant role in regulating KC expression. Gel-shift and supershift analyses demonstrated the involvement of three NF-kappaB subunits, p65, p50 and c-Rel, with p65 as the major subunit in the NF-kappaB dimer complex. Additionally, immunohistochemistry and western blot analyses confirmed the presence of p65 in the nucleus with its transactivation domain phosphorylated at serine 536. In summary, this is the first report to show that the tumor promoter OKA can stimulate the de novo synthesis and secretion of KC, and that this stimulation is mediated through the NF-kappaB pathway in JB6 cells.

Expression and function of connexins in the epidermis, analyzed with transgenic mouse mutants

Eight different connexins are expressed in mouse epidermis with overlapping expression patterns in different epidermal layers. Analyses of mice with deficiency or modifications of distinct connexins yielded insights into the large variety of connexins in the epidermis. Connexin43 (Cx43) deficiency in mouse epidermis resulted in a significant acceleration of wound closure. Truncation by 125 amino acid residues of the Cx43 C-terminal region led to an altered epidermal expression pattern of Cx43 and defective development of the epidermal water barrier in transgenic mice, although the truncated Cx43 protein could still form open gap junctional channels in transfected HeLa cells. Thus, the phenotypic abnormalities observed in mice with truncated Cx43 protein (Cx43K258Stop) are more likely due to defective regulation of this protein rather than the closed Cx43 channel. Our studies of connexin-deficient mice revealed an extensive redundancy of connexins expressed in mouse epidermis. Epidermal connexins seem to form two functional groups in which deficiency of one connexin isoform can be compensated by other connexin isoforms of the same group.

Mitogen-regulated protein/proliferin mRNA induction following single applications of tumor promoters to murine skin

Mitogen-regulated protein/proliferin (mrp/plf) gene family transcripts rise in abundance as a response to diverse chemical and physical agents that promote morphological transformation in the murine C3H/10T1/2 cultured cell model of multi-step carcinogenesis. To determine if proliferin genes respond to tumor promoters in vivo, RNA was extracted from the whole skin of SENCAR mice after single applications of 2 or 20 microg 12-O-tetradecanoylphorbol-13-acetate (TPA); 3.2 or 32 nmole), 20 or 40 mg benzoyl peroxide (BPO; 83, 165 micromole), or acetone vehicle alone (2.72 mmole). RNA samples were prepared from treated skin areas, 2-48 h after painting. Mrp/plf-mRNA was not detected in Northern blot hybridizations, but large increases in mRNAs for ornithine decarboxylase gene and mRNA (odc), v-jun oncogene-related transcription factor gene and mRNA (junB), egr1 (early growth response protein gene and mRNA) were measured relative to beta 2 microglobulin gene and mRNA (b2m) mRNA in response to TPA. BPO induced small relative changes in these mRNAs. Reverse transcriptase (RT)-polymerase chain reactions (PCR) detected fully-processed MRP/plf-mRNA 16-48 h after TPA treatments in five of six animals, and in three of six BPO-treated animals. The MRP/plf-mRNA species expressed in the skin were predominantly plf1 and mrp3 as determined by gene-specific restriction enzyme sites within the RT-PCR products. Expression was either undetectable or found at low levels in acetone-painted controls and was not detected during the anagen phase of the normal hair growth cycle in unpainted animals. These results demonstrate that mrp/plf-mRNA is differentially expressed in murine skin in response to mechanistically distinct tumor promoters and has potential utility as a short-term biomarker for tumor promoting effects in chemical carcinogenesis.

Disrupted traffic of connexin 43 in human testicular seminoma cells: overexpression of Cx43 induces membrane location and cell proliferation decrease

Connexins, the constitutive proteins of gap junctions, are considered to be tumour suppressive agents and are often impaired in the tumourigenic processes. In the present study, the expression of connexin 43 (Cx43), which is involved in the control of spermatogenesis through Sertoli/germ cell coupling, has been investigated in human testicular seminoma cells (tumours and the JKT-1 cell line). Cx43 was immunolocalized in the Golgi apparatus without membrane expression and was detected by immunoblotting in JKT-1 as exclusive 70 kD bands. No mutation could be found by sequencing the transcript obtained by RT-PCR. Transfection with a Cx43-V5 vector reproduced the same gel shift, identifying these 70 kD bands as Cx43. The Cx43-70 kD bands were also expressed in normal testicular tissue, associated with the classical 43 kD isoforms. Stable transfection of JKT-1 with a Cx43-GFP vector allowed restoration of Cx43 membrane expression, functional cell coupling, and inhibition of the cell proliferation rate. Storage of Cx43 in the Golgi apparatus may correspond during spermatogenesis to an intermittent physiological process that becomes permanent in malignant seminoma cells as a result of the tumourigenic process. By preventing Cx43 membrane expression, this disrupted traffic may itself participate in tumour promotion.

The relationship between connexins, gap junctions, tissue architecture and tumour invasion, as studied in a novel in vitro model of HPV-16-associated cervical cancer progression

Disruption of gap junctional intercellular communication (GJIC) and/or connexins (gap junction proteins) is frequently reported in malignant cell lines and tumours. Certain human papillomaviruses (HPV) associated with the development of cancers, especially of the cervix, have previously been reported to downregulate GJIC in vitro. There is also evidence for reduced gap junctions in cervical dysplasia. However, many squamous hyperproliferative conditions, including HPV-induced warts, often show extensive upregulation of certain connexins. The association between HPV and GJIC, and the mechanism and consequence of deregulated GJIC in cervical tumour progression, remains unclear. Therefore, using a variety of nonmalignant and malignant cell lines and an organotypic raft-culture system, we investigated the relationship between HPV, gap junctions and tumour progression. Established cervical tumour cell lines carrying HPV were unable to communicate via gap junctions (when assayed by dye-transfer techniques). This correlated with lack of connexin protein expression, while transfection with connexins 26 or 43 led to functional gap junction membrane plaques. On the other hand, immortal but nonmalignant cell lines that contained episomal or integrated HPV-16, but required feeder-layer and growth-factor support, were consistently well coupled, and expressed multiple connexins at membrane junctions. In vitro selection of feeder-layer and growth-factor-independent variants eventually lead to loss of GJIC, which correlated with loss of membrane and increased cytoplasmic connexin 43 localization. However, this was preceded by loss of differentiation and stromal invasion, as assayed on the organotypic raft-culture model. Using this model, a comparison between noncoupled, well-coupled and connexin-transfected cell lines revealed no firm correlation between GJIC and dysplasia, but GJIC appeared to favour increased stratification. These findings demonstrate that loss of GJIC is frequent, but appears to occur more as a consequence of, rather than being the cause of, epithelial dysplasia, and may be influenced by, but is not directly attributable to, HPV.

Recent advances in the study of epigenetic effects induced by the phycotoxin okadaic acid

Okadaic acid (OA) is a phycotoxin produced by dinoflagellates. It accumulates in the digestive tracts of shellfish causing diarrhetic shellfish poisoning (DSP) in consumers. OA is a tumour promoter, and an inhibitor of both protein phosphatases and protein synthesis. OA induces DNA adducts, suggesting it may be carcinogenic. Since the Ames test without S(9) was negative, but a mutagenesis test was positive in mammalian cells, the question as to whether its molecular mechanism is genotoxic or epigenetic became unavoidable. Therefore, experiments were performed to search for epigenetic effects, since evidence for DNA-adduct formation using the gamma-(32)P-ATP post-labelling method was not obtained. We found that OA is a potent inducer of lipid peroxidation in human intestinal cells (Caco-2) at low concentrations (0.75-7.5 ng/ml versus IC50 of 15 ng/ml) with increased rates of 8-OH-dG and m(5)dC formation causing CG to AT transversion mutations and gene deregulation, respectively. The transcription and translation of connexin 43-specific mRNA were inhibited, and 3H-uridine incorporation in RNA was concomitantly increased. Consequently gap junction intracellular communication (GJIC) was inhibited, making possible cellular anarchic proliferation. Higher OA concentrations also disorganized the cellular cytoskeleton, since both actin and tubulin formations were impaired. Our results suggest that OA may induce tumours via an epigenetic mechanism.

Polyamines regulate gap junction communication in connexin 43-expressing cells

The control of cell-cell communication through gap junctions is thought to be crucial in normal tissue function and during various stages of tumorigenesis. However, few natural regulators of gap junctions have been found. We show here that increasing the activity of ornithine decarboxylase, or adding polyamines to the outside of cells, increases the level of gap junction communication between various epithelial cells. Conversely, reduction of ornithine decarboxylase activity decreases the level of gap junction communication. This regulation is dependent upon the expression of connexin 43 (Cx43 or Cxalpha1), which is a major connexin expressed in many different cell types, and involves an increase in Cx43 and its cellular re-distribution.

Changes in connexin 43 protein expression in human endometrial carcinoma

The expression of connexin 43 was studied using immunohistochemical and Western blot analyses on cell lines of endometrial epithelial origin. Connexin proteins were examined because decreases in their expression and function have been correlated with carcinogenesis. The cell lines were chosen to represent increasing grades of endometrial cancer progression starting from FEEC (fetal endometrial epithelial cells; transformed with SV40 large T antigen) to HEC-1A (stage 1A endometrial carcinoma) to RL-95-2 (grade 2 endometrial carcinoma). Parallel studies using connexin 43 polyclonal antibodies for both Western blots and immunofluorescence showed that the levels of connexin 43 expression were normal endometrial stromal cells = FEEC > HEC-1A > RL-95-2. Consequently, we applied the immunofluorescence assay to analyze paraffin-embedded uterine sections from hysterectomy specimens of patients with normal endometrium and from patients diagnosed with grade 1, 2, and 3 endometrial cancer. Using five different cases from each category, we found an inverse correlation between connexin 43 expression and tumor grade. Our data indicate that connexin 43 expression may be useful as an adjunctive marker of progression for endometrial carcinoma.

TGF-beta3, but not TGF-beta1, protects keratinocytes against 12-O-tetradecanoylphorbol-13-acetate-induced cell death in vitro and in vivo

We have examined the role that individual TGF-beta isoforms, and in particular TGF-beta3, play in control of epidermal homeostasis. Mice with a knockout mutation of the TGF-beta3 gene die a few hours after birth. A full-thickness skin grafting approach was used to investigate the postnatal development and homeostatic control of the skin of these mice. Grafted skin of mice with a disruption of the TGF-beta3 gene developed similarly to grafts of wild type and TGF-beta1 knockout animals. However, a strikingly different response was observed after acute treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). When exposed to TPA, the grafted skin of wild type and TGF-beta1 knockout mice underwent a hyperplastic response similar to that of normal mouse skin. In marked contrast, TPA treatment of TGF-beta3 knockout grafts induced widespread areas of keratinocyte cell death. Analysis of cultured keratinocytes treated with purified TGF-beta isoforms revealed that TGF-beta3 plays a direct and specific function in protecting keratinocytes against TPA-induced cell death. The protective function of TGF-beta3 on TPA-induced cell death was not because of general suppression of the signaling pathways triggered by this agent, as ERK1/2 activation occurred to a similar if not greater extent in TGF-beta3-treated versus control keratinocytes. Instead, TGF-beta3 treatment led to a significant reduction in TPA-induced c-Jun N-terminal kinase activity, which was associated and possibly explained by specific counteracting effects of TGF-beta3 on TPA-induced disruption of keratinocyte focal adhesions.

Reduced connexin43 expression in high-grade human brain glioma cells

BACKGROUND AND OBJECTIVES

Connexin43 (cx43), a gap junction protein, is implicated in the suppression of tumor cell growth. Numerous cancer cells show a reduction or loss of cx43 expression compared to their normal counterparts. Our previous studies suggest that cx43 expression is decreased in a variety of human brain tumor cell lines. To further investigate the role of cx43 in the development of human gliomas, we performed the present study on human glioma grades I-IV.

METHODS

Immunohistochemistry was performed on paraffin-embedded tissue sections of 18 human gliomas to analyze the expression levels of cx43 in different stages of human gliomas.

RESULTS

High levels of cx43 were observed in all normal brain tissue and in glioma grades I and II. In contrast, the expression of cx43 was very weak in grade III gliomas and almost undetectable in grade IV gliomas.

CONCLUSIONS

Our data support the hypothesis that reduction of cx43 is involved in the progression of human gliomas.

Modulation of gap junction expression during transient hyperplasia of rat epidermis

Retinoids and phorbol esters have profound effects on proliferation and differentiation of epidermal keratinocytes when applied topically on rodent skin. Since both agents also modulate gap junction (GJ)-mediated cell-cell communication, we have examined the effects of all-trans retinoic acid (RA) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on the expression of alpha1 (Cx43) and beta2 (Cx26) connexins, the two major gap junction gene products in mature rat epidermis. In fully differentiated, mature epidermis, alpha1 is expressed in the lower, less differentiated portion, while beta2 is localized in upper, more differentiated layers. Dorsal skin of 21-day old rats was treated topically with a single dose of RA, TPA or vehicle alone and used for histological and molecular analyses at different time points. Keratinocytes in interfollicular epidermis were examined for proliferation and differentiation using specific antibodies for keratins (K10, K14) and proliferating cell nuclear antigen (PCNA). An increase in epidermal thickness was noticed within 4 hours after the application of RA or TPA. This increase, however, appeared to be primarily due to hypertrophy, since no substantial changes were observed in the proliferative index of epidermal keratinocytes. PCNA immunoreactivity significantly increased after 8 hours treatment of RA or TPA, suggesting a hyperproliferative growth response. Epidermal hyperplasia was confirmed by monitoring the expression patterns of K10 and K14 in RA- or TPA-treated skin. RA-induced hyperplasia lasted longer as compared to TPA induction. Changes in keratin phenotypes were paralleled by an increase in alpha1 and beta2 connexin expression as well as their colocalization in same epidermal layers. Differences in hyperplastic growth response kinetics were also confirmed at the connexin level, with beta2 antigen sustained for longer and at higher levels in suprabasal layers of RA-treated skin. Overall, this type of connexin expression resembled that observed in the non-differentiated rat epidermis during embryonic development. An increase in alpha1 and beta2 connexin abundance was also observed at the protein and RNA levels. At 96 hours after RA or TPA treatment, expression of both connexins was similar to that of the control epidermis. Taken together, these findings suggest that a higher level of GJ-mediated cell-cell communication, is required for the maintenance of homeostasis during periods of rapid epidermal growth and differentiation.