8-Cl-cAMP affects glioma cell-cycle kinetics and selectively induces apoptosis

Feasibility of Photodynamic Therapy for Glioblastoma with the Mitochondria-Targeted Photosensitizer Tetramethylrhodamine Methyl Ester (TMRM)

One of the most challenging problems in the treatment of glioblastoma (GBM) is the highly infiltrative nature of the disease. Infiltrating cells that are non-resectable are left behind after debulking surgeries and become a source of regrowth and recurrence. To prevent tumor recurrence and increase patient survival, it is necessary to cleanse the adjacent tissue from GBM infiltrates. This requires an innovative local approach. One such approach is that of photodynamic therapy (PDT) which uses specific light-sensitizing agents called photosensitizers. Here, we show that tetramethylrhodamine methyl ester (TMRM), which has been used to asses mitochondrial potential, can be used as a photosensitizer to target GBM cells. Primary patient-derived GBM cell lines were used, including those specifically isolated from the infiltrative edge. PDT with TMRM using low-intensity green light induced mitochondrial damage, an irreversible drop in mitochondrial membrane potential and led to GBM cell death. Moreover, delayed photoactivation after TMRM loading selectively killed GBM cells but not cultured rat astrocytes. The efficacy of TMRM-PDT in certain GBM cell lines may be potentiated by adenylate cyclase activator NKH477. Together, these findings identify TMRM as a prototypical mitochondrially targeted photosensitizer with beneficial features which may be suitable for preclinical and clinical translation.

E2F1-mediated DNA damage is implicated in 8-Cl-adenosine-induced chromosome missegregation and apoptosis in human lung cancer H1299 cells

Although E2F1-mediated DNA double-stranded breaks (DSBs) and tetraploid have been extensively studied, the role of E2F1 in mitotic catastrophe is still unknown. We have previously shown that 8-chloro-adenosine (8-Cl-Ado) induces DNA DSBs and aberrant mitosis in human lung cancer cells, followed by delayed apoptosis. Here, we demonstrate that E2F1-mediated DNA damage is implicated in 8-Cl-Ado-induced chromosome missegregation and apoptosis in lung cancer H1299 cells. We showed that E2F1 was accumulated upon 8-Cl-Ado-induced DNA DSBs. Induction of E2F1 by 8-Cl-Ado caused DNA damage in cycling cells including M cells. In contrast, silencing of E2F1 expression decreased 8-Cl-Ado-induced DNA DSBs, particularly eliminated E2F1-mediated mitotic DNA damage. Over-expression of E2F1 and/or 8-Cl-Ado exposure resulted in aberrant mitotic spindles and chromosome segregation errors. Furthermore, over-expression of E2F1 expression enhanced 8-Cl-Ado-induced apoptosis. Together, our data indicate that E2F1-mediated DNA damage, in particular mitotic DNA damage, is an important fraction of 8-Cl-Ado-induced DNA damage, which is implicated in 8-Cl-Ado-induced mitotic catastrophe and delayed apoptosis. Induction of E2F1 by 8-Cl-Ado may contribute at least partly to the drug-inhibited proliferation of cancer cells.

E2F1 enhances 8-chloro-adenosine-induced G2/M arrest and apoptosis in A549 and H1299 lung cancer cells

The E2F1 transcription factor is a well known regulator of cell proliferation and apoptosis, but its role in response to DNA damage is less clear. 8-Chloro-adenosine (8-Cl-Ado), a nucleoside analog, can inhibit proliferation in a variety of human tumor cells. However, it is still elusive how the agent acts on tumors. Here we show that A549 and H1299 cells formed DNA double-strand breaks after 8-Cl-Ado exposure, accompanied by E2F1 upregulation at protein level. Overexpressed wild-type (E2F1-wt) colocalized with double-strand break marker γ-H2AX and promoted G2/M arrest in 8-Cl-Ado-exposed A549 and H1299, while expressed S31A mutant of E2F1 (E2F1-mu) significantly reduced ability to accumulate at sites of DNA damage and G2/M arrest, suggesting that E2F1 is required for activating G2/M checkpoint pathway upon DNA damage. Transfection of either E2F1-wt or E2F1-mu plasmid promoted apoptosis in 8-Cl-Ado-exposed cells, indicating that 8-Cl-Ado may induce apoptosis in E2F1-dependent and E2F1-independent ways. These findings demonstrate that E2F1 plays a crucial role in 8-Cl-Ado-induced G2/M arrest but is dispensable for 8-Cl-Ado-induced apoptosis. These data also suggest that the mechanism of 8-Cl-Ado action is complicated.

Identification of a 2-cell stage specific inhibitor of the cleavage of preimplantation mouse embryos synthesized by rat hepatoma cells as 5′-deoxy-5′-methylthioadenosine

Rat hepatoma Reuber H-35 cells produce a unique compound designated as Fr.B-25, a 2-cell stage-specific inhibitor of the cleavage of preimplantation mouse embryos culturedin vitro. Here, we identified Fr.B-25 as a purine nucleoside, 5′-deoxy-5′-methylthioadenosine (MTA), by mass spectroscopic analysis. All of the biological activities examined of authentic MTA on the development of mouse zygotes were indistinguishable from those of Fr.B-25. The mechanism of MTA action in the development of preimplantation mouse embryos was probably different from those of hypoxanthine and adenosine, which are well-characterized purine nucleosides that act as inhibitors of the cleavage of mouse 2-cell embryos. From the shared molecular and biological properties of Fr.B-25 and MTA, we concluded that Fr.B-25 is MTA. To the best of our knowledge, this is the first delineation of the effect of MTA on the development of preimplantation mammalian embryos culturedin vitro.

Adenosine-induced caspase-3 activation by tuning Bcl-XL/DIABLO/IAP expression in HuH-7 human hepatoma cells

Extracellular adenosine disrupted mitochondrial membrane potentials in HuH-7 cells, a Fas-deficient human hepatoma cell line, and the effect was inhibited by the adenosine transporter inhibitor dipyridamole or by overexpressing Bcl-X(L). Adenosine downregulated the expression of mRNAs and proteins for Bcl-X(L) and inhibitor of apoptosis protein 2 (IAP2) to directly inhibit caspase-3, -7, and -9, but it otherwise upregulated the expression of mRNA and protein for DIABLO, an inhibitor of IAPs. Those adenosine effects were attenuated by dipyridamole. Caspase-3 and -8 were implicated in adenosine-induced HuH-7 cell death, and adenosine actually activated caspase-3 without caspase-9 activation. The caspase-3 activation was inhibited by overexpressing Bcl-X(L) or IAP2. Taken together, the results of the present study indicate that intracellularly transported adenosine activates caspase-3 by neutralizing caspase-3 inhibition due to IAP as a result of decreased IAP2 expression and reduced IAP activity in response to increased DIABLO expression and perhaps DIABLO release from damaged mitochondria, in addition to caspase-8 activation. This represents further insight into adenosine-induced HuH-7 cell apoptotic pathway.

Elevation of intracellular cyclic AMP inhibits NF-kappaB-mediated thymosin beta4 expression in melanoma cells

Thymosin beta4 (Tbeta4) is a major actin-sequestering protein that has been implicated in the growth, survival, motility, and metastasis of certain tumors and is considered an indicator for malignant progression. Therefore, identifying compounds that can downregulate Tbeta4 expression is very important for the development of anti-cancer chemotherapies. In this study, we investigated the effects of elevated cAMP on Tbeta4 expression and the metastatic potential of murine B16 melanoma cells. In addition, we also dissected the mechanism underlying cAMP-mediated Tbeta4 suppression. We found that treatment with the cAMP-inducing compounds alpha-MSH (alpha-melanocyte stimulating hormone) and IBMX (3-isobutyl-1-methylxanthine) significantly suppressed Tbeta4 expression and regulated EMT-associated genes through the suppression of NF-kappaB activation in B16F10 cells. Along with decreased Tbeta4 expression, the in vitro invasiveness and anchorage-independent growth in a semi-solid agar of these cells were also inhibited. In animal experiments, the metastatic potential of the alpha-MSH- or IBMX-treated B16F10 melanoma cells was decreased compared to untreated control cells. Collectively, our data demonstrate that elevated intracellular cAMP significantly suppresses Tbeta4 expression and reduces MMP-9 activity, which leads to decreased metastatic potential. Moreover, suppression of NF-kappaB activation by alpha-MSH or IBMX is critical for inhibiting Tbeta4 expression.

Extracellular adenosine induces apoptosis in Caco-2 human colonic cancer cells by activating caspase-9/-3 via A(2a) adenosine receptors

BACKGROUND

Extracellular adenosine has been shown to induce apoptosis in a variety of cells via an intrinsic pathway linked to adenosine uptake into cells and the ensuing signaling cascades and an extrinsic pathway linked to adenosine receptors. The present study was designed to understand the mechanism underlying adenosine-induced apoptosis of Caco-2 human colonic cancer cells.

METHODS

To observe cell viability, an MTT assay was carried out in Caco-2 cells untransfected or transfected with the A(2a) adenosine receptor pcDNA3.1. Apoptotic cell death was assessed with flow cytometry using propidium iodide and annexin V and internucleosomal DNA fragmentation analysis. Activities of caspase-3, -8, and -9 were measured using a caspase fluorometric assay kit. Mitochondrial membrane potentials were monitored using a DePsipher kit. Expression of adenosine receptors was examined with the reverse transcription-polymerase chain reaction (RT-PCR) method.

RESULTS

Extracellular adenosine induced Caco-2 cell apoptosis in a concentration-dependent (1-20 mM) and treatment time-dependent (24-72 h) manner. The adenosine effect was inhibited by DMPX, an inhibitor of A(2a) adenosine receptors and SQ22536, an inhibitor of adenylate cyclase. CGS21680, an agonist of A(2a) adenosine receptors, and forskolin, an adenylate cyclase activator, mimicked the adenosine action. Caco-2 cell death was still induced by overexpressing A(2a) adenosine receptors, and adenosine further promoted the cell death. Adenosine disrupted mitochondrial membrane potentials and activated caspase-9 and -3, but not caspase-8.

CONCLUSIONS

Extracellular adenosine induces apoptosis in Caco-2 cells by activating caspase-9 and the downstream effector caspase caspase-3 in association with mitochondrial damage via A(2a) adenosine receptors.

Inhibition of CHK1 kinase by Gö6976 converts 8-chloro-adenosine-induced G2/M arrest into S arrest in human myelocytic leukemia K562 cells

8-Chloro-cAMP (8-Cl-cAMP) and its metabolite 8-chloro-adenosine (8-Cl-Ado) inhibit cell growth by 8-Cl-Ado-converted 8-Cl-ATP that targets cell-cycle control and RNA metabolism. However, the cell-cycle checkpoint pathways remain to be identified. Recent studies have shown that 8-Cl-cAMP administration and 8-Cl-Ado exposure may damage chromosomal DNA in vivo and in vitro. In this study, we demonstrate that 8-Cl-Ado-induced DNA damage activates G2/M phase checkpoint, which is associated with ATM-activated CHK1-CDC25C-CDC2 pathway joined by BRCA1-CHK1 branch in apoptosis-resistant human myelocytic leukemia K562 (p53-null) cells. Inhibition of CHK1 kinase by Gö6976, an inhibitor of CHK1 activity, can promote DNA damage and lead to the activation of CHK2, converting G2/M checkpoint into intra-S-phase checkpoint in which two parallel branches, the ATM-CHK2-CDC25A-CDK2 and the ATM-NBS1/SMC1 cascades, are involved. These observations may provide aid in better understanding of the mechanisms of 8-Cl-cAMP and 8-Cl-Ado actions and in potential design of the combined therapy.

The combination of gamma ionizing radiation and 8-Cl-cAMP induces synergistic cell growth inhibition and induction of apoptosis in human prostate cancer cells

The antiproliferative and cytotoxic potential of the nucleotide analog 8-Cl-cAMP was tested in PC-3 and DU145 metastatic human prostate cancer cells. The drug was examined as the only therapeutic agent and in combination with ionizing irradiation (IR). Highly synergistic effects of IR and 8-Cl-cAMP were observed in both cell lines when examined by the MTT viability and BrdU proliferation assays. The combination of IR and 8-Cl-cAMP at clinically relevant doses exerted substantial growth inhibition. The combination of IR and 8-Cl-cAMP caused a significant disturbance in the distribution of cell cycle phases. Cell cycle arrest in the sub-G0/G1 phase predominated in both cell lines. The most striking observation was a significant increase in apoptotic PC-3 and DU145 cells. The DU145 cells were three times more sensitive to the combined treatment than PC-3 cells. The initial resistance to IR-induced apoptosis in these p53-deficient prostate cancer cell lines was overcome through an alternative proapoptotic pathway induced by 8-Cl-cAMP. Considering the low effective doses of treatments, improved tumor eradication rates and minimal undesirable side effects, the combination of IR and 8-Cl-cAMP could be the therapy of choice in treating prostate cancer.

Intracellularly transported adenosine induces apoptosis in HuH-7 human hepatoma cells by downregulating c-FLIP expression causing caspase-3/-8 activation

Extracellular adenosine induced apoptosis of HuH-7 cells, a Fas-deficient human hepatoma cell line. The adenosine action was inhibited by dipyridamole, an adenosine transporter inhibitor, or 5'-amino-5'-deoxyadenosine, an inhibitor of adenosine kinase to convert from adenosine to AMP, but it was not affected by inhibitors for adenosine A(1), A(2a), A(2b), and A(3) adenosine receptors. Adenosine activated caspase-3 and -8, but not caspase-9, in HuH-7 cells, and the activation was abolished by dipyridamole. In the real-time RT-PCR and Western blot analysis, extracellular adenosine downregulated mRNA and protein levels for c-FLIP, and the effect was suppressed by dipyridamole. Furthermore, overexpression of c-FLIP short in HuH-7 cells inhibited adenosine-induced caspase-8 activity. Taken together, these results suggest that intracellularly transported adenosine, perhaps converted AMP as the ensuing event, activates caspase-8 and the downstream effector caspase caspase-3 by neutralizing caspase-8 inhibition due to c-FLIP as a consequence of decreased c-FLIP expression, leading to apoptosis. This extends our understanding of adenosine-induced molecular apoptotic pathways.

8-Chloro-adenosine inhibits growth at least partly by interfering with actin polymerization in cultured human lung cancer cells

A key feature of actin is its ability to bind and hydrolyze ATP. 8-Chloro-adenosine (8-Cl-Ado), which can be phosphorylated to the moiety of 8-Cl-ATP in living cells, inhibits tumor cell proliferation. Therefore we tested the hypothesis that 8-Cl-Ado can interfere with the dynamic state of actin polymerization. We found that 8-Cl-Ado inhibited the growth of human lung cancer cell line A549 and H1299 in culture, and arrested the target cells in G2/M phase evidenced by fluorescence-activated cell sorting (FACS). Immunocytochemistry showed that the normal organization of microfilaments was disrupted in 8-Cl-Ado-exposed cells, which is accompanied by the decrease of cell size and the alteration of cell shape, and by aberrant mitosis and apoptosis in targeted cells. Furthermore, in vitro light scattering assays revealed that 8-Cl-ATP could directly inhibit the transition of G-actin to F-actin. DNase I inhibition assays showed that the G/F-actin ratio, a surrogate marker of actin polymerization status in living cells, was significantly increased in 8-Cl-Ado-exposed A549 and H1299 cells, compared to the G/F-actin ratio in unexposed cells. Taken together, these results indicate that 8-Cl-Ado exposure can alter the dynamic properties of actin polymerization, disrupt the dynamic instability or the rearrangement ability of actin filaments. Therefore, our data suggest that 8-Cl-Ado may exert its cytotoxicity at least partly by interfering with the dynamic instability of microfilaments, which may correlate with its inhibitory effects on cell proliferation and cell death.

Insulin-like growth factor binding protein-4 (IGFBP-4) is a novel anti-angiogenic and anti-tumorigenic mediator secreted by dibutyryl cyclic AMP (dB-cAMP)-differentiated glioblastoma cells

cAMP has been shown to reverse the transformed phenotype of various cancer cells. Human glioblastoma U87MG cells exposed to 500 microM dB-cAMP for 6 days showed reduced proliferation, attenuated invasiveness, and inability to induce angiogenic responses in human brain endothelial cells (HBECs) grown in Matrigeltrade mark. VEGF was the principal mediator of angiogenic actions of U87MG conditioned media (CM), since VEGF neutralizing antibody completely inhibited U87MG-induced angiogenic responses and no detectable levels of IGF, bFGF, and PlGF were found in U87MG CM. VEGF release was induced ( approximately 20%) in dB-cAMP-treated U87MG cells, suggesting a simultaneous induction of anti-angiogenic mediators. Down-stream effectors of dB-cAMP actions in U87MG were investigated by microarray gene expression analysis. Detected increases in differentiation genes, staniocalcin-1 and Wnt-5a, and angiogenesis-related genes, PAI-1, SPARC, IGFBP-4, IGFBP-7, PAPP-A, and PRSS-11 in dB-cAMP-treated U87MG cells were validated by real-time PCR, Western blot, and/or ELISA. A subsequent series of experiments identified IGFBP-4 as the principal anti-angiogenic mediator secreted by glioblastoma cells in response to dB-cAMP. Human recombinant IGFBP-4 inhibited the angiogenic response of HBEC induced by U87MG CM, whereas anti-human IGFBP-4 antibody restored the pro-angiogenic activity of dB-cAMP-treated U87MG CM. Since neither U87MG nor HBEC cells secreted detectable levels of IGF-I, and there are no known cellular IGFBP-4 receptors, the anti-angiogenic effect of IGFBP-4 was likely IGF-I-independent and indirect. IGFBP-4 also antagonized angiogenic effects of VEGF(165), PlGF, and bFGF, and reduced U87MG colony formation in soft-agar. IGFBP-4 is a novel dB-cAMP-induced anti-angiogenic and anti-tumorigenic mediator that may be a promising candidate for glioblastoma therapy.

The combination of sulfinosine and 8-Cl-cAMP induces synergistic cell growth inhibition of the human neuroblastoma cell line in vitro

To identify purine analogs that could be effective in treating neuroblastomas, we tested the anticancer properties of sulfinosine, 8-Cl-cAMP and 8-Cl-adenosine in the SK-N-SH cell line. First we examined the effects of these three agents on cell growth inhibition and cell viability by the BrdU and Sulforhodamine B assay. Treatment of SK-N-SH cells with increasing concentrations of these compounds led to a significant inhibition of cell proliferation and decrease of cell viability in a time- and dose-dependent manner at micromolar concentration (<10 microm). Treatment with a combination of sulfinosine and 8-Cl-cAMP resulted in synergistic effects on growth inhibition, cell cycle arrest and induction of apoptosis. Flow-cytometric analysis showed that 8-Cl-cAMP arrested the cells in the G0/G1 phase and sulfinosine blocked cell cycle progression at the G2/M stage, in contrast to the combined effects of both agents that did not arrest growth at any particular phase of the cell cycle. Further analysis of apoptosis induction demonstrated an increase from 17 to 24% of both early and late apoptotic cells and a very low percentage of necrotic cells. These results indicate that apoptosis was the predominant type of cell death after treatment of SK-N-SH cells with both substances, as well as with their combinations.

Bacillus anthracis oedema toxin as a cause of tissue necrosis and cell type-specific cytotoxicity

Oedema factor (OF) and protective antigen (PA) are secreted by Bacillus anthracis, and their binary combination yields oedema toxin (OT). Following PA-mediated delivery to the cytosol, OF functions as an adenylate cyclase generating high levels of cAMP. To assess OT as a possible cause of tissue damage and cell death, a novel approach was developed, which utilized a developing zebrafish embryo model to study toxin activity. Zebrafish embryos incubated with OT exhibited marked necrosis of the liver, cranium and gastrointestinal tract, as well as reduced swim bladder inflation. The OT-treated embryos survived after all stages of development but succumbed to the toxin within 7 days. Additional analysis of specific cell lines, including macrophage and non-macrophage, showed OT-induced cell death is cell type-specific. There was no discernible correlation between levels of OF-generated cAMP and cell death. Depending on the type of cell analysed, cell death could be detected in low levels of cAMP, and, conversely, cell survival was observed in one cell line in which high levels of cAMP were found following treatment with OT. Collectively, these data suggest OT is cytotoxic in a cell-dependent manner and may contribute to disease through direct cell killing leading to tissue necrosis.

The modulation of apoptosis by cyclic AMP involves Akt and epidermal growth factor receptor

Adenosine 3',5'-cyclic monophosphate (cAMP) and transforming growth factor-beta are important regulators of many biological processes. In this study we investigated the effect and its potential mechanism of cAMP on transforming growth factor-beta1- and serum deprivation-induced apoptosis in Mv1Lu cells. Transforming growth factor-beta1 treatment or serum deprivation induces apoptotic response in Mv1Lu cells. Forskolin, a cAMP-elevating agent, or 8-Bromo-cAMP (8-B-cAMP), a cell permeable cAMP analogue, inhibited the cell proliferation and markedly enhanced apoptosis induced by transforming growth factor-beta1, but completely suppressed serum deprivation-induced apoptosis. Furthermore, forskolin decreased the Akt phosphorylation, and the inhibition of phosphatidylinositol-3 kinase by LY294002 sensitized Mv1Lu cells to transforming growth factor-beta1-induced apoptosis. In addition, forskolin treatment induced tyrosine phosphorylation of epidermal growth factor receptor. Inhibition of epidermal growth factor receptor by specific inhibitor PD153035 blocked the cAMP-mediated suppression of serum deprivation-induced apoptosis. The results indicate that cAMP exerts its opposite effects in transforming growth factor-beta1- and serum deprivation-induced apoptosis via a mechanism involving the modulation of signaling components of phosphatidylinositol-3-kinase/Akt and epidermal growth factor receptor in Mv1Lu cells.

Exposure of human lung cancer cells to 8-chloro-adenosine induces G2/M arrest and mitotic catastrophe

8-Chloro-adenosine (8-Cl-Ado) is a potent chemotherapeutic agent whose cytotoxicity in a variety of tumor cell lines has been widely investigated. However, the molecular mechanisms are uncertain. In this study, we found that exposure of human lung cancer cell lines A549 (p53-wt) and H1299 (p53-depleted) to 8-Cl-Ado induced cell arrest in the G2/M phase, which was accompanied by accumulation of binucleated and polymorphonucleated cells resulting from aberrant mitosis and failed cytokinesis. Western blotting showed the loss of phosphorylated forms of Cdc2 and Cdc25C that allowed progression into mitosis. Furthermore, the increase in Ser10-phosphorylated histone H3-positive cells revealed by fluorescence-activated cell sorting suggested that the agent-targeted cells were able to exit the G2 phase and enter the M phase. Immunocytochemistry showed that microtubule and microfilament arrays were changed in exposed cells, indicating that the dynamic instability of microtubules and microfilaments was lost, which may correlate with mitotic dividing failure. Aberrant mitosis resulted in mitotic catastrophe followed by varying degrees of apoptosis, depending on the cell lines. Thus, 8-Cl-Ado appears to exert its cytotoxicity toward cells in culture by inducing mitotic catastrophe.

8-Cl-cAMP and its metabolite, 8-Cl-adenosine induce growth inhibition in mouse fibroblast DT cells through the same pathways: protein kinase C activation and cyclin B down-regulation

8-Chloro-cyclic AMP (8-Cl-cAMP) is known to be most effective in inducing growth inhibition and differentiation of a number of cancer cells. Also, its cellular metabolite, 8-Cl-adenosine was shown to induce growth inhibition in a variety of cell lines. However, the signaling mechanism that governs the effects of 8-Cl-cAMP and/or 8-Cl-adenosine is still uncertain and it is not even sure which of the two is the key molecule that induces growth inhibition. In this study using mouse fibroblast DT cells, it was found that adenosine kinase inhibitor and adenosine deaminase could reverse cellular growth inhibition induced by 8-Cl-cAMP and 8-Cl-adenosine. And 8-Cl-cAMP could not induce growth inhibition in the presence of phosphodiesterase (PDE) inhibitor, but 8-Cl-adenosine could. We also found that protein kinase C (PKC) inhibitor could restore this growth inhibition, and both the 8-Cl-cAMP and 8-Cl-adenosine could activate the enzymatic activity of PKC. Besides, after 8-Cl-cAMP and 8-Cl-adenosine treatment, cyclin B was down-regulated and a CDK inhibitor, p27 was up-regulated in a time-dependent manner. These results suggest that it is not 8-Cl-cAMP but 8-Cl-adenosine which induces growth inhibition, and 8-Cl-cAMP must be metabolized to exert this effect. Furthermore, there might exist signaling cascade such as PKC activation and cyclin B down-regulation after 8-Cl-cAMP and 8-Cl-adenosine treatment.

8-Cl-Adenosine enhances 1,25-dihydroxyvitamin D3-induced growth inhibition without affecting 1,25-dihydroxyvitamin D3-stimulated differentiation of primary mouse epidermal keratinocytes

Background

Epidermal keratinocytes continuously proliferate and differentiate to form the mechanical and water permeability barrier that makes terrestrial life possible. In certain skin diseases, these processes become dysregulated, resulting in abnormal barrier formation. In particular, skin diseases such as psoriasis, actinic keratosis and basal and squamous cell carcinomas are characterized by hyperproliferation and aberrant or absent differentiation of epidermal keratinocytes. We previously demonstrated that 8-Cl-adenosine (8-Cl-Ado) can induce keratinocyte growth arrest without inducing differentiation.

Results

To determine if this agent might be useful in treating hyperproliferative skin disorders, we investigated whether 8-Cl-Ado could enhance the ability of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], a known keratinocyte differentiating agent and a clinical treatment for psoriasis, to inhibit keratinocyte growth. We found that low concentrations of 8-Cl-Ado and 1,25(OH)₂D₃ appeared to act additively to reduce proliferation of primary mouse epidermal keratinocytes. However, another agent (transforming growth factor-beta) that triggers growth arrest without inducing differentiation also coincidentally inhibits differentiation elicited by other agents; inhibition of differentiation is suboptimal for treating skin disorders, as differentiation is often already reduced. Thus, we determined whether 8-Cl-Ado also decreased keratinocyte differentiation induced by 1,25(OH)₂D₃, as measured using the early and late differentiation markers, keratin 1 protein levels and transglutaminase activity, respectively. 8-Cl-Ado did not affect 1,25(OH)₂D₃-stimulated keratin 1 protein expression or transglutaminase activity.

Conclusions

Our results suggest that 8-Cl-Ado might be useful in combination with differentiating agents for the treatment of hyperproliferative disorders of the skin.

cAMP Dose-dependently prevents palmitate-induced apoptosis by both protein kinase A- and cAMP-guanine nucleotide exchange factor-dependent pathways in beta-cells

Lipid accumulation in pancreatic beta-cells is thought to cause its dysfunction and/or destruction via apoptosis. Our studies show that incubation of the beta-cell line RINm5F with the saturated free fatty acids (FFA) palmitate caused apoptosis based on increases in caspase 3 activity, Annexin V staining, and cell death. Furthermore, exposure of RINm5F cells to cAMP-increasing agents, 3-isobutyl-1-methylxanthine (IBMX), and forskolin completely abolished palmitate-mediated caspase 3 activity and significantly inhibited Annexin V staining and cell death. The cyclic AMP analogs cpt-cAMP and dibutyryl-cAMP mimicked the protective effects of IBMX and forskolin, suggesting that cAMP is the mediator of the anti-apoptotic effects. The protective action of IBMX and forskolin was rapid and did not appear to require gene transcription or new protein synthesis. However, these protective effects were clearly independent of protein kinase A (PKA) because of the lack of inhibition by the PKA inhibitors H-89 and KT5720. In attempts to identify this PKA-independent mechanism, we found that the newly developed cAMP analog 8CPT-2Me-cAMP, which selectively activates the cAMP-dependent guanine nucleotide exchange factor (cAMP-GEF) pathway, mimicked the protective effects of IBMX and forskolin, suggesting that the cAMP-GEF pathway is involved. In addition, both glucagon-like peptide (GLP-1) and its receptor agonist, Exenatide, inhibited palmitate-mediated caspase 3 activation in a dose-dependent manner. Unexpectedly, H-89 partially reversed the protective effects of GLP-1 and Exenatide, suggesting that PKA may play a role in the protective effects of these incretins. To explain these seemingly conflicting results, we demonstrated that low concentrations of cAMP produced by GLP-1 and Exenatide preferentially activate the PKA pathway, whereas higher cAMP concentrations produced by IBMX and forskolin activate the more dominant cAMP-GEF pathway. Taken together, these results indicate that intracellular concentrations of cAMP may play a key role in determining divergent signaling pathways that lead to antiapoptotic responses.