Multiple Signalling Pathways Interact in the Regulation of Nerve Growth Factor Production in L929 Fibroblasts

Fibroblasts are one of several cell types producing nerve growth factor (NGF) in neuronal targets. In previous studies we found that NGF production is up-regulated by 12-O-tetradecanoylphorbol 13-acetate (TPA) and serum, down-regulated by corticosterone, and unaffected by dibutyryl-cyclic AMP (db-cyclic AMP) in fibroblasts. As fibroblasts in vivo are likely to be exposed to regulatory effects by more than one of these agents at any given time, we examined the effects of combinations of them on NGF production using L929 fibroblasts as a model system. TPA and serum together stimulated NGF production 10-fold more than either agent alone. Corticosterone reduced NGF mRNA and NGF production to less than 10% of basal levels whether or not TPA or serum, or both, were present but not in the presence of the glucocorticoid antagonist RU486. Corticosterone did not increase the rate of NGF mRNA degradation. Forskolin and db-cyclic AMP prevented NGF mRNA induction by TPA and serum without changing basal levels. TPA induced c-fos and junB mRNAs transiently and preceding NGF mRNA induction but c-jun mRNA remained undetectable. Forskolin enhanced the induction of both junB and c-fos mRNA whereas corticosterone prolonged junB mRNA induction. Thus, TPA induction of NGF mRNA is modulated differentially by corticosterone and cyclic AMP. c-fos and junB may play a role in the underlying mechanisms.     

Phorbol ester induces manganese-superoxide dismutase in tumor necrosis factor-resistant cells.

The effects of phorbol ester (TPA) and other known stimulators such as tumor necrosis factor (TNF), interleukin-1, and lipopolysaccharide on induction of mRNA for manganese-superoxide dismutase (Mn-SOD) were investigated in various cell lines. TPA enhanced Mn-SOD mRNA expression in TNF-resistant cell lines including HeLa cells, in which the other reagents also induced expression of the gene, but did not affect TNF-sensitive cells, in which the other stimulators did not alter expression of the gene. HeLa cells which had been desensitized to TPA by pretreatment with TPA for 24 h expressed Mn-SOD mRNA at a slightly higher level than the cells without TPA treatment. TPA-pretreated cells stimulated with TNF, however, expressed Mn-SOD mRNA at about twice the level of TNF-stimulated, TPA-untreated cells. When protein synthesis was inhibited by cycloheximide during TPA pretreatment, TNF no more enhanced the Mn-SOD mRNA accumulation. These data suggest that at least two separate signal-transducing pathways are involved in expression of this gene. One is triggered by protein kinase C activation itself in the absence of new protein synthesis. The other can be activated by stimulation with TNF, interleukin-1, or lipopolysaccharide and in which a protein factor that can be induced by TPA treatment is involved.     

Bombesin induces c-fos and c-myc expression in quiescent Swiss 3T3 cells. Comparative study with other mitogens

We have studied the effect of the potent mitogen bombesin on the expression of c-fos and c-myc genes in quiescent mouse fibroblasts. We have demonstrated that bombesin rapidly induces a transient expression of c-fos mRNA followed by a more protracted elevation in c-myc mRNA levels. The intensity of the induction of expression of both proto-oncogenes depended on the dose of bombesin used. Prolonged treatment of the cells with TPA, which causes a selective decrease in protein kinase C activity, partially inhibited the induction of c-fos and c-myc gene expression by bombesin, similar to what has been observed with PDGF. However, a dramatic inhibition of the mitogenic response to bombesin--but not to PDGF--was found in TPA-treated cells. In contrast, TPA-treated cells showed an increased response to EGF with regard to proto-oncogene expression. The role of protein kinase C and Ca2+-dependent pathways in proto-oncogene induction by bombesin is discussed.     

Position-independent expression of a human nerve growth factor-luciferase reporter gene cloned on a yeast artificial chromosome vector.

Two yeast artificial chromosomes containing the entire human nerve growth factor gene were isolated and mapped. By homologous recombination a luciferase gene was precisely engineered into the coding portion of the NGF gene and a neomycin selection marker was placed adjacent to one of the YAC telomeres. Expression of the YAC-based NGF reporter gene and a plasmid-based NGF reporter gene were compared with the regulation of endogenous mouse NGF protein in mouse L929 fibroblasts. In contrast to the plasmid-based reporter gene, expression and regulation of the YAC-based reporter gene was independent of the site of integration of the transgene. Basic fibroblast growth factor and okadaic acid stimulated expression of the YAC transgene, whereas transforming growth factor-beta and dexamethasone inhibited it. Although cyclic AMP strongly stimulated production of the endogenous mouse NGF, no effect was seen on the human NGF reporter genes. Downregulation of the secretion of endogenous mouse NGF already occurred at an EC50 of 1-2 nM dexamethasone, but downregulation of the expression of NGF reporter genes occurred only at EC50 of 10 nM. This higher concentration was also required for upregulation of luciferase genes driven by the dexamethasone-inducible promoter of the mouse mammary tumor virus in L929 fibroblasts.     

Suppression of IkappaBalpha expression is necessary for c-Jun N-terminal kinase-mediated enhancement of Fas cytotoxicity

The role of c-Jun N-terminal kinase (JNK) in the regulation of Fas-mediated cell death was investigated. Murine L929 fibroblasts were pretreated with anisomycin for 1 h to activate JNK, followed by exposure to anti-Fas antibodies/actinomycin D (ActD) for 16-24 h. Compared to untreated controls, the induction of JNK activation failed to raise cellular sensitivity to anti-Fas/ActD killing. Notably, a significant increase in anti-Fas/ActD killing as induced by JNK preactivation was observed in L929 cells which were engineered to suppress IkappaBalpha protein expression by antisense mRNA. Restoration of the IkappaBalpha protein level in these cells by ectopic expression of a cDNA construct abolished the JNK-increased anti-Fas/ActD killing. Despite the suppression of IkappaBalpha, no constitutive p65 (RelA) NF-kappaB nuclear translocation was observed in the IkappaBalpha-antisense cells. Also, inhibition of NF-kappaB by curcumin failed to inhibit the JNK-increased Fas cytotoxicity, suggesting that NF-kappaB is not involved in the observed effect. Most interestingly, culturing of L929 cells on extracellular protein matrices resulted in partial suppression of IkappaBalpha expression and constitutive JNK and p42/44 MAPK activation. Upon stimulation with anisomycin, these matrix protein-stimulated cells further exhibited reduced IkappaBalpha expression and p42/44 MAPK activation, as well as became sensitized to JNK-increased anti-Fas/ActD killing. Again, ectopic expression of IkappaBalpha in these cells abolished the enhanced anti-Fas/ActD killing effect. Together, these results indicate that suppression of IkappaBalpha expression is essential for JNK-mediated enhancement of Fas cytotoxicity.     

Retinoic acid and phorbol ester synergistically up-regulate IL-8 expression and specifically modulate protein kinase C-epsilon in human skin fibroblasts.

Phorbol ester (TPA) and retinoic acid (RA) are two potent immunomodulatory agents whose actions are mediated through distinct signal transduction pathways involving protein kinase C (PKC) and nuclear RA receptors, respectively. We have investigated the interactions between these two pathways in the regulation of expression of the inflammatory cytokine IL-8 in human skin fibroblasts. TPA (as previously reported) and RA both induced IL-8 mRNA and protein in a time- and dose-dependent manner. IL-8 mRNA induction by TPA (10 nM) was maximal (15-fold) within 6 h, and returned to baseline within 24 h of treatment, although maximal induction (10-fold) by RA (1 microM) did not occur until 24 h posttreatment. Induction of IL-8 by TPA was blocked by 1-(5-isoquinolinyl-sulfonyl)-2-methylpiperazine, which inhibits PKC and cAMP-dependent protein kinases (PKA), but not by N-(2-ganidinoethyl)-5-isoquinoline sulfonamide, which preferentially inhibits PKA, consistent with the participation of PKC in the induction of IL-8 by TPA. In contrast, induction of IL-8 by RA was inhibited by both 1-(5-isoquinoline sulfonamide and N-(2-gamidinoethyl)-5-isoquinoline sulfonamide, suggesting the participation of PKA in the induction of IL-8 by RA. However, activation of PKA by addition of cAMP analogues was not sufficient to induce IL-8 expression. Induction of IL-8 by RA also did not appear to be mediated indirectly through induction of IL-1, because addition of IL-1R antagonist did not block IL-8 induction by RA. RA and TPA added in combination synergistically enhanced expression of IL-8 mRNA, measured at 6 (2-fold) and 24 h (10-fold) posttreatment. To investigate the mechanism of this synergy, the effect of TPA and RA on fibroblast PKC activation and PKC isozyme levels were determined. TPA, either alone or together with RA, but not RA alone, stimulated phosphorylation of an endogenous 80-kDa PKC substrate. Dermal fibroblasts expressed three PKC isozymes (alpha, (delta, and (epsilon). TPA, but not RA, down-regulated PKC-alpha, neither TPA or RA affected the level of PKC-delta, and both TPA and RA down-regulated PKC-epsilon. This latter effect was enhanced 2-fold by addition of RA and TPA together. These data suggest that modulation of PKC-epsilon may be a common participant in the regulation of IL-8 expression by TPA and RA.     

Cell cycle dependent regulation of cdc2 mRNA in mouse fibroblasts: Requirement of protein synthesis and of continued mitogenic stimulation

In the chemically transformed mouse fibroblasts (BP-A31) placed in a serum-free medium, the cdc2 mRNA content decreases in parallel with the cessation of [³H]thymidine incorporation. Extinction of the cdc2 gene expression is also observed in BP-A31 cells overexpressing the human c-myc oncogene. At quies-cence, the cdc2 gene expression can be reinduced with serum or with other mitogens such as insulin or 12-O-tetradecanoyl phorbol 13-acetate (TPA). The kinetics of induction is characterized by a lag period which differs according to the mitogen used and reflects the length of the G1 phase (4–6 h with insulin or serum, 9–12 h with TPA). The cdc2 mRNA accumulation is prevented when protein synthesis is blocked with cycloheximide, also if the drug is added at a time when the synthesis of cdc2 mRNA is already under way. Similarly, removal of the mitogen leads to a cessation of the cdc2 mRNA accumulation. These results suggest that the increased expression of the cdc2 gene is mediated by (a) short-lived, growth factor-regulated protein(s). © 1993 Wiley-Liss, Inc.