Liquid Biopsy in Lung Cancer: Biomarkers for the Management of Recurrence and Metastasis

Liquid biopsies have emerged as a promising tool for the detection of metastases as well as local and regional recurrence in lung cancer. Liquid biopsy tests involve analyzing a patient's blood, urine, or other body fluids for the detection of biomarkers, including circulating tumor cells or tumor-derived DNA/RNA that have been shed into the bloodstream. Studies have shown that liquid biopsies can detect lung cancer metastases with high accuracy and sensitivity, even before they are visible on imaging scans. Such tests are valuable for early intervention and personalized treatment, aiming to improve patient outcomes. Liquid biopsies are also minimally invasive compared to traditional tissue biopsies, which require the removal of a sample of the tumor for further analysis. This makes liquid biopsies a more convenient and less risky option for patients, particularly those who are not good candidates for invasive procedures due to other medical conditions. While liquid biopsies for lung cancer metastases and relapse are still being developed and validated, they hold great promise for improving the detection and treatment of this deadly disease. Herein, we summarize available and novel approaches to liquid biopsy tests for lung cancer metastases and recurrence detection and describe their applications in clinical practice.

Sequence-Based Platforms for Discovering Biomarkers in Liquid Biopsy of Non-Small-Cell Lung Cancer

Lung cancer detection and monitoring are hampered by a lack of sensitive biomarkers, which results in diagnosis at late stages and difficulty in tracking response to treatment. Recent developments have established liquid biopsies as promising non-invasive methods for detecting biomarkers in lung cancer patients. With concurrent advances in high-throughput sequencing technologies and bioinformatics tools, new approaches for biomarker discovery have emerged. In this article, we survey established and emerging biomarker discovery methods using nucleic acid materials derived from bodily fluids in the context of lung cancer. We introduce nucleic acid biomarkers extracted from liquid biopsies and outline biological sources and methods of isolation. We discuss next-generation sequencing (NGS) platforms commonly used to identify novel biomarkers and describe how these have been applied to liquid biopsy. We highlight emerging biomarker discovery methods, including applications of long-read sequencing, fragmentomics, whole-genome amplification methods for single-cell analysis, and whole-genome methylation assays. Finally, we discuss advanced bioinformatics tools, describing methods for processing NGS data, as well as recently developed software tailored for liquid biopsy biomarker detection, which holds promise for early diagnosis of lung cancer.

Emergence and dissemination of three mild outbreaks of Neisseria gonorrhoeae with high-level resistance to azithromycin in Barcelona, 2016-18

BACKGROUND

Neisseria gonorrhoeae (NG) isolates with high-level azithromycin resistance (HL-AziR) have emerged worldwide in recent decades, threatening the sustainability of current dual-antimicrobial therapy.

OBJECTIVES

This study aimed to characterize the first 16 NG isolates with HL-AziR in Barcelona between 2016 and 2018.

METHODS

WGS was used to identify the mechanisms of antimicrobial resistance, to establish the MLST ST, NG multiantigen sequence typing (NG-MAST) ST and NG sequence typing for antimicrobial resistance (NG-STAR) ST and to identify the clonal relatedness of the isolates with other closely related NG previously described in other countries based on a whole-genome SNP analysis approach. The sociodemographic characteristics of the patients included in the study were collected by comprehensive review of their medical records.

RESULTS

Twelve out of 16 HL-AziR isolates belonged to the MLST ST7823/NG-MAST ST5309 genotype and 4 to MLST ST9363/NG-MAST ST3935. All presented the A2059G mutation in all four alleles of the 23S rRNA gene. MLST ST7823/NG-MAST ST5309 isolates were only identified in men who have sex with women and MLST ST9363/NG-MAST ST3935 were found in MSM. Phylogenomic analysis revealed the presence of three transmission clusters of three different NG strains independently associated with sexual behaviour.

CONCLUSIONS

Our findings support the first appearance of three mild outbreaks of NG with HL-AziR in Spain. These results highlight the continuous capacity of NG to develop antimicrobial resistance and spread among sexual networks. The enhanced resolution of WGS provides valuable information for outbreak investigation, complementing the implementation of public health measures focused on the prevention and dissemination of MDR NG.

Allosteric modulation of protease-activated receptor signaling

The protease-activated receptors (PARs) are G protein-coupled receptors (GPCRs) that are uniquely activated by proteolysis. PARs mediate hemostasis, thrombosis, inflammation, embryonic development and progression of certain malignant cancers. The family of PARs include four members: PAR1, PAR2, PAR3 and PAR4. PARs harbor a cryptic ligand sequence within their N-terminus that is exposed following proteolytic cleavage. The newly formed PAR Nterminus functions as a tethered ligand that binds intramolecularly to the receptor to trigger transmembrane signaling. This unique mechanism of activation would indicate that regardless of the activating protease, cleavage of PARs would unmask a tethered ligand sequence that would induce a similar active receptor conformation and signaling response. However, this is not the case. Recent studies demonstrate that PARs can be differentially activated by synthetic peptide agonists, proteases or through dimerization, that ultimately result in distinct cellular responses. In some cases, allosteric modulation of PARs involves compartmentalization in caveolae, plasma membrane microdomains enriched in cholesterol. Here, we discuss some mechanisms that lead to allosteric modulation of PAR signaling.

Persistent transactivation of EGFR and ErbB2/HER2 by protease-activated receptor-1 promotes breast carcinoma cell invasion

Hyperactivation of ErbB signaling is implicated in metastatic breast cancer. However, the mechanisms that cause dysregulated ErbB signaling and promote breast carcinoma cell invasion remain poorly understood. One pathway leading to ErbB activation that remains unexplored in breast carcinoma cell invasion involves transactivation by G-protein-coupled receptors (GPCRs). Protease-activated receptor-1 (PAR1), a GPCR activated by extracellular proteases, is overexpressed in invasive breast cancer. PAR1 is also proposed to function in breast cancer invasion and metastasis, but how PAR1 contributes to these processes is not known. In this study, we report that proteolytic activation of PAR1 by thrombin induces persistent transactivation of EGFR and ErbB2/HER2 in invasive breast carcinoma, but not in normal mammary epithelial cells. PAR1-stimulated EGFR and ErbB2 transactivation leads to prolonged extracellular signal-regulated kinase-1 and -2 signaling and promotes breast carcinoma cell invasion. We also show that PAR1 signaling through Galpha(i/o) and metalloprotease activity is critical for ErbB transactivation and cellular invasion. Finally, we demonstrate that PAR1 expression in invasive breast carcinoma is essential for tumor growth in vivo assessed by mammary fat pad xenografts. These studies reveal a critical role for PAR1, a receptor activated by tumor-generated proteases, in hyperactivation of ErbB signaling that promotes breast carcinoma cell invasion.

Protease-activated receptor-1 down-regulation: a mutant HeLa cell line suggests novel requirements for PAR1 phosphorylation and recruitment to clathrin-coated pits

Protease-activated receptor-1 (PAR1), a G protein-coupled receptor (GPCR) for thrombin, is irreversibly activated by a proteolytic mechanism, then internalized and degraded in lysosomes. The latter is critical for temporal fidelity of thrombin signaling. Toward understanding PAR1 down-regulation, we first investigated the pathway of PAR1 internalization. Activated PAR1 was rapidly recruited to clathrin-coated pits, where it colocalized with transferrin receptor (TfnR). Dominant-negative dynamin and clathrin hub mutants both blocked PAR1 internalization. Blockade of PAR1 internalization with dynamin K44A also inhibited activation-dependent PAR1 degradation. Thus activated PAR1 internalizes via clathrin-coated pits together with receptors that recycle and is then sorted away from such receptors and delivered to lysosomes. In the course of these studies we identified a mutant HeLa cell line, designated JT1, that was defective in PAR1 internalization. PAR1 signaled robustly in JT1 cells but was not phosphorylated or recruited to clathrin-coated pits after activation. Internalization of TfnR was intact in JT1 cells and internalization of beta(2)-adrenergic receptor, a GPCR that internalizes and recycles, was present but perhaps reduced. Taken together, these studies suggest that PAR1 is internalized in a dynamin- and clathrin-dependent manner like TfnR and beta(2)-adrenergic receptor but requires a distinct gene product for recruitment into this pathway.

Glycoprotein V-deficient platelets have undiminished thrombin responsiveness and Do not exhibit a Bernard-Soulier phenotype

Adhesion of platelets to extracellular matrix via von Willebrand factor (vWF) and activation of platelets by thrombin are critical steps in hemostasis. Glycoprotein (GP) V is a component of the GPIb-V-IX complex, the platelet receptor for vWF. GPV is also cleaved by thrombin. Deficiency of GPIb or GPIX results in Bernard-Soulier syndrome (BSS), a bleeding disorder in which platelets are giant and have multiple functional defects. Whether GPV-deficiency might also cause BSS is unknown as are the roles of GPV in platelet-vWF interaction and thrombin signaling. We report that GPV-deficient mice developed normally, had no evidence of spontaneous bleeding, and had tail bleeding times that were not prolonged compared with wild-type mice. GPV-deficient platelets were normal in size and structure as assessed by flow cytometry and electron microscopy. GPV-deficient and wild-type platelets were indistinguishable in botrocetin-mediated platelet agglutination and in their ability to adhere to mouse vWF A1 domain. Platelet aggregation and ATP secretion in response to low and high concentrations of thrombin were not decreased in GPV-deficient platelets compared with wild-type. Our results show that (1) GPV is not necessary for GPIb expression and function in platelets and that GPV deficiency is not likely to be a cause of human BSS and (2) GPV is not necessary for robust thrombin signaling. Whether redundancy accounts for the lack of phenotype of GPV-deficiency or whether GPV serves subtle or as yet unprobed functions in platelets or other cells remains to be determined.

The cytoplasmic tails of protease-activated receptor-1 and substance P receptor specify sorting to lysosomes versus recycling

The G protein-coupled receptor (GPCR) for thrombin, protease-activated receptor-1 (PAR1), is activated when thrombin cleaves its amino-terminal exodomain. The irreversibility of this proteolytic mechanism raises the question of how desensitization and resensitization are accomplished for thrombin signaling. PAR1 is phosphorylated, uncoupled from signaling, and internalized after activation like classic GPCRs. However, unlike classic GPCRs, which internalize and recycle, activated PAR1 is sorted to lysosomes. To identify the signals that specify the distinct sorting of PAR1, we constructed chimeras between PAR1 and the substance P receptor. Wild-type substance P receptor internalized and recycled after activation; PAR1 bearing the cytoplasmic tail of the substance P receptor (P/S) behaved similarly. By contrast, wild-type PAR1 and a substance P receptor bearing the cytoplasmic tail of PAR1 (S/P) sorted to lysosomes after activation. Consistent with these observations, PAR1 and the S/P chimera were effectively down-regulated by their respective agonists as assessed by both receptor protein levels and signaling. Substance P receptor and the P/S chimera showed little down-regulation. These data suggest that the cytoplasmic tails of PAR1 and substance P receptor specify their distinct intracellular sorting patterns after activation and internalization. Moreover, by altering the trafficking fates of PAR1 and substance P receptor, one can dictate the efficiency with which a cell maintains responsiveness to PAR1 or substance P receptor agonists over time.

Termination of signaling by protease-activated receptor-1 is linked to lysosomal sorting

The irreversible proteolytic mechanism by which protease-activated receptor-1 (PAR1), the G protein-coupled receptor (GPCR) for thrombin, is activated raises the question of how it is shut off. Like classic GPCRs, activated PAR1 is rapidly phosphorylated and internalized, but unlike classic GPCRs, which recycle, internalized PAR1 is sorted to lysosomes. A chimeric PAR1 bearing the substance P receptor's cytoplasmic carboxyl tail sequestered and recycled like wild-type substance P receptor. In cells expressing this chimera, signaling in response to the PAR1-activating peptide SFLLRN ceased as expected upon removal of this agonist. Strikingly, however, when the chimera was activated proteolytically by thrombin, signaling persisted even after thrombin was removed. This persistent signaling was apparently due to "resignaling" by previously activated receptors that had internalized and recycled back to the cell surface. Thus the cytoplasmic carboxyl tail of PAR1 specifies an intracellular sorting pattern that is linked to its signaling properties. In striking contrast to most GPCRs, sorting of activated PAR1 to lysosomes rather than recycling is critical for terminating PAR1 signaling-a trafficking solution to a signaling problem.

Role of the thrombin receptor's cytoplasmic tail in intracellular trafficking. Distinct determinants for agonist-triggered versus tonic internalization and intracellular localization

The G protein-coupled thrombin receptor is activated by an irreversible proteolytic mechanism and, perhaps as a result, exhibits an unusual trafficking pattern in the cell. Naive receptors tonically cycle between the cell surface and a protected intracellular pool, whereas receptors cleaved and activated at the cell surface internalize and move to lysosomes. Toward understanding how these trafficking events are regulated, we examined a series of receptor mutants. A receptor with alanine substitutions at all potential phosphorylation sites in the cytoplasmic tail failed to display agonist-triggered internalization but, like wild type receptor, displayed robust signaling, tonic cycling, and localization to both the cell surface and an intracellular pool. A truncation mutant that lacked most of the cytoplasmic tail also signaled robustly, lacked phosphorylation, and was defective in agonist-triggered internalization. However, in contrast to the specific phosphorylation site mutant, the truncation mutant did not display tonic cycling and localized exclusively to the cell surface. An analysis of a series of truncation mutants localized residues important for receptor trafficking to a 10-amino acid stretch in its cytoplasmic tail. These data suggest that phosphorylation may trigger internalization of activated thrombin receptors but that a second phosphorylation-independent signal mediates tonic internalization of naive receptors. They further suggest that maintenance of the intracellular pool of naive thrombin receptors requires tonic receptor internalization.

The cloned thrombin receptor is necessary and sufficient for activation of mitogen-activated protein kinase and mitogenesis in mouse lung fibroblasts. Loss of responses in fibroblasts from receptor knockout mice

The mitogenic activity of thrombin on fibroblasts and smooth muscle cells may contribute to embryonic development and normal wound healing, and it may also play a role in pathological responses to vascular injury. To examine the importance of thrombin signaling in vivo and to define the cloned thrombin receptor's role, we disrupted the thrombin receptor gene (tr) in mice. Platelets from tr-/- mice responded normally to thrombin, but tr-/- fibroblasts showed no thrombin-induced calcium mobilization or phosphoinositide hydrolysis. Thus distinct thrombin receptors act in different tissues. This study focuses on the role of the thrombin receptor in thrombin-induced mitogenesis and mitogen-activated protein (MAP) kinase activation in mesenchymal cells. Thrombin and thrombin receptor agonist peptide both stimulated DNA synthesis and MAP kinase activation in fibroblasts derived from wild-type mice. These responses were selectively lost in fibroblasts from tr-/- mice. Activation of the cloned thrombin receptor is therefore necessary and sufficient for thrombin-induced mitogenesis and MAP kinase activation in mouse lung fibroblasts. The tr-/- mouse thus provides a valuable model for defining the role of thrombin-induced proliferative events in vivo. Because thrombin-induced MAP kinase activation was attributable to a single receptor expressed at natural levels, mouse lung fibroblasts presented an opportunity to define the pathways that normally mediate activation of MAP kinase by the thrombin receptor. Elimination of phorbol-sensitive protein kinase C by prolonged exposure to phorbol ester only partially inhibited MAP kinase activation by thrombin but completely blocked c-Raf kinase activation. Pertussis toxin partially inhibited MAP kinase activation by thrombin but had no significant effect on c-Raf kinase activation. Thus in mouse lung fibroblasts, one thrombin receptor utilizes two pathways for MAP kinase activation: one is protein kinase C- and c-Raf dependent, and a second is Gi-dependent and c-Raf-independent.

Human Ste20 homologue hPAK1 links GTPases to the JNK MAP kinase pathway

BACKGROUND

The Rho-related GTP-binding proteins Cdc42 and Rac1 have been shown to regulate signaling pathways involved in cytoskeletal reorganization and stress-responsive JNK (Jun N-terminal kinase) activation. However, to date, the GTPase targets that mediate these effects have not been identified. PAK defines a growing family of mammalian kinases that are related to yeast Ste20 and are activated in vitro through binding to Cdc42 and Rac1 (PAK: p21 Cdc42-/Rac-activated kinase). Clues to PAK function have come from studies of Ste20, which controls the activity of the yeast mating mitogen-activated protein (MAP) kinase cascade, in response to a heterotrimeric G protein and Cdc42.

RESULTS

To initiate studies of mammalian Ste20-related kinases, we identified a novel human PAK isoform, hPAK1. When expressed in yeast, hPAK1 was able to replace Ste20 in the pheromone response pathway. Chemical mutagenesis of a plasmid encoding hPAK1, followed by transformation into yeast, led to the identification of a potent constitutively active hPAK1 with a substitution of a highly conserved amino-acid residue (L107F) in the Cdc42-binding domain. Expression of the hPAK1(L107F) allele in mammalian cells led to specific activation of the Jun N-terminal kinase MAP kinase pathway, but not the mechanistically related extracellular signal-regulated MAP kinase pathway.

CONCLUSIONS

These results demonstrate that hPAK1 is a GTPase effector controlling a downstream MAP kinase pathway in mammalian cells, as Ste20 does in yeast. Thus, PAK and Ste20 kinases play key parts in linking extracellular signals from membrane components, such as receptor-associated G proteins and Rho-related GTPases, to nuclear responses, such as transcriptional activation.

A direct role for protein kinase C and the transcription factor Jun/AP-1 in the regulation of the Alzheimer's beta-amyloid precursor protein gene

Overexpression of the beta-amyloid precursor protein gene (beta-APP) may contribute to the abnormal generation of beta-amyloid protein in Alzheimer's disease. We demonstrate using a human glial cell line (1321N1) that activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate (PMA) increases beta-APP mRNA levels, induces known components of the transcription factor activator protein-1 (AP-1), and increases protein-DNA binding activity to AP-1 sequences within the beta-APP promoter. A beta-APP promoter-luciferase reporter gene is transcriptionally activated by PMA, as well as by expression of constitutively activated PKC or by expression of c-Jun. Further characterization suggests that the distal but not the proximal AP-1 recognition site binds nuclear proteins regulated by PKC, and that the AP-1 binding activity is likely to be composed of Jun-Jun homodimers rather than Jun-Fos heterodimers. Additional studies demonstrate that a single copy of the distal AP-1 site fused to a heterologous promoter is sufficient to confer a response to PMA. Mutagenesis of this site in the beta-APP promoter renders it unresponsive to c-Jun and attenuates transcriptional activation by PMA. We suggest that cellular mediators that activate PKC, particularly those that induce significant increases in c-Jun, may up-regulate expression of the beta-APP gene and consequently affect production and processing of this protein.

Biphasic increase in c-jun mRNA is required for induction of AP-1-mediated gene transcription: differential effects of muscarinic and thrombin receptor activation

Activation of either muscarinic cholinergic or thrombin receptors increases phosphoinositide turnover, Ca2+ mobilization, and redistribution of protein kinase C and induces rapid transient increases in c-fos mRNA and c-jun mRNA in 1321N1 cells. To determine whether the increases in c-fos and c-jun mRNA induced by carbachol and thrombin are sufficient to stimulate AP-1-mediated transactivation, 1321N1 cells were transfected with a reporter carrying two copies of the tetradecanoyl phorbol acetate response element and the firefly luciferase gene. Thrombin was significantly more effective than carbachol at stimulating AP-1-mediated transactivation. To identify the factors underlying the difference in AP-1 activity induced by carbachol and thrombin, members of the fos and jun families which encode components of AP-1 were examined. Carbachol and thrombin have similar effects on expression of c-fos, fosB, fra-2, junB, and junD, both acutely and over a 24-h time course. However, whereas carbachol leads only to transient induction of c-jun (maximal at 0.5 h), thrombin induces a biphasic increase in c-jun mRNA--an initial peak at 0.5 h and a second, more-prolonged increase at 12 h. Thrombin but not carbachol also induces a late increase in fra-1 mRNA, which peaks at 12 h. The secondary increase in c-jun mRNA is associated with marked increases in c-Jun protein levels and AP-1 DNA-binding activity. The late induction of c-jun and fra-1 mRNA can be prevented by adding the antagonist hirudin 30 min after thrombin, which results in loss of thrombin-stimulated increases in c-Jun protein, AP-1 DNA-binding activity, and AP-1-mediated transactivation. These findings suggest that rapid and transient conduction of c-fos and c-jun mRNA is insufficient to induce prominent changes in gene transcription, while the sustained increase in c-jun mRNA and perhaps the late induction of fra-1 mRNA are required for generation of AP-1 DNA-binding activity and transactivation through AP-1.

Screening for candidate hepatic growth factors by selective portal infusion after canine Eck's fistula

Completely diverting portacaval shunt (Eck's fistula) in dogs causes hepatocyte atrophy, disruption of hepatocyte organelles, fatty infiltration and low-grade hyperplasia. The effect of hepatic growth regulatory substances on these changes was assessed by constantly infusing test substances for four postoperative days after Eck's fistula into the detached left protal vein above the shunt. The directly infused left lobes were compared histopathologically with the untreated right lobes. In what has been called an hepatotrophic effect, stimulatory substances prevented the atrophy and increased hepatocyte mitoses. Of the hormones tested, only insulin was strongly hepatotrophic; T3 had a minor effect, and glucagon, prolactin, angiotensin II, vasopressin, norepinephrine and estradiol were inert. Insulin-like growth factor, hepatic stimulatory substance, transforming growth factor-alpha and hepatocyte growth factor (also known as hematopoietin A) were powerfully hepatotrophic, but epidermal growth factor had a barely discernible effect. Transforming growth factor-beta was inhibitory, but tamoxifen, interleukin-1 and interleukin-2 had no effect. The hepatotrophic action of insulin was not altered when the insulin infusate was mixed with transforming growth factor-beta or tamoxifen. These experiments show the importance of in vivo in addition to in vitro testing of putative growth control factors. They illustrate how Eck's fistula model can be used to screen for such substances and possibly to help delineate their mechanisms of action.

Screening for candidate hepatic growth factors by selective portal infusion after canine Eck's fistula

Completely diverting portacaval shunt (Eck's fistula) in dogs causes hepatocyte atrophy, disruption of hepatocyte organelles, fatty infiltration and low-grade hyperplasia. The effect of hepatic growth regulatory substances on these changes was assessed by constantly infusing test substances for four postoperative days after Eck's fistula into the detached left protal vein above the shunt. The directly infused left lobes were compared histopathologically with the untreated right lobes. In what has been called an hepatotrophic effect, stimulatory substances prevented the atrophy and increased hepatocyte mitoses. Of the hormones tested, only insulin was strongly hepatotrophic; T3 had a minor effect, and glucagon, prolactin, angiotensin II, vasopressin, norepinephrine and estradiol were inert. Insulin-like growth factor, hepatic stimulatory substance, transforming growth factor-alpha and hepatocyte growth factor (also known as hematopoietin A) were powerfully hepatotrophic, but epidermal growth factor had a barely discernible effect. Transforming growth factor-beta was inhibitory, but tamoxifen, interleukin-1 and interleukin-2 had no effect. The hepatotrophic action of insulin was not altered when the insulin infusate was mixed with transforming growth factor-beta or tamoxifen. These experiments show the importance of in vivo in addition to in vitro testing of putative growth control factors. They illustrate how Eck's fistula model can be used to screen for such substances and possibly to help delineate their mechanisms of action.

c-fos and c-jun are induced by muscarinic receptor activation of protein kinase C but are differentially regulated by intracellular calcium

It has become increasingly clear that agents classically thought to act as neurotransmitters can also alter gene expression. To understand the early events by which neurotransmitters could effect genetic responses, we have studied the induction of two immediate early genes, c-fos and c-jun. These genes encode proteins that form a dimeric complex (AP-1) active as a transcriptional factor. Using the stable acetylcholine analog carbachol to activate muscarinic receptors (mAChR) in a glial cell line (1321N1), we show that c-fos and c-jun mRNA levels are transiently increased, reaching a maximum at 30 min after agonist addition. Experiments in which the actions of carbachol are blocked by adding atropine at various times demonstrate that only 1.5 min of agonist stimulation is needed to give maximal increases in c-fos or c-jun mRNA at 30 min. These results suggest that events previously shown to occur in the first minute of mAChR occupation (the mobilization of intracellular Ca2+, activation of protein kinase C) are sufficient for induction of these immediate early genes. In cells in which protein kinase C has been down-regulated, carbachol no longer stimulates c-fos or c-jun expression, indicating a critical role for protein kinase C in these responses. In cells loaded with bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) to buffer increases in cytosolic [Ca2+], mAChR-mediated induction of c-fos is markedly reduced; in contrast there is enhanced c-jun expression. The strong enhancement of c-jun induction by carbachol in BAPTA-treated cells is due at least in part to mRNA stabilization. Experiments using phorbol ester (phorbol 12-myristate 13-acetate) in combination with the Ca2+ ionophore ionomycin confirm that activation of protein kinase C induces c-fos and c-jun expression and that a concomitant increase in cytosolic [Ca2+] potentiates the induction of c-fos while repressing that of c-jun. The data suggest that the ability of neurotransmitters or growth factors to mobilize Ca2+ would modulate the effect of concomitant protein kinase C activation on AP-1 generation and consequent target gene expression.