The regulatory subunit of cAMP-dependent protein kinase as a target for chemotherapy of cancer and other cellular dysfunctional-related diseases

Serodiagnosis of multiple cancers using an extracellular protein kinase A autoantibody-based lateral flow platform

Extracellular protein kinase A autoantibody (ECPKA-AutoAb) has been suggested as a universal cancer biomarker due to its higher amounts in serum of several types of cancer patients than that of normal individuals. Herein, we first developed a lateral flow immunoassay (LFIA) tool, using a sandwich format, toward ECPKA-AutoAb in human serum. For this format, 3G2 as a capture antibody was identified using hybridoma technique and a series of screenings where it showed superior capacity to recognize Enzo PKA catalytic subunit alpha (Cα), compared to other PKA antibodies and antigens. Using these components, we performed sandwich ELISA toward a mimic and real sample of ECPKA-AutoAb. As per the results, limit of detection (LOD) was found to be 135 ng/mL and ECPKA-AutoAb levels were higher in various cancer patients than in normal individuals like previous studies. Based on these results, we applied this sandwich format into LFIA tool and found that the LOD of the fabricated LFIA tool showed about 3.8 ng/mL using spiked PKA-Ab, which is significantly improved compared to the LOD of sandwich ELISA. Also, the developed LFIA tool demonstrated a remarkable ability to detect significant differences in ECPKA-AutoAb levels between normal and cancer patients within 15 min, showing a potential for point-of-care (PoC) detection. One interesting point is that our LFIA strip contains an additional conjugation pad II, named because of its position behind the conjugation pad, in which PKA Cα is dried, enabling a sandwich format.

Characteristics of extracellular cyclic AMP-dependent protein kinase as a biomarker of cancer in dogs

OBJECTIVE

Early and proper diagnosis of cancer is the most critical factor for the survival and treatment of veterinary cancer patients. In this study, we evaluated extracellular cyclic AMP-dependent protein kinase A (ECPKA) level in serum as a useful cancer biomarker in dogs.

METHODS

ECPKA levels were detected in sera from dogs with cancers (n = 48), benign tumours (n = 18), and non-tumour diseases (n = 102) as well as healthy control dogs (n = 54) utilizing enzyme-linked immunosorbent assay (ELISA).

RESULTS

Sera from dogs bearing various types of cancer exhibited markedly increased levels of ECPKA by up to 7.1-, 8.8-, and 10.9-fold compared with those from dogs harbouring benign tumours, dogs with non-tumour diseases, and healthy control dogs, respectively (P < .0001). In addition, serum ECPKA level did not show statistically significant correlation with gender, breed, or age of dogs or their non-cancerous disease conditions.

CONCLUSION

Our data strongly propose that detection of serum ECPKA level is a potential and specific diagnostic tool for cancer in dogs.

Dichotomous role of protein kinase A type I (PKAI) in the tumor microenvironment: a potential target for 'two-in-one' cancer chemoimmunotherapeutics

An emerging trend in cancer chemoimmunotherapeutics is to develop 'two-in-one' therapies, which directly inhibit tumor growth and progression, as well as enhance anti-tumor immune surveillance. Protein kinase A (PKA) is a cAMP-dependent protein kinase that mediates signal transduction of G-protein coupled receptors (GPCRs). The regulatory subunit of PKA exists in two isoforms, RI and RII, which distinguish the PKA isozymes, PKA type I (PKAI) and PKA type II (PKAII). The differential expression of both PKA isozymes has long been linked to growth regulation and differentiation. RI/PKAI is particularly implicated in cellular proliferation and neoplastic transformation. Emerging experimental and pre-clinical data also indicate that RI/PKAI plays a key role in tumor-induced immune suppression. More briefly, RI/PKAI possesses a dichotomous role in the tumor microenvironment: not only contributes to tumor growth and progression, but also takes part in tumor-induced suppression of the innate and adaptive arms of anti-tumor immunosurveillance. This review specifically discusses this dichotomous role of RI/PKAI with respect to 'two-in-one' chemoimmunotherapeutic manipulation. The reviewed experimental and pre-clinical data provide the proof of concept validation that RI/PKAI may be regarded as an attractive target for a new, single-targeted, 'two hit' chemoimmunotherapeutic approach against cancer.

Structural insights into mis-regulation of protein kinase A in human tumors

The extensively studied cAMP-dependent protein kinase A (PKA) is involved in the regulation of critical cell processes, including metabolism, gene expression, and cell proliferation; consequentially, mis-regulation of PKA signaling is implicated in tumorigenesis. Recent genomic studies have identified recurrent mutations in the catalytic subunit of PKA in tumors associated with Cushing's syndrome, a kidney disorder leading to excessive cortisol production, and also in tumors associated with fibrolamellar hepatocellular carcinoma (FL-HCC), a rare liver cancer. Expression of a L205R point mutant and a DnaJ-PKA fusion protein were found to be linked to Cushing's syndrome and FL-HCC, respectively. Here we reveal contrasting mechanisms for increased PKA signaling at the molecular level through structural determination and biochemical characterization of the aberrant enzymes. In the Cushing's syndrome disorder, we find that the L205R mutation abolishes regulatory-subunit binding, leading to constitutive, cAMP-independent signaling. In FL-HCC, the DnaJ-PKA chimera remains under regulatory subunit control; however, its overexpression from the DnaJ promoter leads to enhanced cAMP-dependent signaling. Our findings provide a structural understanding of the two distinct disease mechanisms and they offer a basis for designing effective drugs for their treatment.

ABC transporters and neuroblastoma

Neuroblastoma is the most common cancer of infancy and accounts for 15% of all pediatric oncology deaths. Survival rates of high-risk neuroblastoma remain less than 50%, with amplification of the MYCN oncogene the most important aberration associated with poor outcome. Direct transcriptional targets of MYCN include a number of ATP-binding cassette (ABC) transporters, of which ABCC1 (MRP1), ABCC3 (MRP3), and ABCC4 (MRP4) are the best characterized. These three transporter genes have been shown to be strongly prognostic of neuroblastoma outcome in primary untreated neuroblastoma. In addition to their ability to efflux a number of chemotherapeutic drugs, evidence suggests that these transporters also contribute to neuroblastoma outcome independent of any role in cytotoxic drug efflux. Endogenous substrates of ABCC1 and ABCC4 that may be potential candidates affecting neuroblastoma biology include molecules such as prostaglandins and leukotrienes. These bioactive lipid mediators have the ability to influence biological processes contributing to cancer initiation and progression, such as angiogenesis, cell signaling, inflammation, proliferation, and migration and invasion. ABCC1 and ABCC4 are thus potential targets for therapeutic suppression in high-risk neuroblastoma, and recently developed small-molecule inhibitors may be an effective strategy in treating aggressive forms of this cancer, as well as other cancers that express high levels of these transporters.

Epidermal growth factor receptor targeting in cancer: a review of trends and strategies

The epidermal growth factor receptor (EGFR) is a cell-surface receptor belonging to ErbB family of tyrosine kinase and it plays a vital role in the regulation of cell proliferation, survival and differentiation. However; EGFR is aberrantly activated by various mechanisms like receptor overexpression, mutation, ligand-dependent receptor dimerization, ligand-independent activation and is associated with development of variety of tumors. Therefore, specific EGFR inhibition is one of the key targets for cancer therapy. Two major approaches have been developed and demonstrated benefits in clinical trials for targeting EGFR; monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs). EGFR inhibitors like, cetuximab, panitumumab, etc. (mAbs) and gefitinib, erlotinib, lapatinib, etc. (TKIs) are now commercially available for treatment of variety of cancers. Recently, many other agents like peptides, nanobodies, affibodies and antisense oligonucleotide have also shown better efficacy in targeting and inhibiting EGFR. Now a days, efforts are being focused to identify molecular markers that can predict patients more likely to respond to anti-EGFR therapy; to find out combinatorial approaches with EGFR inhibitors and to bring new therapeutic agents with clinical efficacy. In this review we have outlined the role of EGFR in cancer, different types of EGFR inhibitors, preclinical and clinical status of EGFR inhibitors as well as summarized the recent efforts made in the field of molecular EGFR targeting.

Selective inhibition of cell death in malignant vs normal B-cell precursors: implications for cAMP in development and treatment of BCP-ALL

B-cell precursor acute lymphoblastic leukemia (BCP-ALL) is the most commonly occurring pediatric cancer. Despite its relatively good prognosis, there is a steady search for strategies to improve treatment effects and prevent the undesired side effects on normal cells. In the present paper, we demonstrate a differential effect of cyclic adenosine monophosphate (cAMP) signaling between normal BCPs and BCP-ALL blasts, pointing to a potential therapeutic window allowing for manipulation of cAMP signaling in the treatment of BCP-ALL. By studying primary cells collected from pediatric BCP-ALL patients and healthy controls, we found that cAMP profoundly decreased basal and DNA damage-induced p53 levels and cell death in malignant cells, whereas normal BCP counterparts displayed slightly augmented cell death when exposed to cAMP-increasing agents. We did not find evidence for a selection process involving generation of increased basal cAMP levels in BCP-ALL cells, but we demonstrate that paracrine signaling involving prostaglandin E2-induced cAMP generation has the potential to suppress p53 activation and cell death induction. The selective inhibitory effect of cAMP signaling on DNA damage-induced cell death in BCP-ALL cells appears to be an acquired trait associated with malignant transformation, potentially allowing the use of inhibitors of this pathway for directed killing of the malignant blasts.

PKA knockdown enhances cell killing in response to radiation and androgen deprivation

The therapeutic efficacy of Gem®231, a second generation antisense molecule targeted to the RIα subunit of PKA(RIα) (AS-PKA), administered in combination with androgen deprivation (AD) and radiation therapy (RT), was examined in androgen sensitive (LNCaP) and insensitive (PC3) cell lines. Apoptosis was assayed by Caspase 3 + 7 activity and Annexin V binding. AS-PKA significantly increased apoptosis in vitro from RT (both lines), with further increases in LNCaP cells grown in AD medium. In LNCaP cells, AD increased phosphorylated mitogen activated protein-kinase (pMAPK), which was reduced by AS-PKA relative to the mismatch (MM) controls. AS-PKA also reduced pMAPK levels in PC3 cells. Cell death was measured by clonogenic survival assays. In vivo, LNCaP cells were grown orthotopically in nude mice. Tumor kinetics were measured by magnetic resonance imaging and serum prostate-specific antigen. PC3 cells were grown subcutaneously and tumor volume assessed by caliper measurements. In PC3 xenografts, AS-PKA caused a significant increase in tumor doubling time relative to MM controls as a monotherapy or in combination with RT. In orthotopic LNCaP tumors, AS-PKA was ineffective as a monotherapy; however, it caused a statistically significant increase in tumor doubling time relative to MM controls when used in combination with AD, with or without RT. PKA(RIα) levels in tumors were quantified via immunohistochemical (IHC) staining and image analysis. IHC measurements in LNCaP cells exhibited that AS-PKA reduced PKA(RIα) levels in vivo. We demonstrate for the first time that AS-PKA enhances cell killing androgen sensitive prostate cancer cells to AD ± RT and androgen insensitive cells to RT.

Protein kinase A activation confers resistance to trastuzumab in human breast cancer cell lines

PURPOSE

Trastuzumab is a monoclonal antibody targeted to the Her2 receptor and approved for treatment of Her2-positive breast cancer. Among patients who initially respond to trastuzumab therapy, resistance typically arises within 1 year. BT/Her(R) cells are trastuzumab-resistant variants of Her2-positive BT474 breast cancer cells. The salient feature of BT/Her(R) cells is failure to downregulate phosphoinositide 3-kinase/Akt signaling on trastuzumab binding. The current work addresses the mechanism of sustained signaling in BT/Her(R) cells, focusing on the protein kinase A (PKA) pathway.

EXPERIMENTAL DESIGN

We performed microarray analysis on BT/Her(R) and BT474 cell lines to identify genes that were upregulated or downregulated in trastuzumab-resistant cells. Specific genes in the PKA pathway were quantified using reverse transcription-PCR and Western hybridization. Small interfering RNA transfection was used to determine the effects of gene knockdown on cellular response to trastuzumab. Electrophoretic mobility shift assays were used to measure cyclic AMP-responsive element binding activity under defined conditions. Immunohistochemistry was used to analyze protein expression in clinical samples.

RESULTS

BT/Her(R) cells had elevated PKA signaling activity and several genes in the PKA regulatory network had altered expression in these cells. Downregulation of one such gene, the PKA-RIIalpha regulatory subunit, conferred partial trastuzumab resistance in Her2-positive BT474 and SK-Br-3 cell lines. Forskolin activation of PKA also produced significant protection against trastuzumab-mediated Akt dephosphorylation. In patient samples, PKA signaling appeared to be enhanced in residual disease remaining after trastuzumab-containing neoadjuvant therapy.

CONCLUSIONS

Activation of PKA signaling may be one mechanism contributing to trastuzumab resistance in Her2-positive breast cancer. We propose a molecular model by which PKA confers its effects.

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.

Extracellular activity of cyclic AMP-dependent protein kinase as a biomarker for human cancer detection: distribution characteristics in a normal population and cancer patients

The overexpression of cyclic AMP (cAMP)-dependent protein kinase (PKA) has been reported in patients with cancer, and PKA inhibitors have been tested in clinical trials as a novel cancer therapy. The present study was designed to characterize the population distribution of extracellular activity of cAMP-dependent protein kinase (ECPKA) and its potential value as a biomarker for cancer detection and monitoring of cancer therapy. The population distribution of ECPKA activity was determined in serum samples from a Chinese population consisting of a total of 603 subjects (374 normal healthy volunteers and 229 cancer patients). The serum ECPKA was determined by a validated sensitive radioassay, and its diagnostic values (including positive and negative predictive values) were analyzed. The majority of normal subjects (>70%) have undetectable or very low levels of serum ECPKA. In contrast, the majority of cancer patients (>85%) have high levels of ECPKA. The mean ECPKA activity in the sera of cancer patients was 10.98 units/mL, 5-fold higher than that of the healthy controls (2.15 units/mL; P < 0.001). In both normal subjects and cancer patients, gender and age had no significant influence on the serum ECPKA. Among factors considered, logistic analysis revealed that the disease (cancer) is the only factor contributing to the elevation of ECPKA activity in cancer patients. In conclusion, ECPKA may function as a cancer marker for various human cancers and can be used in cancer detection and for monitoring response to therapy with other screening or diagnostic techniques.

Activation of protein kinase A (PKA) signaling mitigates the antiproliferative and antiinvasive effects of alpha-difluoromethylornithine in breast cancer cells

We have shown that alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, the first and rate-limiting enzyme in polyamine synthesis, has significant antiproliferative and antiinvasive effects in breast cancer cells. We have also reported that these antitumor effects are associated with activation of multiple signaling pathways, including STAT-3, STAT-1, Jun-N-Terminal kinase (JNK), and Mitogen activated protein kinase (MAPK), the latter being found to mediate its antiinvasive action in MDA-MB-435 cells. The present experiments were designed to test the effect of DFMO on the protein kinase A (PKA) pathway and determine its biological significance. We found that DFMO administration (1 mM) to MDA-MB-435 breast cancer cells significantly increased cAMP response element (CRE)-binding protein (CREB) phosphorylation as well as the transactivation of pCRE-luc, a CREB-dependent promoter activated by PKA. To determine the significance of this biochemical effect of DFMO, we used the PKA inhibitor H89 which, as expected, suppressed in a dose-dependent manner (1 and 10 microM) basal and DFMO-induced CREB phosphorylation in our system. Administration of H89 alone was able to suppress proliferation of MDA-MB-435 cells when used at a concentration (10 microM) shown to completely block basal CREB phosphorylation. At concentrations of 0.5 and 1 muM, H89 treatment, while having no antiproliferative effect of its own, potentiated in a dose-dependent fashion the growth inhibitory action of a suboptimal concentration of DFMO (0.01 mM). Ten micromoles of H89 reduced invasiveness of MDA-MB-435 cells in matrigel by approximately 40% (an effect similar to that of 1 mM DFMO). The combination treatment further reduced invasiveness by approximately 80% (P < 0.01 versus the individual treatments). H89 treatment (10 microM) partially reduced DFMO-induced phosphorylation of STAT-3 but not that of STAT-1, Extracellular regulated kinase (ERK), and JNK. In conclusion, our results indicate that PKA signaling exerts proproliferative and proinvasive effects in our experimental system. Therefore, its activation by DFMO represents a compensatory mechanism which should be blocked in order to maximize the antitumor action of the drug. Our data are also consistent with the notion that STAT-3 activation by DFMO is at least in part mediated through the PKA pathway.

Regulation of rat alveolar type 2 cell proliferation in vitro involves type II cAMP-dependent protein kinase

To elucidate the role of cAMP and different cAMP-dependent protein kinases (PKA; A-kinase) in lung cell proliferation, we investigated rat alveolar type 2 cell proliferation in relation to activation or inhibition of PKA and PKA regulatory subunits (RIIalpha and RIalpha). Both the number of proliferating type 2 cells and the level of different regulatory subunits varied during 7 days of culture. The cells exhibited a distinct peak of proliferation after 5 days of culture. This proliferation peak was preceded by a rise in RIIalpha protein level. In contrast, an inverse relationship between RIalpha and type 2 cell proliferation was noted. Activation of PKA increased type 2 cell proliferation if given at peak RIIalpha expression. Furthermore, PKA inhibitors lowered the rate of proliferation only when a high RII level was observed. An antibody against the anchoring region of RIIalpha showed cell cycle-dependent binding in contrast to antibodies against other regions, possibly related to altered binding to A-kinase anchoring protein. Following activation of PKA, relocalization of RIIalpha was confirmed by immunocytochemistry. In conclusion, it appears that activation of PKA II is important in regulation of alveolar type 2 cell proliferation.

Oncogenes as novel targets for cancer therapy (part II): Intermediate signaling molecules

This is the second part of a four-part review on potential therapeutic targeting of oncogenes. The previous part introduced the new technologies responsible for the advancement of oncogene identification, target validation, and drug design. Because of such advances, new specific and more efficient therapeutic agents can be developed for cancer. This part of the review continues the exploration of various oncogenes, which we have grouped within seven categories: growth factors, tyrosine kinases, intermediate signaling molecules, transcription factors, cell cycle regulators, DNA damage repair genes, and genes involved in apoptosis. Part I included a discussion of growth factors and tyrosine kinases. This portion of the review covers intermediate signaling molecules and the various strategies used to inhibit their expression or decrease their activities.

Differential activity profile of cAMP-dependent protein kinase isoforms during long-term memory consolidation in the crab Chasmagnathus

The isoforms of cAMP-dependent protein kinase (PKA) show distinct biochemical properties and subcellular localization, suggesting different physiological functions, and conferring the fine-tuning between the activation of cAMP-PKA cascade and the cellular response. The critical role of PKA in memory and synaptic plasticity has been extensively demonstrated both in vertebrates and invertebrates, but the role of PKA isoforms is a matter of debate. Here we present experimental data showing differential PKA activation profiles after two different experiences: an instance of associative contextual learning (context-signal learning) and a single exposure to a novel context, both in the learning and memory model of the crab Chasmagnathus. Differences were found in the temporal course of activation and in the involvement of PKA isoforms. We found increased PKA activity immediately and 6 h after context-signal training correlating with the critical periods during which pharmacological inhibition of PKA disrupts memory formation. In contrast, PKA activity increased immediately but not 6 h after single exposure to a novel context. The amounts of PKA I and PKA II holoenzymes were analyzed to determine changes in holoenzyme levels and/or differential activation induced by both experiences. Results indicate that context-induced PKA activation is at least in part due to PKA II, and that PKA activation 6 h after context-signal learning coincides with an increase in the total level of PKA I. Considering the higher sensitivity of PKA I to cAMP, its increment can account for the PKA activation found 6 h after training and is proposed as a novel mechanism providing the prolonged PKA activation during memory consolidation.

Antisense therapeutics: from theory to clinical practice

The use of antisense (AS) oligonucleotides as therapeutic agents was proposed as far back as the 1960s/1970s when the AS strategy was initially developed. However, it has taken almost a quarter of a century for this potential to be realized. The last few years has seen a rapid increase in the number of AS molecules progressing past Phase I in clinical trials, due in part to our increased knowledge of their structure and chemistry. Here, we describe the most prominent of these modifications with respect to clinical applicability. However, the main focus of this review is clinical application, with a focus on cancer. We will discuss in detail both the status of the current AS clinical trials and the molecules that are likely to be the targets of the next group of AS molecules entering the clinic.

Minor participation of cAMP on the protein kinase phosphorylation of mitochondrial and cytosolic fractions from Ascaris suum: a comparative study with porcine heart muscle

In contrast to porcine heart muscle in which cAMP effectively activated the phosphorylation of cytosolic proteins, cAMP exerted a minor effect on the phosphorylation of proteins from the soluble fraction of Ascaris suum muscle. Similarly, cAMP did not enhance the kinase activity in the mitochondrial membranes from porcine heart and A. suum, although major differences in protein phosphorylation were observed between both fractions. However, cAMP-dependent protein kinases (PKA) were evidenced in the parasitic soluble mitochondrial fraction, since the phosphorylation of histone IIA and kemptide was augmented in this fraction, in the presence of cAMP. An increase in the phosphorylation of exogenously added A. suum phosphofructokinase was also obtained when cAMP was added to the parasite soluble mitochondrial fraction. The phosphorylation of phosphofructokinase by this fraction was inhibited when kemptide and cAMP were included in the reaction mixture, suggesting substrate competition for the same PKA. Although PKI (6-22), a reported inhibitor of the catalytic subunit of mammalian cAMP-dependent PKAs, did not affect the endogenous phosphorylation of proteins in the various A. suum fractions, an inhibition on the phosphorylation of exogenously added kemptide and phosphofructokinase was observed when PKI (6-22) was incubated with the parasite mitochondrial soluble fraction.

Anti-proliferative effects of 8-chloro-cAMP and other cAMP analogs are unrelated to their effects on protein kinase A regulatory subunit expression

Conflicting reports have attributed 8-chloro-cAMP (Cl-cAMP)-mediated inhibition of tumor cell growth to either a toxic 8-chloro-adenosine (Cl-AdR) breakdown product or a Cl-cAMP-mediated decrease in ratio of Type I to Type II regulatory (R) subunits of protein kinase A (PKA). Using the MCF-7 human breast cancer and S49 mouse lymphoma cell lines as models, we show that the effects of Cl-cAMP and other cAMP analogs on growth and R subunit expression are unrelated. MCF-7 cell growth was insensitive to most analogs and inducers of cAMP, but was potently inhibited by Cl-cAMP acting through uptake and phosphorylation of its Cl-AdR breakdown product. Possible roles of adenosine receptors or P(2) purinoceptors in these Cl-cAMP-mediated growth effects were ruled out by studies with agonists and antagonists. Cholera toxin markedly decreased the ratio of Type I to Type II R subunits in MCF-7 cells without affecting growth, while growth inhibitory concentrations of Cl-cAMP or Cl-AdR had insignificant effects on this ratio. In S49 cells, where PKA activation is known to inhibit cell growth, PKA-deficient mutants retained sensitivity to both Cl-cAMP and the related 8-bromo-cAMP. Adenosine kinase (AK)-deficient S49 cells were inhibited only by higher concentrations of these 8-halogenated cAMP analogs. Of the commonly used cAMP analogs, only 8-(4-chlorophenylthio)-cAMP acted purely as a cyclic nucleotide-having no effect on PKA-deficient cells, but strongly inhibiting both wild-type and AK-deficient cells. Where growth inhibitory concentrations of most cAMP analogs reduced RI expression in the AK-deficient mutant, a functionally equivalent concentration of (N(6), O(2'))dibutyryl-cAMP maintained or increased this expression.

7-Benzylamino-6-chloro-2-piperazino-4-pyrrolidino-pteridine, a potent inhibitor of cAMP-specific phosphodiesterase, enhancing nuclear protein binding to the CRE consensus sequence in human tumour cells

The cAMP-specific phosphodiesterase isoenzyme family PDE4 represents the highest cAMP-hydrolysing activity in many human cancer cell lines including the human large cell lung carcinoma cell line LXFL529L. Treatment of LXFL529L cells with the potent PDE4 inhibitor 7-benzylamino-6-chloro-2-piperazino-4-pyrrolidino-pteridine (DC-TA-46) induces dose-dependent growth inhibition. Cells are arrested in the G(1)-phase of the cell cycle and the induction of apoptosis is observed. In this study, we investigated the effect of DC-TA-46 on downstream elements of the cAMP-pathway. DC-TA-46 mediated inhibition of PDE4 activity in LXFL529L cells resulted in an increase of the intracellular cAMP level and significant induction of the activity of protein kinase A (PKA). The regulatory PKA subunit RIalpha was predominantly expressed in LXFL529L cells. In contrast to effects induced by cAMP analogues like 8-Cl-cAMP, the expression of the regulatory subunits of PKA remained unaffected by DC-TA-46. Treatment of LXFL529L cells with DC-TA-46 enhanced the binding of nuclear proteins to the cAMP-responsive element (CRE) consensus sequence TGACGTCA in a time- and dose-dependent manner, indicating the activation of transcription factors by PKA phosphorylation.

Omega-3 fatty acids decrease protein kinase expression in human breast cancer cells

We report that 5-day exposure to physiological concentrations of eicosapentaenoic and docosahexaenoic acids resulted in a strong decrease in expression of the RIalpha regulatory subunit of protein kinase A and the PKC-alpha isozyme of protein kinase C in the human breast cancer cell line MDA-MB-231.

8-Cl-adenosine mediated cytotoxicity and sensitization of T-lymphoblastic leukemia cells to TNFalpha-induced apoptosis is via inactivation of NF-kappaB

These data show that 8-Cl-cAMP is cytotoxic to the lymphoblastic leukemia cell line CEM and its vinblastine selected multidrug resistant derivative, CEM/VLB100 although PKA was not involved in these effects. The cytotoxic effects of 8-Cl-cAMP was abrogated by cotreatment with either ADA or IBMX which indicated a degradation form of 8-Cl-cAMP was needed for this cytotoxicity. CEM and CEM/VLB100 cells displayed a notable sensitivity to 8-Cl-adenosine-induced growth inhibition and apoptosis. 8-Cl-adenosine increased the cytosolic levels of IkappaBalpha which prevented NF-kappaB nuclear translocation. 8-Cl-adenosine also prevented TNFalpha-induced IkB decay and NF-kappaB activation in CEM and CEM/VLB100 cells.

Structural and functional analysis of the cAMP binding domain from the regulatory subunit of Mucor rouxii protein kinase A

The cAMP binding domain of the regulatory subunit (R) of Mucor rouxii protein kinase A was cloned. The deduced amino acid sequence was highly homologous in sequence and in size to the corresponding region in fungal and higher eukaryotic regulatory subunits (47-54%), but particularly homologous (62%) to Blastocladiella emersonii, a fungus classified in a different phylum. Amino acids reported to be important for interaction with cAMP, for cooperativity between the two cAMP binding domains, in the general folding of the domain, and for interaction with the catalytic subunit were conserved in all the fungal sequences. Based on either sequence or functional behavior, the M. rouxii R subunit cannot be classified as being more similar to RI or RII of mammalian systems. The M. rouxii protein sequence was modeled using as template the coordinates of the crystallized bovine regulatory subunit type Ialpha. The quality of the model is good. The two backbones could be perfectly overlapped, except for two loop regions of high divergence. The alpha helix C of domain A, proposed to have a strong interaction with the catalytic subunit, contains a leucine replacing a basic residue (arginine or lysine) commonly found in RI or RII. The domains A and B of the M. rouxii regulatory subunit were overexpressed as fusion proteins with GST. GST domain B protein was inactive. GST domain A was active; the kinetic parameters of affinity toward cAMP analogs, site selectivity, and dissociation kinetics of bound cAMP were analogous to the properties of the domain in the whole regulatory subunit.

Mechanism of activation of cAMP-dependent protein kinase: in Mucor rouxii the apparent specific activity of the cAMP-activated holoenzyme is different than that of its free catalytic subunit

Kinetic constants for peptide phosphorylation by the catalytic subunit of the dimorphic fungus Mucor rouxii protein kinase A were determined using 13 peptides derived from the peptide containing the basic consensus sequence RRASVA, plus kemptide, S6 peptide, and protamine. As a whole, although with a greater Km, the order of preference of the peptides by the M. rouxii catalytic subunit was similar to the one displayed by mammalian protein kinase A. Particularly significant is the replacement of serine by threonine in the basic peptide RRATVA, which impaired its role as a substrate of M. rouxii catalytic subunit. Mucor rouxii protein kinase A is a good model in which to study the mechanism of activation since cAMP alone is not enough to promote activation and dissociation. Four peptides were selected for the study of holoenzyme activation under conditions in which the enzymatic activity was not proportional to the holoenzyme concentration: RRASVA, RRRRASVA, KRRRLSSRA (S6 peptide), and LRRASLG (kemptide); protamine was used as reference. Differential activation degree was observed depending on the peptide used and on cAMP concentration. Ratios of activity between different substrates displayed by the holoenzyme under the above conditions did not reflect the one expected for the free catalytic subunit. The degree of inhibition of the holoenzyme activity by an active peptide derived from the thermostable protein kinase inhibitor was dependent on the substrate used and on the holoenzyme concentration, while it was found to be independent of these two parameters for free catalytic subunit. Polycation modulation of holoenzyme activation by cAMP was also dependent on the polycation itself and on the peptide used as substrate. The observed kinetic differences between holoenzyme and free catalytic subunit were decreased or almost abolished when working at low enzyme or at high cAMP concentrations. Two hypotheses compatible with the results are discussed: substrate participation in the dissociation process and/or holoenzyme activation without dissociation.

Cyclic nucleotide analogs as biochemical tools and prospective drugs

Cyclic AMP (cAMP) and cyclic GMP (cGMP) are key second messengers involved in a multitude of cellular events. From the wealth of synthetic analogs of cAMP and cGMP, only a few have been explored with regard to their therapeutic potential. Some of the first-generation cyclic nucleotide analogs were promising enough to be tested as drugs, for instance N(6),O(2)'-dibutyryl-cAMP and 8-chloro-cAMP (currently in clinical Phase II trials as an anticancer agent). Moreover, 8-bromo and dibutyryl analogs of cAMP and cGMP have become standard tools for investigations of biochemical and physiological signal transduction pathways. The discovery of the Rp-diastereomers of adenosine 3',5'-cyclic monophosphorothioate and guanosine 3',5'-cyclic monophosphorothioate as competitive inhibitors of cAMP- and cGMP-dependent protein kinases, as well as subsequent development of related analogs, has proven very useful for studying the molecular basis of signal transduction. These analogs exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity. Furthermore, better understanding of signaling pathways and ligand/protein interactions has led to new therapeutic strategies. For instance, Rp-8-bromo-adenosine 3',5'-cyclic monophosphorothioate is employed against diseases of the immune system. This review will focus mainly on recent developments in cyclic nucleotide-related biochemical and pharmacological research, but also highlights some historical findings in the field.

8-chloro-cAMP inhibits smooth muscle cell proliferation in vitro and neointima formation induced by balloon injury in vivo

OBJECTIVES

The aims of the present study were to assess 1) the effect of 8-C1-cAMP (cyclic-3'-5'-adenosine monophosphate) on vascular smooth muscle cell (VSMC) proliferation in vitro and 2) the efficacy of systemic administration of 8-C1-cAMP on neointimal formation after balloon injury in vivo.

BACKGROUND

Neointimal formation after vascular injury is responsible for restenosis after arterial stenting. Recently, 8-C1-cAMP, a cAMP analogue that induces growth arrest, has been safely administered in phase I studies in humans.

METHODS

The effect of 8-C1-cAMP on cell proliferation was first assessed on SMCs in vitro. To study the effects of cAMP in vivo, balloon injury was performed in 67 rats using a 2F Fogarty balloon catheter.

RESULTS

The 8-C1-cAMP markedly inhibited VSMC proliferation in vitro, reduced protein kinase A (PKA) RIalpha subunit expression, and induced PKA RIIbeta subunit expression. In addition, 8-C1-cAMP reduced, in a dose-dependent manner, neointimal area and neointima/media ratio after balloon injury. The proliferative activity, assessed by proliferating nuclear cell antigen immunostaining, revealed a reduction of proliferative activity of VSMCs in vivo in the 8-C1-cAMP group. Moreover, the systemic administration of 8-C1-cAMP did not affect renal function, blood pressure and heart rate.

CONCLUSIONS

We conclude that 8-C1-cAMP potently inhibits VSMC proliferation in vitro and reduces neointima formation by balloon injury in vivo after systemic administration. These data may have a clinical relevance in designing future strategies to prevent restenosis after arterial stenting and perhaps after percutaneous transluminal coronary angioplasty.

Inhibition of cell growth and proliferation in human glioma cells and normal human astrocytes induced by 8-Cl-cAMP and tiazofurin

8-Cl-cAMP and tiazofurin (TR) are anti-tumor agents that besides their antiproliferative effect, also induce differentiation of tumor cells. Although, these agents exert a profound effect on the same events of tumor cell life, it is thought that 8-Cl-cAMP and TR act by modulating the signal transduction pathway through distinct mechanisms. We have compared their effect on two human glioma cell lines (U87 MG and U251 MG) and examined if there is selectivity in their action toward normal human astrocytes.

Constitutive levels of cAMP-dependent protein kinase activity determine sensitivity of human multidrug-resistant leukaemic cell lines to growth inhibition and apoptosis by forskolin and tumour necrosis factor alpha

The cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) signal pathway regulates cell proliferation, differentiation and cell death. It may also regulate the multidrug resistance (MDR) phenotype in leukaemic cells. These data showed that MDR1+ K/Dau600 cells exhibited a higher basal level of PKA activity than MDR- parental cells. The significance of this on tumour necrosis factor alpha (TNFalpha)-induced apoptosis and cytostasis was investigated further. In comparison with MDR1- parental cells, K/Dau600 cells had a higher expression of PKA regulatory subunit RIalpha and nuclear catalytic subunit PKAcalpha. They were also more susceptible to inhibition of proliferation and induction of apoptosis by TNFalpha and/or forskolin, but this could be attenuated by H89. An increase in cAMP was associated with the apoptosis in the K/Dau600 cell line. Forskolin inactivated NF-kappaB in K/Dau600 cells but not in K562 cl. 6 cells, whereas TNF activated NF-kappaB in K562 cl.6 cells but not in K/Dau600 cells. 8-Cl-cAMP exhibited similar inhibitory effects on the proliferation of all of the cell lines used via its metabolite 8-Cl-adenosine, which indicates that these effects were independent of residual PKA or cAMP. Therefore, the differential sensitivity to apoptosis and/or growth inhibition could be mediated via cAMP, partly through PKA via NF-kappaB and partly by PKA-independent pathways.

Vasoactive intestinal polypeptide stimulates the proliferation of HaCaT cell via TGF-alpha

It is well known that psoriasis, an immunogenetic cutaneous disorder whose major pathogenic findings are epidermal hyperplasia and T-cell infiltration, is aggravated by psychological stresses. Although the exact mechanism is not yet clarified, antidromic secretion of neuropeptides by cutaneous nerve fibers is thought to be involved. In this study, we examined the effect and mechanism of vasoactive intestinal polypeptide (VIP), one of the major neuropeptides, on the proliferation of HaCaT cell which is a spontaneous, immortalized, human keratinocyte cell line. Twenty-four and 48 h after its addition, 1 pM to 100 nM of VIP increased the number of cells cultured with/without serum. We indirectly verified VIP(1)R on the surface of HaCaT cell based on the proliferative ability of various VIP families such as VIP, PACAP and secretin, and increased PKA level 30 min after stimulation. However, because H-89, a PKA inhibitor, did not inhibit the proliferative potential of VIP, its mitogenicity is not medicated through VIP(1)R. One nM VIP produced the TGF-alpha protein which is a strong mitogen of keratinocytes and increased in the psoriatic lesion 2.25 times more compared with the control. Genistein, a tyrosine kinase inhibitor, abrogated the mitogenic activity of VIP. Like VIP, VIP fragments, VIP(1-12) and VIP(10-28) also acted as a mitogen for HaCaT cells through the same mechanism. Collectively, our studies clearly show that VIP and its fragments stimulate keratinocyte growth, not through increased cAMP level, but through increased TGF-alpha protein production.

Antitumor activity and pharmacokinetics of a mixed-backbone antisense oligonucleotide targeted to the RIalpha subunit of protein kinase A after oral administration

Overexpression of the RIalpha subunit of cAMP-dependent protein kinase (PKA) has been demonstrated in various human cancers. PKA has been suggested as a potential target for cancer therapy. The goal of the present study was to evaluate an anti-PKA antisense oligonucleotide (mixed-backbone oligonucleotide) as a therapeutic approach to human cancer treatment. The identified oligonucleotide inhibited the growth of cell lines of human colon cancer (LS174T, DLD-1), leukemia (HL-60), breast cancer (MCF-7, MDA-MB-468), and lung cancer (A549) in a time-, concentration-, and sequence-dependent manner. In a dose-dependent manner, the oligonucleotide displayed in vivo antitumor activity in severe combined immunodeficient and nude mice bearing xenografts of human cancers of the colon (LS174T), breast (MDA-MB-468), and lung (A549). The routes of drug administration were intraperitoneal and oral. Synergistic effects were found when the antisense oligonucleotide was used in combination with the cancer chemotherapeutic agent cisplatin. The pharmacokinetics of the oligonucleotide after oral administration of (35)S-labeled oligonucleotide into tumor-bearing mice indicated an accumulation and retention of the oligonucleotide in tumor tissue. This study further provides a basis for clinical studies of the antisense oligonucleotide targeted to the RIalpha subunit of PKA (GEM 231) as a cancer therapeutic agent used alone or in combination with conventional chemotherapy.

Protein kinase A regulatory subunits in colon cancer

The protein kinase A (PKA) is classified as type I or II depending on the association of the catalytic subunit with either the R(I) or R(II) regulatory subunits. Alterations in the levels of these regulatory subunits and PKA activity itself appear to affect cellular proliferation and tumorigenesis. We examined colorectal tumor specimens from 45 patients to investigate the potential role of cAMP-related signaling molecules in regulating tumorigenesis. Western blot analysis (PKA subunit protein levels) and in vitro kemptide phosphorylation assays (PKA catalytic subunit activity) were performed on human colorectal tumor tissue homogenates. R(I)beta protein levels were decreased 200% in ascending and 50% in descending colonic tumors compared to adjacent mucosa. R(II) protein levels were decreased 77% in descending colonic tumors but no change was observed in ascending colonic tumors compared to adjacent mucosa. PKA activity and the absolute amount of catalytic subunit protein in ascending and descending tumors were unchanged compared to adjacent mucosa. Differences in cAMP-related signaling molecules exist between neoplastic and normal colorectal tissues. These differences may not only serve as potential therapeutic targets for chemotherapeutic agents, but also lead to the identification of novel regulatory mechanisms involved in cellular proliferation and tumorigenesis.

Type II cAMP-dependent protein kinase regulates electrogenic ion transport in rabbit collecting duct

cAMP mediates many of the effects of vasopressin, prostaglandin E2, and beta-adrenergic agents upon salt and water transport in the renal collecting duct. The present studies examined the role of cAMP-dependent protein kinase (PKA) in mediating these effects. PKA is a heterotetramer comprised of two regulatory (R) subunits and two catalytic (C) subunits. The four PKA isoforms may be distinguished by their R subunits that have been designated RIalpha, RIbeta, RIIalpha, and RIIbeta. Three regulatory subunits, RIalpha, RIIalpha, and RIIbeta, were detected by immunoblot and ribonuclease protection in both primary cultures and fresh isolates of rabbit cortical collecting ducts (CCDs). Monolayers of cultured CCDs grown on semipermeable supports were mounted in an Ussing chamber, and combinations of cAMP analogs that selectively activate PKA type I vs. PKA type II were tested for their effect on electrogenic ion transport. Short-circuit current (Isc) was significantly increased by the PKA type II-selective analog pairs N6-monobutyryl-cAMP plus 8-(4-chlorophenylthio)-cAMP or N6-monobutyryl-cAMP plus 8-chloro-cAMP. In contrast the PKA type I-selective cAMP analog pair [N6-monobutyryl-cAMP plus 8-(6-aminohexyl)-amino-cAMP] had no effect on Isc. These results suggest PKA type II is the major isozyme regulating electrogenic ion transport in the rabbit collecting duct.

Up-regulated EGF receptors undergo to rapid internalization and ubiquitin-dependent degradation in human cancer cells exposed to 8-Cl-cAMP

8-Cl-cAMP, a cAMP analogue that antagonizes type I cAMP-dependent protein kinase, is a novel anti-tumor agent presently under investigation in clinical trials. Herein we report the effects of this agent on epidermal growth factor receptor expression and degradation in human KB cancer cells. Exposure to 10 microM 8-Cl-cAMP for 48 h induced a 65% increase in epidermal growth factor receptor surface expression while the receptor synthesis was 22-fold enhanced. Analysis of epidermal growth factor-dependent receptor internalization in 8-Cl-cAMP-treated cells showed a higher endocytosis rate as well as an accelerated degradation which occurred together with an increased receptor ubiquitination. The enhanced degradation of epidermal growth factor receptor correlated with the lack of epidermal growth factor-induced proliferation and mitogen-activated protein kinase stimulation. The disregulation of epidermal growth factor receptor internalization and ubiquitin-dependent degradation could underlay a new mechanism of the anti-tumor activity of 8-Cl-cAMP suggesting its combination with agents that disrupt epidermal growth factor receptor signalling.

Cooperative inhibition of renal cancer growth by anti-epidermal growth factor receptor antibody and protein kinase A antisense oligonucleotide

BACKGROUND

The expression of epidermal growth factor receptor (EGFR) and type I cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKAI) is associated with neoplastic transformation. By use of human renal cancer cell lines (i.e., 769-P, ACHN, A498, and SW839), we investigated the antiproliferative activity and the antitumor activity of an anti-EGFR humanized chimeric mouse monoclonal antibody, MAb C225, and a novel mixed backbone 18-mer antisense oligonucleotide, HYB 190, that targets expression of the RIalpha regulatory subunit of PKAI.

METHODS

The antiproliferative activity of MAb C225 and oligonucleotide HYB 190, alone or in combination, on different renal cancer cell lines was determined by monitoring cell growth in soft agar. In addition, the induction of apoptosis by treatment with the anti-EGFR antibody and/or antisense PKAI oligonucleotides was evaluated by flow cytometric analysis of fragmented DNA. The antitumor activity of MAb C225 and oligonucleotide HYB 190 was determined in athymic mice bearing established ACHN tumor xenografts. Cell proliferation and tumor growth data were evaluated for statistical significance using Student's t test; reported P values are two-sided.

RESULTS

MAb C225 and oligonucleotide HYB 190 inhibited colony formation in soft agar in a dose-dependent manner for all renal cancer cell lines tested. We observed a potentiation of growth inhibition and induction of apoptosis when 769-P cells and ACHN cells were treated with both agents. Combination treatment with MAb C225 and oligonucleotide HYB 190 caused regression of ACHN tumor xenografts, whereas single-agent treatment only delayed tumor growth.

CONCLUSION

The combination of anti-EGFR MAb C225 and ited cooperative antiproliferative effects and cooperative antitumor effects on EGFR and PKAI-expressing human renal cancer cell lines.

Physiological exposure to melatonin supersensitizes the cyclic adenosine 3',5'-monophosphate-dependent signal transduction cascade in Chinese hamster ovary cells expressing the human mt1 melatonin receptor

Here, we report the effects of short exposure to melatonin on the human mt1 (h mt1) melatonin receptor-mediated signaling in Chinese hamster ovary (CHO) cells, and the consequences of an exposure that resembles the physiological pattern of melatonin release on cAMP-mediated signal transduction. Short exposure (10 min) of h mt1 melatonin receptors to melatonin (400 pM) inhibited forskolin-stimulated cAMP formation, cAMP-dependent protein kinase activity, and phosphorylation of the cAMP response element-binding protein. However, treatment of mt1-CHO cells with melatonin in a manner that closely mimics the in vivo activation of melatonin receptors (i.e. 400 pM melatonin for 8 h to mimic darkness) resulted in a supersensitization of the cAMP-dependent signal transduction cascade during the period of withdrawal (i.e. 16 h without melatonin to mimic the light cycle of a diurnal photoperiod). During the period of withdrawal, forskolin induced a time-dependent (1-16 h) increase in cAMP formation (approximately 200% of control cells). This effect of melatonin was dependent on the presence of the h mt1 melatonin receptor, as no potentiation of forskolin-induced cAMP formation was observed in CHO cells transfected only with the neomycin resistance plasmid. The time-dependent increase in forskolin-stimulated cAMP levels resulted in a potentiation of cAMP-dependent protein kinase activity 1 h after withdrawal (approximately 130% of control cells; P < 0.05) and in the number of cells containing the phosphorylated form of cAMP response element-binding protein (approximately 75% of cells at 1 and 16 h compared with 30% in control cells; P < 0.05). An increase in the undissociated state (G alphabetagamma) of Gi proteins may underlie this phenomenon as demonstrated by the increase in pertussis toxin-catalyzed ADP-ribosylation of G proteins (217 +/- 48% of control; P < 0.05) after melatonin withdrawal. This increase in the ribosylation was not due to an up-regulation of Galpha(i) protein, as no significant change in Galpha(i) protein levels occurred at this time. We demonstrated that activation of the h mt1 melatonin receptor in a manner that resembles the physiological pattern of melatonin exposure alters signaling, as potentiation of cAMP-mediated signal transduction events is observed after hormone withdrawal. The CHO cells expressing the human melatonin receptor may provide an in vitro cellular model in which to investigate the putative signaling mechanisms leading to gene regulation by melatonin.

Moderate dietary protein and energy restriction modulate cAMP-dependent protein kinase activity in rat liver

Very low protein diets result in a desensitization of hepatic cAMP signaling in rats, which is characterized by a loss of cAMP-dependent protein kinase (PKA) activity and type I regulatory subunit (RI). Here we have tested whether more moderate protein restriction (Trial 1) or energy restriction (Trial 2) also modulates hepatic PKA quantity and activity. In trial 1, weanling rats were allowed free access to diets containing normal protein (15%, AL-NP), moderately restricted protein (12.5%, AL-MP) and low protein (7.5%, AL-LP); in trial 2, rats were allowed free access to diet containing 15% (AL-NP) or 0.5% protein (very low protein, AL-VLP) or were energy restricted by pair-feeding a diet isonitrogenous to AL-NP but at 65% of the energy intake (ER-IN) for 14 d. Body weights were lower (P < 0.05) by d 14 in all restricted groups compared with the AL-NP group. The quantity of cytosolic RI was lower (P < 0.05) in AL-LP and AL-VLP, but not in AL-MP or ER-IN, compared with AL-NP. In contrast, there was no effect of diet on RI in the particulate fraction. RII was not changed by moderate and low protein diets in either the cytosol or particulate fraction. However, type II regulatory subunit (RII) was greater in the cytosol of ER-IN and in the particulate fraction of AL-VLP (P < 0.05) compared with AL-NP. Specific activity of PKA was lower in the cytosol and particulate fraction (P < 0.05) in the AL-VLP and ER-IN groups compared with the AL-NP group. In contrast, specific activity of PKA was maintained in cytosol from AL-LP, but lower in the particulate fraction (P < 0.05) compared with AL-NP. In summary, protein restricted-diets lower RI subunit in the cytosol; however, only in rats fed very low protein diets is this loss of RI associated with lower cytosolic PKA activity. In contrast, energy restriction lowers PKA activity in the cytosol and particulate fractions, independent of signficant reduction in RI or RII subunits. Taken together, these data indicate that moderate protein and energy restrictions have differential effects on activity and quantity of PKA.

Cisplatin resistance in cyclic AMP-dependent protein kinase mutants

The emergence of cisplatin resistance poses a major problem to the successful treatment of a variety of human malignancies. Therefore, understanding the molecular mechanisms that underlie cisplatin resistance could significantly improve the clinical efficacy of this cytotoxic agent. Various studies have described that cellular sensitivity to cisplatin can be influenced by several signal transduction pathways. In this review, we examine the role of the cyclic AMP-dependent protein kinase (PKA) in the modulation of drug resistance in cancer. By a somatic mutant genetic approach, the role of PKA in the development of resistance to chemotherapeutic agents has been investigated. A series of mutants with decreased PKA activity was examined for their sensitivity to cisplatin. PKA mutants with defective regulatory (RIalpha) subunits, but not altered catalytic (C) subunits, exhibit increased resistance to cisplatin, as well as other DNA-damaging agents. Furthermore, since RIalpha subunit mutants show enhanced DNA repair we, therefore, hypothesize that functional inactivation of PKA may result in increased recognition and repair of cisplatin lesions. Alternatively, it seems likely that mutation of the RIalpha subunit may affect cellular sensitivity to various anticancer drugs, suggesting that the RIalpha subunit may have other physiological functions in addition to inhibiting the kinase activity of the C subunit. Therefore, exploitation of cyclic AMP levels or functional alteration of the R subunit may potentiate the cytotoxicity of chemotherapeutic agents and circumvent drug resistance in cancer. More importantly, the altered pattern and mechanism of drug resistance may offer the opportunity to investigate novel regulatory functions of the RIalpha subunit of PKA.

Induction of differentiation of the human histocytic lymphoma cell line U-937 by hypericin

Hypericin, a photosensitizing plant pigment, was found to be a potent inducer of differentiation of human myeloid leukemia U-937 cells. At a concentration of 0.2 microM, hypericin exhibited 50% growth inhibition. An effect on cell differentiation by hypericin was assessed by its ability to induce phagocytosis of latex particles, and to reduce nitroblue tetrazolium (NBT). Approximately 51% of 0.2 microM hypericin-treated cells were stained with NBT and 63% showed phagocytic activity. In order to establish whether hypericin induces differentiation of U-937 cells to macrophage or granulocyte, esterase activities and cell sizes were measured. When U-937 cells were treated with 0.2 microM and 0.15 microM of hypericin, the alpha-naphthyl acetate esterase activity was increased by 38.4% and 48.1%, respectively, but naphthol AS-D chloroacetate esterase activity was not influenced. The size of hypericin-treated cells in terms of cell mass was larger than that observed in untreated cells as determined by flow cytometry. Protein kinase C (PKC) inhibitor, NA-382, decreased the NBT reducing activity of hypericin, whereas a cAMP-dependent protein kinase A (PKA) inhibitor, H-89, did not show any influence on the differentiation. These results indicate that hypericin triggers differentiation toward monocyte/macrophage lineage by PKC stimulation.

Down-regulation of type I protein kinase A by transfection of human breast cancer cells with an epidermal growth factor receptor antisense expression vector

MDA-468 human breast cancer cells overexpress the EGFR and exhibit a functional TGFalpha-EGFR autocrine pathway. Loss of EGFR expression following stable transfection with an antisense EGFR cDNA containing plasmid down-regulates type I cAMP-dependent protein kinase (PKAI) expression with acquisition of cell growth resistance to the PKAI inhibitor 8-Cl-cAMP. These results suggest that PKAI expression and function are controlled by a TGFalpha-EGFR autocrine pathway in human breast cancer cells overexpressing the EGFR.

Synergistic inhibition of human cancer cell growth by cytotoxic drugs and mixed backbone antisense oligonucleotide targeting protein kinase A

Protein kinase A type I plays a key role in neoplastic transformation, conveying mitogenic signals of different growth factors and oncogenes. Inhibition of protein kinase A type I by antisense oligonucleotides targeting its RIalpha regulatory subunit results in cancer cell growth inhibition in vitro and in vivo. A novel mixed backbone oligonucleotide HYB 190 and its mismatched control HYB 239 were tested on soft agar growth of several human cancer cell types. HYB 190 demonstrated a dose-dependent inhibition of colony formation in all cell lines whereas the HYB 239 at the same doses caused a modest or no growth inhibition. A noninhibitory dose of each mixed backbone oligonucleotide was used in OVCAR-3 ovarian and GEO colon cancer cells to study whether any cooperative effect may occur between the antisense and a series of cytotoxic drugs acting by different mechanisms. Treatment with HYB 190 resulted in an additive growth inhibitory effect with several cytotoxic drugs when measured by soft agar colony formation. A synergistic growth inhibition, which correlated with increased apoptosis, was observed when HYB 190 was added to cancer cells treated with taxanes, platinum-based compounds, and topoisomerase II selective drugs. This synergistic effect was also observed in breast cancer cells and was obtained with other related drugs such as docetaxel and carboplatin. Combination of HYB 190 and paclitaxel resulted in an accumulation of cells in late S-G2 phases of cell cycle and marked induction of apoptosis. A cooperative effect of HYB 190 and paclitaxel was also obtained in vivo in nude mice bearing human GEO colon cancer xenografts. These results are the first report of a cooperative growth inhibitory effect obtained in a variety of human cancer cell lines by antisense mixed backbone oligonucleotide targeting protein kinase A type I-mediated mitogenic signals and specific cytotoxic drugs.

Serotonin modulates immune function in T cells from HIV-seropositive subjects

We have shown earlier increased intracellular levels of cAMP in peripheral lymphocytes from HIV-seropositive subjects and that a chemically induced decrease in this level increases cell proliferation and cytotoxicity. Others have shown that serotonin indirectly decreases intracellular cAMP levels in normal peripheral lymphocytes. In this study, we show that addition of serotonin decreases intracellular levels of cAMP in lymphocytes from HIV-seropositive subjects and significantly increases the proliferative capacity in vitro. However, the effect of serotonin varies with the initial proliferative response; e.g., these with the highest initial responses have the highest increases. An increase in IL-2 production may be a part of this mechanism since addition of serotonin to in vitro cultures of PHA-stimulated cells increases the expression of mRNA for IL-2 and IFN-gamma. The effect on lymphocyte proliferation was most likely mediated through the serotonin 5HT1a receptor because similar results could be obtained by using DPAT, a specific activator of this receptor. Changes in the expression of 5HT1a receptors as judged by the expression of mRNA could not explain why serotonin in vitro had a stronger enhancing effect on cell proliferation in some HIV-seropositive individuals than in others.

Gonadotropin-releasing hormone and pituitary adenylate cyclase-activating polypeptide affect levels of cyclic adenosine 3',5'-monophosphate-dependent protein kinase A (PKA) subunits in the clonal gonadotrope alphaT3-1 cells: evidence for cross-talk between PKA and protein kinase C pathways

We have shown previously that protein kinase A (PKA) subunit levels are regulated by activation of PKA or protein kinase C (PKC) in anterior pituitary cells. GnRH also influenced PKA subunit levels, suggesting that hormonal regulation occurs in gonadotrophs, and therefore, we have reexamined this question using the clonal gonadotrope-derived cell line (alphaT3-1 cells). Western blot analysis, using specific immunoaffinity purified immunoglobulins, revealed expression of catalytic (Cat) and regulatory type I (RI) and type II (RII) subunits of PKA in these cells. Activation of adenylyl cyclase (AC) with forskolin, or of PKC with tetradecanoyl phorbol acetate (TPA), caused a rapid (detectable at 0.5-1 h) and concentration-dependent loss of all PKA subunits. Forskolin (10-100 microM) reduced Cat and RI by 60% and RII by 30%, whereas TPA (0.1-1 microM) reduced Cat and RII by 50% and RI by 40%. Simultaneous activation of PKA and PKC caused the expected dose-dependent reductions in Cat, and the effects of forskolin or TPA were nearly additive. RI and RII were reduced similarly by 10 nM TPA, whereas 100 nM TPA tended to prevent the reduction of RI or RII caused by forskolin. GnRH, which activates phosphoinositidase C and not AC in these cells, caused a clear loss of Cat or RII at all concentrations tested and of RI at 0.1 nM. Pituitary adenylate cyclase-activating polypeptide 38, which acts via PVR-1 receptors to stimulate both phosphoinositidase C and AC in these cells, also caused a clear dose-dependent decrease in Cat, RI, and RII, although higher concentrations were needed for the latter effects. Together, the data demonstrate that catalytic and regulatory subunits of PKA are subject to both hormonal and receptor-independent regulation in alphaT3-1 cells, reinforcing the possibility that such effects occur in nonimmortalized gonadotropes. Whereas the effects of PKA activators very likely involve proteolytic degradation of the dissociated PKA holoenzyme, the effects of TPA and GnRH occur in the absence of cAMP elevation by unknown mechanisms. Whatever the mechanisms involved, the data reveal a mechanism for cross-talk between phosphoinositidase C and AC-mediated hormonal signals, in which PKC activation seems to play a pivotal role.

Ex vivo cytotoxic drug evaluation by DiSC assay to expedite identification of clinical targets: results with 8-chloro-cAMP

There is a pressing need to reduce the time and cost of developing new cytotoxic agents and to accurately identify clinically active agents at an early stage. In this study, the differential staining cytotoxicity (DiSC) assay was used to assess the efficacy of the novel antitumour cAMP analogue, 8-chloro-cAMP (8-Cl-cAMP) (and its metabolite 8-Cl-adenosine) against 107 fresh specimens of human neoplastic and normal cells. Diagnoses included chronic and acute leukaemias, myeloma, non-Hodgkin's lymphoma (NHL) and miscellaneous solid tumours. The aim was to identify targets for subsequent phase I, II and III trials. 8-Cl-cAMP was tested at 4-985 microM, along with standard chemotherapeutic drugs. 8-Cl-cAMP and its metabolite caused no morphologically observable cell differentiation but induced dose-dependent cytotoxicity. Compared with untreated patients, previously treated chronic lymphocytic leukaemia (CLL) patients showed no increase in ex vivo resistance to 8-Cl-cAMP (P = 0.878); minimal cross-resistance with other cytotoxic drugs was detected. Compared with normal cells (mean LC90 = 1803 microM), 8-Cl-cAMP showed significant ex vivo activity against CLL (117.0 microM; P < 0.0001) and NHL (140.0 microM; P < 0.0001), of which eight were mantle cell NHL (84.7 microM), and greatest activity against cells from patients with acute myeloid leukaemia (AML; mean LC90 = 24.3 microM; in vitro therapeutic index 74-fold, P < 0.0001). Solid tumour specimens were comparatively resistant to 8-Cl-cAMP. The results highlight the clinical potential of 8-Cl-cAMP, point to several new phase I, II and III trial possibilities and provide a rationale for the inclusion of ex vivo cytotoxic drug evaluation in the drug development process.

Recent status of the antisense oligonucleotide approaches in oncology

Antisense oligonucleotides designed to complement a region of a particular messenger RNA may inhibit gene expression potentially through sequence-specific hybridization. Their inhibiting effect has been shown in a variety of in vitro and in vivo models in oncology, whereas much rarer clinical trials have been carried out. Rigorous demonstration of in vitro and in vivo specific effects upon their targets is mandatory before their use as drugs in cancer therapy.