Types of cyclic AMP binding proteins in human breast cancers

Tissue biomarkers for prostate cancer radiation therapy

Prostate cancer is the most common cancer and second leading cause of cancer deaths among men in the United States. Most men have localized disease diagnosed following an elevated serum prostate specific antigen test for cancer screening purposes. Standard treatment options consist of surgery or definitive radiation therapy directed by clinical factors that are organized into risk stratification groups. Current clinical risk stratification systems are still insufficient to differentiate lethal from indolent disease. Similarly, a subset of men in poor risk groups need to be identified for more aggressive treatment and enrollment into clinical trials. Furthermore, these clinical tools are very limited in revealing information about the biologic pathways driving these different disease phenotypes and do not offer insights for novel treatments which are needed in men with poor-risk disease. We believe molecular biomarkers may serve to bridge these inadequacies of traditional clinical factors opening the door for personalized treatment approaches that would allow tailoring of treatment options to maximize therapeutic outcome. We review the current state of prognostic and predictive tissue-based molecular biomarkers which can be used to direct localized prostate cancer treatment decisions, specifically those implicated with definitive and salvage radiation therapy.

Downregulation of phosphodiesterase 4B (PDE4B) activates protein kinase A and contributes to the progression of prostate cancer

BACKGROUND

Prostate cancer is the most commonly diagnosed non-cutaneous cancer in American men. Unfortunately, few successful therapies for castration-resistant prostate cancer (CRPC) exist. The protein kinase A (PKA) pathway is a critical mediator of cellular proliferation and differentiation in various normal and cancerous cells. However, the PKA activity and the mechanism of regulation in CRPC remain unclear. Then, in this study, we intended to reveal the PKA activity and the mechanism of regulation in CRPC.

METHODS

Western blotting, quantitative real-time polymerase chain reaction, cytotoxicity analysis, and cell proliferation assay were used to resolve the regulatory role of PKA in prostate cancer cell line, LNCaP and their derivatives.

RESULTS

cAMP-specific phosphodiesterase 4B (PDE4B) was downregulated and the PKA pathway was activated in castration-resistant LNCaP derivatives (CxR cells). Rolipram activated the PKA pathway via inhibition of PDE4B, resulting in AR transactivation while the PKA inhibitor, H89 reduced AR transactivation. In response to hydrogen peroxide and in hydrogen peroxide-resistant LNCaP derivatives (HPR50 cells) PDE4B was decreased and as a result PKA activity was increased. Moreover, PDE4B expression was reduced in advanced prostate cancer and PDE4B knockdown promoted castration-resistant growth of LNCaP cells.

CONCLUSIONS

Oxidative stress may suppress PDE4B expression and activate the PKA pathway. The PDE4B/PKA pathway contributed to progression of androgen-dependent prostate cancer to CRPC. This pathway may represent an attractive therapeutic molecular target.

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.

The importance of protein kinase A in prostate cancer: relationship to patient outcome in Radiation Therapy Oncology Group trial 92-02

PURPOSE

We previously reported that protein kinase A type I (PKA(RIalpha)) overexpression was predictive of outcome in prostate cancer patients treated with radiotherapy (RT) +/- short-term androgen deprivation (STAD) on Radiation Therapy Oncology Group (RTOG) protocol 86-10. Here, we attempt to verify our prior findings and test the hypothesis that the relationship of the length of AD to patient outcome is affected by PKA(RIalpha) overexpression.

EXPERIMENTAL DESIGN

There were 313 cases in the RTOG 92-02 study cohort with available tissue and suitable staining by immunohistochemistry. Median follow-up was 10.1 years. The intensity of PKA(RIalpha) staining intensity was quantified manually and by image analysis. Multivariate analyses were done for overall mortality using Cox proportional hazards models and for local failure, biochemical failure, distant metastasis, and cause-specific mortality using Fine and Gray's regression models.

RESULTS

The expression levels of PKA(RIalpha), determined by manual and image analysis, were strongly correlated (P < 0.0001). In the multivariate analyses, manual-quantified and image analysis-quantified PKA(RIalpha) staining intensities were independent predictors of distant metastasis (P < 0.01), local failure (P < 0.05), and biochemical failure (P

CONCLUSIONS

PKA(RIalpha) overexpression has been shown in two RTOG trials to be associated with an increased risk of failure after AD + RT. In this series of contemporary high-risk patients, PKA(RIalpha) overexpression was associated with diminished response to LTAD + RT relative to STAD + RT, suggesting that such patients would be ideal for a PKA(RIalpha) knockdown strategy.

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MESH Headings

• Adult
• Aged
• Aged, 80 and over
• Androgen Antagonists/therapeutic use
• Biomarkers, Tumor/biosynthesis
• Cohort Studies
• Combined Modality Therapy
• Cyclic AMP-Dependent Protein Kinase Type I/biosynthesis
• Humans
• Male
• Middle Aged
• Multivariate Analysis
• Proportional Hazards Models
• Prostatic Neoplasms/drug therapy
• Prostatic Neoplasms/enzymology
• Prostatic Neoplasms/radiotherapy
• Radiotherapy Dosage
• Treatment Outcome

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Grants

• CA-101984-01 NCI NIH HHS
• CA-006927 NCI NIH HHS
• R01 CA101984-05 NCI NIH HHS
• R01 CA101984 NCI NIH HHS
• P30 CA006927 NCI NIH HHS

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Affiliation(s)

Alan Pollack Department of Radiation Oncology, University of Miami Miller School of Medicine, Miami, Florida 33136, USA.
Kyounghwa Bae
Li-Yan Khor
Tahseen Al-Saleem
M Elizabeth Hammond
Varagur Venkatesan
Roger W Byhardt
Sucha O Asbell
William U Shipley
Howard M Sandler

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Protein kinase A RI-alpha predicts for prostate cancer outcome: analysis of radiation therapy oncology group trial 86-10

PURPOSE

The RI-alpha regulatory subunit of protein kinase A type 1 (PKA) is constitutively overexpressed in human cancer cell lines and is associated with active cell growth and neoplastic transformation. This report examined the association between PKA expression and the endpoints of biochemical failure (BF), local failure (LF), distant metastasis (DM), cause-specific mortality (CSM), and overall mortality in men treated with radiotherapy, with or without short-term androgen deprivation in Radiation Therapy Oncology Group trial 86-10.

METHODS AND MATERIALS

Pretreatment archival diagnostic tissue samples from 80 patients were stained for PKA by immunohistochemical methods from a parent cohort of 456 cases. PKA intensity was scored manually and by image analysis. The Cox proportional hazards model for overall mortality and Fine and Gray's regression models for CSM, DM, LF and BF were then applied to determine the relationship of PKA expression to the endpoints.

RESULTS

The pretreatment characteristics of the missing and determined PKA groups were not significantly different. On univariate analyses, a high PKA staining intensity was associated with BF (image analysis, continuous variable, p = 0.022), LF (image analysis, dichotomized variable, p = 0.011), CSM (manual analysis, p = 0.037; image analysis, continuous, p = 0.014), and DM (manual analysis, p = 0.029). On multivariate analyses, the relationships to BF (image analysis, continuous, p = 0.03), LF (image analysis, dichotomized, p = 0.002), and DM remained significant (manual analysis, p = 0.018). In terms of CSM, a trend toward an association was seen (manual analysis, p = 0.08; image analysis, continuous, p = 0.09).

CONCLUSION

PKA overexpression was significantly related to patient outcome and is a potentially useful biomarker for identifying high-risk prostate cancer patients who might benefit from a PKA knockdown strategy.

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.

Proliferation of transformed somatotroph cells related to low or absent expression of protein kinase a regulatory subunit 1A protein

The two regulatory subunits (R1 and R2) of protein kinase A (PKA) are differentially expressed in cancer cell lines and exert diverse roles in growth control. Recently, mutations of the PKA regulatory subunit 1A gene (PRKAR1A) have been identified in patients with Carney complex. The aim of this study was to evaluate the expression of the PKA regulatory subunits R1A, R2A, and R2B in a series of 30 pituitary adenomas and the effects of subunit activation on cell proliferation. In these tumors, neither mutation of PRKAR1A nor loss of heterozygosity was identified. By real-time PCR, mRNA of the three subunits was detected in all of the tumors, R1A being the most represented in the majority of samples. By contrast, immunohistochemistry documented low or absent R1A levels in all tumors, whereas R2A and R2B were highly expressed, thus resulting in an unbalanced R1/R2 ratio. The low levels of R1A were, at least in part, due to proteasome-mediated degradation. The effect of the R1/R2 ratio on proliferation was assessed in GH3 cells, which showed a similar unbalanced pattern of R subunits expression, and in growth hormone-secreting adenomas. The R2-selective cAMP analog 8-Cl cAMP and R1A RNA silencing, stimulated cell proliferation and increased Cyclin D1 expression, respectively, in human and rat adenomatous somatotrophs. These data show that a low R1/R2 ratio promoted proliferation of transformed somatotrophs and are consistent with the Carney complex model in which R1A inactivating mutations further unbalance this ratio in favor of R2 subunits. These results suggest that low expression of R1A protein may favor cAMP-dependent proliferation of transformed somatotrophs.

Antisense DNAs as targeted genetic medicine to treat cancer

Nucleic acid therapies represent a direct genetic approach for cancer treatment. Such an approach takes advantage of mechanisms that activate genes known to confer a growth advantage to neoplastic cells. The ability to block the expression of these genes allows exploration of normal growth regulation. Progress in antisense technology has been rapid, and the traditional antisense inhibition of gene expression is now viewed on a genomic scale. This global view has led to a new vision in antisense technology, the elimination of nonspecific and undesirable side effects, and ultimately, the generation of more effective and less toxic nucleic acid medicines. Several antisense oligonucleotides are in clinical trials, are well tolerated, and are potentially active therapeutically. Antisense oligonucleotides are promising molecular medicines for treating human cancer in the near future.

Regulatory subunits of PKA and breast cancer

Overexpression of the R subunits of PKA (in particular, RI) is associated with high proliferation in normal breast, malignant transformation in the breast, poor prognosis in established breast cancer, and resistance to antiestrogens. These data, together with the observation that successful antiestrogen therapy is associated with reduced expression of RI mRNA, suggest that targeting R subunits is an appropriate therapeutic strategy for breast cancer. Initial experimental results, using antisense RI oligonucleotides, are promising in terms of reducing the growth rate of breast cancer cells and xenografts. While clinical trials designed to target RI subunits have yet to be established (and interventions as preventative measures are even more distant), the concept of these approaches to prevent and treat breast cancer should be developed and exploited.

Nuclear translocation of the catalytic subunit of protein kinase A induced by an antisense oligonucleotide directed against the RIalpha regulatory subunit

The regulatory (R) subunits of cAMP-dependent protein kinase (PKA) are implicated in the regulation of cell proliferation and differentiation. There are two isoforms of PKA that are distinguished by two types of R subunit, RI and RII. Evidence suggests that RI is associated with proliferation and RII is associated with cell differentiation. Previous work in this laboratory has demonstrated that depletion of the RIalpha subunit by treatment with an antisense oligonucleotide (ODN) induces differentiation in leukemia cells and growth arrest and apoptosis in epithelial cancer cells. Using the prostate cancer cell line PC3M as a model system, we have developed a cell line that overexpresses a retroviral vector construct containing the RIalpha antisense gene. This cell line has been characterized and the effectiveness of the construct determined. In the work presented here, we demonstrate by immunocytochemistry that treatment with RIalpha antisense ODN induces translocation of the Calpha subunit of PKA to the nucleus of PC3M prostate cancer cells. The translocation of Calpha triggered by exogenous antisense ODN treatment mirrors that observed in cells endogenously overexpressing the antisense gene. Triggering the nuclear translocation of the Calpha subunit of PKA in the cell may be an important mechanism of action of RIalpha antisense that regulates cell growth independent of adenylate cyclase and cellular cAMP levels. The nuclear localization of the Calpha subunit of PKA may be an essential step in revealing the mechanism whereby this critical kinase regulates cell growth.

Antisense DNAs as multisite genomic modulators identified by DNA microarray

Antisense oligodeoxynucleotides can selectively block disease-causing genes, and cancer genes have been chosen as potential targets for antisense drugs to treat cancer. However, nonspecific side effects have clouded the true antisense mechanism of action and hampered clinical development of antisense therapeutics. Using DNA microarrays, we have conducted a systematic characterization of gene expression in cells exposed to antisense, either exogenously or endogenously. Here, we show that in a sequence-specific manner, antisense targeted to protein kinase A RIalpha alters expression of the clusters of coordinately expressed genes at a specific stage of cell growth, differentiation, and activation. The genes that define the proliferation-transformation signature are down-regulated, whereas those that define the differentiation-reverse transformation signature are up-regulated in antisense-treated cancer cells and tumors, but not in host livers. In this differentiation signature, the genes showing the highest induction include genes for the G proteins Rap1 and Cdc42. The expression signature induced by the exogenously supplied antisense oligodeoxynucleotide overlaps strikingly with that induced by endogenous antisense gene overexpression. Defining antisense DNAs on the basis of their effects on global gene expression can lead to identification of clinically relevant antisense therapeutics and can identify which molecular and cellular events might be important in complex biological processes, such as cell growth and differentiation.

Cloning of human preprotachykinin-I promoter and the role of cyclic adenosine 5'-monophosphate response elements in its expression by IL-1 and stem cell factor

Preprotachykinin-I gene (PPT-I) encodes several peptides with organ-specific functions that link the neuroendocrine-immune-hemopoietic axis. We cloned upstream of the initiation site of human PPT-I promoter and identified consensus sequences for two cAMP response elements (CRE). PPT-I is induced by cytokines including those that signal through the cAMP pathway. Therefore, we studied the role of the two CRE in IL-1alpha and stem cell factor (SCF) stimulation of bone marrow stroma because both cytokines induce endogenous PPT-I in these cells and activate the cAMP pathway. Furthermore, bone marrow stroma expresses the transcription factors regulated by the cAMP pathways such as the repressor (ICERIIgamma) and activator (CREMtau). Mutagenesis of the two CRE and/or cotransfection with vectors that express ICERIIgamma or CREMtau indicated that the two CRE have major roles in PPT-I expression. The two CRE are also required for optimal promoter activity by SCF and IL-1alpha. A particular cytokine could concomitantly induce PPT-I and the high affinity G protein-coupled receptor for PPT-I peptides, NK-1R. We showed that SCF, a representative cytokine, induced PPT-I and NK-1R leading to autocrine and/or paracrine cell activation. Because NK-1R activates cAMP through the G protein, the results suggest that the presence of CRE sequences within PPT-I promoter could be important in the regulation of PPT-I expression by cytokines, irrespective of their ability to signal through cAMP. As PPT-I is implicated in hemopoietic regulation, immune responses, breast cancer, and other neural functions, these studies add to the basic biology of these processes and could provide targets for drug development.

Oligonucleotide sequence-specific inhibition of gene expression, tumor growth inhibition, and modulation of cAMP signaling by an RNA-DNA hybrid antisense targeted to protein kinase A RIalpha subunit

The primary mediator of cAMP action in mammalian cells is cAMP-dependent protein kinase (PKA). There are two types of PKA, type I (PKA-I) and type II (PKA-II), which share a common catalytic subunit but contain distinct regulatory subunits, RI and RII, respectively. Evidence suggests that increased expression of RIalpha/PKA-I correlates with neoplastic cell growth. Here, we show that sequence-specific oligonucleotide inhibition of RIalpha expression results in inhibition of growth and modulation of cAMP signaling in cancer cells. The antisense promoted growth inhibition in a time-dependent, concentration-dependent, and sequence-dependent manner in human cancer cells in monolayer culture, and it inhibited colony formation in soft agar and tumor growth in nude mice. Among the cancer cells are LS-174T, HCT-15, and Colo-205 colon carcinoma cells; A-549 lung carcinoma cells; LNCaP prostate adenocarcinoma cells; Molt-4 leukemia cells; and Jurkat T lymphoma cells. Northern blot and immunoprecipitation analyses revealed that the growth inhibitory effect of the antisense correlated with a decrease in RIalpha expression at both the mRNA and protein levels. Pulse-chase experiments revealed that the antisense-directed inhibition of RIalpha expression resulted in compensatory changes in expression of the isoforms of R and C subunits and cAMP signaling in a cell type-specific manner. These results demonstrate that cAMP is ubiquitous in the regulation of cell growth and that the antisense oligonucleotide, which inhibits the synthesis of the RIalpha subunit of PKA, can be targeted to a single gene for treatment of cancer in a variety of cell types.

Extracellular protein kinase A as a cancer biomarker: its expression by tumor cells and reversal by a myristate-lacking Calpha and RIIbeta subunit overexpression

Overexpression of cAMP-dependent protein kinase (PKA) type I isozyme is associated with cell proliferation and neoplastic transformation. The presence of PKA on the external surface of LS-174T human colon carcinoma cells has been shown. Here, we show that cancer cells of various cell types excrete PKA into the conditioned medium. This extracellular PKA (ECPKA) is present in active, free catalytic subunit (C subunit) form, and its activity is specifically inhibited by PKA inhibitory protein, PKI. Overexpression of the Calpha or RIalpha subunit gene of PKA in an expression vector, which up-regulates intracellular PKA type I, markedly up-regulates ECPKA expression. In contrast, overexpression of the RIIbeta subunit, which eliminates PKA type I, up-regulates PKA type II, and reverts the transformed phenotype, down-regulates ECPKA. A mutation in the Calpha gene that prevents myristylation allows the intracellular PKA up-regulation but blocks the ECPKA increase, suggesting that the NH(2)-terminal myristyl group of Calpha is required for the ECPKA expression. In serum of cancer patients, the ECPKA expression is up-regulated 10-fold as compared with normal serum. These results indicate that the ECPKA expression is an ordered cellular response of a living cell to actively exclude excess intracellular PKA molecules from the cell. This phenomenon is up-regulated in tumor cells and has an inverse relationship with the hormone dependency of breast cancer. Thus, the extracellular PKA may serve as a potential diagnostic and prognostic marker for cancer.

8-Cl-cAMP antagonizes mitogen-activated protein kinase activation and cell growth stimulation induced by epidermal growth factor

The growth factor-activated mitogenic pathways are often disregulated in tumour cells and, therefore, they can provide specific molecular targets for novel anti-tumour approaches. 8-Chloro-cAMP (8-Cl-cAMP), a synthetic cAMP analogue, is a novel anti-tumour agent that has recently undergone clinical evaluation. We investigated the effects of 8-Cl-cAMP on the epidermal growth factor (EGF)/EGF receptor (EGF-R) signalling in human epidermoid cancer KB cells, which are responsive to the mitogenic stimulus of EGF. We found that the growth-promoting activity of EGF was completely abolished when EGF treatment was performed in combination with 8-Cl-cAMP. The inhibition of the EGF-induced proliferation by 8-Cl-cAMP was paralleled by the blockade of the EGF-stimulated activation of mitogen-activated protein kinases (MAPK), ERK-1 and ERK-2. Conversely, we found an increase of EGF-R expression and EGF-R tyrosine phosphorylation when KB cells were growth inhibited by 8-Cl-cAMP. Moreover, the activity of Raf-1 and MEK-1 protein kinases, the activators upstream MAPK in the phosphorylation cascade induced by EGF, was not modified in 8-Cl-cAMP-treated cells. We concluded that the impairment of KB cell response to EGF, induced by 8-Cl-cAMP, resides in the specific inhibition of MAPK/ERKs activity while the function of the upstream elements in the EGF-R signalling is preserved.

Ala99ser mutation in RI alpha regulatory subunit of protein kinase A causes reduced kinase activation by cAMP and arrest of hormone-dependent breast cancer cell growth

Expression of the RIalpha regulatory subunit of protein kinase A type I is increased in human cancer cell lines, in primary tumors, in cells after transformation, and in cells upon stimulation of growth. Ala99 (the pseudophosphorylation site) of human RIalpha was replaced with Ser (RIalpha-p) for the structure-function analysis of RIalpha. MCF-7 hormone-dependent breast cancer cells were transfected with an expression vector for the wild-type RIalpha or mutant RIalpha-p. Overexpression of RIalpha-P resulted in suppression of protein kinase A type II, the isozyme of type I kinase, production of kinase exhibiting reduced cAMP activation, and inhibition of cell growth showing an increase in G0/G1 phase of the cell cycle and apoptosis. The wild-type RIalpha overexpression had no effect on protein kinase A isozyme distribution or cell growth. Overexpression of protein kinase A type II regulatory subunit, RIIbeta, suppressed RIalpha and protein kinase A type I and inhibited cell growth. These results show that the growth of hormone-dependent breast cancer cells is dependent on the functional protein kinase A type I.

Antisense oligonucleotide inhibition of serine/threonine kinases: an innovative approach to cancer treatment

The identification of genes that confer a growth advantage on neoplastic cells and the understanding of the genetic mechanism(s) responsible for their activation have made possible a direct genetic approach to cancer treatment using nucleic acid therapeutics. Moreover, the ability to block the expression of individual genes that promote carcinogenesis provides a powerful tool to explore the molecular basis of normal growth regulation, as well as the opportunity for therapeutic intervention. One technique for turning off a single activated gene is the use of antisense oligodeoxynucleotides and their analogs for inhibition of gene expression. The serine/threonine kinases are involved in mediating intracellular responses to external signals, such as growth factors, hormones, and neurotransmitters, and are involved in cell proliferation and oncogenesis. Described herein are recent studies supporting the potential use of oligonucleotides targeting these kinases as chemotherapeutic agents for cancer treatment. The serine/threonine kinases included here are protein kinase A, protein kinase C, and c-raf-1 kinase.

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.

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.

Elevation of protein kinase A and protein kinase C activities in malignant as compared with normal human breast tissue

Because of their central role in the transduction of extracellular signals, protein kinases A (PKA) and C (PKC) are critical enzymes in the control of cellular proliferation and differentiation. We have measured the catalytic activity of PKA and PKC, as well as the regulatory subunit expression for PKA, in paired samples of normal and malignant breast tissue from 13 patients with breast cancer. Paired non-parametric (Wilcoxon) analysis revealed significantly higher values for both basal (P = 0.0002) and total (P = 0.0002) PKA catalytic activity in malignant compared with normal breast in all 13 paired tissue samples. Expression of both R1- and RII-PKA regulatory subunits were also higher in malignant tissue from 12 (P = 0.0005) and 9 (P = 0.01) of the 13 pairs, respectively. However, the degree of RI-subunit overexpression in malignant tissue was greater than that of the RII-subunit, as demonstrated by an increase in the RI/RII subunit ratio in 10 of the 13 paired samples (P = 0.017). Total PKC catalytic activity was elevated in 11 of the 13 malignant tissue specimens when compared with corresponding normal breast tissue (P = 0.01). This was accounted for by an increase in Ca(2+)-dependent PKC activity (P = 0.01), there being no significant increase in Ca(2+)-independent PKC activity. These data suggest that the activities of both PKA and PKC signalling pathways are intrinsically higher in malignant compared with normal breast tissue and these may therefore represent targets for interventive treatment of breast cancer.

Cyclic adenosine 3',5'-monophosphate-binding proteins in human ovarian cancers

The aims of the present study were to characterise an assay for cAMP-binding proteins in ovarian cancer and then to measure levels in a series of tumours with a view to developing a potential prognostic indicator for this disease. Levels and types of binding proteins have been measured in cytosols from 50 ovarian tumours. Binding proteins were detected in all tumours but, as calculated from Scatchard analysis, binding levels ranged from 267 to 12,037 fmol per mg of cytosol protein (mean value of 4248 fmol mg-1). Dissociation constants of binding varied between 0.4 x 10(-8) and 5.9 x 10(-8) (mean value 2.3 x 10(-8)). Types of binding protein were detected by incubation with the photoaffinity ligand 8-N3-[32P]cAMP, followed by polyacrylamide gel electrophoresis and autoradiography. Labelled proteins with molecular weights of 52, 48, 43, 39 and 37 kDa were identified in the cytosols. The proportion and pattern of bands detected varied between different cytosols. The significance of these findings awaits clinical follow-up of the patients.