Site-selective 8-chloroadenosine 3',5'-cyclic monophosphate inhibits transformation and transforming growth factor alpha production in Ki-ras-transformed rat fibroblasts

Metabolism, Biochemical Actions, and Chemical Synthesis of Anticancer Nucleosides, Nucleotides, and Base Analogs

Nucleoside, nucleotide, and base analogs have been in the clinic for decades to treat both viral pathogens and neoplasms. More than 20% of patients on anticancer chemotherapy have been treated with one or more of these analogs. This review focuses on the chemical synthesis and biology of anticancer nucleoside, nucleotide, and base analogs that are FDA-approved and in clinical development since 2000. We highlight the cellular biology and clinical biology of analogs, drug resistance mechanisms, and compound specificity towards different cancer types. Furthermore, we explore analog syntheses as well as improved and scale-up syntheses. We conclude with a discussion on what might lie ahead for medicinal chemists, biologists, and physicians as they try to improve analog efficacy through prodrug strategies and drug combinations.

The cAMP analogs have potent anti-proliferative effects on medullary thyroid cancer cell lines

The oncogenic activation of the rearranged during transfection (RET) proto-oncogene has a main role in the pathogenesis of medullary thyroid cancer (MTC). Several lines of evidence suggest that RET function could be influenced by cyclic AMP (cAMP)-dependent protein kinase A (PKA) activity. We evaluated the in vitro anti-tumor activity of 8-chloroadenosine-3',5'-cyclic monophosphate (8-Cl-cAMP) and PKA type I-selective cAMP analogs [equimolar combination of the 8-piperidinoadenosine-3',5'-cyclic monophosphate (8-PIP-cAMP) and 8-hexylaminoadenosine-3',5'-cyclic monophosphate (8-HA-cAMP) in MTC cell lines (TT and MZ-CRC-1)]. 8-Cl-cAMP and the PKA I-selective cAMP analogs showed a potent anti-proliferative effect in both cell lines. In detail, 8-Cl-cAMP blocked significantly the transition of TT cell population from G2/M to G0/G1 phase and from G0/G1 to S phase and of MZ-CRC-1 cells from G0/G1 to S phase. Moreover, 8-Cl-cAMP induced apoptosis in both cell lines, as demonstrated by FACS analysis for annexin V-FITC/propidium iodide, the activation of caspase-3 and PARP cleavage. On the other hand, the only effect induced by PKA I-selective cAMP analogs was a delay in G0/G1-S and S-G2/M progression in TT and MZ-CRC-1 cells, respectively. In conclusion, these data demonstrate that cAMP analogs, particularly 8-Cl-cAMP, significantly suppress in vitro MTC proliferation and provide rationale for a potential clinical use of cAMP analogs in the treatment of advanced MTC.

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.

Integrating signals between cAMP and the RAS/RAF/MEK/ERK signalling pathways. Based on the anniversary prize of the Gesellschaft für Biochemie und Molekularbiologie Lecture delivered on 5 July 2003 at the Special FEBS Meeting in Brussels

One of the hallmarks of cAMP is its ability to inhibit proliferation in many cell types, but stimulate proliferation in others. Clearly cAMP has cell type specific effects and the outcome on proliferation is largely attributed to crosstalk from cAMP to the RAS/RAF/mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK pathway. We review the crosstalk between these two ancient and conserved pathways, describing the molecular mechanisms underlying the interactions between these pathways and discussing their possible biological importance.

Minireview: PRKAR1A: normal and abnormal functions

The type 1alpha regulatory subunit (RIalpha) of cAMP-dependent protein kinase (PKA) (coded by the PRKAR1A gene) is the main component of type I PKA, which regulates most of the serine-threonine kinase activity catalyzed by the PKA holoenzyme in response to cAMP. Carney complex (CNC), or the complex of spotty skin pigmentation, myxomas, and endocrine overactivity, is a multiple endocrine (and not only) neoplasia syndrome that is due to PRKAR1A-inactivating mutations. The R1alpha protein and PRKAR1A mRNA have been found to be up-regulated in a series of cell lines and human and rodent neoplasms, suggesting this molecule's involvement in tumorigenesis and its potential role in cell cycle regulation, growth, and/or proliferation. Alterations in PKA activity elicit a variety of effects depending on the tissue, developmental stage, degree of differentiation, and cAMP levels. In addition, RIalpha may have functions independent of PKA. The presence of inactivating germline mutations and the loss of its wild-type allele in some CNC lesions indicate that PRKAR1A might function as a tumor suppressor gene in these tissues, but could PRKAR1A be a classic tumor suppressor gene? Probably not, and this review explains why.

DeltaRaf-1:ER* bypasses the cyclic AMP block of extracellular signal-regulated kinase 1 and 2 activation but not CDK2 activation or cell cycle reentry

Elevation of cellular cyclic AMP (cAMP) levels inhibits cell cycle reentry in a variety of cell types. While cAMP can prevent the activation of Raf-1 and extracellular signal-regulated kinases 1 and 2 (ERK1/2) by growth factors, we now show that activation of ERK1/2 by DeltaRaf-1:ER is insensitive to cAMP. Despite this, DeltaRaf-1:ER-stimulated DNA synthesis is still inhibited by cAMP, indicating a cAMP-sensitive step downstream of ERK1/2. Although cyclin D1 expression has been proposed as an alternative target for cAMP, we found that cAMP could inhibit DeltaRaf-1:ER-induced cyclin D1 expression only in Rat-1 cells, not in CCl39 or NIH 3T3 cells. DeltaRaf-1:ER-stimulated activation of CDK2 was strongly inhibited by cAMP in all three cell lines, but cAMP had no effect on the induction of p21(CIP1). cAMP blocked the fetal bovine serum (FBS)-induced degradation of p27(KIP1); however, loss of p27(KIP1) in response to DeltaRaf-1:ER was less sensitive in CCl39 and Rat-1 cells and was completely independent of cAMP in NIH 3T3 cells. The most consistent effect of cAMP was to block both FBS- and DeltaRaf-1:ER-induced expression of Cdc25A and cyclin A, two important activators of CDK2. When CDK2 activity was bypassed by activation of the ER-E2F1 fusion protein, cAMP no longer inhibited expression of Cdc25A or cyclin A but still inhibited DNA synthesis. These studies reveal multiple points of cAMP sensitivity during cell cycle reentry. Inhibition of Raf-1 and ERK1/2 activation may operate early in G(1), but when this early block is bypassed by DeltaRaf-1:ER, cells still fail to enter S phase due to inhibition of CDK2 or targets downstream of E2F1.

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.

Protein kinase A as target for novel integrated strategies of cancer therapy

We have studied the role of protein kinase A (PKA) in neoplastic transformation, apoptosis, and angiogenesis and its relationship with other signaling molecules, as a basis for developing novel therapeutic strategies. We demonstrated the involvement of PKA type I (PKA-I) in the transduction of mitogenic signals from different sources and demonstrated functional and structural interactions between PKA-I and the activated epidermal growth factor receptor (EGFR). We contributed to the identification and development of several selective inhibitors of PKA-I, such as 8-Cl-cAMP and a hybrid DNA/RNA antisense oligonucleotide of a novel class (AS-PKA-I) and of EGFR, including mAbC225 and ZD1839 (Iressa). All these agents have been investigated in cancer patients. We demonstrated the therapeutic potential of the combined blockade of PKA-I and EGFR, reporting a synergistic antitumor effect when their inhibitors are used in combination. We have also shown that PKA-I and EGFR inhibitors are able to cooperate with selected class of cytotoxic drugs and with ionizing radiation, causing a synergistic inhibition of tumor growth in vitro and in vivo, accompanied by inhibition of expression of growth and angiogenic factors and by suppression of vessel production. Moreover, PKA-I is implicated in a bcl-2-dependent apoptotic pathway, and we have recently reported a cooperative antitumor and proapoptotic effect of AS-PKA-I in combination with an AS-bcl-2. Finally, we have shown that AS-PKA-I also has antitumor and antiangiogenic effects following oral administration and that they can be greatly enhanced in combination with oral ZD1839 and oral taxanes.

Cyclic AMP blocks cell growth through Raf-1-dependent and Raf-1-independent mechanisms

It is widely accepted that cyclic AMP (cAMP) can block cell growth by phosphorylating Raf-1 on serine 43 and inhibiting signaling to extracellular signal-regulated protein kinase. We show that the suppression of Raf-1 by cAMP is considerably more complex than previously reported. When cellular cAMP is elevated, Raf-1 is phosphorylated on three residues (S43, S233, and S259), which work independently to block Raf-1. Both Ras-dependent and Ras-independent processes are disrupted. However, when cAMP-insensitive versions of Raf-1 are expressed in NIH 3T3 cells, their growth is still strongly suppressed when cAMP is elevated. Thus, although Raf-1 appears to be an important cAMP target, other pathways are also targeted by cAMP, providing alternative mechanisms that lead to suppression of cell growth.

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.

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.

Mammary gland specific hEGF receptor transgene expression induces neoplasia and inhibits differentiation

The epidermal growth factor receptor (EGFR) is overexpressed in about 48% of human breast cancer tissues. To analyse the role of the EGFR in mammary tumor development we generated transgenic mice expressing the human EGFR under the control of either the MMTV-LTR (MHERc) or the beta-lactoglobulin promoter (BLGHERc). The BLGHERc-transgene was expressed exclusively in the female mammary gland, whereas the MHERc transgene was expressed more promiscuously in other organs, such as ovary, salivary gland and testis. Female virgin and lactating transgenic mice of both strains have impaired mammary gland development. Virgin EGFR transgenic mice developed mammary epithelial hyperplasias, whereas in lactating animals progression to dysplasias and tubular adenocarcinomas was observed. In both strains the number of dysplasias increased after multiple pregnancies. The transgene expression pattern was heterogeneous, but generally restricted to regions of impaired mammary gland development. Highest EGFR transgene expression was observed in adenocarcinomas. By using a whole mount organ culture system to study the differentiation potential of the mammary epithelium, we observed a reduced number of fully developed alveoli and a decrease in whey acidic protein expression. Taken together, EGFR overexpression results in a dramatic effect of impaired mammary gland development in vitro as well as in vivo, reducing the differentiation potential of the mammary epithelium and inducing epithelial cell transformation.

Molecular biology of pancreatic cancer; oncogenes, tumour suppressor genes, growth factors, and their receptors from a clinical perspective

Pancreatic cancer represents the fourth leading cause of cancer death in men and the fifth in women. Prognosis remains dismal, mainly because the diagnosis is made late in the clinical course of the disease. The need to improve the diagnosis, detection, and treatment of pancreatic cancer is great. It is in this type of cancer, in which the mortality is so great and the clinical detection so difficult that the recent advances of molecular biology may have a significant impact. Genetic alterations can be detected at different levels. These alterations include oncogene mutations (most commonly, K-ras mutations, which occur in 75% to more than 95% of pancreatic cancer tissues), tumour suppressor genes alterations (mainly, p53, p16, DCC, etc.), overexpression of growth factors (such as EGF, TGF alpha, TGF beta 1-3, aFGF, bTGF, etc.) and their receptors (i.e., EGF receptor, TGF beta receptor I-III, etc.). Insights into the molecular genetics of pancreatic carcinogenesis are beginning to form a genetic model for pancreatic cancer and its precursors. These improvements in our understanding of the molecular biology of pancreatic cancer are not simply of research interest, but may have clinical implications, such as risk assessment, early diagnosis, treatment, and prognosis evaluation.

Protein kinase A-Ialpha subunit-directed antisense inhibition of ovarian cancer cell growth: crosstalk with tyrosine kinase signaling pathway

Expression of the RIalpha subunit of cAMP-dependent protein kinase type I is increased in human cancers in which an autocrine pathway for epidermal growth factor-related growth factors is activated. We have investigated the effect of sequence-specific inhibition of RIalpha gene expression on ovarian cancer cell growth. We report that RIalpha antisense treatment results in a reduction in RIalpha expression and protein kinase A type I, and inhibition of cell growth. The growth inhibition was accompanied by changes in cell morphology and appearance of apoptotic nuclei. In addition, EGF receptor, c-erbB-2 and c-erbB-3 levels were reduced, and the basal and EGF-stimulated mitogen-activated protein kinase activities were reduced. Protein kinase A type I and EGF receptor levels were also reduced in cells overexpressing EGF receptor antisense cDNA. These results suggest that the antisense depletion of RIalpha leads to blockade of both the serine-threonine kinase and the tyrosine kinase signaling pathways resulting in arrest of ovarian cancer cell growth.

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.

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.

Elevation of intracellular cyclic adenosine monophosphate inhibits the epidermal growth factor signal transduction pathway and cellular growth in pancreatic adenocarcinoma cell lines

BACKGROUND

The epidermal growth factor (EGF) signal transduction pathway, frequently activated in pancreatic cancer, is an important regulator of cellular growth and transformation. This study examined whether activation of the cyclic adenosine monophosphate protein kinase A pathway may inhibit the EGF signal transduction pathway in pancreatic cancer cell lines.

METHODS

Human pancreatic cancer lines BxPC-3 and AsPC-1 were stimulated with EGF, forskolin, or both. Forskolin is a compound that increases cyclic adenosine monophosphate levels. Assays of cell lines were then obtained for cellular growth (MTT assay), anchorage-independent growth (soft agar), and EGF-induced mitogen-activated protein kinase activation as measured by an in-gel kinase assay.

RESULTS

Treatment with forskolin resulted in inhibition of EGF-induced activation of mitogen-activated protein kinase activity (BxPC-3 78% inhibition and AsPC-1 70% inhibition, p < 0.005), diminished cellular proliferation (BxPC-3 92% inhibition and AsPC-1 86% inhibition, p < 0.001), and formation of colonies in soft agar (BxPC-3 98% inhibition and AsPC-1 76% inhibition, p < 0.001). Forskolin did not inhibit EGF receptor autophosphorylation or tyrosine kinase signaling in response to EGF.

CONCLUSIONS

Forskolin-induced inhibition of mitogen-activated protein kinase is associated with diminished pancreatic cancer cell proliferation in vitro. Use of strategies to increase cyclic adenosine monophosphate levels may have therapeutic application in pancreatic cancer.

Protein kinase A-directed antisense restrains cancer growth: sequence-specific inhibition of gene expression

Increased expression of the RI alpha subunit of cAMP-dependent protein kinase type I has been shown in human cancer cell lines, in primary tumors, in cells after transformation, and in cells upon stimulation of growth. The sequence-specific inhibition of RI alpha gene expression by an antisense oligodeoxynucleotide results in the differentiation of leukemia cells and growth arrest of cancer cells of epithelial origin. A single-injection RI alpha antisense treatment in vivo also causes a reduction in RI alpha expression and inhibition of tumor growth. Tumor cells behave like untransformed cells by making less protein kinase type I. The RI alpha antisense, which produces a biochemical imprint for growth control, requires infrequent dosing to restrain neoplastic growth in vivo.

Growth inhibition by 8-chloro cyclic AMP of human HT29 colorectal and ZR-75-1 breast carcinoma xenografts is associated with selective modulation of protein kinase A isoenzymes

Significant dose-related inhibition of growth of HT29 human colorectal cancer xenografts and ZR-75-1 breast cancer xenografts in immune-suppressed mice was induced by the cyclic AMP analogue, 8-chloroadenosine 3',5'-cyclic monophosphate (8-Cl-cyclic AMP) when given by alzet mini-pumps over a 7-day period at doses of either 50 or 100 mg/kg/day. Levels and types of cyclic AMP binding proteins were measured by ligand binding and photoaffinity labelling, respectively, in tumours harvested at the end of the treatment period. Compared with levels in tumours from control animals, values of tumour cyclic AMP binding proteins from treated animals were significantly reduced. These effects were associated with an apparent modulation of the types of cyclic AMP binding proteins, 8-Cl-cyclic AMP-treated xenografts displaying a reduced ratio of RI/RII isoforms compared with untreated control tumours.

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.

Flow-cytometric detection of the RI alpha subunit of type I cAMP-dependent protein kinase in human cells

cAMP-dependent protein kinase (PKA) is composed of two genetically distinct catalytic (C) and regulatory (R) subunits. There are two different classes of PKA, designated as type I and type II, which contain distinct R subunits (RI or RII, respectively) but share a common C subunit. Enhanced expression of type I PKA has been correlated with cell proliferation and neoplastic transformation. Detection of the different PKA subunits is usually performed by photoaffinity labeling with 8-N3-32P-cAMP or by radioimmunolabeling techniques. Both techniques are time consuming and require a high number of cells and the use of radioactive reagents. Using the MCF-10A normal human mammary cell line infected with a recombinant retroviral vector containing the human RI alpha gene (MCF-10A RI alpha), we have developed a flow-cytometric assay to detect the intracellular content of RI alpha protein in human cells. MCF-10A and MCF-10A RI alpha cells were fixed in 1.5% paraformaldehyde at 37 degrees C for 15 min and permeabilized by methanol and acetone (1:1) at -20 degrees C for 5 min before staining with a specific IgG2a MoAb followed by a FITC-conjugate rabbit-anti mouse IgG. This procedure was also successfully utilized to recognize RI alpha protein content in human peripheral blood lymphocytes. Flow-cytometric detection of the RI alpha subunit in human cells is feasible and allows the study of the role of type I PKA in cell growth and neoplastic transformation.

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

Three separate experimental approaches, using site-selective cAMP analogs, antisense strategy and retroviral vector-mediated gene transfer, have provided evidence that two isoforms, the RI- and RII-regulatory subunits of cAMP-dependent protein kinase, have opposite roles in cell growth and differentiation; RI being growth stimulatory while RII is a growth-inhibitory and differentiation-inducing protein. As RI expression is enhanced during chemical or viral carcinogenesis, in human cancer cell lines and in primary human tumors, it is a target for cancer diagnosis and therapy. 8-Cl-cAMP and RI antisense oligodeoxynucleotide, those that effectively down-regulate RI alpha and up-regulate RII beta, provide new approaches toward the treatment of cancer. This approach to modulation of RI vs RII cAMP transducers may also be beneficial toward therapy of endocrine or cellular dysfunction-related diseases where abnormal signal transduction of cAMP is critically involved.

Inhibition of human pancreatic cancer cell (MIA PaCa-2) growth by cholera toxin and 8-chloro-cAMP in vitro

The effects of cholera toxin (CT) and 8-chloro-cAMP (8-Cl-cAMP) on cell growth were investigated using two human pancreatic carcinoma cell lines (MIA PaCa-2, Panc-1). CT, which catalyses the ADP ribosylation of Gs, suppresses the proliferation of MIA PaCa-2(PC) cells. CT at the low dose of 0.1 pg ml-1 was inhibitory of PC cell growth, and the maximum suppression (70%) was achieved at a CT concentration of 100 pg ml-1. This phenomenon was reversible. The production of cAMP by CT (100 pg ml-1) in PC cells was enhanced 320-fold compared with the control. In addition, cAMP analogues (8-Cl-cAMP, 8-Br-cAMP) and forskolin decreased the growth rate of PC cells in a dose-dependent manner. These results support the view that CT suppresses PC cell growth by stimulating cAMP production. Conversely, Panc-1 cells were far less sensitive to CT in cell growth and cAMP production. 8-Cl-cAMP was also less effective on Panc-1 cell growth. The binding of an insulin-like growth factor (IGF)-I and transforming growth factor (TGF)-alpha, which has been shown to stimulate PC cell growth in an autocrine manner, to PC cells was not modified in cells treated with CT or 8-Cl-cAMP. The results suggest that the inhibitory actions of these substances do not occur at the level of the receptor for IGF-I or EGF/TGF-alpha. We have previously shown that phorbol esters, which decrease the binding of TGF-alpha to PC cells, has an anti-proliferative activity on these tumour cells. Inhibited cell growth by maximum suppressive dose of CT or 8-Cl-cAMP was further inhibited by TPA. In addition, an oncogene product of K-ras which is commonly activated in pancreatic cancer, was increased by CT and 8-Cl-cAMP. It is concluded that CT and 8-Cl-cAMP inhibit PC cell growth, presumably in a similar manner, and their mechanism(s) of action may be different from that of TPA. The anti-proliferative effect of CT or 8-Cl-cAMP was enhanced by TPA, implying that the combination of these substances results in increased inhibition of the PC cell growth.

Down-regulation of RI alpha subunit of cAMP-dependent protein kinase induces growth inhibition of human mammary epithelial cells transformed by c-Ha-ras and c-erbB-2 proto-oncogenes

MCF-10A is a spontaneously immortalized, non-transformed human mammary epithelial cell line. We have recently obtained MCF-10A clones (MCF-10A HE cells) that are transformed following over-expression of both a human point-mutated c-Ha-ras and the c-erbB-2 proto-oncogenes. Two isoforms of the cAMP-dependent protein kinase (cAK) have been described in mammalian cells. Enhanced levels of type-I cAK (cAKI) are generally found in tumor cells. To determine whether inhibition of cAKI expression may interfere with ras and erbB-2 oncogene-induced transformation of human mammary epithelial cells, we have tested the effects of 2 agents that specifically down-regulate cAKI, such as 8-chloro-cAMP and an anti-sense oligodeoxynucleotide targeted against the RI alpha regulatory subunit of cAKI on MCF-10A HE cells. Treatment of MCF-10A HE cells with 8-chloro-cAMP induces a dose-dependent growth inhibition under both monolayer and soft-agar growth conditions, that is correlated with an accumulation of MCF-10A HE cells in G0/G1 phases of the cell cycle and a reduction of the number of cells in S phase. In contrast, 8-chloro-cAMP has no effect on MCF-10A cell growth. Furthermore, 8-chloro-cAMP treatment of MCF-10A HE cells induces a 4- to 6-fold reduction in p185erbB-2 expression and brings p21ras expression to levels comparable to those found in MCF-10A cells. Treatment of MCF-10A HE cells with an RI alpha anti-sense oligodeoxynucleotide determines a comparable inhibition of both anchorage-dependent and anchorage-independent cell growth. Our results suggest that cAKI may act as a mediator of ras and erbB-2 oncogene action in human breast cells and that interference with cAKI action provides a potential tool for inhibiting the growth-promoting effects of these oncogenes.

Role of site-selective cAMP analogs in the control and reversal of malignancy

Two isoforms of cAMP receptor protein, RI and RII, the regulatory subunits of cAMP-dependent protein kinase, transduce opposite signals, the RI being stimulatory and the RII being inhibitory of cell proliferation. In normal cells RI and RII exist at a specific physiological ratio whereas in cancer cells such physiological balance of these receptor proteins is disrupted. Reversal and suppression of malignancy can be achieved when the physiologic ratio of these intracellular signal transducers of cAMP is restored as shown by the use of site-selective cAMP analogs, antisense oligodeoxynucleotides or gene transfer, suggesting new approaches to cancer control.

Suppression of malignancy targeting the intracellular signal transducing proteins of cAMP: the use of site-selective cAMP analogs, antisense strategy, and gene transfer

An hypothesis has been presented suggesting that two isoforms of cAMP receptor proteins are crucial effectors in tumorigenesis. The evidence in support of this hypothesis shows that: (1) cAMP transduces dual controls, both positive and negative, on cell growth and differentiation. (2) Such dual controls are respectively governed by two isoforms of cAMP receptor proteins, the type I and type II regulatory subunits of cAMP-dependent protein kinase. (3) In normal physiology, the functional balance of these cAMP receptor isoforms is strictly controlled to meet either stimulation or inhibition of cell growth as it is required, whereas such control is lost in cancer cells. (4) Cancer cells can also be made to differentiate and acquire growth control when the functional balance of these intracellular signal transducers of cAMP is restored by the use of site-selective cAMP analogs, antisense strategy, or gene transfer, suggesting new approaches to cancer therapy.