The significance of cyclic AMP and cyclic GMP in cancer treatment

Effects of dimaprit on growth and differentiation of human promyelocytic cell line, HL-60

The human promyelocytic cell line HL-60, differentiates in response to a variety of agents including dibutyryl cAMP and agents which increase intracellular cAMP concentrations (phosphodiesterase inhibitors, PGE2, and cholera toxin). HL-60 is also known to be rich in H2 -histamine sensitive adenylate cyclase activity. The present study was therefore designed to test the effects of H2-stimulation on growth and differentiation of HL-60 using the potent H2 agonist dimaprit. Dimaprit markedly increased cAMP production in a dose-dependent manner reaching maximal levels after 30-60 minutes. Intracellular cAMP levels decreased thereafter and by 24 hours were approximately 2-3 fold increased above control. Intracellular cAMP levels were not altered by dimaprit (10(-7)M to 10(-4)M) at 4 days in culture compared to either untreated HL-60 cells or dimethylsulfoxide (DMSO) (1.3%) treated cells. While exponential growth was unaltered by dimaprit (10(-7)M to 10(-4)M) as compared to control, dimaprit induced i) morphologic maturation to the myelocyte and metamyelocyte form with no differentiation seen beyond the metamyelocyte even after 6 days in culture, ii) increased NBT reductase activity and iii) dose-dependent increase in lysozyme activity which could be completely blocked by cimetidine, a specific H2 antagonist. Dimaprit-induced differentiation of HL-60 cells was associated with an initial but transient increase in intracellular cAMP production. Maturation beyond the metamyelocyte stage was not observed. Acquisition of NBT reductase and lysozyme activity correlated with morphologic maturation.

Granulocyte functions during maturation of human promyelocytic leukemia cells

Human promyelocytic leukemia cells (HL-60) mature into functional granulocytes in response to in vitro treatment with several classes of chemical agents. Compounds that increase intracellular adenosine 3':5' -cyclic monophosphate induce a modified program of maturation in which the cells demonstrate functional properties characteristic of mature phagocytic cells while remaining morphologically immature. We compared the developmental programs initiated by two well-studied inducers, retinoic acid and dimethyl sulfoxide, with the programs initiated by two inducers known to raise intracellular adenosine 3':5' -cyclic monophosphate; N6, O2-dibutyryl adenosine 3':5' -cyclic monophosphate and the combination of prostaglandin E2 and theophylline. In response to the increase in intracellular adenosine 3':5' -cyclic monophosphate, the cells ceased proliferation, expressed chemotactic receptors and demonstrated stimulated enzyme release within 24 h. Chemotaxis, adherence, NBT reduction and superoxide production appeared by 72 h, although the cells remained unchanged morphologically. A similar developmental program was induced by dimethylsulfoxide, but appearance of the markers was delayed by 48 h. Expression of these markers was delayed and incomplete in response to retinoic acid.

Protamine inhibits adenylate cyclase activity: a possible reason for the toxicity of protamine

Protamine is an effective inhibitor of the various activated forms of adenylate cyclase of liver plasma membranes. Inhibition of adenylate cyclase may account for its toxic but not its antitumor effects.

Caffeine (1,3,7-trimethylxanthine), a temperature promoter of DMBA-induced rat mammary gland carcinogenesis

The administration of caffeine to the drinking water (250 mg/l and 500 mg/l) of female Sprague-Dawley rats after treatment with 7,12-dimethylbenzanthracene (DMBA) (5 mg i.g.) resulted in an increase in mammary carcinoma incidence. The confidence levels of statistical significance for the groups of rats receiving the 250-mg and 500-mg doses of caffeine were greater than 0.70 and greater than 0.99, respectively. This increase in mammary carcinoma incidence was observed when caffeine treatment was initiated commencing 3 days after DMBA treatment and continued for 21 weeks. This effect was also observed when caffeine treatment was initiated commencing 20 weeks after DMBA treatment and continued for 6 weeks in rats relatively refractory to carcinogen treatment (mammary tumor-free at onset of treatment) and in rats relatively sensitive to the carcinogen (mammary tumor bearing at onset of caffeine treatment). Caffeine treatment of rats prior to and during carcinogen treatment did not significantly affect mammary carcinoma incidence. Thus caffeine consumption has been shown to significantly enhance the promoting phase but is without effect on the initiating phase of this carcinogenic process.

Cyclic nucleotide-induced maturation of human promyelocytic leukemia cells

Myeloid differentiation in vitro is characterized by the sequential appearance of morphological, functional, and biochemical markers of maturation. We examined the effect of agents that increased the intracellular concentration of adenosine 3'5'-cyclic monophosphate on the expression of these markers by human promyelocytic leukemia cells (HL60). Cells treated with 500 muM N(6),O(2)-dibutyryl adenosine 3'5'-cyclic monophosphate expressed formyl peptide and complement receptors, reduced nitroblue tetrazolium, adhered to substrate, demonstrated chemotaxis and stimulated lysosomal enzyme release, rapidly ceased proliferation, and assumed the morphology of myelocytes and metamyelocytes. Prostaglandin E(2) (100 nM) and theophyllin (500 muM) induced similar functional changes but the cells did not mature beyond the myelocyte stage. Cholera toxin (1 or 50 nM) induced formyl-peptide receptor expression and adherence, but the cells did not reduce nitroblue tetrazolium, continued to proliferate, and were unchanged morphologically. Formyl-peptide receptor expression was the earliest marker of these modified programs of maturation. The receptor appeared within 2 h after treatment and increased linearly for 72 h. Receptor expression was dependent on new protein synthesis. At 48 h, Scatchard analysis demonstrated 2.4 x 10(5) receptors/ cell with a K(D) of 1.3 nM. In contrast to induction of HL60 differentiation by dimethyl sulfoxide, retinoic acid, or phorbol myristate acetate, the developmental programs initiated by agents that raised intracellular adenosine 3'5'-cyclic monophosphate shared several unique features: (a) plasma membrane maturation was dissociated from morphological maturation; (b) no latent period was evident following induction-the earliest membrane marker was expressed within 2 h; (c) commitment to terminal differentiation was delayed.

Cytostatic and reverse-transforming therapies of cancer - a brief review and future prospects

Cytostatic cancer therapies are defined as those which retard cancer growth by intervening selectively in the disordered control mechanisms responsible for malignant behavior, without producing direct cytotoxicity. A number of possible approaches to cytostatic therapy are now under investigation and are briefly reviewed here - cyclic AMP congeners and promoters, prostaglandin precursors, thioproline, benzaldehyde, retinoids, interferon, chalones, antineoplastons, protease inhibitors, polar solvents. Suggestions for the further development of cytostatic therapies are offered, and a role for cytostatic therapy within a broad global concept of cancer therapy is proposed.

Potentiation of the growth inhibitory effects of adenosine 3',5'-monophosphate analogues by homocysteine

The growth inhibitory effects of N6-monobutyryl adenosine 3',5'-monophosphate (mbcAMP) and N6,O2'-dibutyryl adenosine 3',5' monophosphate (dbcAMP) towards Walker carcinoma in vitro are significantly potentiated by the addition of L-homocysteine to the culture medium. This effect is not seen with L-cysteine or when exogenous cAMP or prostaglandin E2(PGE2) replace the butyrylated cyclic nucleotide. Combinations of mbcAMP or dbcAMP and L-homocysteine significantly inhibit nucleic acid methylations. Both the butyrylated cyclic nucleotides cause an elevation of the intracellular level of S-adenosyl-L-homocysteine (SAH), a potent inhibitor of S-adenosyl-L-methionine (SAM) dependent methyltransferases, and this is significantly enhanced in combination with L-homocysteine. The increase in SAH level produced by such combinations is proportional to the inhibition of methyl group incorporation into 5-methyl cytosine and 7-methyl guanine. These results suggest that L-homocysteine potentiates accumulation of SAH in the presence of mbcAMP and dbcAMP and that the resultant inhibition of methylation accounts for the enhanced growth inhibition.

Effect of the antitumour agent 5-(3,3-dimethyl-1-triazeno imidazole-4-carboxamide (DTIC) on cyclic AMP levels in intact MH1C1 hepatoma cells

Addition of the cytostatic agent 5-(3,3-dimethyl-1-triazeno)-imidazole-carboxamide (DTIC, dacarbazin) to MH1C1 hepatoma cells, alone or in combination with adrenaline, was shown to elevate intracellular cyclic AMP. The cyclic AMP level was increased from basal values of about 0.40 to about 0.75 pmol/mg protein 10 min. after DTIC addition. Accumulation of cyclic AMP in response to a supramaximal concentration of adrenaline is amplified in a dose-dependent way by 0.1-2 mM DTIC. The effect is apparently due to inhibition of cyclic AMP breakdown, since DTIC inhibits the low-Km form of the cyclic AMP phosphodiesterase without significantly affecting the adenylate cyclase activity of MH1C1 homogenates.