A comparative study on the metabolism of N 6 , O 2′ -dibutyryl adenosine 3′,5′-monophosphate in various rat hepatoma cell lines in culture

Effect of adenosine 3':5'-cyclic monophosphate and inhibition of protein kinase A on heat sensitivity in H35 hepatoma cells

PURPOSE

To investigate the role of the cyclic adenosine 3':5'-monophosphate (AMP) signal transductions pathway in heat-induced cell death and the development of thermotolerance.

METHODS AND MATERIALS

Reuber H35 rat hepatoma cells were heated after preincubation with various compounds known to modulate the cyclic AMP signal transduction pathway. Cell survival was determined by colony-forming ability.

RESULTS

Preincubation of H35 cells with forskolin, a stimulator of adenylate cyclase, in combination with IBMX (3-isobutyl-1-methylxanthine), an inhibitor of cyclic AMP phosphodiesterase, results in thermosensitization. Similar results are obtained with various cyclic AMP analogs. Maximum thermosensitization occurs with 0.5 mM dibutyryl cyclic AMP (DBcAMP) after a preincubation period of 5 h and heating in the presence of the drug. The same relative degree of thermosensitization is found with 8-Cl-cAMP, but at a 10-fold lower concentration. Thermosensitization by DBcAMP is prevented by H89, a specific inhibitor of cyclic AMP-dependent protein kinase (PKA). Without additional cyclic AMP-inducing factors, H89 induces thermoprotection. None of the drug treatments are cytotoxic at 37 degrees C. DBcAMP does not affect the development of heat-induced thermotolerance but it reduces its expression to an extent similarly found in the observed thermosensitization in nonthermotolerant cells.

CONCLUSION

The results strongly indicate that the cyclic AMP signal transduction pathway is involved in the process of heat-induced cell death. DBcAMP reduces the expression of thermotolerance, but does not affect its induction.

Inhibition of hepatoma cell growth by analogs of adenosine and cyclic AMP and the influence of enzymes in mammalian sera

The following evidence suggests that inhibition of hepatoma cell (HTC) growth by cyclic nucleotides is an adenosine-like effect that is greatly modified by the type and treatment of serum used in the culture medium and is probably not mediated by cyclic AMP-dependent protein kinase: 1) Heating serum reduces its phosphodiesterase content, thereby slowing metabolism of cyclic AMP and reducing the inhibition of HTC cell growth by cyclic AMP; 2) Using medium that contains phosphodiesterase but lacks adenosine deaminase causes adenosine to accumulate from cyclic AMP and increases the toxicity of cyclic AMP; 3) Uridine or cytidine reverses the growth inhibition caused by adenosine, 5'-AMP or cyclic AMP; 4) adenosine, 5'-AMP and N6-(delta 2-isopentenyl) adenosine are more toxic for HTC cells than is cyclic AMP, and N6,O2-dibutyryl cyclic AMP is not toxic; and 5) N6,O2'-dibutyryl cyclic AMP inhibits growth of Reuber H35 cells, but uridine prevents this inhibition of growth. We conclude that most, if not all, of the inhibitory effects of cyclic AMP and N6,O2'-dibutyryl cyclic AMP on HTc and Reuber H35 hepatoma cell growth are due to the generation of toxic metabolites.

Positive and negative cAMP-mediated control of tyrosine aminotransferase synthesis in Reuber H35 hepatoma cells

Induction of L-tyrosine:2-oxoglutarate aminotransferase (EC 2.6.1.5) by N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) in Reuber H35 hepatoma cells reaches a maximum value between 3-5 h after addition of Bt2cAMP and subsequently decreases in the continuous presence of Bt2cAMP. We have investigated the kinetics of the increase, i.e. induction, and the decrease, i.e. the repressed state, of the tyrosine-aminotransferase-synthesizing system under these conditions. Our experimental results are as follows. 1. The repressed state of the tyrosine-aminotransferase-synthesizing system is not caused by a decrease in the intracellular cAMP concentration. 2. The repressed state is inhibited by actinomycin D (while induction is not inhibited). 3. During the repressed state no effect of dexamethasone on tyrosine aminotransferase synthesis is found, while during induction Bt2cAMP and dexamethasone act synergistically. 4. Longer starvation of the cells in serum-free medium has no influence on the kinetics of the induction/repressed state curve. From these results we have concluded that the mechanism of the transition to the repressed state of the tyrosine-aminotransferase-synthesizing system is essentially different from the mechanism of deinduction which occurs after removal of the inducer. Moreover, the repressed state of the system is a phenomenon which is induced by Bt2cAMP separately from induction at a different level of protein synthesis.

Further evidence for translational regulation of tyrosine aminotransferase synthesis by dibutyryl cyclic AMP in Reuber H35 hepatoma cells

Cyclic AMP derivatives increase the rate of synthesis of tyrosine aminotransferase in Reuber H35 hepatoma cells. Various studies lend support to the hypothesis that cyclic AMP increases the synthesis of tyrosine aminotransferase by acting at a posttranscriptional site. The presence of a limited non-translatable pool of tyrosine aminotransferase mRNA prior to the formation of the translatable tyrosine aminotransferase mRNA implicates a possible site of action of cyclic AMP. We compared the capacity of N6,O2'-dibutyryl cyclic AMP to induce tyrosine aminotransferase synthesis when untranslatable tyrosine aminotransferase mRNA sequences are present or absent. The transition of a condition in which non-translatable tyrosine aminotransferase mRNA sequences were present to a condition in which they were absent was established by preinduction of Reuber H35 cells with dexamethasone, followed by addition of actinomycin D. In the time period thereafter, the amount of non-translatable mRNA decreased and 1.5-2 h after addition of actinomycin D, only translatable tyrosine aminotransferase mRNA was present. It can be seen that the induction of tyrosine aminotransferase synthesis by dibutyryl cyclic AMP follows the normal decrease of tyrosine aminotransferase mRNA. We present evidence that dibutyryl cyclic AMP in Reuber H35 hepatoma cells regulates tyrosine aminotransferase synthesis at a posttranscriptional site independent of the pool of non-translatable tyrosine aminotransferase mRNA sequences, but influencing the efficiency of translation of active tyrosine aminotransferase mRNA.

Intracellular protein degradation in growing, in density-inhibited, and in serum-restricted fibroblast cultures

Exponentially growing Balb/3T3 mouse fibroblasts contain protein populations with slow and fast turnover. These two stability classes were labelled selectively with 3H-leucine. The intracellular degradation of the proteins was then followed as the release into the medium of radioactive leucine. The degradation rate of both stability classes of protein is increased by about 55% in cultures whose growth is inhibited by high cell density. Serum-deprivation, which also halts cell growth, accelerates protein breakdown to a smaller extent, the increases for relatively stable and unstable proteins being 30% and 13%, respectively. The density-dependent increase in protein breakdown is also found in BHK21 cells but not in chick fibroblasts. Protein degradation in Balb/3T3 cells transformed by simian virus 40 is affected by serum-deprivation but not by cell density. The proteins which are relatively stable during growth were shown to become less stable in density-inhibited or serum-deprived cultures, and vice versa. Cycloheximide inhibits degradation to a variable extent. Dibutyryl adenosine-3',5'-cyclic monophosphate has no effect on the protein degradation under the conditions investigated here.