Cyclic AMP-resistant mutants of S49 mouse lymphoma cells hemizygous for expression of regulatory subunit of type I cyclic AMP-dependent protein kinase

Spectrum of spontaneous missense mutations causing cyclic AMP-resistance phenotypes in cultured S49 mouse lymphoma cells differs markedly from those of mutations induced by alkylating mutagens

Mutants of S49 mouse lymphoma cells resistant to cytolysis by analogs of cyclic AMP (cAMP) generally have missense mutations in the gene encoding the regulatory subunit of cAMP-dependent protein kinase. We have compared the mutations in 95 spontaneous isolates with those in 60 mutagen-induced isolates by sequence analysis of amplified cDNAs. Twenty-nine single basepair substitutions in 19 codons produced selectable phenotypes. The spontaneous mutant spectrum was dominated by a CpG transition hotspot in the codon for Arg334. This and other nearby CpG sites were found to be methylated in genomic S49 cell DNA by restriction enzyme analyses. Most of the remaining spontaneous mutants had either G-C-->C-G or T-A-->G-C transversions, which have been associated with damage caused by oxygen radicals. In contrast, the majority of mutants induced with the alkylating mutagens ethyl methanesulfonate (EMS) and N-methyl-N'-nitro-N-nitrosoguanidine had G-C-->A-T mutations at non-CpG sites; in addition, EMS induced several A-T-->G-C, A-T-->T-A, and G-C-->T-A substitutions. A single ICR191-induced mutant analyzed had a unique A-T-->G-C lesion. A number of spontaneous and mutagen-induced isolates had closely linked double or triple substitutions, and two isolates had tandem triple substitutions.

Linked spontaneous CG----TA mutations at CpG sites in the gene for protein kinase regulatory subunit

CG----TA transitions at CpG sequences account for many human point mutations and are thought to result from hydrolytic deamination of 5-methylcytosine residues in these sites. The gene for regulatory subunit of murine cyclic AMP-dependent protein kinase has two closely linked CpG sites, one of which is a strong hotspot for spontaneous CG----TA mutations leading to cyclic AMP resistance in S49 mouse lymphoma cells. About 5% of mutants with a spontaneous mutation at this CpG site had also acquired a second CG----TA mutation at the nearby CpG site. The two mutations were always at first positions of the Arg codons in which they occurred, and they were always together in a single regulatory subunit allele. Their linked appearance could be attributed to neither the selection conditions nor the preexistence of one mutation in the target cells. The high frequency of these double mutants suggests that their lesions result not from hydrolytic deamination but rather from an endogenous enzymatic mechanism.

Linked spontaneous CG----TA mutations at CpG sites in the gene for protein kinase regulatory subunit

CG----TA transitions at CpG sequences account for many human point mutations and are thought to result from hydrolytic deamination of 5-methylcytosine residues in these sites. The gene for regulatory subunit of murine cyclic AMP-dependent protein kinase has two closely linked CpG sites, one of which is a strong hotspot for spontaneous CG----TA mutations leading to cyclic AMP resistance in S49 mouse lymphoma cells. About 5% of mutants with a spontaneous mutation at this CpG site had also acquired a second CG----TA mutation at the nearby CpG site. The two mutations were always at first positions of the Arg codons in which they occurred, and they were always together in a single regulatory subunit allele. Their linked appearance could be attributed to neither the selection conditions nor the preexistence of one mutation in the target cells. The high frequency of these double mutants suggests that their lesions result not from hydrolytic deamination but rather from an endogenous enzymatic mechanism.

Modulation of IFN-mediated Ly-6E antigen induction by cAMP in a T cell lymphoma: opposite effects on the responses to IFN-gamma and IFN-alpha/beta

This study was initiated to examine the role of cyclic nucleotides in the regulation of the expression of the Ly-6E cell surface Ag by IFN. As a model system, we used the YAC T cell lymphoma where this Ag is constitutively absent but is highly inducible by both IFN-gamma and IFN-alpha/beta. Treatment with cAMP or cGMP analogs did not cause Ly-6E expression in the absence of IFN. However, treatment with cAMP analogs, but not with cGMP analogs, markedly altered Ly-6E expression triggered by IFN, both at the mRNA and at the cell surface protein levels. Interestingly, these effects depended on whether Ly-6E induction was mediated by IFN-gamma or IFN-alpha/beta. Thus, the response to IFN-gamma was enhanced up to tenfold, whereas the response to IFN-alpha/beta was suppressed by 90-95%. Similar differential modulations of Ly-6E induction were also exerted by forskolin and cholera toxin, which are known to elevate intracellular cAMP concentration through distinct mechanisms. A YAC cell variant (C10) was isolated, where cAMP analogs or cAMP inducers could not modify Ly-6E induction. Although resistant to the biological effect of cAMP, the C10 mutant exhibited normal IFN-mediated Ly-6E responses. Moreover, in this mutant, Ly-6E induction was still affected by the PKC activator PMA, as in wild-type YAC cells. Altogether, our data demonstrate that cAMP (and cGMP) is not directly involved as second messenger in Ly-6E induction mediated by IFNs. However, a rise of cAMP modulates in an opposite fashion the Ly-6E-inducing actions of IFN-gamma and IFN-alpha/beta, which suggests that the two types of IFN utilize separate biochemical pathways to regulate Ly-6E expression.

Analysis of the dominance of mutations in cAMP-binding sites of murine type I cAMP-dependent protein kinase in activation of kinase from heterozygous mutant lymphoma cells

Structural lesions in cAMP-binding sites of regulatory (R) subunit of cAMP-dependent protein kinase caused identical increases in apparent constants for cyclic nucleotide-dependent kinase activation in preparations from cells that were hemizygous or heterozygous for mutant R1 subunit expression. No wild-type kinase activation was observed in extracts from heterozygous mutant cells. This "dominance" was investigated by characterizing expression of wild-type and mutant R1 subunits and properties of protein kinase from S49 mouse lymphoma cell mutants heterozygous for expression of wild-type R1 subunits and R1 subunits with a lesion (Glu200) that inactivates cAMP-binding site A. By both studies of cAMP dissociation and two-dimensional gel analysis, wild-type R subunits comprised about 35% of total R1 subunits in heterozygous mutants. Synthesis of wild-type and mutant R1 subunits was equivalent, but wild-type subunits were degraded preferentially. Hydroxylapatite chromatography revealed a novel R1 subunit-containing species from heterozygous mutant preparations whose elution behavior suggested a trimeric kinase consisting of an R1 subunit dimer and one catalytic (C) subunit. Wild-type R1 subunit was found only in dimer and "trimer" peaks; the tetrameric kinase peak contained only mutant R1 subunit. It is concluded that C subunit binds preferentially to mutant R1 subunit in heterozygous cells forming either tetrameric kinase with mutant R1 subunit homodimers or trimeric kinase with R1 subunit heterodimers. This preferential binding results both in suppression of wild-type kinase activation and differential stabilization of mutant R1 subunits.

Cyclic nucleotide-dependent protein kinases

The actions of several hormones and neurotransmitters evoke signal transduction pathways which rapidly elevate the cytosolic concentrations of the intracellular messengers, cAMP and cGMP. The cyclic-nucleotide dependent protein kinases, cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG), are the major intracellular receptors of cAMP and cGMP. These enzymes become active upon binding respective cyclic nucleotides and modulate a diverse array of biochemical events through the phosphorylation of specific substrate proteins. The focus of this review is to describe the progress made in understanding the structure and function of both PKA and PKG.