Chemoprevention with protein kinase A RIalpha antisense in DMBA-mammary carcinogenesis.

Cancer is potentially preventable disease. A surprising variety of intracellular pathways can be a target for chemoprevention. Earlier it was discovered that cAMP-mediated system can play important role in prevention of DMBA-mammary carcinogenesis. There are two types of cAMP-dependent protein kinases (PKA), type I (PKA-I) and type II (PKA-II), which share a common catalytic (C) subunits, but contain distinct regulatory (R) ones, RI versus RII, respectively. Evidence suggests that increased expression of PKA-I and its regulatory subunit (RIalpha) correlates with tumorogenesis and tumor growth. It was found that downregulation of RIalpha by 21-mer antisense oligonucleotide led to growth arrest of cancer cells. The effect of RIalpha antisense oligonucleotide correlated with a decrease in RIalpha protein and a concomitant increase in RIIbeta protein level. It was shown that antisense RIalpha can protect in a sequence-specific manner from 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinogenesis. At 90 days after DMBA intubation, RIalpha-antisense-treated rats exhibited significantly lower number of tumors per rat, than untreated control animals. The antisense also delayed the first tumor appearance. An increase in RIalpha and PKA-I levels in the mammary gland and liver preceded tumor production, and antisense downregulation of RIalpha restored normal levels of PKA-I and PKA-II in these tissues. Antisense RIalpha in the liver induced the phase II enzymes, glutathione S-transferase and quinone oxidoreductase, c-fos protein, and activator protein-1 (AP-1)- and cAMP response element (CRE)-directed transcription. In the mammary gland, antisense RIalpha promoted DNA repair processes. In contrast, the CRE transcription-factor decoy could not mimic these effects of antisense RIalpha. The results demonstrate that RIalpha antisense produces dual anticarcinogenic effects : (a) increasing DMBA detoxification in the liver by increasing phase II enzyme activities, increasing CRE-binding-protein phosphorylation and enhancing CRE- and AP-1 directed transcription; and (b) activating DNA repair processes in the mammary gland by downregulating of PKA-1.