Counteracting effects of E1a transformation on cAMP growth inhibition

Several signaling molecules have been identified which act as inhibitors of epithelial cell growth. The mechanisms for this negative growth regulation are still poorly understood. In the case of TGF-β, inhibition of keratinocyte cell growth can be totally prevented by transformation with an intact early region 1a (E1a) oncogene. We show here that E1a-transformed keratinocytes become also partially resistant to growth inhibition by elevated 3′,5′-cyclic adenosine monophosphate (cAMP) levels, as induced by treatment with forskolin, dibutyryl-cAMP, 8Br-cAMP, or 8C1-cAMP. Resistance to cAMP is due to interference of E1a with signaling pathways downstream of protein kinase A (PKA) activation, as intracellular cAMP levels and PKA activity were found to be similar in control and E1a-transformed cells. Induction of c-fos expression by 8Br-cAMP occurs at the same time in both cell lines. Interestingly however, this effect is maintained longer in the case of E1a-transformed cells compared to the control. A truncated E1a mutant which is still able to bind to the p105-Rb gene product, p107, and p60/cyclin A, induces cAMP resistance at levels which are only slightly lower than those induced by an intact E1a oncogene. In contrast, an E1a mutant which binds only to a p300 cellular protein and induces a substantial level of TGF-β resistance fails to induce cAMP resistance. Thus, E1a transformation counteracts the growth-inhibitory effects of cAMP as well as TGF-β, but to a different degree and through an only partially overlapping mechanism.