Effects of adenosine and 2'-deoxyadenosine on epidermal keratinocyte proliferation: its relation to cyclic AMP formation.

Although it has been reported that adenosine has an inhibitory effect on keratinocyte proliferation at both G2 and S phases of the cell cycle, its relation to cyclic AMP formation through the adenylate cyclase system has been less well characterized. In order to determine the precise mechanism of the adenosine effect, another physiologic adenine nucleoside, 2'-deoxyadenosine was employed. 2'-Deoxyadenosine was shown to be remarkably different from adenosine in its ability to stimulate the epidermal adenylate cyclase; whereas adenosine markedly increased cyclic AMP levels of pig epidermis, deoxyadenosine had a much weaker effect on the cyclic AMP levels of the skin. Using several parameters of cell proliferation, comparison was made between the effects of these two compounds. Pig keratinocyte explant culture system was employed for the measurement of outgrowth and mitosis. Mitosis was determined after 72-h incubation (to monitor the overall cell proliferation inhibition) and 4-h incubation (to monitor G2 phase inhibition) with the chemicals. Pig skin keratome slice system was employed for [3H]thymidine uptake measurement. Both adenosine and deoxyadenosine were shown to have marked inhibitory effects on keratinocyte out-growth, [3H]thymidine uptake, and keratinocyte mitosis. The effects of deoxyadenosine on outgrowth and [3H]thymidine uptake were greater than that of adenosine. The inhibitory effect of adenosine and deoxyadenosine on mitosis were about the same in both 4-h and 72-h incubation systems. Thus deoxyadenosine, which is a much weaker stimulator of epidermal adenylate cyclase, was also shown to be as potent an inhibitor of keratinocyte proliferation as adenosine. These results further substantiate the view that cyclic AMP elevating agents (such as adenosine and deoxyadenosine) might not necessarily reveal their inhibitory effects on keratinocyte proliferation through their effects of cyclic AMP formation.