Control of cell division: a unifying hypothesis.

A constant feature of the initiation of cell division in a number of different cells is a rise in the intracellular level of calcium. The importance of cyclic nucleotides may depend on the way they interact with calcium. Cyclic AMP is apparently not an essential regulator of cell division but through its ability to modulate the intracellular level of calcium this cyclic nucleotide can exert profound effects on cell growth. In some systems (liver and salivary glands) cyclic AMP seems to augment the calcium signal whereas in others (lymphocytes and fibroblasts) it opposes calcium and can thus inhibit cell division. A rise in the level of calcium may be responsible for the parallel increase in cyclic GMP level which is usually associated with the stimulus to divide. An appealing feature of this calcium hypothesis is that it can account for the growth characteristics revealed by fibroblasts in tissue culture or embryonic cells during development. In both cases there is an initial phase of exponential growth during which I have proposed that the high level of calcium at mitosis persists into early G1 to provide the signal for the next division. In order to account for the sudden cessation of cell division at confluency, or at a specific stage during development, it is necessary to postulate that there is something different about the final mitosis which sets it apart from earlier mitoses. It is proposed that as the cells leave the last mitosis the level of calcium falls much more rapidly than it did during preceeding mitoses perhaps as a result of a more rapid rise in the level of cyclic AMP. This rapid rise in cyclic AMP level may have a dual function. Not only will it lower the level of calcium thus preventing further division, but it may also stimulate differentiation. Many of the embryonic cells which differentiate into specialized cells (lymphocytes, liver, salivary gland) retain the ability to divide if provided with appropriate stimuli. Although the nature of these stimuli vary considerably, they all seem to act by elevating the intracellular level of calcium.