Polyamines and cellular regulation: perspectives.

The polyamines are low molecular mass aliphatic amines that can be synthesized by both prokaryotic and eukaryotic cells. The most common are putrescine, spermidhe and spermine (Fig. I ). The intracellular concentrations of these polyamines vary greatly with the proliferative status of the cell, and also in response to a wide variety of stimuli [ 1-31, Induction of polyamine biosynthesis, a delicately regulated process, is a very early event in cell proliferation. IntracelMar polyamine levels usually increase before changes in DNA, RNA or protein can be detected, and depletion of intracellular polyamines, by inhibition of their biosynthesis or by precursor starvation, results in a cessation of growth, which can be restored by addition of exogenous polyamines to the treated cells. The regulatory effects of polyamines on gene expression, in particular on growth-related genes [4], is well documented, as is their role in cellular differentiation [5, 61. Similarly, there is a growing body of literature showing that polyamines increase not only the rates but also the fidelity of transcription and translation [7-91. Newer areas of interest that will be discussed here are the polyamine-dependent protein kinases; polyamine transport; the ability of polyamines to modulate intracellular free calcium, and the altered polyamine oxidase activity and/or levels of acetylated polyamines in tumour cells.