Effects of cystine or glutamine restriction on human diploid fibroblasts in culture.

Amino-acid restriction was studied using a strain of human diploid fibroblasts (HDF) isolated from fetal skin. HDF populations were maintained at reduced proliferation rates by lowering cystine or glutamine concentrations of Eagle's MEM. Effects of this treatment on mean cell diameters, total cell protein and specific activity of acid phosphatase were assessed. Threshold levels of cystine required for maintenance were found to be markedly influenced by type and/or batch of serum used irrespective of their free cystine content. A 96-hr exposure of passage 25 cells to medium 0.01 or 0.005 mM in cystine resulted in an increased mean cell diameter and a greater heterogeneity of cell size. This was accompanied by an increase in the total cellular protein. A 48-hr exposure, followed by return to control serial cultivation, induced persistent change in mean cell diameter without affecting the total life span (49 passages over 165 days). The mean size of untreated cells increased with serial passage reaching a size similar to treated cells only at passage 44. Acid-phosphatase specific activities were higher in cells maintained under conditions restrictive in cystine. None of the above changes were observed when glutamine concentrations were reduced to 0.0025 mM, a level required to inhibit division rates. The possible relevance of these findings to the phenomenon of in vitro cell senescence is discussed.