Growth regulation of cells grown in suspension culture

Abstract

VARIOUS aspects of a cell's growth cycle can be manipulated in tissue culture. For example, the initiation of DNA synthesis and, in some cases, culture growth, can be enhanced by serum¹ and platelet² components, polypeptides purified from growth-conditioned medium³ and urine⁴, insulin⁵, corticosteroids⁶, prostaglandins⁷, cyclic nucleotides (see, for example, ref. 8) and proteolytic enzymes⁹. In addition, at least three purified proteins can initiate DNA synthesis in fibroblast-like cells—nerve growth factor¹⁰, epidermal growth factor¹¹ and fibroblast growth factor^12,13. Disturbance of the culture medium directly over the cultured cells also induces DNA synthesis¹⁴. Fibroblast-like cells have been used in most of these studies, and they grow tightly associated with the culture dish and are strongly anchorage dependent for division^15,16. Given the strong anchorage dependence of the test cells, two alternatives could account for the induction of DNA synthesis and growth by the diversity of conditions and “factors” outlined above. The first would ascribe the effects of these compounds to interactions on the cell surface with a suitable receptor. These are hormone-like interactions, thought to be exemplified by compounds such as insulin and purified growth factors¹. The second alternative represents interactions which function directly through alterations of cell anchorage. For example, the growth stimulating effects of the proteases are presumably affected by modification of the cell–substratum (anchorage) interaction¹⁷. To distinguish between these two alternatives, it should be sufficient to assay the effects of serum and a purified growth factor on the initiation of DNA synthesis in an anchorage-dependent cell line, and on a variant which grows in suspension culture. If serum and the growth factor were equally effective in both cell lines, then a disturbance in anchorage could not account for the mitogenic activity, ruling out the second alternative. The first, hormone-like, mechanism, however, would be excluded if the growth factor was unable to induce DNA synthesis in the cells grown in suspension. The following experiments support this latter possibility by showing that fibroblast growth factor^12,13 does not affect the growth of myoblast cells grown in suspension culture.