Mammalian cells can be classified into two types based upon whether or not they show growth response to ethanolamine (Etn) in culture. The content of phosphatidylethanolamine (PE) in phospholipid and incorporation of radioactive Etn into the cells were examined in the Etn-responsive and -nonrespponsive cells in order to elucidate the mechanisms of growth stimulation by Etn. In all Etn-responsive cells tested, 5 pM Etn significantly altered the composition of cellular phospholipid compared to that grown without Etn, while Etn-nonresponsive cells had a similar phospholipid composition whether the growth medium contained Etn or not. Using two rat mammary carcinoma cell lines, 64-24 (responsive type) and 22-1 (nonresponsive type), further studies were carried out. In 64-24 cells there was a proportional increase in PE content as the dosage of Etn in the medium was increased. The increase in PE content leveled off at 10 pM. Further, the increase in PE content was correlated with increased rate of growth. In contrast, PE content or growth rate did not change at all in 22-1 cells. In 64-24 cells radioactive Etn (0.1-50 pM) was incorporated four-to five-fold more efficiently into phospholipid, and the aqueous pool of precursors of PE was ten times less as compared to 22-1 cells, indicating that Etn-responsive cells utilize Etn supplied in the medium to synthesize PE far more efficiently than Etn-nonresponsive cells. De novo synthesis of PE must not be sufficient to support optimum growth in Etnresponsive cells.
Ethanolamine (Etn) is a structural component of phosphatidylethanolamine (PE), which is the second most abundant phospholipid in mammalian cell membranes. However, not until recently has the importance of Etn in growth of mammalian cells in culture been recognized. Traditionally, culture medium contained the phospholipid polar head groups, choline, inositol, and serine, but not Etn, indicating that in ordinary culture conditions the cells did not require Etn to grow.
We first discovered that a rat mammary carcinoma 64-24 cell line required phosphoethanolamine (PEtn) or Etn to grow in culture, while another rat mammary carcinoma cell line, 22-1, did not (Kano-Sueoka et al., 1979; Kano-Sueoka and Errick, 1981). Among several mammary carcinoma lines of rat and human we tested, a human breast carcinoma T-47D cell line also was found to be growth responsive to Etn (Kano-Sueoka and Errick, 1981). Further, in primary cultures of normal rat mammary epithelial cells, thymidine incorporation was significantly stimulated by PEtn, indicating that normal mammary epithelial cells are growth responsive to Etn or PEtn (Kano-Sueoka and Errick, 1980). Examination of various cell types other than mammary cells revealed that rat hepatoma cells, fibroblast cells of hamster, mouse, and rat, rat neuronal and glial cells were nonresponsive to Etn, but that a mouse hybridoma line produced by mouse plasmacytoma and mouse splenocyte was growth responsive to Etn (Kano-Sueoka and Errick, 1982). Indeed, all the hybridoma lines of mouse and rat