Thyroid hormone dependent GH1 rat
pituitary tumor cell growth in serum-free chemically defined medium required a serum-derived mediator (i.e.,
thyromedin) which was identified as
transferrin [Sirbasku, D.A., Stewart, B.H., Pakala, R., Eby, J.E., Sato, H., & Roscoe, J.M. (1990) Biochemistry 30, 295-304]. The
transferrin isolated was consistent with the equine R or D variants and was biologically active only as
apotransferrin (apoTf). To determine if other variants of horse
transferrin also were thyromedins, a purification was developed which yielded seven separate forms. Initially, only four of these had activity when assayed in standard "
iron salts containing" medium (ED50 values of 290-1160 nM). To further assess activity, the
iron contents of all seven were altered either by saturation with
ferric ammonium citrate or by
citrate/
acid depletion of the
metal ion. Thereafter, potencies were compared in "
iron salts containing" and "
iron salts reduced" media. All seven variants proved to be active as apoTf. Bioassays in which apoTf was maximized showed ED50 values of 2.1-3.8 nM. Conversely, assays in which thyromedins were converted to Tf.2Fe showed no activity. Previously, the only known physiological function of apoTf was that of a carrier/detoxifier of
iron; this study indicates a new role in
hormone-dependent pituitary cell growth.