Thyroid hormone dependent pituitary tumor cell growth in serum-free chemically defined culture. A new regulatory role for apotransferrin.

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.