Apotransferrin stimulation of thyroid hormone dependent rat pituitary tumor cell growth in serum-free chemically defined medium: role of FE(III) chelation.

Triiodothyronine (T3) dependent growth of GH1 rat pituitary tumor cells in serum-free defined culture requires apotransferrin (apoTf) (Sirbasku et al.: Mol. Cell. Endocrinol., 77:C47-C55, 1991). Diferric transferrin (2Fe.Tf) also is necessary as an iron source (Eby et al.: Anal. Biochem., 203:317-325, 1992). Further, T3 dependence is prevented by soluble Fe(III) addition to the medium (Sato et al.: In Vitro Cell. Dev. Biol., 27A:599-602, 1991). While our data suggested that apoTf caused growth by chelation of Fe(III), direct evidence was required. We used urea polyacrylamide gel electrophoresis along with autoradiography and Western immunoblotting to measure the Fe(III) content of growing GH1 cell cultures and identify the apoTf, mono-metal transferrins and 2Fe.Tf present. We found that apoTf per se did not cause growth but instead chelated inhibitory levels of Fe(III). In fact, apoTf need not be present at all provided that Fe(III) is reduced to < or = 0.6 microM. In addition, other protein and non-protein Fe(III) chelators were shown to be as effective as apoTf. Here, we report that pituitary cells are completely inhibited by > or = 1.2 microM Fe(III), which are concentrations which might be expected in many culture media and usually are not thought to influence growth. The high sensitivity of pituitary cells to Fe(III) suggests further study to determine what cellular functions are affected and how they interfere with thyroid hormone dependence.