The long range goal of this research program is to understand the molecular basis of thyroid hormone action. This application addresses the specific question of how the thyroid hormones promote cell growth. Thyroid hormone is an absolute requirement for the growth of GH4C1 rat pituitary cells if the cells are plated at low density inn medium depleted of thyroid hormones. We have shown that thyroid hormones promote GH4C1 cell division by stimulating the cells to secrete an autocrine growth factor. This growth factor totally replaces the requirement for thyroid hormone. We have also isolated a variant line of cells that is able to grow normally without any added thyroid hormone but has otherwise normal thyroid hormone responsivity. We plan to purify and characterize the actions of the thyroid hormone induced growth factor secreted by pituitary cells. We will first optimize the conditions for maximal secretion of the activity. We will then carry out basic biochemical characterization of the growth factor or factors, which will give an indication of molecular size and structure. The growth factor will be purified by conventional biochemical techniques using either growth promotion or stimulated incorporation of radioactive thymidine or uptake of radioactive amino acids as a bioassay during purification. In parallel, we will attempt to radioactively label the growth factor and identify candidate peptides. This will be facilitated by using the mutant able to grow in the absence of thyroid hormones; this line should secrete growth factor with or without thyroid hormones, whereas the parental line should secrete growth factor only in the presence of the hormone. If a putative growth factor is identified it will be used for the generation of antibodies. Purified thyroid hormone induced growth factor will be used to study the mechanism of action in the GH4C1 cell line. We will determine whether the growth factor has an intrinsic thyromimetic activities, beyond growth stimulation, and study the cellular changes following addition of the growth factor to quiescent cells.