Cellular Transfection of GH Receptor cDNA to Study GH Action

The many cellular actions of growth hormone (GH) are caused by GH binding to a membrane bound GH receptor. Although much still remains to be learned, progress is now being made towards a deeper understanding of the mechanism of GH action. In vitro systems are essential tools for studying signal transduction by hormones and different GH responsive cell systems such as primary liver cells (Toilet et al. 1991) and preadipocytes (Catalioto et al. 1990) have been described. An alternative approach to study GH function in vitro is to transfect GH receptor cDNA and thereby create cells expressing GH receptors. The strategy of transfecting cells with receptor cDNA allows studies of GH induced changes in cellular signal pathways and in addition, mutated forms of the GH receptor can be introduced into cells to identify receptor domains critical for function. The need for this type of model system to study GH action is further motivated by the fact that several cell lines lose their GH receptors under in vitro conditions. For example, the liver is a major GH responsive tissue where GH receptors are abundant; however, in many hepatoma cell lines we have studied the receptor levels are low. This is probably because cellular de-differentiation is associated with a loss of GH receptors and in agreement with this, fetal and transformed livers have low receptor levels (Mathews et al. 1989 Levinovitz et al. 1990). Below, the approach of cellular transfection of GH receptor cDNA will be discussed.