The insulin-like factors (IGFs) have dramatic and complex effects on the stimulation of growth and differentiation of many tissues, including skeletal muscle. The goals of this project are 1) to investigate the effect of constitutive IGF gene expression in skeletal muscle cells on the regulation of a representative muscle-specific gene (Myosin Light Chain 1/3), and 2) to exploit this information to develop a potential therapy for diabetes-related disorders by controlled expression of IGFs in re-implanted myoblasts. Recent evidence from several laboratories, including our own, suggests an important role for both IGF I and II in myogenic differentiation. By introducing IGF expression vectors, under the control of constitutive regulatory elements, into myoblasts under growth or differentiation-promoting culture conditions, we will assess their potential to affect the myogenic phenotype, by their ability to modulate the expression of a co-transfected reporter gene driven by transcriptional regulatory elements from the MLC locus. These elements have already been well defined in my laboratory as targets for the MyoD protein family, and other less well defined muscle regulatory factors. From these studies we will determine the feasibility of our second aim, in which myoblasts carrying IGF expression vectors under the control of selected MLC regulatory elements will be generated in culture and implanted into diabetic mouse skeletal muscle beds. The proven potential of injected myoblasts to persist indefinitely by fusing to existing non-proliferating myocytes is based on three considerations: 1) the normal regenerative properties of muscle tissues, 2) the ability of differentiated muscle cells to synthesize, process and secrete IGFs, and 3) observations that IGFs stimulate glucose transport through their own receptors in human muscle preparations, and increase glucose disposal in insulin-resistant patients. These properties make skeletal muscle an ideal target tissue for gene therapy of diabetes-related pathologies. The permanent introduction of IGF genes, under the appropriate muscle-specific regulation, directly into intact skeletal muscle tissue may ultimately serve as a prototype for the myoblast-mediated delivery of secreted gene products as a potential treatment of genetic diseases in general.
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