Paracrine IGF/GFBP Interactions In Vivo

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DESCRIPTION (provided by applicant): Insulin-like growth factors (IGFs) regulate cell proliferation, survival and differentiation. These actions are carried out by locally produced IGFs as well as IGFs derived from the circulation. IGF partition between plasma and cellular interstitial compartments is likely accomplished in part by members of a family of high affinity IGF binding proteins. We believe the function of paracrine IGF-I and of the IGFBPs is best explored after tissue-specific perturbations in vivo, since their activity appears to be recruited after injury or other triggers for tissue remodeling. SMC of the vascular wall, intestine and urinary bladder retain the ability to proliferate and to remodel the extracellular matrix when subjected to particular pathophysiological challenges. IGF-I is prominent among them, as it stimulates SMC growth and survival in vitro and in vivo. After balloon endoarterial injury, there is a marked induction of IGF-I gene expression. Moreover, transgenic mice with targeted overexpression of IGF-I in SMC have enhanced neointimal formation through increased proliferation and cell migration after carotid artery injury. Despite this, the requirement for paracrine production of IGF-I as a mediator of injury responses has not been conclusively established, nor is the role of other factors determining local IGF-I bioavailability. In this proposal we will address some of these questions using mouse genetic strategies. We will pursue the following aims: 1) Determine whether lgflr is required for the smooth muscle response to injury: We will first characterize the effects of smooth muscle-specific deletion of the IGF type I receptor (Igflr) on growth, migration and neointimal formation after arterial injury. 2) Determine the requirement of paracrine production of lGF-1 in response to vascular injury: We will explore this question in SMP8-cre-ERT2 mice crossed with homozygous loxP-Igfl mice. Injury experiments will be done after tamoxifen-induced deletion of the IgfI alleles and depletion of local IGF-I tissue stores. 3) Determine the requirement for local IGFBP expression in smooth muscle response to injury: We will first explore this by targeted deletion of lGFBP-4 in smooth muscle cells in a tamoxifen-dependent manner. We will test the hypothesis that SMP8-cre-ERT2/loxP-lgfbp-4 mice will have diminished smooth muscle tissue stores of IGF-I, and that this will be associated with impaired responses to carotid artery injury. 4) Determine the requirement for local expression of the zinc metalloproteinase PAPP-A in smooth muscle responses to injury: Here we will test the hypothesis that after tamoxifen-induced recombination of floxed PAPP-A, IGFBP protease activity will be abolished or dampened, IGFBP-4 stabilized, resulting in IGF-I sequestration and impaired SMC migration and proliferation.