Abstract Negative energy balance (calorie restriction, CR) inhibits, while positive energy balance enhances tumor promotion using the two-stage skin carcinogenesis model. Biochemical studies have demonstrated that CR reduced, while diet-induced obesity (DIO) increased insulin-like growth factor 1 receptor (IGF-1R) and epidermal growth factor receptor (EGFR) activation and downstream signaling (i.e., Akt and mTOR) following TPA treatment. Additional studies performed using the liver IGF-1 deficient (LID) mouse model (i.e., 75% reduction in circulating IGF-1) demonstrated reduced susceptibility to epithelial carcinogenesis and attenuated IGF-1R and EGFR signaling during tumor promotion, similar to CR mice. Taken together, these findings suggest that dietary energy balance, primarily through its effects on levels of circulating IGF-1, modulates epithelial carcinogenesis and tumor promotion through diet-induced changes in IGF-1R and EGFR signaling and receptor crosstalk. To determine the impact of IGF-1 on IGF-1R and EGFR signaling and receptor crosstalk, Western blot analyses, immunoprecipiation and qPCR analyses were performed using C50 cells (a nontumorigenic keratinocyte cell line) stimulated with IGF-1. IGF-1 treatment of serum starved cells induced rapid phosphorylation (within 5 minutes) of the IGF-1R, as well as the EGFR and ErbB2. Additional experiments were conducted to evaluate mechanisms underlying the effects of IGF-1 on EGFR activation. Immunoprecipitation experiments demonstrated that IGF-1 induced an association between the IGF-1R and the EGFR. Furthermore, qPCR analysis demonstrated that IGF-1 induced HB-EGF and amphiregulin mRNA expression. Current experiments are exploring the possibility that IGF-1 treatment also leads to increased ectodomain shedding of membrane bound EGFR ligands. Further studies were conducted to examine the impact of caloric consumption on IGF-1R and EGFR crosstalk in vivo. For these experiments, ICR female mice were maintained on either a 30% CR or 60Kcal% fat (DIO) regimen for 15 weeks, and then treated with a single application of either acetone or 3.4 nmol TPA. CR reduced, while DIO increased the interaction between the IGF-1R and the EGFR in the epidermis of TPA treated mice. Furthermore, CR reduced, while DIO increased mRNA expression of EGFR ligands both in the presence and absence of TPA treatment. Additional experiments are in progress to confirm that IGF-1 levels, per se, are directly responsible for these diet-induced changes in IGF-1R/EGFR receptor crosstalk. Collectively, the current data suggest that IGF-1 levels and the activation status of the IGF-1R modulate IGF-1R and EGFR receptor crosstalk. Furthermore, diet-induced changes in IGF-1R/EGFR receptor crosstalk subsequently modulate downstream signaling to Akt and mTOR, thus contributing, at least in part, to the effect of dietary energy balance on skin tumor promotion. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 815. doi:10.1158/1538-7445.AM2011-815