Weight control via dietary calorie restriction (DCR) has a profound inhibitory effect on carcinogenesis in animal models. Previous studies conducted in a two-stage mouse skin carcinogenic model have demonstrated a common hypothesis that DCR inhibits carcinogenesis by suppressing tumor promoter phorbol ester-induced AP-1 transcriptional activation via blocking PKC-Raf-ERK signal transduction pathways. A novel hypothesis to be tested in this grant proposal is that DCR lowers circulating metabolic hormone levels (e.g. IGF-1, insulin, and leptin) and thereby abrogates hormone-dependent Ras-Raf-ERK-AP-1 signaling which contributes to the inhibition of carcinogenesis. This new hypothesis will also be tested in an alternative weight control approach to DCR by increasing energy expenditure via treadmill exercise. We and others have demonstrated that the plasma levels of IGF-1, insulin, and leptin are significantly reduced in IDCR mice. We recently found a similar reduction of these hormones in treadmill-exercised mice that pair-fed at the same amount as their ad libitum-fed sedentary counterpart. These observations, coupled with our preliminary data that an inhibition of activated Ras signaling down to ERK and AP-1 inactivation in the epidermis of DCR mice was attenuated by IGF-1 implantation, indicate a requirement of IGF-1 reduction for the tumor prevention by DCR. Considering that AP-1 transcriptional activation is critical for mouse skin tumor promotion and all the three hormones are able to activate Ras-ERK-AP-1 signaling, we anticipate that the reduction of the three hormones observed in weight control mice may thus contribute individually or interactively to tumor prevention. The themes of the three specific aims are as follows:
Aim 1 will determine whether DCR and/or exercise target the Ras-ERK-AP-1 signaling pathways;
Aim 2 will test whether the reduction of circulating levels of IGF-1 alone or in combination with insulin and leptin contributes towards the blockage of the Ras-ERK-AP-1 signaling induced by DCR and/or exercise;
and Aim 3 will test whether the restoration of reduced hormone levels in DCR and exercised mice abrogates the inhibition of skin carcinogenesis. The results will provide a mechanistic understanding of DCR and exercise on cancer prevention and facilitate the identification of the targeted pathways by weight control via decreasing calorie intake or increasing physical activity.
