ABSTRACT: Age plays a major role in tumor incidence and is an important consideration when modeling the carcinogenesis process or estimating cancer risks. Epidemiological data show that from adolescence through middle age, cancer incidence increases with age. This effect is commonly attributed to a lifetime accumulation of cellular, particularly DNA, damage. However, during middle-age, the incidence begins to decelerate and, for many tumor sites, it actually decreases at sufficiently advanced ages. We investigated if the observed deceleration and potential decrease in incidence could be attributed to a decreased capacity of older hosts to support tumor progression, and whether HZE (high atomic number (Z), high energy (E)) radiation differentially modulates tumor progression in young versus middle-age hosts, issues relevant to estimating carcinogenesis risk for astronauts. Lewis lung carcinoma (LLC) cells were injected into syngeneic mice (143 and 551 days old), which were then subject to whole-body 56Fe irradiation (1GeV/amu). Three findings emerged: 1) among unirradiated animals, substantial inhibition of tumor progression and significantly decreased tumor growth rates were seen for middle-aged mice compared to young mice; 2) whole-body 56Fe irradiation (1GeV/amu) inhibited tumor progression in both young and in middle-aged mice (with greater suppression seen in case of young animals), with little effect on tumor growth rates; and 3) 56Fe irradiation (1GeV/amu) suppressed tumor progression in young mice, to a degree not significantly different than transiting from young to middle-aged. Thus, 56Fe irradiation (1GeV/amu) acted similar to aging with respect to tumor progression. We further investigated the molecular underpinnings driving the radiation modulation of tumor dynamics in young and middle-aged mice. Through global gene expression analysis, the key players, FASN, AKT1, and the CXCL12/CXCR4 complex, were determined to be contributory. In sum, these findings demonstrate a reduced capacity of middle-aged hosts to support the progression phase of carcinogenesis and identify molecular factors contributory to HZE radiation modulation of tumor progression as a function of age. For genome-wide expression profiling of tumor tissue, Mouse WG-6 BeadArray chips (Illumina, San Diego, CA) were used. Total RNA was amplified with the Ambion Illumina TotalPrep Amplification Kit (Ambion, Austin, TX) and labeled from all replicate biological samples for each condition. The number of tumor sample replicates used from each condition is as follows: 10 samples from young unirradiated mice, 8 samples from young irradiated mice, 7 samples from middle-aged unirradiated mice, 5 samples from middle-aged irradiated mice. Total RNA was isolated and purified using Trizol (Invitrogen) or RNeasy (Qiagen), quantified and qualified using Agilent Bioanalyzer (Agilent) and samples were deemed suitable for amplification and hybridization if they had O.D. 260/280 = 1.7 - 2.1, 28s/18s = 2:1, RIN (RNA integrity number) >7. Total RNA of 500ng per sample was amplified using Ambion TotalPrep (Ambion), and 1.5µg of the product was loaded onto the chips. Following hybridization at 55C, the chips were washed and then scanned using the Illumina iScan (Illumina), and the data were analyzed using GenomeStudio (Illumina). Data were first analyzed for gene expression and then culled for present genes (genes that meet the criteria of detection p-value < 0.05). Expression above background was included in an expressed genes working data set for further analyses. Rank variant normalization was applied to the data before extensive analysis. Differential gene expression analysis was used to compare to the reference group, young unirradiated mice, and genes were then evaluated and validated.
