“Temporal control of tumor growth in nocturnal mammals: impact of the circadian system”

Abstract

4Glioblastoma multiforme is the most aggressive brain tumor; however, little is known about the impact of the 5 circadian system on the tumor formation, growth and treatment. We investigated day/night differences in tumor 6 growth after injecting A530 glioma cells isolated from malignant peripheral nerve sheath tumor of NPcis 7 (Trp53 +/-; Nf1 +/-) mice. Tumors generated in the sciatic nerve zone of C57BL/6 mice injected early at night in 8 the light/dark cycle or in constant darkness, showed higher growth rates than in animals injected diurnally. 9Similar nocturnal increases were observed when injecting B16 melanoma cells or when 10 mice received knocked-down clock gene Bmal1 cells. Moreover, treatment with a low-dose of the proteasome 11 inhibitor Bortezomib (0.5 mg/kg) in tumor-bearing animals, displayed higher efficacy when administered at 12 night. Results suggest the existence of a precise temporal control of tumor growth and of drug efficacy in which 13 the host state and susceptibility are critical. 14 15At the molecular level, the circadian clock operates through a transcriptional/translational feedback loop that 1 involves a group of regulatory transcription factors -clock genes and proteins-including Clock, Bmal1, NPAS2, 2 Periods (Per) and Cryptochromes (Cry), among others [5,6]. Once translated, these proteins display activity as 3 activators (CLOCK, BMAL1, NPAS2) or repressors (PER, CRY) of gene transcription and are capable of 4 driving clock gene rhythms and rhythmic clock-controlled gene expression under a 24 h cycle through a 5 consensus sequence E-box at the promoters of target genes [5]. A secondary cycle involves the nuclear receptor 6 REV-ERBα/β (REV): when expression levels of this receptor are high, it binds to RORE sites present in 7 promoter and enhancer regions of the Clock and Bmal1 genes, repressing its transcription. In contrast, when the 8 REV-ERBs levels are low, another nuclear receptor of retinoic acid, ROR, binds to the RORE site of promoters 9 and may activate Clock and Bmal1 transcription [7,8]. Per1 and 2 are involved in the synchronization of the 10 molecular clock after stimulation by early response transcription factors whose activity is controlled by 11 extracellular signals such as hormones, second messengers, temperature or neurotransmitters. Among these 12 transcription factors are cAMP response elements and protein-CRE binding (CREB) elements, HSF1 that binds 13 to heat shock response elements (HSEs), serum response factors (SRF) that bind to response elements serum 14 (SREs) and glucocorticoid receptors (GR) that bind to glucocorticoid response elements (GREs) [9]. At the 15 cellular level, a redox / metabolic clock has been reported to drive peroxiredoxin (PRX) oxidation cycles and 16shown to be highly conserved through evolution. These redox oscillators are present in all kingdoms of life and 17 even when they interact with the transcriptional molecular clock they can still function in the absence of 18 transcription, as occurs in enucleated cells [10]. 19Through t...