Cryptochrome Function beyond the Clock: Cryptochromes Influence Cancer Growth

Abstract

Cancer has been studied for many centuries, yet only relatively has recently cancer been studied in the context of circadian rhythms. Circadian rhythms entrain many physiological and behavioral processes to the daily solar cycle. Main conclusions from research regarding cancer and circadian rhythms are that the cell and circadian cycles can dynamically interact on transcriptional and post-translational levels. The mammalian circadian rhythm is generated in part by four core clock proteins through a transcriptional feedback loop consisting of the activating transcription factors CLOCK and BMAL1 and their transcriptional repressors cryptochrome (CRY1-2) and period (PER 1-3). CRY1 and CRY2 can function transcriptionally outside of circadian rhythm generation as seen by genome-wide analysis of DNA binding of the core clock proteins and can also act as transcriptional repressors of a wide number of nuclear hormone receptors.

One mechanism to reset the molecular clock is through protein degradation. Of note, CRY1 and CRY2 are degraded by the SKP-CULLIN-FBOX (SCF) family E3 ligase complex containing the F-box and leucine rich repeat 3 (FBXL3) substrate adaptor (SCF^FBXL3). Here, we examine whether CRY1 and/or CRY2 can function outside the circadian clock through participating in proteolysis of proteins involved in cancer, through acting as an adapter for the SCF^FBXL3 complex, such as c-MYC, TLK2, and E2F family transcription factors. We find that CRY2 or both CRYs, respectively, can specifically act as an adapter for SCF^FBXL3 stimulated degradation of c-MYC and TLK2, and CRY1 and CRY2 can regulate protein stability of a subset of E2F proteins. These findings provide insight into how proteins that generate circadian rhythms can affect protein stability and therefore function in cancer.

Additionally, we investigated whether CRY2 mutations found in human cancers could correlate to aberrant cell growth and disrupted circadian rhythms to fuel cancer progression, as some studies have shown that disrupted circadian rhythms can alter cancer progression. Our data suggests that two CRY2 mutants (D347H/D325H and S532L/S510L) influence cell growth via suppression of the P53 pathway, and D347H/D325H and D467N/D445N cannot restore circadian rhythms in vitro. Overall, this suggests that CRYs, and by extension circadian rhythms, can play an important role in regulating cell growth.