Background: Circadian coordination in mammals is accomplished, in part, by coordinate, rhythmic expression of a series of circadian clock genes in the central clock within the suprachiasmatic nuclei (SCN) of the hypothalamus. These same genes are also rhythmically expressed each day within each peripheral tissue.
Methods: We measured tumor size, tumor cell cyclin E protein, tumor cell mitotic index, and circadian clock gene expression in liver and tumor cells at six equispaced times of day in individual mice of a 12-h light, 12-h dark schedule.
Results: We demonstrate that C3HFeJ/HeB mice with transplanted syngeneic mammary tumor maintain largely normal circadian sleep/activity patterns, and that the rate of tumor growth is highly rhythmic during each day. Two daily 2.5-fold peaks in cancer cell cyclin E protein, a marker of DNA synthesis, are followed by two daily up-to-3-fold peaks in cancer cell mitosis (one minor, and one major peak). These peaks are, in turn, followed by two prominent daily peaks in tumor growth rate occurring during mid-sleep and the second, during mid-activity. These data indicate that all therapeutic targets relevant to tumor growth and tumor cell proliferation are ordered in tumor cells within each day. The daily expression patterns of the circadian clock genes Bmal1, mPer1, and mPer2, remain normally circadian coordinated in the livers of these tumor bearing mice. Bmal1 gene expression remains circadian rhythmic in cancer cells, although damped in amplitude, with a similar circadian pattern to that in normal hepatocytes. However, tumor cell mPer1 and mPer2 gene expression patterns fail to maintain statistically significant daily rhythms.
Conclusion: We conclude that, if core circadian clock gene expression is essential to gate tumor cell proliferation within each day, then there may be substantial redundancy in this timing system. Alternatively, the daily ordering of tumor cell clock gene expression may not be essential to the daily gating of cancer cell DNA synthesis, mitosis and growth. This would indicate that host central SCN-mediated neuro-humoro-behavioral controls and/or daily light-induced changes in melatonin or peripherally-induced rhythms such as those resulting from feeding, may be adequate for the daily coordination of cancer cell expression of proliferation related therapeutic targets.