In vivo flow cytometry reveals a circadian rhythm of circulating tumor cells

Xi Zhu,Fuli Zhang,Hao He,Chengying Xie,Kai Pang,Meilu Tian,Yuanzhen Suo,Nan Ding,Xunbin Wei,Yuting Fu,Xiaofu Weng

doi:10.1038/s41377-021-00542-5

Abstract

Circulating tumor cells (CTCs) is an established biomarker of cancer metastasis. The circulation dynamics of CTCs are important for understanding the mechanisms underlying tumor cell dissemination. Although studies have revealed that the circadian rhythm may disrupt the growth of tumors, it is generally unclear whether the circadian rhythm controls the release of CTCs. In clinical examinations, the current in vitro methods for detecting CTCs in blood samples are based on a fundamental assumption that CTC counts in the peripheral blood do not change significantly over time, which is being challenged by recent studies. Since it is not practical to draw blood from patients repeatedly, a feasible strategy to investigate the circadian rhythm of CTCs is to monitor them by in vivo detection methods. Fluorescence in vivo flow cytometry (IVFC) is a powerful optical technique that is able to detect fluorescent circulating cells directly in living animals in a noninvasive manner over a long period of time. In this study, we applied fluorescence IVFC to monitor CTCs noninvasively in an orthotopic mouse model of human prostate cancer. We observed that CTCs exhibited stochastic bursts over cancer progression. The probability of the bursting activity was higher at early stages than at late stages. We longitudinally monitored CTCs over a 24-h period, and our results revealed striking daily oscillations in CTC counts that peaked at the onset of the night (active phase for rodents), suggesting that the release of CTCs might be regulated by the circadian rhythm.

Highlights

Circulating tumor cells (CTCs) have aroused wide attention as indicators of metastasis[1,2,3]
CTCs monitored by fluorescence in vivo flow cytometry (IVFC) were not distributed homogeneously in the circulation To explore the temporal distribution of CTCs in the circulation, we established a fluorescence in vivo flow cytometer (Fig. 1a) and an orthotopic nude mouse model of human prostate cancer (Fig. 1b)
When a GFPexpressing CTC passed through the focused laser sheet across an artery, its fluorescence was excited and collected by a photomultiplier tube (PMT)[22,26]

Summary

Introduction

Circulating tumor cells (CTCs) have aroused wide attention as indicators of metastasis[1,2,3]. CTCs are very rare cells that are shed by solid tumors and circulate in the vasculature. Analyses of CTCs have provided insights into the mechanism of metastasis, thereby facilitating the development of early diagnosis, prognosis evaluation, and antimetastatic therapeutic strategies[4,5]. The temporal distribution and circulation dynamics of CTCs are important for understanding the mechanisms underlying tumor cell dissemination. Studies have revealed that the circadian rhythm may disrupt the growth of tumors[6,7,8], it is generally unclear whether the circadian rhythm controls the trafficking of CTCs. The circadian rhythm is the internal molecular clock that oscillates with a periodicity of 24 h under the entrainment of external or environmental cues

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Conclusion