Investigating the Impact of a Primary Tumor on Metastasis and Dormancy Using MRI: New Insights into the Mechanism of Concomitant Tumor Resistance

Abstract

Dormant cancer cells, also referred to as quiescent, slowly cycling or “nonproliferative” cells, are believed to contribute to tumor recurrence and present a therapeutic problem because they are nonresponsive to current therapies that target proliferating cells. Concomitant tumor resistance (CTR) is the ability of a primary tumor to restrict the growth of secondary metastases. In this paper, we investigate these 2 cancer concepts using cellular magnetic resonance imaging (MRI). A new model for CTR is presented where a primary mammary fat pad tumor is generated using a human breast cancer cell line (231) and breast cancer brain metastases are generated using a cell line derived from 231 to be brain metastatic (231-BR). Iron oxide particles are used to label the 231BR cells to allow for tracking of the proliferating cells, which form metastases, and the nonproliferating cells, which remain dormant in the brain. Bioluminescence and fluorescence-activated cell sorting are used to validate the MRI data. The presence of a primary 231 mammary fat pad tumor inhibited the formation of MRI-detectable 231BR brain metastases. More iron-retaining cells persisted in the brains of mice with a primary tumor. Bioluminescence and fluorescence-activated cell sorting provide evidence that signal voids detectable by MRI on day 0 represent live, iron-labeled cells in the brain. This work shows that retention of iron by nonproliferative cancer cells can be exploited to monitor the fate of this important cell population in vivo, and it points to a new mechanism for CTR, the enhancement of dormancy by a primary tumor.

Safe harbor-targeted CRISPR-Cas9 homology-independent targeted integration for multimodality reporter gene-based cell tracking

“…These noninvasive cell tracking tools are invaluable for understanding mechanisms of disease progression and the evaluation of treatments in preclinical animal models. Important examples in cancer research include the tracking of therapeutic stem cells (9)(10)(11); tracking immune cell migration, cancer progression, and metastasis (12)(13)(14); and evaluating tumor response to novel anticancer therapeutics (15,16). More recently, the use of reporter genes to track therapeutic cells has been translated into the clinic.…”

Comparing the fate of brain metastatic breast cancer cells in different immune compromised mice with cellular magnetic resonance imaging

“…For example, in Parkins et al [39] we investigated this mouse model using luciferase-positive 231BR cells and measured a strong correlation on day 0 between the number of signal voids detected in day 0 brain MR images and the brain signal measured in bioluminescence images, which is only detected from viable cells, providing evidence that signal voids represent live iron-labeled cells. In Hamilton et al [41] we used fluorescence activated cell sorting (FACS) to successfully isolate live 231BR cells that were GFP-positive and labeled with red fluorescent (Flash Red) MPIO from the brains of mice. The GFP+ Flash Red+ cells were collected and expanded in vitro.…”

“…The signal voids which do persist in the brain over time are thought to represent iron-retaining, non-proliferative or dormant cancer cells. These cells have been shown to contribute to tumor recurrence [41]. It appears that both nude and NSG mice are capable of clearing dead cells from the brain.…”