Positron emission tomography detection of human endothelial cell and fibroblast monolayers: effect of pretreament and cell density on 18FDG uptake

Abstract

Background

The non-destructive assessment and characterization of tridimensional (3D) cell and tissue constructs in bioreactors represents a challenge in tissue engineering. Medical imaging modalities, which can provide information on the structure and function of internal organs and tissues in living organisms, have the potential of allowing repetitive monitoring of these 3D cultures in vitro. Positron emission tomography (PET) is the most sensitive non-invasive imaging modality, capable of measuring picomolar amounts of radiolabeled molecules. However, since PET imaging protocols have been designed almost exclusively for in vivoinvestigations, suitable methods must be devised to enable imaging of cells or tissue substitutes. As a prior step to imaging 3D cultures, cell radiotracer uptake conditions must first be optimized.

Methods

In this study, human umbilical vein endothelial cells (HUVEC) and human fibroblasts were cultured at different densities and PET was used to non-destructively monitor their glycolytic activity by measuring ¹⁸F-fluorodeoxyglucose ( ¹⁸FDG) uptake. Various cell preconditioning protocols were investigated by adjusting the following parameters to optimize ¹⁸FDG uptake: glucose starvation, insulin stimulation, glucose concentration, ¹⁸FDG incubation time, cell density and radiotracer efflux prevention.

Results

The conditions yielding optimal ¹⁸FDG uptake, and hence best detection sensitivity by PET, were as follows: 2-hour cell preconditioning by glucose deprivation with 1-hour insulin stimulation, followed by 1-hour ¹⁸FDG incubation and 15-minute stabilization in standard culture medium, prior to rinsing and PET scanning.

Conclusions

A step-wise dependence of ¹⁸FDG uptake on glucose concentration was found, but a linear correlation between PET signal and cell density was observed. Detection thresholds of 36 ± 7 and 21 ± 4 cells were estimated for endothelial cells and fibroblasts, respectively.