Cell-Based Sensing: From 2D to 3D Cell Culture
Authors: Yang Liju , Trivedi Goral , Adcock Audrey F. , Tyson William 
Published: February 2020

Cell-based biosensors use living cells or tissues as sensing element to monitor physiological and functional changes induced by external stimuli. They have become an important pillar of drug discovery process, to provide a simple, fast and cost-effective tool to avoid large-scale and cost-intensive animal testing. The sensing element---cultured cells, is the most critical part of a cell-based biosensor. To date, almost all cell-based biosensors use traditional 2 dimensional (2D) monolayer cells cultured on flat and rigid substrate as the sensing element. Although the time-honored 2D cell culture has proven to be a valuable method for cell-based studies, its limitations have been increasingly recognized. In in vivo environment, almost all cells are surrounded by other cells and extracellular matrix (ECM) in a 3D fashion. As a result, 2D cell culture tests sometimes give unsatisfactorily misleading and non-predictive data for in vivo responses. On the other hand, 3D cell culture provides a more physiologically relevant environment for cells and allows the study of cellular responses in a setting that resembles in vivo environments. The 3D structure not only influences the spatial organization of the cell surface receptors engaged in interactions with surrounding cells, but also induce the physical constraint to cells. These spatial and physical aspects in 3D affect the signal transduction from the outside to the inside of cells, and ultimately influence on gene expression and cellular behaviors. Compared to 2D cell culture, 3D culture replicates more accurately the actual microenvironment where cells reside in tissue and therefore the behavior of cells in 3D culture reflects closely the in vivo responses.
This study focused on the adoption of 3D cell cultures to cell-based biosensors, aiming to provide more in vivo-like experimental results for drug discovery. In this study, we established the conditions for growing cancer cells into 3D spheroids on Matrigel for two different cancer cell lines: oral cancer cell line CAL 27 and prostate cancer cell line DU 145. We systematically investigated the cellular responses to different anticancer drugs in 3D culture in comparison to those of 2D culture, within the same cell line and between different cell lines. We also examined the difference in expressions of drug-action related factors in 3D and 2D cultures. The results showed that the cell proliferation rate in 3D culture on Matrigel in comparison to 2D culture was cell line dependent, as we observed the proliferation of CAL27 cells was enhanced in 3D whereas the proliferation DU 145 cells we reduced in 3D. Sensitivity of cellular responses to drugs in 3D relative to 2D was drug-action and cell line dependent. For DU 145 cells, 3D cell culture model was more resistant to Docetaxel, but less resistant to Rapamycin as compared to 2D cell culture model. And surprisingly, for CAL 27 cells, 3D cell culture model was less resistant to both bleomycin and erlotinib as compared to 2D cell culture. Further examination of drug-action related factors indicated that the cellular responses of 3D culture were correlated with the expression of drug action related biomarkers as compared to 2D cell culture.

Acknowledgement: The research is supported by NSF (CBET #1159871)