Polyacrylamide gel substrates that simulate the mechanical stiffness of normal and malignant neuronal tissues increase protoporphyin IX synthesis in glioma cells

Abstract

Protoporphyrin IX (PPIX) produced following the administration of exogenous 5d-aminolevulinic acid is clinically approved for photodynamic therapy and fluorescence-guided resection in various jurisdictions around the world. For both applications, quantification of PPIX forms the basis for accurate therapeutic dose calculation and identification of malignant tissues for resection. While it is well established that the PPIX synthesis and accumulation rates are subject to the cell’s biochemical microenvironment, the effect of the physical microenvironment, such as matrix stiffness, has received little attention to date. Here we studied the proliferation rate and PPIX synthesis and accumulation in two glioma cell lines U373 and U118 cultured under five different substrate conditions, including the conventional tissue culture plastic and polyacrylamide gels that simulated tissue stiffness of normal brain (1 kPa) and glioblastoma tumors (12 kPa). We found that the proliferation rate increased with substrate stiffness for both cell lines, but not in a linear fashion. PPIX concentration was significantly higher in cells cultured on tissue-simulating gels than on the much stiffer tissue culture plastic for both cell lines. These findings, albeit preliminary, suggest that the physical microenvironment might be an important determinant of tumor aggressiveness and PPIX synthesis in glioma cells.

Photodynamic Therapy and the Biophysics of the Tumor Microenvironment

“…Evaluation of PpIX production in these two cell lines on substrates of varying stiffnesses showed that the mechanical properties of the underlying matrix affected each cell line differently. In the U118 cell line, a stiffer microenvironment led to increased PpIX accumulation with no change in the rate of cell growth . In contrast, substrate stiffness had no significant effect on PpIX accumulation in the U373 cell line (Fig.…”

“…In contrast, substrate stiffness had no significant effect on PpIX accumulation in the U373 cell line ( Fig. 5b) (13). In another study by Ulrich et al, micromechanical cues from the ECM on glioma cells were evaluated by testing various ECM substrates with different mechanical and biochemical properties.…”

“…(a) Summary of the pathways involved in targeting by cellular and tissue modulation, and (b) increased PpIX accumulation with higher substrate stiffness in glioma cells .…”

“…An emerging area of investigation is the role of mechanical stress on PpIX production and accumulation. A study by Niu et al using two glioma cell lines of bulk/solid tumors, U373 and U118, showed that cell proliferation rate increased in a nonlinear fashion as substrate stiffness increased. Their motivation was to study cell proliferation, and PpIX synthesis in glioma cells on substrates simulated the tissue stiffnesses of normal brain (1 kPa) and glioblastoma (12 kPa) .…”

“…Cells that are poorly differentiated and rapidly proliferating, as is common in many tumors, may PpIX produced in mitochondria, some relocalizes to the plasma membrane and lysosomes (169) Figure 5. (a) Summary of the pathways involved in targeting by cellular and tissue modulation, and (b) increased PpIX accumulation with higher substrate stiffness in glioma cells (13).…”