Abstract

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising agent for cancer therapy due to its selective induction of apoptosis in malignant cells. However, acquired resistance remains a significant clinical challenge. This study investigates the mechanism underlying the reversible increase in resistance to TRAIL-induced apoptosis observed in A-431 human epidermoid carcinoma cells when cultured to high confluence. We hypothesized that this density-dependent resistance is mediated by changes in the cell surface expression of the death receptors DR4 and DR5, which are essential for TRAIL signaling. Using flow cytometry and Western blot analysis, we demonstrated a significant, reversible downregulation of both DR4 and DR5 receptor expression on the cell membrane of confluent A-431 cultures compared to subconfluent cells. This decrease in receptor density directly correlated with a pronounced reduction in sensitivity to TRAIL-mediated cell death. These findings highlight a critical role for cell-cell contact and culture density in modulating the extrinsic apoptotic pathway, suggesting that the cellular microenvironment can rapidly alter the apoptotic threshold of carcinoma cells. This mechanism represents a potential target for therapeutic strategies aimed at overcoming TRAIL resistance in solid tumors.