Triphenyl phosphate promotes lipid accumulation in human mesenchymal stem cells through metabolic stress pathways.

Organophosphate esters (OPEs), used as both plasticizers and flame retardants, are increasingly recognized for their potential to disrupt cellular homeostasis and emerging evidence indicates their ability to induce metabolic stress in diverse biological systems. This study investigated the impact of Triphenyl Phosphate (TPhP) on human mesenchymal stem cell (hMSC) differentiation and cell stress. hMSCs were treated with DMSO (vehicle control), 10 µM rosiglitazone (a PPARγ agonist), or 25 µM TPhP for 14-21 days. Following treatment, cells were stained to assess nuclear integrity, lipid accumulation, and mineralization. Immunocytochemistry (ICC) for osteogenic and adipogenic markers was used to determine whether TPhP promotes adipogenesis under osteogenic conditions. TPhP exposure led to significant lipid accumulation without activation of adipogenic differentiation programs and markedly reduced mineralization. In particular, co-treatment with PPARγ inhibitors did not mitigate lipid accumulation or modulate the expression of adipogenic regulators. Gene expression analysis following TPhP exposures revealed upregulation of pathways involved in lipid biogenesis, triglyceride synthesis, phospholipid metabolism and endoplasmic reticulum (ER) stress, with minimal changes to classical adipogenic markers. These findings indicate that TPhP disrupts the metabolic balance between osteogenesis and lipid metabolism, with the potential to promote ectopic lipid deposition within the bone forming environment. This dysregulation is relevant for the development of metabolic disorders, including obesity, type 2 diabetes, and metabolic bone diseases, including osteoporosis.