Cultured cells activate IRE1 during attachment and flattening after routine passaging

During endoplasmic reticulum (ER) stress, the ER membrane protein IRE1 initiates the regulated splicing of <i>Xbp1</i> mRNA, leading to the production of a potent transcription factor that helps cells restore proteostasis. We report that <i>Xbp1</i> is also spliced following the routine passaging of mouse MC3T3-E1 cells, without the addition of canonical ER stressors. This splicing was independent of the type of dissociation buffer used to release cells from the surface, but was reduced when cells were plated on non-adherent culture dishes. These findings suggest that certain cultured mammalian cells induce an unfolded protein response during reattachment and spreading after passaging.