Regulation of calreticulin expression during induction of differentiation in human myeloid cells: Evidence for remodeling of the endoplasmic reticulum

Induction of differentiation of HL-60 human myeloid cells profoundly affected expression of calreticulin, a Ca²⁺-binding endoplasmic reticulum chaperone. Induction with Me₂SO or retinoic acid reduced levels of calreticulin protein by ∼60% within 4 days. Pulse-chase studies indicated that labeled calreticulin decayed at similar rates in differentiated and undifferentiated cells (t_1/2 ∼4.6 days), but the biosynthetic rate was lt;10% of control after 4 days. Differentiation also induced a rapid decline in calreticulin mRNA levels (90% reduction after 1 day) without a decrease in transcript stability (t_1/2 ∼5 h). Nuclear run-on analysis demonstrated rapid downregulation of gene transcription (21% of control at 2 h). Differentiation also greatly reduced the Ca²⁺ content of the cells (25% of control), although residual Ca²⁺ pools remained sensitive to thapsigargin, ionomycin, and inositol trisphosphate. Progressive decreases were also observed in levels of calnexin and ERp57, whereas BiP/ GRP78 and protein disulfide isomerase were only modestly affected. Ultrastructural studies showed a substantial reduction in endoplasmic reticulum content of the cells. Thus, terminal differentiation of myeloid cells was associated with decreased endoplasmic reticulum content, selective reductions in molecular chaperones, and diminished intracellular Ca²⁺ stores, perhaps reflecting an endoplasmic reticulum remodeling program as a prominent feature of granulocytic differentiation.