Insulin‐dependent signaling regulates azurophil granule‐selective macroautophagy in human myeloblastic cells

We show that insulin‐dependent signals regulate azurophil granule‐selective macroautophagy in human myeloid cells. Depletion of insulin from an insulin‐transferrin‐supplemented serum‐free medium caused growth retardation of myeloblastic HL‐60 cells, in which sequestration of electronic‐dense cytoplasmic materials by autophagosomes was observed. Positive immunoreactivity with anti‐CD68, anti‐cathepsin D, and anti‐myeloperoxidase antibodies indicated that the sequestrated materials were azurophil granules, the granulocyte/macrophage lineage‐specific lysosome‐like particles. By contrast, other organelles, including the mitochondria, endoplasmic reticulum, and Golgi apparatus remained intact, indicating that the macroautophagy selectively targeted azurophil granules. The addition of insulin induced rapid activations of p70 S6K and Akt, and the cells were rescued from macroautophagy. Rapamycin, an inhibitor of mammalian target of rapamycin, did not block the insulin‐mediated rescue from macroautophagy, although it nullified the activation of p70 S6K and cell growth. Low doses of LY294002, a phosphatidyl‐inositol‐3‐kinase inhibitor, which abolished cell growth and p70 S6K activity but did not influence Akt activity, did not block the insulin‐mediated rescue either. By contrast, low doses of Akt‐specific inhibitors, which inhibited neither cell growth nor p70 S6K activity, completely blocked the insulin‐mediated rescue from macroautophagy. Thus, insulin‐dependent signals are responsible for the control of azurophil granule‐selective macroautophagy via Akt‐dependent pathways, while p70 S6K ‐dependent pathways promote cell growth.