Mitochondrial alterations accompany forced differentiation in acute myeloid leukemia.

Abstract

Leukemia is characterized by blocked hematopoietic differentiation. Blocked differentiation results in the uncontrolled proliferation of immature, malignant myeloblasts. To counteract myeloblast proliferation, forced differentiation has been studied as a possible treatment for various types of leukemia -- most notably, acute promyelocytic leukemia (APL). In APL, standard of care encompasses treatment with all trans retinoic acid (ATRA), administered alongside chemotherapy or arsenic trioxide. Although the combination of ATRA with chemotherapy achieves complete remission in ~ 90% of APL patients, relapse remains a major clinical problem. For example, treatment with ATRA alone induces only temporary remission, with relapse usually occurring within six months. In vitro studies of differentiation in leukemia cells have shown that the cells seem to take on the phenotype of a terminally differentiated cell, but it is unclear whether the biochemical processes intrinsic to the cell return to normal. While changes in mitochondrial content and function are known to occur with hematopoietic differentiation, the impacts of ATRA-induced differentiation on mitochondrial bioenergetics has yet to be explored. To address this gap in knowledge, a human AML-M2 cell line, HL-60, was treated with ATRA, and the resulting changes in proliferation, morphology, and mitochondrial function and protein expression were investigated. In response to ATRA, cellular proliferation halted by day three of treatment and histology confirmed altered morphological appearance, consistent with myeloid differentiation. In treated cells, mitochondrial respiration was decreased; however, mitochondrial content was unchanged. Interestingly, the respiratory profile of ATRA-differentiated HL60 cells differed substantially from that of healthy primary human granulocytes. These findings were corroborated by differences in size and morphological appearance between the ATRA-treated HL-60 cells and primary human granulocytes. Although further investigation is necessary to fully elucidate the effects of ATRA-induced differentiation of HL-60 cells, these findings suggest that the clinical success of ATRA is likely not the result of APL terminal differentiation.