Differentiation of Hodgkin lymphoma cells by reactive oxygen species and regulation by heme oxygenase-1 through HIF-1α

We previously indicated that Hodgkin lymphoma (HL) cells contain a small side population (SP) that differentiate into a large major population (MP) with giant Hodgkin and Reed-Sternberg (H and RS)-like cells. However, its molecular mechanisms are not fully understood. In this study, we found that intracellular reactive oxygen species (ROS) are low in the SP compared to the MP. Hydrogen peroxide induces large H- and RS-like cells in HL cell lines, but induces cell death in unrelated lymphoid cell lines. Microarray analyses revealed the enrichment of upregulated genes under hypoxic conditions in the SP compared to the MP, and we verified that the SP cells are hypoxic. Hypoxia inducible factor (HIF)-1 alpha was preferentially expressed in the SP. CoCl2, a HIF-1 alpha stabilizer, blunted the effect of hydrogen peroxide. Heme oxygenase-1 (HO-1), a scavenger of ROS, was triggered by HIF-1 alpha. The effect of hydrogen peroxide was inhibited by HO-1 induction, whereas it was promoted by HO-1 knockdown. HO-1 inhibition by zinc protoporphyrin promoted the differentiation and increased ROS. These results stress the unique roles of ROS in the differentiation of HL cells. Immature HL cells are inhibited from differentiation by a reduction of ROS through the induction of HO-1 via HIF-1 alpha. The breakdown of this might cause the accumulation of intracellular ROS, resulting in the promotion of HL cell differentiation.

Automated summary

This study found that Hodgkin lymphoma cells with low intracellular ROS levels in the small side population differentiate into large major population cells with high ROS levels. The hypoxic conditions led to upregulated genes and increased expression of HIF-1 alpha in the SP cells. HO-1 induction inhibited the effect of ROS, while HO-1 knockdown promoted ROS levels and cell differentiation in HL cells.