233 The role of the MR 7 DISCUSSION In the current study, we found that hydrocortisone is a potent steroid to induce steroidinduced cell death and that this steroid-induced cell death can be achieved solely by activation of the mineralocorticoid receptor (MR). By selectively inducing either MR (NR3C2 gene) or glucocorticoid receptor (GR, NR3C1 gene) expression in MR/GR-naive Reh cells, we observed that hydrocortisone induces the expression of NR3C1 via both the GR and MR, and that hydrocortisone can induce significant cell death in these cells. In contrast, dexamethasone mainly induced NR3C1 transcription via the GR, but was not able to induce NR3C2 or other downstream target genes including BIM, GILZ or FKBP5 via the MR, despite the fact that it could induce cell death in NR3C2-overexpressing Reh cells that lack a functional NR3C1 receptor. These results highlight that the MR is potentially capable of inducing leukemic cell death after activation by steroid treatment. The finding that hydrocortisone is the most potent steroid in both our in vitro NR3C1 and NR3C2 models is interesting, since the glucocorticoid activity of hydrocortisone is reported to be inferior compared to other steroids. Conventionally, dexamethasone is reported to be the most potent steroid, with high glucocorticoid, but no mineralocorticoid activity.24 However, this is based on anti-inflammatory and Na+-retaining potency of the different steroids. Several in vitro studies in primary ALL samples with different cellular backgrounds showed that dexamethasone had higher antileukemic activity compared to either prednisone or hydrocortisone than the traditionally used equivalent doses.9,25-27 In contrast to this previous research, hydrocortisone seems more efficient in inducing steroid-induced cell death than prednisolone and dexamethasone in both our RehNR3C1 and RehNR3C2 cell line models. This may be related to the high expression of NR3C1 and NR3C2 in our models compared to PDX or patient samples. Indeed, in the primary patient and PDX samples, hydrocortisone appeared less cytotoxic compared to dexamethasone or prednisone and the expression of NR3C2 was minor compared to the induced expression levels in our RehNR3C2 model (Supplemental Figure 4B). The inter-patient variability in steroid sensitivity is high, with cytotoxicity values sometimes varying more than 1000fold among patient samples.25,26 It therefore cannot be excluded that certain patients respond better to other steroids than the traditionally used dexamethasone or prednisone. Moreover, our Reh cell line model underscores the potential of hydrocortisone to induce steroid-induced cell death in leukemic cells. It is known that ALL patients with a t(12;21)(p13;q22) lesion, which leads to the ETV6-RUNX1 fusion gene, also known as TEL/AML1, have an excellent prognosis.23,28-30 Interestingly, this pre-B ALL subgroup has a relatively high expression of both NR3C1 and NR3C2 (Figure 3A). It is therefore tempting to speculate that the MR contributes to the overall
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