Endocrine Abstracts

Introduction: Glucocorticoids are used for treatment of various hematological malignancies such as Acute Myeloid Leukemia (AML). Glucocorticoids are known to induce apoptosis by regulating the expression of key anti- or pro-apoptotic genes. Vitamin D induces apoptosis and cell cycle arrest in various types of cancer cells. Despite recent advances in treatment of AML, the poor response rate to therapy remains a big challenge.Therefore, discovering new factors that can affect therapeutic response to Glucocorticoid therapy is mandatory.

Aim: I) to investigate the effect of 1,25(OH)₂vitamin D₃ (VitD) on glucocorticoid-induced AML cells apoptosis and/or cell cycle arrest, II)to determine whether this effect is mediated through activation of Notch-1 signaling.

Methods: Kasumi-1 cells were either incubated for 72 hours with dexamethasone (Dex-10⁻⁶M and 10⁻⁷M) alone or pre-incubated with VitD (10⁻⁷-10⁻⁹M) for 24 h. MTS assay was performed to quantify viable cells whereas apoptosis was measured by flow cytometry (Annexin V/PI staining). The mRNA expression of Mcl-1, Noxa, Bcl-2, Bax, p21, Notch-1 and Notch-2 genes implicated in apoptosis and cell cycle arrest, was evaluated by qPCR. Notch-1 and cleaved Notch-1 (NICD-active-form) protein levels were measured by Western blot.

Results: The results of MTS and FACS indicate that Dex induced apoptosis in Kasumi cells dose-dependently, however this effect was attenuated significantly in cells pre-incubated with VitD. Pre-incubation of cells with VitD for 24h followed by co-incubation with Dex for further 72h increased the Bcl-2/Bax ratio significantly via reducing the Bax mRNA expression while did not change the mRNA MCL-1/NOXA ratio compared to those cells incubated with Dex alone. P21 mRNA expression was reduced significantly in cells pre-incubated with VitD compared to cells incubated with Dex alone. The Notch-1 and its intracellular domain protein levels was not detectable in the presence of dexamethasone at lower concentration. The protein level of Notch-1 as well as NICD were significantly increased in cells pre-incubated with VitD (10⁻⁸M) and then co-incubated with Dex (10⁻⁷M) as compared to cells incubated with Dex (10⁻⁷M) alone.

Conclusions: VitD exerts inhibitory effect against Dex induced-apoptosis in Kasumi-1 cells via increasing Bcl2/Bax ratio, while MCL-1/NOXA ratio did not seem to play important role. VitD can induce cell survival by inhibition of GC-mediated cell cycle arrest through activation of Notch-1 signaling, which could in turn result in reduction of p21 expression.