Cytotoxicity and Differentiating Effect of the Poly(ADP-Ribose) Polymerase Inhibitor Olaparib in Myelodysplastic Syndromes.

Myelodysplastic syndromes (MDS) are highly heterogeneous myeloid diseases, characterized by frequent genetic/chromosomal aberrations. Olaparib is a potent, orally bioavailable poly(ADP-ribose) polymerase 1 (PARP1) inhibitor with acceptable toxicity profile, designed as targeted therapy for DNA repair defective tumors. Here, we investigated olaparib activity in primary cultures of bone marrow mononuclear cells collected from patients with MDS (n = 28). A single treatment with olaparib induced cytotoxic effects in most samples, with median IC50 of 5.4 µM (2.0-24.8 µM), lower than plasma peak concentration reached in vivo. In addition, olaparib induced DNA damage as shown by a high proportion of γH2AX positive cells in samples with low IC50s. Olaparib preferentially killed myeloid cells causing a significant reduction of blasts and promyelocytes, paralleled by an increase in metamyelocytes and mature granulocytes while sparing lymphocytes that are not part of the MDS clone. Consistently, flow cytometry analysis revealed a decrease of CD117+/CD123+ immature progenitors (p < 0.001) and induction of CD11b+/CD16+ (p < 0.001) and CD10+/CD15+ (p < 0.01) neutrophils. Morphological and immunophenotypic changes were associated with a dose-dependent increase of PU.1 and CEBPA transcription factors, which are drivers of granulocytic and monocytic differentiation. Moreover, the combination of olaparib with decitabine resulted in augmented cytotoxic and differentiating effects. Our data suggest that olaparib may have therapeutic potential in MDS patients.

Mutant p53 improves cancer cells' resistance to endoplasmic reticulum stress by sustaining activation of the UPR regulator ATF6.

Missense mutations in the TP53 gene are frequent in human cancers, giving rise to mutant p53 proteins that can acquire oncogenic properties. Gain of function mutant p53 proteins can enhance tumour aggressiveness by promoting cell invasion, metastasis and chemoresistance. Accumulating evidences indicate that mutant p53 proteins can also modulate cell homeostatic processes, suggesting that missense p53 mutation may increase resistance of tumour cells to intrinsic and extrinsic cancer-related stress conditions, thus offering a selective advantage. Here we provide evidence that mutant p53 proteins can modulate the Unfolded Protein Response (UPR) to increase cell survival upon Endoplasmic Reticulum (ER) stress, a condition to which cancer cells are exposed during tumour formation and progression, as well as during therapy. Mechanistically, this action of mutant p53 is due to enhanced activation of the pro-survival UPR effector ATF6, coordinated with inhibition of the pro-apoptotic UPR effectors JNK and CHOP. In a triple-negative breast cancer cell model with missense TP53 mutation, we found that ATF6 activity is necessary for viability and invasion phenotypes. Together, these findings suggest that ATF6 inhibitors might be combined with mutant p53-targeting drugs to specifically sensitise cancer cells to endogenous or chemotherapy-induced ER stress.