tors CDKN1A and CDK4, which confirms its role in conjunction with KLF4. Of note, with restored platinum sensitivity from ALDH1A1 knockdown, expression of the pro-apoptotic factor BAX was upregulated nearly 3.95-fold. ALDH1A1 knockdown demonstrated increased number of cells in early apoptotic phase compared to the control. After exposure of cells PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19674025 to 1.0 mM staurosporine for 6 hours, a dramatic increase in early apoptotic cells was observed in ALDH1A1 deficient cells compared to their ALDH1A1 proficient counter parts . These data suggests that ALDH1A1 knockdown cells are more susceptible to BAXinduced apoptosis, which has been well described in the inhibition of p21 cell cycle checkpoint. ALDH1A1-mediated platinum resistance correlates to altered DNA repair networks Intact cell cycle checkpoints and DNA damage response signaling mechanisms are important for the cell’s ability to counter with different kinds of genomic insults and orderly progression of cell cycle. However, a common feature of cancer cells is altered PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19673813 regulation of these signaling cascades to acquire additional genetic changes required for re-differentiation and survival thereby display therapeutic resistance. Likewise, ALDH1A1 cells displayed altered regulation of KFL4/p21 mediated cell cycle checkpoint DMXB-A cost mechanism, which primarily directs the inhibition of G1 to S and G2 to M progression in response to DNA damage to allow more time for the cell to repair. Considering the association of PARP-1 in repair of carboplatin induced DNA damage, we evaluated its involvement in ALDH1A1 cells. PARP-1 levels progressively increased up to 45 minutes following carboplatin treatment. However, downregulation of ALDH1A1 resulted in significant decrease in total PAR levels compared to ALDH1A1 proficient cells. ALDH1A1 Maintains Stem-Like Properties by Altered DNA Repair Networks Several recent studies on breast cancer stem-cells indicate altered regulation of DNA repair networks, particularly an inverse relationship between ALDH1A1 status and BRCA1 gene expression. Moreover, in response to DNA damage, BRCA1 is known to govern cell cycle checkpoints and choice of the DNA repair pathways to timely repair of these DNA lesions. Consistent with breast cancer stem-cell data, Western blot analysis revealed an inverse relationship between ALDH1A1 and BRCA1 expression in A2780/CP70 cells, suggesting ALDH1A1 expressing ovarian cancer stem-like cells more likely to lose or express low levels of BRCA1. Further analysis revealed down regulation of ALDH1A1 induced spontaneous DNA damage response by expressing c-H2AX protein. This is also coincided with the diminished levels of excision repair protein, replication checkpoint kinase protein 1 and other replication stress associated Fanconi anemia -BRCA gene products FANCD2 and FANCJ. Together these data suggest that ovarian cancer stem-like cells may maintain therapeutic resistance by expressing ALDH1A1 and depletion of which, abrogates G1 and S-phase checkpoints leading to replication stress. Though ALDH1A1 depleted cells accumulated in S and G2 phases, the phosphorylation status of replication checkpoint protein Chk1 and expression of replication fork associated FA pathway proteins FANCD2 and FANCJ were affected. This is also conferred by induction of c-H2AX, a marker for DSB and reduced cell survival. Since Chk1 and FA proteins are important for the replication checkpoint and the stability of stalled replication forks, the spontaneous DNA damage response