Abstract.We have recently shown that mRNA and protein of PHLDA1 (pleckstrin-homology-like domain family A, member 1) were by far the most upregulated molecules upon treatment of IMR-32 cells with the anti-GD2 ganglioside monoclonal antibody 14G2a. Hence, we decided to study functions of PHLDA1 using human neuroblastoma IMR-32 cells as a model. Here, we show that constitutive expression of mRNA and protein of the PHLDA1 gene in IMR-32 cells was inversely correlated with transcript of the AURKA gene and Aurora A oncoprotein. Next, we silenced PHLDA1 expression in IMR-32 cells using an shRNA interference method. We report that IMR-32 cells with stable downregulation of PHLDA1 showed enhanced cellular ATP levels and an increase in mitochondrial membrane potential, as compared to control and non-transduced cells. We demonstrated that downregulation of PHLDA1 leads to a significant increase in expression of Aurora A and TRKB that are markers of poor prognosis in neuroblastoma. Also, we measured an increase in Aurora A and Akt kinases phosphorylation in the cells. Most importantly, PHLDA1-silenced cells were less susceptible to apoptosis than control cells, as shown by the lower expression of cleaved caspase-3 and PARP as well as a decreased activity of caspase-3 and -7. Our study negatively correlates expression of PHLDA1 and Aurora A in IMR-32 cells and sheds new light on functions of PHLDA1 in the neuroblastoma tumor cells, suggesting its role as a pro-apoptotic protein. Additionally, our results show possible links of the protein to regulation of features of mitochondria and formation of autophagosomes.
IntroductionPHLDA1 was identified as a gene involved in Fas (CD95) expression and apoptosis induced after an anti-TCR antibody binding to mouse T cell hybridoma cells (1). In humans the gene is located on the long arm of the chromosome 12 [12q15 (2)] and encodes a 401 amino acid (aa) protein (45 kDa) named pleckstrin-homology-like domain family A, member 1 (PHLDA1, synonyms include: DT1P1B11, PHRIP, 'prolinehistidine rich protein', TDAG51, based on www.genecards. org).Several research groups have aimed to characterize involvement of PHLDA1 in biology of cancer cells, including its function in apoptosis. Thus, Neef et al measured levels of mRNA of PHLDA1 in three sets of cell lines, derived from paired samples of primary and metastatic melanoma tumors, and reported that PHLDA1 was downregulated in the latter. PHLDA1 was detected by the authors in benign melanocytic nevi, and its level was shown to decrease with progression of malignant melanomas. Also, constitutive expression of the PHLDA1 protein in a melanoma cell line Mel Rif has led to cell growth reduction along with an increase in basal apoptosis and sensitivity to doxorubicin and camptothecin (3). In 2006, Hayashida et al showed that PHLDA1 is a heat shock inducible gene regulated by HSF1 and when overexpressed in HeLa cells has pro-apoptotic functions (4). In yet another report, Nagai et al have reported that downregulation of PHLDA1 is a strong predictor of ...