Histone modifications coordinate the chromatin localization of key regulatory factors in mitosis. For example, mitotic phosphorylation of Histone H3 threonine-3 (H3T3ph) by Haspin creates a binding site for the chromosomal passenger complex (CPC). However, how these histone modifications are spatiotemporally controlled during the cell cycle is unclear. Here we show that Plk1 binds to Haspin in a Cdk1-phosphorylation-dependent manner. Reducing Plk1 activity decreases the phosphorylation of Haspin and inhibits H3T3ph, particularly in prophase, suggesting that Plk1 is required for initial activation of Haspin in early mitosis. These studies demonstrate that Plk1 can positively regulate CPC recruitment in mitosis.

Apart from b-catenin accumulation, loss of 3p21 is one of the most frequent genetic alterations in numerous malignancies including nasopharyngeal carcinoma (NPC). Herein, we characterized a novel candidate tumor suppressor gene (TSG) CAC-NA2D3, a voltage-dependent subunit alpha 2 delta 3 of a calcium channel complex. Downregulation of CACNA2D3 was frequently detected in primary NPCs and NPC cell lines compared with their nontumorigenic counterparts. Attenuated CACNA2D3 expression may be associated with loss of heterozygosity (LOH) at intragenic single-nucleotide polymorphism sites (rs589281, rs1449325 and rs6797113) and/or epigenetic silencing by methylation and histone deacetylation. Given the extensive effects of calcium in cancer, we then investigated the tumor suppressive role and underlying mechanism of CACNA2D3 in the development and progression of NPC. CACNA2D3 was stably transfected into NPC cell lines (C666 and SUNE1) at levels comparative with the normal nasopharynx, alongside siRNA-mediated silencing in an immortalized nasopharyngeal epithelial cell line (NP69) to conduct in vivo and in vitro functional assays. Our findings show that CACNA2D3-mediated increase in intracellular calcium (Ca21) can induce mitochondrial-mediated apoptosis and activation of NLK (through the Wnt/Ca21 pathway) to antagonize Wnt signaling-mediated anchorage-dependent and independent cell proliferation (via CCND1 and CMYC), invasion (via MMP7) and epithelial-to-mesynchemal transition (via SNAIL). As the expression pattern of calcium channels and their degree of functionality can change with the progression of cancer, CACNA2D3 may indeed be a promising biomarker for NPC. Our study also warrants further exploration in the potential therapeutic use of existing epigenetic targeting drugs (e.g., 5-azacytidine, SAHA) to reconstitute CACNA2D3-associated tumor suppression in NPC.

No abstract