Bee venom treatment induced β-actin to localise throughout the nuclei of cancer cells in accordance with an increase in phosphorylation of H2AX, but significantly found in the cytoplasmic form in non-cancerous cells in accordance with a decrease in γH2AX nuclear accumulation. The histone H2AX used is a 14 kDa member of the H2A histone family, which is evolutionarily conserved at the C-terminus in eukaryotes. Serine 139 within this motif is rapidly phosphorylated in response to double-stranded DNA damage and apoptosis, and its phosphorylated form is known as γH2AX. Phosphorylation reaches its maximum level at 1–3 minutes after DNA damage occurs, and hundreds of γH2AX molecules are released during each double-strand DNA break (DSBs) (Schmid, Zlobinskaya, & Multhoff, 2012) and the H2AX phosphorylation is the most fundamental modification involved in DNA damage (Sedelnikova, Pilch, Redon, & Bonner, 2003; Sonoda, Hochegger, Saberi, Taniguchi, & Takeda, 2006; Stucki et al., 2005). Therefore the immunostaining pattern of γH2AX has been used for a marker of DSBs and genomic instability in cancer cells (Hamer et al., 2003; Ji et al., 2017; Nagelkerke & Span, 2016; Redon, Dickey, Bonner, & Sedelnikova, 2009). But, H2AX phosphorylation has been also observed in undamaged cells in human (Meyer et al., 2013). To determine whether bee venom has a genotoxic effect on NIH3T3, MDA-MB-231 and HEPG2 cells, immunostaining was performed for γH2AX after bee venom treatment for 24 h, and γH2AX staining was decreased in normal cells while increased in cancer cells after bee venom. The level of DSBs increases with cellular death, chromosomal aberrations, mutations and initiation of pathological effects such as cancer so that repairing of DSBs is critical and essential to prevent carcinogenesis (Kasparek & Humphrey, 2011). Histone modifications associated with DNA damage allow DNA repair factors to access damaged sites of DNA (Waterman, Haber, & Smolka, 2020). H2AX is mostly found in nuclues, but It has been previously reported that phosphorylated H2AX was also accumulated within the cytoplasm in some DNA damage induced by such overactivation of a nerve growth receptor (Jung & Kim, 2010, 2011). In each cells some of γH2AX protein were found in cytoplasm after bee venom. But, the cellular pathway for γH2AX protein accumulation after bee venom needs further investigation.
Cytoskeleton is structured by various proteins such as microtubules, actin and intermediate filaments (Fletcher & Mullins, 2010). The cytoskeletal proteins have important roles at cell movement, biochemical process of cell, and survival of the cell (Ong et al., 2020). Cancer cells have altered skeletal machinery that causes cell invasion and metastasis (Pawlak & Helfman, 2001). Actins have three isoforms including α-, β- (Β-actin), and γ-actin and also they are involved in forming microfilaments in the cell (Kavallaris, 2010; Rao & Li, 2004). While normal cells strictly establish cell architecture, mobility and adhesion with actin crosslinks, cancer cells disrupt the actin cytoskeleton arrangement and change the nuclear:cytoplasmic ratio in cells. This facilitates the metastasis by inducing flexible, mobile and fast movement of cancer cells (Ong et al., 2020; Rao & Li, 2004; Tojkander, Gateva, & Lappalainen, 2012).
Defects in cellular morphogenesis are accompanied by uncontrolled migration, acquisition of invasive features, and genomic instability in carcinogenesis. Cancer cells have abnormalities in each cellular compartment as in nucleus, cytoplasm and membranes. Cytoplasmic rearrangement mediate cell mobility and its abnormalities are one of the key events in cancer cells. For example, actin and tubulin proteins involve in the regulation of intracellular compartments, cell polarity and contractility. During morphogenesis, they determine cell shape and polarity and also promote stable cell-cell and cell-matrix junctions through interactions with cadherin and integrin proteins, respectively (Eden, Rohatgi, Podtelejnikov, Mann, & Kirschner, 2002). Additionally they activate chromosomal segregation and cell division during mitosis. This study showed that β-actin protein is localized both in the cytoplasm and in the nucleus depending on the bee venom response in different cells. The findings support previous studies that have shown the localisation of β-actin within both cytoplasm and nucleus (Caridi et al., 2018; Cerutti, 2019; Schrank et al., 2018). β-actin in the cytoplasm is involved in cell movement, formation of the skeleton and cell division, while β-actin in the nucleus is involved in the G2 checkpoint where especially the repair of DSBs occurs (Schrank et al., 2018). In addition, it has been determined that β-actin has various structural and functional roles in the nuclear structure, such as nuclear matrix assembly, chromatin remodeling, transcription and mRNA processing (Bohnsack, Stuven, Kuhn, Cordes, & Gorlich, 2006). Mass spectrometry (Bohnsack et al., 2006) and immunoreactivity experiments (Hu, Wu, & Hernandez, 2004) have showed that β-actin is a nuclear isoform associated with heterogeneous nuclear ribonucleoproteins (hnRNPs) and RNA polymerase complexes. It has been also shown that different polymerization states of β-actin coexist in the nucleus (Schoenenberger et al., 2005). Fluorescent staining studies have revealed that approximately 20% of the nuclear actin has a dynamic pattern (McDonald, Carrero, Andrin, de Vries, & Hendzel, 2006). Our results showed that bee venom differentially induced the re-organisation of cytoskeleton but similarly induced DNA damage suggesting that bee venom selectively can stimulate β-actin-mediated DNA repair within the nucleus in cancer cells. Nuclear form of β-actin has been recently shown to mediate re-organisation of nucleolus after DNA repair particularly in ribosomal DNA sequences which is completed after UV-induced damage (Caridi et al., 2018). Another study showed that γH2AX phosphorylation increased in cells after radiation, followed by the binding of NMI (nuclear myosin 1) to ribosomal DNA (rDNA), and subsequent β-actin molecules were recruited into damaged cell nuclei (Cerutti, 2019) after phosphorylated H2AX-mediated binding of NMI to the damaged site (Caridi et al., 2018). NMI and β-actin are required for appropriate rearrangement of the nucleolus after the completion of the DNA repair machinary. Results showed that genotoxicity increased after bee venom in cancer cells but not in normal cells, and β-actin localised within the nuclei in cancer cells. This suggests that β-actin may recruit to damaged site within the nucleus in coordination with γH2AX in the cell nucleus to manage DNA damage. The results in this study indicate that after bee venom treatment, β-actin was accumulated more in the nuclei of cancer cells and but in the cytoplasm of normal cells. Thus, it suggests that bee venom may act by interfering with chromatin changes and mRNA processing of β-actin in cancer cells.