Cancer cells exhibit a variety of features indicative of atypical nuclei. However, the molecular mechanisms underlying these phenomena remain to be elucidated. The linker of nucleoskeleton and cytoskeleton (LINC) complex, a nuclear envelope protein complex consisting mainly of the SUN and nesprin proteins, connects nuclear lamina and cytoskeletal filaments and helps to regulate the size and shape of the nucleus. Using immunohistology, we found that a nuclear lamina component, lamin A/C and all of the investigated LINC complex components, SUN1, SUN2, and nesprin-2, were downregulated in human breast cancer tissues. In the majority of cases, we observed lower expression levels of these analytes in samples' cancerous regions as compared to their cancer-associated noncancerous regions (in cancerous regions, percentage of tissue samples exhibiting low protein expression: lamin A/C, 85% [n = 73]; SUN1, 88% [n = 43]; SUN2, 74% [n = 43]; and nesprin-2, 79% [n = 53]). Statistical analysis showed that the frequencies of recurrence and HER2 expression were negatively correlated with lamin A/C expression (P < 0.05), and intrinsic subtype and ki-67 level were associated with nesprin-2 expression (P < 0.05). In addition, combinatorial analysis using the above four parameters showed that all patients exhibited reduced expression of at least one of four components despite the tumor's pathological classification. Furthermore, several cultured breast cancer cell lines expressed less SUN1, SUN2, nesprin-2 mRNA, and lamin A/C compared to noncancerous mammary gland cells. Together, these results suggest that the strongly reduced expression of LINC complex and nuclear lamina components may play fundamental pathological functions in breast cancer progression.

Global histone modification patterns correlate with tumor phenotypes and prognostic factors in multiple tumor types. Recent studies suggest that aberrant histone modifications play an important role in cancer. However, the effects of global epigenetic rearrangements on cell functions remain poorly understood. In this study, we show that the histone H3 lysine 9 (H3K9) methyltransferase SUV39H1 is clearly involved in regulating cell migration in vitro. Overexpression of wild-type SUV39H1, but not enzymatically inactive SUV39H1, activated migration in breast and colorectal cancer cells. Inversely, migration was reduced by knockdown of SUV39H1 or chemical inhibition by chaetocin. In addition, H3K9 trimethylation (H3K9me3) was specifically increased in invasive regions of colorectal cancer tissues. Moreover, the presence of H3K9me3 positively correlated with lymph node metastasis in colorectal cancer patients. Furthermore, overexpression of SUV39H1 drove tumorigenesis in mouse, resulting in a considerable decrease in survival rate. These data indicate that H3K9 trimethylation plays an important role in human colorectal cancer progression, possibly by promoting collective cell invasion. (Cancer Sci 2013; 104: 889-895) G lobal changes in the epigenetic landscape are a hallmark of cancer.(1,2) For example, genome-wide loss of lysine 16 acetylation and lysine 20 trimethylation of histone H4 (H4K16ac and H4K20me3, respectively) is observed in multiple types of human cancers. (3,4) Moreover, global histone modification patterns predict clinical outcome, and correlate with tumor phenotypes and prognostic factors, in tumors including breast, prostate, lung, and gastric cancer.(5-10) Although somatic mutation is widely accepted as the origin of cancer, recent studies suggest that epigenetic alterations may be key initiating events, and that genetic and epigenetic alterations interact at all stages of cancer development and cancer progression. (11,12) Importantly, epigenetic aberrations, unlike genetic mutations, are potentially reversible and can potentially be restored to their normal state by epigenetic therapy. (13,14)

The LINC complex is a multifunctional protein complex that is involved in various processes at the nuclear envelope, such as nuclear migration, mechanotransduction and chromatin tethering in the meiotic phase. However, it remains unknown how these functions are regulated in different cell contexts. An inner nuclear membrane component of the LINC complex, SUN1, is ubiquitously expressed. The human SUN1 gene produces over 10 variants by alternative splicing. Although functions of SUN1 are relatively well characterized, functional differences among SUN1 splice variants are poorly characterized. LINC complex components are associated with a wide range of human diseases; therefore, it is important to understand the functional diversity among SUN1 splice variants. Here, we identified a novel human SUN1 splice variant, SUN1_888. overexpression of the SUN1 splice variants, SUN1_888 or SUN1_785, but not the predominant isoform, SUN1_916, activated directional cell migration. Knockdown of SUN1_888 suppressed cell migration; in contrast depletion of SUN1_916 activated cell migration. In addition, all of investigated SUN1 splicing variants rescued cell migration in SUN1 knock out cell. These results indicate that redundant and non-redundant functions of SUN1 splice variant in directional cell migration and suggest that variable LINC complexes with distinct task may exit. Furthermore, in contrast to previous studies, we showed association between SUN1 and B-type lamins. Interestingly, B-type lamin preferentially interacts with SUN1 but not SUN2. These results suggest that tissue-specific SUN1 variants variably interact with nucleoplasmic partners and allow variable assembly of LINC complexes that can be assigned to distinct tasks.

IntroductionLoss of histone H4 lysine 20 trimethylation (H4K20me3) is associated with multiple cancers, but its role in breast tumors is unclear. In addition, the pathological effects of global reduction in H4K20me3 remain mostly unknown. Therefore, a major goal of this study was to elucidate the global H4K20me3 level in breast cancer tissue and investigate its pathological functions.MethodsLevels of H4K20me3 and an associated histone modification, H3 lysine 9 trimethylation (H3K9me3), were evaluated by immunohistochemistry in a series of breast cancer tissues. Univariate and multivariate clinicopathological and survival analyses were performed. We also examined the effect of overexpression or knockdown of the histone H4K20 methyltransferases, SUV420H1 and SUV420H2, on cancer-cell invasion activity in vitro.ResultsH4K20me3, but not H3K9me3, was clearly reduced in breast cancer tissue. A reduced level of H4K20me3 was correlated with several aspects of clinicopathological status, including luminal subtypes, but not with HER2 expression. Multivariate analysis showed that reduced levels of H4K20me3 independently associated with lower disease-free survival. Moreover, ectopic expression of SUV420H1 and SUV420H2 in breast cancer cells suppressed cell invasiveness, whereas knockdown of SUV420H2 activated normal mammary epithelial-cell invasion in vitro.ConclusionsH4K20me3 was reduced in cancerous regions of breast-tumor tissue, as in other types of tumor. Reduced H4K20me3 level can be used as an independent marker of poor prognosis in breast cancer patients. Most importantly, this study suggests that a reduced level of H4K20me3 increases the invasiveness of breast cancer cells in a HER2-independent manner.

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