Senescence and cell cycle control

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

Knockdown of moesin expression accelerates cellular senescence of human dermal microvascular endothelial cells

PURPOSE

Endothelial cells maintain the homeostasis of blood, which consists of plasma and cellular components, and regulate the interaction between blood and the surrounding tissues. They also have essential roles in vascular permeability, the circulation, coagulation, inflammation, wound healing, and tissue growth. The senescence of endothelial cells is closely related to the aging of the adjacent tissues and to age-related vascular disease. Recently, the expression of moesin was found to be decreased in elderly human dermal microvascular endothelial cells (HDMECs), and an association between moesin and senescence has been suggested. This study examined the functional role of moesin in cellular senescence.

MATERIALS AND METHODS

To study the effects of decreased moesin expression on cellular senescence and metabolism, HDMECs were transfected with short hairpin-RNA (shRNA) lentivirus to silence moesin gene expression. In addition, specimens from young and old human skin were stained with antimoesin and anti-p16 antibodies as an in vivo study.

RESULTS

Using shRNAlentivirus, moesin knock-down HDMECs developed characteristics associated with aging and expressed senescence associated-beta-galactosidase during early passages. They also showed increased p16 expression, decreased metabolic activity, and cell growth retardation. Human skin tissue from elderly persons showed decreased moesin expression and increased p16 expression.

CONCLUSION

These findings suggest that there is a functional association between moesin expression and cellular senescence. Further study of the functional mechanism of moesin in the cytoskeleton and cellular senescence is needed. In addition, this study provides a useful model for developing anti-aging treatments.

Cell-specific responses to loss of cyclin-dependent kinases

Inactivation of cyclin-dependent kinases (Cdks) and/or cyclins in mice has changed our view of cell cycle regulation. In general, cells are far more resistant to the loss of Cdks than originally anticipated, suggesting widespread compensation among the Cdks. Early embryonic cells are, so far, not sensitive to the lack of multiple Cdks or cyclins. In contrast, differentiated cells are more dependent on Cdk/cyclin complexes and the functional redundancy is more limited. Our challenge is to better understand these cell-type specific differences in cell cycle regulation that can be used to design efficient cancer therapy. Indeed, tumor cells seem to respond to inhibition of Cdk activities, however, with different outcome depending on the tumor cell type. Tumor cells share some proliferation features with stem cells, but appear more sensitive to loss of Cdk activity, somewhat resembling differentiated cells. We summarize the current knowledge of cell cycle regulation in different cell types and highlight their sensitivity to the lack of Cdk activities.

Regulatory networks: Linking microarray data to systems biology

Gene regulation and aging are intrinsically linked and these links often reach directly to transcription factors and their actions in gene regulation. However, it is very difficult to follow all the individual directions such factors can affect. Therefore, the opposite approach became more popular recently, i.e. observing the endpoints of all these actions. Microarrays are the preferred technology to monitor large-scale changes in transcripts across whole genomes. The trade-off for being able to survey whole genome transcriptomes is that the results are mere observations, which do not directly reveal the underlying mechanisms that represent the real link to transcription factors and their actions. Fortunately, a combination of knowledge mining (including but not restricted to literature mining) with genomics analyses can be harnessed to elucidate at least some of the regulatory networks orchestrating the transcriptional changes observed by microarray experiments. Thus, a considerable part of the functional system structure of cells and organisms can be revealed, which is a pivotal prerequisite for any meaningful systems biology approach towards aging related phenotypes.