Characterization of two populations of statin and the relationship of their syntheses to the state of cell proliferation

Nonlamin components of the lamina: a paucity of proteins

Current models of nuclear organization propose that nuclear functions are modulated in part by reversible tethering of chromatin loops to structural elements of the nucleoplasm and the nuclear envelope. Lamins are the best-characterized proteins of the lamina portion of the nuclear envelope and are involved in binding chromatin to the inner nuclear membrane. However, they are not a universal feature of eukaryotic nuclei and do not account fully for the putative functions of the lamina in all organisms. It is possible that nonlamin components of the lamina may substitute for lamins in organisms from which they are absent and modify the properties of lamins during development and the cell cycle. We review the properties of the relatively small number of such components that have been reported, including the young arrest (fs(1)Ya) protein of Drosophila, statin, circumferin, and the MAN antigens. The experimental evidence indicates they are a diverse group of proteins, and that at least some have the potential to modulate the interactions of chromatin, lamins, and the nuclear membranes.Key words: nuclear envelope, lamina, YA protein, statin, circumferin.

Characterization of senescence- and apoptosis-dependent forms of terminin as derived from a precursor found in replicating and nonreplicating cells

Previously we have reported the production of a monoclonal antibody (Mab 1.2) which recognizes a cytoplasmic protein, terminin, in three different molecular weights: 90 (Tp90), 60 (Tp60), and 30 kDa (Tp30) forms. Further characterization shows that Tp90 is found in young growing and nongrowing quiescent fibroblasts, while Tp60 is found in permanently growth-arrested senescent fibroblasts and Tp30 in cells committed to undergo programmed cell death (apoptosis). In tissue, Tp90 is found in embryonic brain; later, in neonatal brain after terminal differentiation is completed, only Tp60 is found. Tp30 is found in crude liver fractions extracted without the protective action of protease inhibitors. In all these circumstances, Tp90 is mostly seen in the detergent-soluble fraction, while Tp60 and Tp30 are detergent-insoluble. We now report that in cultured fibroblasts, as well as in tissues such as brain and liver, Tp60 and Tp30 are derived from the Tp90 polypeptide, indicated by the fact that only the Tp90 species is identified by both immunoblotting and immunoprecipitation assays, when the cell or tissue extracts are prepared in the presence of protease inhibitors. Further evidence shows that immunoprecipitation of in vitro translation products from brain, liver, and cultured fibroblasts also present a single band of Tp90 polypeptide. Pulse-chase experiments show that during apoptosis, Tp90 is processed to Tp60, and eventually to Tp30. However, when the total protein extracts are fractionated, only Tp90 is found in the detergent-soluble fraction, with diminishing quantities during the time course of apoptosis, and Tp30, in contrast, is found as the only protein species in the insoluble fraction, with increasing quantity during the same time course. Newly processed Tp60 is not found in either of the fractions, reflecting its loss during the fractionation procedure. Limited one-dimensional peptide mapping of Tp90 yields three different bands at 30, 28, and 25 kDa, but only the one at 30 kDa is recognized by Mab 1.2. These results lead us to suggest that terminin protein is synthesized in the Tp90 form, and cleaved to lower molecular weight forms depends upon different physiologic conditions, with Tp60 processed in the terminally differentiated or senescent state and rapidly to Tp30 in apoptosis. Our findings further suggest that Tp90's processing to either Tp60 or Tp30 produces insoluble protein forms. Furthermore, the presence of Tp90 in nonapoptotic (either replicating or nonreplicating) cells may reflect the absence of necessary proteolytic action required for the execution of apoptosis. Future experiments will allow us to determine the nature of this proteolytic action, as well as whether this action is due to the autocatalytic action of Tp90 or by other endogenous proteases, and then to determine the significance of this biochemical action in cells.

Expression of the nuclear membrane protein statin in cycling cells

Statin is a 57 kD protein previously reported to be expressed by cells in G0. We have studied the detailed distribution of statin immunoreactivity in normal human and rat tissues, and correlated this with investigation of in vitro model systems. By laser confocal microscopy, statin immunoreactivity is localized to the nuclear membrane. In contrast to previous reports, using in vitro model systems we found that statin was also expressed by replicating cells as judged by both co-localization with [3H]thymidine-labelled and Ki67-labelled cells. Furthermore, in a nude mouse xenograft model the number of statin-labelled cells exceeded the number of quiescent cells as assessed by both fraction of labelled mitosis methods and labelling with [3H]thymidine and Ki67. We conclude that although there is an association between expression of the 57 kD nuclear membrane protein statin and growth arrest, this is not absolute and it is expressed in a sub-population of cycling cells. The properties of statin closely resemble those of nuclear lamins, members of the intermediate filament family.

Characterization of the absence of an unique DNA-binding protein in senescent but not in their young growing and nongrowing counterparts provides the means to mark the final stage of the cellular aging process

In senescent fibroblasts, the incapability of cell replication is permanent, and may involve the irreversible loss of gene expressions potentiating the engagement in DNA replication. The initial attempt is to identify gene products that are permanently lost in irreversibly growth-arrested cells. In this article, we report the success in identifying a DNA-associated S6 antigen found in the nuclei of growing and growth-arrested young cells, but not in the nuclei of their senescent counterparts. The presence of the S6 antigen is uniform throughout the nucleoplasm, except in the regions of the nucleoli, and found to be associated with condensed chromosomes in mitotic cells. Treatment with RNase does not abolish the antibody staining activity in the nuclei, while treatment with DNase does remove the activity. Equal intensity of S6 antibody staining is observed in transformed, growing, and contact-inhibited young fibroblasts. Significant reduced level of S6 antibody staining activity is found in the nuclei of senescent fibroblasts, indicating the loss of the expression of this protein during cellular aging process. Immunoblotting assay shows that the S6 antigen is of 50 kDa and with a possibility of a 100 kDa as a dimeric precursor. Our results suggests that a permanent turning off of unique gene expression is associated with the onset of senescence or terminal differentiation, and this hypothesis is supported by the characterization of S6 antigen's absence in in vitro-aged fibroblasts.

Expression of cell cycle related proteins--proliferating cell nuclear antigen (PCNA) and statin--during adaptation and de-adaptation of EUE cells to a hypertonic medium

EUE cells adapted to grow for long times in a hypertonic medium have a longer cell cycle than those growing in isotonic medium. To elucidate whether this lengthening involves specific cycle phases to differing extents, the expression of two cycle-related protein, PCNA and statin, was studied by dual parameter flow cytometry of indirect immunofluorescence protein labelling and DNA content. In isotonic medium, most cells, in all the cycle phases, were PCNA positive; in contrast, PCNA negative cells and statin positive cells were very few in number and only fell in the G0/1 range of DNA contents. In hypertonic medium, the frequency of PCNA positive cells was lower, and that of statin positive cells higher, than in isotonic medium, particularly in the G0/1 range of DNA contents: this suggests that a G0 block occurs under long-term hypertonic stress. Consistently, dual parameter flow cytometric measurement of BrdUrd immunofluorescence labelling and DNA content showed that fewer cells entered S phase in hypertonic medium and their progression through the S phase was slower; evidence was also found for the occurrence of a G2 block. These kinetics changes were fully reversible in isotonic medium, thus indicating the adaptive nature of the EUE response to hypertonicity.