Events blocked in prereplicative phase in senescent human diploid cells, TIG-1, following serum stimulation

[Mechanism of cell proliferation--cell cycle, oncogenes, and senescence]

Cell proliferation is regulated through a transition between the G0 phase and cell cycle. We isolated a mammalian temperature-sensitive mutant cell line defective in the function from the G0 phase to cell cycle. Senescent human somatic cells fail to enter into the cell cycle from the G0 phase with stimulation by any growth factor. Telomere shortening was found to be a cause of cellular senescence, and reexpression of telomerase immortalized human somatic cells. Immortalized human somatic cells showed normal phenotypes and were useful not only for basic research but also for clinical and applied fields. The importance of p53 and p21 activation/induction i now well accepted in the signal transduction process from telomere shortening to growth arrest, but the precise mechanism is largely unknown as yet. We found that the MAP kinase cascade and histone acetylase have an important role in the signaling process to express p21. Tumor tissues and cells were found to have strong telomerase activity, while most normal somatic human tissues showed very weak or no activity. Telomerase activity was shown to be a good marker for early tumor diagnosis because significant telomerase activity was detected in very early tumors or even in some precancerous tissues compared with adjacent normal tissues. Telomere/telomerase is a candidate target for cancer chemotherapeutics, and an agent that abrogated telomere functions was found to kill tumor cells effectively by inducing apoptosis whereas it showed no effect on the viability of normal cells.

The regulation of cyclin D1 expression in senescent human fibroblasts

To clarify the molecular mechanisms of cyclin D1 expression during in vitro cellular aging, we investigated the binding of nuclear protein factors to the cyclin D1 gene promoter domain in young and senescent normal human fibroblasts. The cyclin D1 promoter binding activities of nuclear protein factors from young and senescent cells were examined by the gel mobility shift assay. Our findings revealed that (i) the binding of a specific nuclear factor to the enhancer element was very weak in senescent cells; (ii) the binding of a specific nuclear factor to the CRE, which is independent of cell growth, was unchanged between young and senescent cells; (iii) nuclear factors from senescent cells did not bind to the presumptive silencer element; (iv) the binding of specific factors to the Inr (transcription initiation region) and E2F increased with growth stimulation in young cells and was weakly detectable in senescent cells; and (v) the binding of Sp1 to its promoter element occurred only in senescent cells. The analysis of the silencer element by the gel mobility shift assay revealed that the essential sequence required for binding of specific factors to the silencer element was TTTAAT. The molecular weight of the binding factor to the silencer element was determined to be approximately 35 kDa by the Southwestern blotting and UV cross-linking assay. Thus, we postulated that the observed increase of cyclin D1 expression during cellular aging is due to an increase in the binding activity of specific nuclear protein factors to an enhancer element, Sp1, and a decrease in binding to a silencer element in senescent cells.

Enhanced expression of cyclin D1 in senescent human fibroblasts

When human fibroblast, TIG-1, was growth-stimulated with fetal bovine serum, the induction level of cell cycle-dependent genes was generally much lower in senescent cells than in young counterparts. Exceptionally, the expression level of cyclin D1 in senescent cells was constitutively higher than in young cells and further increased after serum stimulation, which was confirmed by Northern and Western blots and immunoprecipitation. This was also true in other human diploid fibroblast lines, TIG-3 and MRC-5. However, cyclin D1-dependent kinase activity was not detected in senescent cells. When sense- or antisense-cyclin D1 cDNA driven by beta-actin promoter was transfected into young TIG-1 cells, the number of appeared colonies from sense-strand transfected cultures was lower than that from antisense-strand-transfected ones. However, clones expressing cyclin D1 at low or undetectable level which were isolated after transfection with antisense-cyclin D1 proliferated up to the same division limit as untransfected and sense-strand transfected cells. Four clones of SV40-transformed TIG-1 expressed cyclin D1 at moderate levels during their extended proliferative lifespan. It appears that, if the extremely overexpressed cyclin D1 could cause an inhibition of cell proliferation at senescent stage, cellular senescence occurs regardless of overexpression of cyclin D1.

Elevated levels of glycoprotein gp200 in progeria fibroblasts

The glycosylation of proteins in fibroblasts from people with the premature ageing disease Hutchinson-Gilford Progeria Syndrome (progeria) was investigated. Protein was prepared from fibroblast cell lines established from skin biopsy taken from progeria patients and control donors. Glycoproteins were labelled by the covalent attachment of the steroid hapten digoxygenin to the sugar group. After separation of total protein by SDS-PAGE and electroblotting onto Immobilon-PTM, glycoproteins were detected by enzyme immunoassay. We have observed a glycoprotein of M(r) 200 kDa which is consistently present in protein preparations from progeria fibroblasts and which is absent, or markedly reduced, in preparations from control fibroblasts. This suggests that it may be useful as a marker for progeria. Similar analysis of progeria lymphoblast and control lymphoblast cultures did not show this altered pattern of glycosylated proteins, indicating that it may be cell-type specific. Glycoproteins were also detected by labelling fibroblasts in vitro with D-[6-3H]glucosamine hydrochloride followed by SDS-PAGE of isolated protein and subsequent fluorography. Profiles of glycoproteins from progeria and control fibroblasts were consistent with those obtained from labelling of carbohydrate groups with digoxygenin. Protease digestion of cell protein verified that the band at M(r) 200 kDa contains a protein core. Characteristic features of progeria primarily involve the connective tissue and include wrinkled and loose skin, loss of soft tissue, thin limbs and stiff joints. Death of progeria patients is usually a result of cardiovascular abnormalities. The most consistent manifestations thus involve the connective tissue. The glycoprotein of M(r) 200 kDa which we have observed in progeria fibroblasts in vitro could reflect a perturbation in glycosylation which may underly the connective tissue defects seen in progeria.

Protein synthesis and the components of protein synthetic machinery during cellular aging

The slowing down of protein synthesis is a change widely observed during the aging of organisms. It has also been claimed that a decline in the rate of protein synthesis occurs during cellular aging. However, the evidence in favour of this view is not clear-cut, and reliable estimates of rates of protein synthesis during cellular aging have yet to be made. Studies on various components of the protein synthetic machinery during cellular aging have revealed a decline in the efficiency and accuracy of ribosomes, an increase in the levels of rRNA and tRNA, and a decrease in the amounts and activities of elongation factors. Detailed studies on the structure and function of ribosomes, tRNA isoacceptor profiles, activities of aminoacyl-tRNA synthetases, levels and activities of initiation factors, rates of protein elongation, and the accuracy of protein synthesis will be needed before the molecular mechanisms of the regulation of protein synthesis during cellular aging can be understood.

Regulation of the retinoblastoma anti-oncogene in aging human diploid fibroblasts

Cell cycle expression of the retinoblastoma (RB) gene was investigated in young and aging IMR 90 human diploid fibroblasts and in the immortalized but untransformed A31 BALB/c 3T3 mouse fibroblasts. RB RNA levels increased about 75% during late G1/S in both the young and old IMR 90 cells. Young and old cells had similar normalized amounts of RB RNA per cell. Kinase C activation did not stimulate RB transcription. The A31 cells showed a constant RB RNA level throughout G0/S. The data is consistent with the hypothesis that RB activity is regulated primarily post-transcriptionally and indicates that a change in the regulation of RB transcription is not part of the senescence phenomenon.

Early kinase C dependent events in aging human diploid fibroblasts

Two early kinase C dependent events, the induction of c-fos transcription and the phosphorylation of an 87 kDa protein, were investigated in aging IMR 90 human diploid fibroblasts. C-fos induction and 87-kDa protein phosphorylation were similarly regulated in aging cells and their young counterparts. The data suggests that G0 exit or the competence phenomenon is intact in aging cells and that the proliferative block occurs later in G0/S.

The effects of in vitro age and culture state on histone variant synthesis in human diploid fibroblasts

Histone variant synthesis patterns from human diploid fibroblast-like cells of different in vitro ages were determined during exponential growth, at confluence, and during low serum arrest. The results are reported as the ratios of H2A variant synthesis (H2A.1 and H2A.2/H2A.x and H2A.z) and H3 variant synthesis (H3.1 and H3.2/H3.3) that have been used to characterize individual cell cycle states. Hydroxyurea was employed in some experiments to reduce S phase cells. The results indicate that high population doubling level (PDL) cells move through the G1 phase of the division cycle during exponential growth and exist in the G0 cell cycle state at confluence and during low serum arrest. Low PDL cells, however, exist in the G1 cell cycle state at confluence and revert to a G0 state only after maintenance as quiescent populations. This would suggest that when stimulated high PDL cells cannot enter into S phase, they revert to a GO cell cycle state.

Histone variant synthesis patterns in confluent populations of human diploid cells of different in vitro ages

Histone variant synthesis patterns have been used to determine the cell cycle state of cultured cells. H2A and H3 histone variant synthesis patterns from confluent populations of human diploid fibroblast-like cells of different in vitro ages were determined and compared as ratios. In the absence of significant levels of DNA synthesis, there was an age related increase in the H2A ratio while the H3 ratio remained constant. These results indicate that confluent populations of older HDF are either in a different state of the cell cycle than younger cells or that they are synthesizing a different set of histone components while being maintained in the same cell cycle state at confluence. The synthesis and deposition of inappropriate histone components may alter chromatin structure in older cells and inhibit their subsequent progression through the cell cycle.