Decline of signal transduction by phospholipase C gamma 1 in IMR 90 human diploid fibroblasts at high population doubling levels

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

1alpha,25(OH)2D3 causes a rapid increase in phosphatidylinositol-specific PLC-beta activity via phospholipase A2-dependent production of lysophospholipid

1alpha,25(OH)(2)D(3) activates protein kinase C (PKC) in rat growth plate chondrocytes via mechanisms involving phosphatidylinositol-specific phospholipase C (PI-PLC) and phospholipase A(2) (PLA(2)). The purpose of this study was to determine if 1alpha,25(OH)(2)D(3) activates PI-PLC directly or through a PLA(2)-dependent mechanism. We determined which PLC isoforms are present in the growth plate chondrocytes, and determined which isoform(s) of PLC is(are) regulated by 1alpha,25(OH)(2)D(3). Inhibitors and activators of PLA(2) were used to assess the inter-relationship between these two phospholipid-signaling pathways. PI-PLC activity in lysates of prehypertrophic and upper hypertrophic zone (growth zone) cells that were incubated with 1alpha,25(OH)(2)D(3), was increased within 30s with peak activity at 1-3 min. PI-PLC activity in resting zone cells was unaffected by 1alpha,25(OH)(2)D(3). 1beta,25(OH)(2)D(3), 24R,25(OH)(2)D(3), actinomycin D and cycloheximide had no effect on PLC in lysates of growth zone cells. Thus, 1alpha,25(OH)(2)D(3) regulation of PI-PLC enzyme activity is stereospecific, cell maturation-dependent, and nongenomic. PLA(2)-activation (mastoparan or melittin) increased PI-PLC activity to the same extent as 1alpha,25(OH)(2)D(3); PLA(2)-inhibition (quinacrine, oleyloxyethylphosphorylcholine (OEPC), or AACOCF(3)) reduced the effect of 1alpha,25(OH)(2)D(3). Neither arachidonic acid (AA) nor its metabolites affected PI-PLC. In contrast, lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) activated PI-PLC (LPE>LPC). 1alpha,25(OH)(2)D(3) stimulated PI-PLC and PKC activities via Gq; GDPbetaS inhibited activity, but pertussis toxin did not. RT-PCR showed that the cells express PLC-beta1a, PLC-beta1b, PLC-beta3 and PLC-gamma1 mRNA. Antibodies to PLC-beta1 and PLC-beta3 blocked the 1alpha,25(OH)(2)D(3) effect; antibodies to PLC-delta and PLC-gamma did not. Thus, 1alpha,25(OH)(2)D(3) regulates PLC-beta through PLA(2)-dependent production of lysophospholipid.

Retinoic acid extends the in vitro life span of normal human oral keratinocytes by decreasing p16(INK4A) expression and maintaining telomerase activity

Retinoic acid (RA) plays an important role in the regulation of cell growth and differentiation. To investigate whether RA extends in vitro the life span of human epithelial cells, we examined the effect of all-trans RA on both the cumulative population-doubling level (PDL) and the replicative senescence of cultured oral keratinocytes. When proliferating oral keratinocytes were cultured in medium containing 1 nM of all-trans RA, the in vitro life span of the cells was increased 1.5- to 1.8-fold compared to the vehicle control and the replicative senescence of the cells was significantly inhibited. Since the replicative senescence of human epithelial cells is associated with a steady increase of p16(INK4A) and a loss of telomerase activity, we expected that RA could delay the replicative senescence of oral keratinocytes by decreasing p16(INK4A) expression and/or inhibiting the loss of telomerase activity. To test this possibility, we examined the expression of replicative senescence-associated genes and the telomerase activities of different PDL numbers of oral keratinocytes exposed to 1 nM of all-trans RA. The protein level of cellular p16(INK4A) in the RA-treated oral keratinocytes was gradually but significantly enhanced by an increased PDL number; however, the level was significantly lower than that of the vehicle control at all of the same PDL numbers. In contrast, the telomerase activity was maintained in oral keratinocytes with increasing PDL numbers induced by RA treatment. Summarizing, these results indicate that RA induces the in vitro life-span extension of oral keratinocytes, which is linked to a decreased cellular level of p16(INK4A) and the maintenance of telomerase activity.

Molecular markers of senescence in fibroblast-like cultures

The loss of replicative capacity in vitro of normal human diploid fibroblasts is a model for studying molecular changes that accompany both regulated growth control and cellular senescence. We describe the molecular phenotype of senescent fibroblasts in terms of markers that are altered with proliferative decline. We describe these markers by analyzing pathways and associated mechanisms related to the responsiveness of proliferatively competent and senescent cells to growth signals including changes in the extracellular environment, growth factors, growth factor receptors, secondary messengers, cell-cycle progression, transcription factors, and the fidelity of DNA synthesis. There is an abundance of molecular markers for senescence in culture at every level of information transfer. Although it seems clear that some alterations in gene expression with senescence are the result of specific changes in upstream events, more global dysregulation of coordinated growth control point to as yet undefined mechanisms.

Potentiated bradykinin-induced increase of 1,2-diacylglycerol generation and phospholipase D activity in human senescent fibroblasts

1. The comparative study of the effect of bradykinin (BK) in young and old IMR-90 human fibroblasts shows that old cells are characterized by a reduced increase in 1,2-diacylglycerol (1,2-DAG) generation upon stimulation after short-term treatment and a significant higher increase after long-term agonist treatment. BK-induced activation of phospholipase D (PLD), the major enzyme involved in sustained 1,2-DAG generation, was 2.5-fold higher in old cells, strongly suggesting that it is involved in the potentiated increase of 1,2-DAG formation. The increased activation of PLD by BK in old cells was specific, since in parallel experiments the effect of thrombin was not significantly different in young and old cells. PLD activity in old cells was only reduced by down-regulation of protein kinase C (PKC) activity, in contrast to what was observed in young cells where it was completely abolished. This indicates that the enzyme activity in old cells was partially PKC-independent. BK was also able to increase the release of [14C]ethanolamine, a water-soluble product of hydrolysis of phosphatidylethanolamine (PtdEtn), through PLD activation in young and old cells. The BK effect was significantly higher in old cells and, very likely, PKC-independent, since phorbol 12-myristate 13-acetate failed to induce PtdEtn hydrolysis. 2. The present results indicate that the PLD/1,2-DAG pathway is specifically potentiated by BK in old fibroblasts, demonstrating that the formation of positive effectors of PKC activation is not necessarily decreased in cellular senescence. It remains to be established whether the increased generation of DAG upon BK stimulation plays any role in the altered PKC signalling pathway which characterizes old fibroblasts.