Differential extracellular matrix gene expression by fibroblasts during their proliferative life span in vitro and at senescence

Generation of Human Neural Stem Cells by Direct Phenotypic Conversion

Human neural stem cells (hNSC) are multipotent adult stem cells. Various studies are underway worldwide to identify new methods for treatment of neurological diseases using hNSC. This chapter summarizes the latest research trends in and fields for application of patient-specific hNSC using direct phenotypic conversion technology. The aim of the study was to analyze the advantages and disadvantages of current technology and to suggest relevant directions for future hNSC research.

Extracellular matrix alterations in senescent cells and their significance in tissue homeostasis

Normal cells after a defined number of successive divisions or after exposure to genotoxic stresses are becoming senescent, characterized by a permanent growth arrest. In addition, they secrete increased levels of pro-inflammatory and catabolic mediators, collectively termed "senescence-associated secretory phenotype". Furthermore, senescent cells exhibit an altered expression and organization of many extracellular matrix components, leading to specific remodeling of their microenvironment. In this review we present the current knowledge on extracellular matrix alterations associated with cellular senescence and critically discuss certain characteristic examples, highlighting the ambiguous role of senescent cells in the homeostasis of various tissues under both normal and pathologic conditions.

HSP90 inhibition blocks ERBB3 and RET phosphorylation in myxoid/round cell liposarcoma and causes massive cell death in vitro and in vivo

Myxoid sarcoma (MLS) is one of the most common types of malignant soft tissue tumors. MLS is characterized by the FUS-DDIT3 or EWSR1-DDIT3 fusion oncogenes that encode abnormal transcription factors. The receptor tyrosine kinase (RTK) encoding RET was previously identified as a putative downstream target gene to FUS-DDIT3 and here we show that cultured MLS cells expressed phosphorylated RET together with its ligand Persephin. Treatment with RET specific kinase inhibitor Vandetanib failed to reduce RET phosphorylation and inhibit cell growth, suggesting that other RTKs may phosphorylate RET. A screening pointed out EGFR and ERBB3 as the strongest expressed phosphorylated RTKs in MLS cells. We show that ERBB3 formed nuclear and cytoplasmic complexes with RET and both RTKs were previously reported to form complexes with EGFR. The formation of RTK hetero complexes could explain the observed Vandetanib resistence in MLS. EGFR and ERBB3 are clients of HSP90 that help complex formation and RTK activation. Treatment of cultured MLS cells with HSP90 inhibitor 17-DMAG, caused loss of RET and ERBB3 phosphorylation and lead to rapid cell death. Treatment of MLS xenograft carrying Nude mice resulted in massive necrosis, rupture of capillaries and hemorrhages in tumor tissues. We conclude that complex formation between RET and other RTKs may cause RTK inhibitor resistance. HSP90 inhibitors can overcome this resistance and are thus promising drugs for treatment of MLS/RCLS.

Randomized, double-blinded, vehicle-controlled, split-face study to evaluate the effects of topical application of a Gold Silk Sericin/Niacinamide/Signaline complex on biophysical parameters related to skin ageing

OBJECTIVE

To investigate the effects of topical application of a Gold Silk Sericin (GSS) complex on biophysical parameters related to skin ageing.

METHODS

A range of non-invasive bioengineering methods were deployed in an 8-week randomized, double-blinded, vehicle-controlled, split-face study among 40 female subjects aged 40-70. Endpoints measured included expert grades of skin condition, stratum corneum (SC) hydration, SC barrier function, elasticity and surface topography.

RESULTS

The GSS complex produced significant single-variable (P < 0.05) improvements in SC hydration, barrier function, elasticity and surface topography compared with the Vehicle control.

CONCLUSION

The GSS complex examined in this study represents an interesting new cosmetic topical technology with which to address multiple aspects of aged/photoaged female facial skin.

Stress-induced senescence exaggerates postinjury neointimal formation in the old vasculature

This study aims to demonstrate the role of stress-induced senescence in aged-related neointimal formation. We demonstrated that aging increases senescence-associated β-galactosidase activity (SA-β-Gal) after vascular injury and the subsequent neointimal formation (neointima-to-media ratio: 0.8 ± 0.2 vs. 0.54 ± 0.15) in rats. We found that senescent cells (SA-β-Gal+ p21+) were scattered throughout the media and adventitia of the vascular wall at day 7 after injury and reached their maximum number at day 14. However, senescent cells only persisted in the injured arteries of aged animals until day 30. No senescent cells were observed in the noninjured, contralateral artery. Interestingly, vascular senescent cells accumulated genomic 8-oxo-7,8-dihydrodeoxyguanine, indicating that these cells were under intense oxidative stress. To demonstrate whether senescence worsens intimal hyperplasia after injury, we seeded matrigel-embedded senescent and nonsenescent vascular smooth muscle cells around injured vessels. The neointima was thicker in arteries treated with senescent cells with respect to those that received normal cells (neointima-to-media ratio: 0.41 ± 0.105 vs. 0.26 ± 0.04). In conclusion, these results demonstrate that vascular senescence is not only a consequence of postinjury oxidative stress but is also a worsening factor for neointimal development in the aging vasculature.

Expression of TARSH gene in MEFs senescence and its potential implication in human lung cancer

To reveal the molecular mechanism of cellular senescence, we have surveyed the genes that are specifically upregulated via MEFs senescence by suppression subtractive hybridization method. We show here that mTARSH was induced particularly in the relative early phase of MEFs cellular senescence. Further structural analysis of mTARSH disclosed five splicing variants shared a common reading frame whose diversity was derived from the SH3-binding motif cluster in the middle of the gene. We also show that mTARSH mRNA predominantly expressed in lung and that conspicuous expression of TARSH was drastically declined in all several lung cancer cell lines we tested. Thus, TARSH presumably represents a trigger gene for evoking cellular senescence, which has also been suggested to be involved in the prevention of tumorigenesis.

Impact of collagen structure on matrix trafficking by human fibroblasts

Biodegradation of collagen biomaterial matrices and the deposition of new collagen extracellular matrix (ECM) are critical to the integration of in vitro bioengineered materials and tissues in vivo. In previous studies, we observed significant impact of collagen matrix structure on primary lung fibroblast behavior in vitro. In the present work, to begin to understand the mechanistic basis for our previous observation, the response of human fibroblasts (IMR-90) to the structural state of collagen matrices was studied with respect to cell proliferation, cell morphology, beta-galactosidase level, and transcript content for collagen (Col-1), matrix metalloproteinases (MMP-1, MMP-2), tissue inhibitors of matrix metalloproteinase (TIMP-1 and TIMP-2). Collagen digestion was assessed quantitatively by uptake of collagen-coated fluorescent beads incorporated in the preformed collagen matrix. Transcript levels related to the deposition of new ECM proteins varied as a function of the structure of the collagen matrix presented to the cells. Col-1 expression was 2-fold higher and expression for MMP-1, MMP-2, TIMP-1, and TIMP-2 increased for cells when grown on 156 microg/cm2 denatured collagen compared with cells grown on tissue culture (TC) plastic. On 156 microg/cm2 nondenatured (native) collagen, Col-1 expression was decreased by half and MMP-2 was increased by 2.5-fold compared with cells grown on TC plastic. On 78 microg/cm2 denatured collagen, Col-1 expression was 80% whereas the MMPs and TIMPs were increased by 1.25- to 2-fold compared with cells grown on TC plastic. On 78 microg/cm2 nondenatured collagen expression of all 5 transcripts was reduced 60-90% of the levels determined for the cells grown on TC plastic. Cell viability, based on cell morphology and beta-galactosidase activity, was improved on the denatured collagen. A higher level of collagen matrix incorporation was observed for cells grown on denatured collagen than on nondenatured collagen or TC plastic. These data suggest that tissue engineering matrices incorporating denatured collagen may promote more active remodeling toward new ECM in comparison to cells grown on nondenatured collagen or cells grown on TC plastic.

Involvement of IL-1 family proteins in p38 linked cellular senescence of mouse embryonic fibroblasts

Senescence of mammalian cultured cells is essentially organized by a machinery of cell division and cellular stresses induced by various extracellular stimuli. Here, we show that in mouse embryonic fibroblasts (MEFs) culture in vitro, expression of an inflammatory cytokine, interleukin-1beta (IL-1beta) and its antagonist, IL-1 receptor antagonist (IL-1Ra) are induced by senescence. The kinetics of IL-1beta-expression was similar to that of p38 activation during MEFs culture. We also found a distinguishable accelerated senescence in cell growth in IL-1Ra deficient MEFs culture. Our results suggest that IL-1beta signaling pathway is involved in activation of p38 linked cellular senescence.

Expression of a membrane-bound form of the ferroxidase ceruloplasmin by leptomeningeal cells

Ceruloplasmin is a key enzyme involved in detoxifying ferrous iron, which can generate free radicals. The secreted form of ceruloplasmin is produced by the liver and is abundant in serum. We have previously identified a membrane-bound glycosylphosphatidylinositol (GPI)-anchored form of ceruloplasmin (GPI-Cp) that is expressed by astrocytes in the central nervous system (CNS) (Patel and David. 1997. J Biol Chem 272:20185-20190). We now provide direct evidence that rat leptomeningeal cells, which cover the surface of the brain, also express GPI-Cp. The expression of GPI-Cp on the surface of these cells increases with postnatal development and is regulated in vitro by cell density, time in culture, and various extracellular matrix molecules. The expression of GPI-Cp also appears to be regulated differently in astrocytes and leptomeningeal cells in vitro. The abundant expression of GPI-Cp on the surface of leptomeningeal cells suggests that these cells play a role in antioxidant defense along the surface of the postnatal CNS possibly by detoxifying the cerebrospinal fluid.

Cellular mechanisms for diminished scarring with aging

The study of an age-dependent spectrum of scar formation is driven by the desire to understand and recapitulate scarless healing. Although focus in the past has been directed toward scarring in the fetus, less exuberant scarring is a common clinical observation in the elderly. Cell turnover is a major contributor to the development of scar tissue and is governed by the proliferative and apoptotic cellular fractions within a healing wound. We hypothesize that the balance between cell proliferation and apoptosis during late stages of excisional wound healing is, at least in part, responsible for age-related variations in scarring potential. Full-thickness 7-mm ulcers (four per ear), exposing bare cartilage, were made on the inner surface of the ear on 12 young and 12 aged New Zealand White rabbits. Analyses were performed at days 15, 21, and 28 postwounding. A previously described Scar Elevation Index was derived from histomorphometric analysis, along with the quantification of epithelial ingrowth and total cellularity. Apoptotic cellular fractions were derived from TdT-mediated dUTP nick end-labeling assay-stained histologic sections; proliferative fractions were derived from proliferating cell nuclear antigen-labeled serial sections. Young rabbits demonstrated significantly greater scar elevation/area. Apoptosis was strongly associated with progress of epithelialization in both groups. Significantly higher proliferative indices were seen in the young and were sustained through day 28, by which time levels had substantially declined in the aged. No differences in apoptotic indices were demonstrated between groups at any time point. The clinical observation of less exuberant scarring with aging is supported by this animal model. Apoptosis follows the progression of epithelialization but does not appear to independently influence scar morphology. A diminished proliferative response during later stages of healing is an important contributing mechanism for the decrease in scar formation seen in the elderly.

OPC-15161 suppresses the proliferation of Tenon's capsule fibroblasts and the production of type I collagen and fibronectin stimulated by TGF-beta1 in vitro

PURPOSE

We investigated the effects of OPC-15161 on the growth of cultured human Tenon's capsule fibroblasts (TCFs), as well as on the production of type I procollagen, fibronectin, and laminin. These effects were examined in the presence or absence of TGF-beta1.

METHODS

Cell proliferation was assayed by counting cell number and assay of DNA synthesis. Cytotoxicity was determined by the MTT method. Matrix components were assayed by enzyme immunoassay of material in the medium and in the cell lysate with or without OPC-15161. Total protein content was determined. Cellular ultrastructure was also evaluated.

RESULTS

Treatment with OPC-15161 (up to 100.0 microg ml(-1)) significantly reduced the proliferation and DNA synthesis of TCFs. No significant decrease in MTT values was observed in confluent TCF cultures with OPC-15161 (up to 100.0 microg ml(-1)). TGF-beta1 enhanced the TCF production of procollagen I and fibronectin. OPC-15161 significantly decreased the procollagen I content in both the medium, in the cell lysate of TGF-beta1-stimulated cells, and fibronectin content in the lysate. OPC-15161 did not affect the laminin or total protein content, either with or without TGF-beta1. No ultrastructural evidence of cytotoxicity was observed.

CONCLUSIONS

OPC-15161 inhibited the proliferation of TCFs, and reduced their production of procollagen I and fibronectin in the presence of TGF-beta1 without evidence of cytotoxicity. OPC-15161 may be useful in inhibiting the excessive fibrosis produced in the wound in response to filtering surgery.

Cell density-dependent regulation of fibronectin splicing at the EDA region in fibroblasts: cell density also modulates the responses of fibroblasts to TGF-beta and cancer cell-conditioned medium

Recently we reported that cancer cell-fibroblast interactions can modulate the expression of fibronectin (FN) isoforms in vitro, i.e. conditioned medium of human rectal adenocarcinoma cell line RCM-1 (RCM-1 CM) stimulated the expression of EDA-containing FN (EDA(+)FN) mRNA by fibroblasts and this stimulation was partly mediated by transforming growth factor-beta (TGF-beta) included in RCM-1 CM. In the present study, cell density was shown to regulate FN splicing at the EDA region in fibroblasts. Fibroblasts plated at a low cell density expressed a significantly higher percentage of EDA(+)FN mRNA than those plated at a high cell density. Moreover, fibroblast cell density modulated the effects of TGF-beta and RCM-1 CM on FN splicing at the EDA region differently. The time courses of their effects were similar to each other at a high cell density. At a low cell density, however, they were different. TGF-beta showed a relatively short-lived stimulation of EDA(+)FN mRNA, with the peak response 24 h after treatment, followed by a decline to the base line by 72 h. On the other hand, RCM-1 CM caused a prolonged stimulation, maintaining almost the maximum responses from 24 to 72 h. Thus, these results at a low cell density indicated the presence of a factor(s) other than TGF-beta in RCM-1 CM that stimulates the expression of EDA(+)FN mRNA directly or modulates the effect of TGF-beta. The use of several different cell densities might help in the search for new factors affecting FN splicing.

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.

A hVti1 homologue: its expression depends on population doubling levels in both normal and SV40-transformed human fibroblasts

A cDNA clone was isolated by differential colony hybridization from a cDNA library prepared from life-extended SV40-transformed human fibroblasts. The clone, tentatively named N-10, was 1272 bp in length coding for 232 amino acids. Northern analysis revealed that the expression level of N-10 was increased in normal senescent and life-extended SV40-transformed fibroblasts than in their young counterparts but was not enhanced by growth arrest. The protein fused to GFP (green fluorescent protein) localized in cytoplasmic granule. Enforced expression of N-10 resulted in premature senescence in young fibroblasts. The deduced amino acid sequence of N-10 was identical to the recently reported hVti1 gene except in one amino acid: Asp24(GAC) was ours and Asn24 (AAC) was reported. Additional base differences were found, so we referred to our sequence as the hVti1 homologue. As hVti1 protein was suggested to be involved in the vesicle transport process, the homologue may be concerned with increased secretion of extracellular matrix and various cytokines associated with cellular senescence.

Increased transcription and modified growth state-dependent expression of the plasminogen activator inhibitor type-1 gene characterize the senescent phenotype in human diploid fibroblasts

The type-1 inhibitor of plasminogen activator (PAI-1) is a major physiologic regulator of pericellular proteolytic activity and, as such, influences matrix integrity, cell-to-substrate adhesion, and cellular proliferation. Excessive accumulation of both PAI-1 mRNA and protein correlates with the progressive acquisition of morphological and growth traits characteristic of the senescent phenotype (Mu and Higgins, 1995, J. Cell. Physiol., 165:647-657). Compared to early-passage IMR-90 human diploid fibroblasts, a late-passage senescence-associated 11-fold elevation in steady-state PAI-1 mRNA content reflected a 15-fold increase in constitutive PAI-1 gene transcription. Differential mRNA stability was not a factor in age-associated PAI-1 overexpression in IMR-90 cells. Upon removal of serum, early-passage human fibroblasts enter into a state of growth arrest with marked down-regulation of PAI-1 synthesis. Rapid induction of both the 3.0- and 2.2-kb PAI-1 mRNA species was evident upon serum-induced "activation" of quiescent early-passage fibroblasts; induced PAI-1 transcripts were maximal at 2 hr post-serum stimulation and declined in late G1 prior to entry into S phase. In contrast, late-passage (p32) fibroblasts maintained a significant level of PAI-1 expression under serum-free culture conditions. Although the PAI-1 gene was further responsive to serum in senescent cells, transcript abundance remained elevated and actually increased over the 12 to 16 hr post-serum addition period (a time when early-passage fibroblasts down-regulate PAI-1 mRNA content). Development of the senescent phenotype in human fibroblasts is associated, therefore, with significant changes in PAI-1 gene regulation. Such reprogramming involves predominantly transcriptional events and results in a marked increase in steady-state PAI-1 transcript abundance involving both the 3.0- and 2.2-kb mRNA species.

Modulation of transcription of the rat fibronectin gene by cell density

The fibronectin (FN) gene is under complex regulatory control in vitro and in vivo. Sequences from the rat FN gene directed efficient expression of a lacZ reporter gene product, beta-galactosidase, in NIH/3T3 mouse fibroblasts. Stable transfectants were generated to facilitate studies of gene regulation by cell growth state. The expression of FN-lacZ constructs increased approximately twofold when cultures attained confluence, relative to total protein. The magnitude of this increase correlates well with that observed for FN mRNA levels and protein synthesis rate. Fragments containing 4.9, 0.9, or 0.3 kbp upstream of the transcription start site are equally responsive to cell density and/or cell contact. Deletion of a cAMP-responsive element enhanced the response, suggesting a negative role for this sequence motif and demonstrating that the FN gene is regulated by cell density at the transcriptional level. The effect of high cell density is apparently different from decreased growth rate, as incubation with low serum did not result in increased expression of the lacZ reporter. Finally, conditioned medium from dense cells did not enhance reporter gene expression in sparse cells, suggesting that the density signal is not transmitted via a soluble factor.

Differential growth state-dependent regulation of plasminogen activator inhibitor type-1 expression in senescent IMR-90 human diploid fibroblasts

The type-1 inhibitor of plasminogen activator (PAI-1) regulates pericellular proteolytic activity functioning, thereby to control matrix integrity, cell growth, and morphology. Subconfluent late-passage IMR-90 human fibroblasts and normal rat kidney (NRK) cells, both at the stage of replicative senescence accumulated 15- to 30-fold more undersurface PAI-1 protein compared to early-passage, actively-proliferating, cultures. Senescence-associated elevations in PAI-1 expression by IMR-90 cells reflected corresponding 11-fold increases in the 3.0- and 2.2-kb PAI-1 mRNA species. The 2.2-kb transcript exhibited a greater age-dependent increase (7.2-fold) compared to the 3.0-kb mRNA (3.7-fold). Since PAI-1 expression is coupled to growth activation in serum-deprived cultures (Ryan and Higgins, 1993, J. Cell. Physiol., 155:376-384), it was important to determine if PAI-1 gene regulation was altered as a function of cellular aging. In contrast to early-passage cultures, senescent IMR-90 fibroblasts did not down-regulate either PAI-1 protein expression or steady-state levels of PAI-1 mRNA transcripts upon serum-deprivation. Late-passage human fibroblasts at their proliferative end-stage, thus, appear to regulate PAI-1 mRNA levels through different mechanisms than do young, actively-proliferating, cells. PAI-1 overexpression during in vitro cellular aging, therefore, may contribute to the acquisition of specific senescence-associated phenotypic traits (e.g., enlarged cell morphology; increased adhesivity) by altering the pericellular proteolytic balance influencing, in turn, the formation or stability of cell-to-substrate attachment complexes.

Molecular hysteresis: residual effects of hormones and glucose on genes during aging

The basic concept of molecular hysteresis may be succintly summarized as follows in the following Limerick:. Hormones behave like Don Juan: They show up, do their thing, then they're gone. But when genes have been kissed Some effects may persist, And the melody still lingers on.

Senescence of aortic endothelial cells in culture: effects of basic fibroblast growth factor expression on cell phenotype, migration, and proliferation

Bovine aortic endothelial cells (BAEC) can be isolated in large numbers without major contamination by other cells and maintained in culture with a limited life span for about 100 population doublings. In order to study phenotypic changes of BAEC during long-term culture, stocks of different passages of BAEC were established and their morphological, migratory, and proliferative properties analyzed. Early-passage BAEC (passages 5-15) rapidly produce dense, cobblestone-like monolayers. Their growth beyond the monolayer configuration is characterized by the formation of an irregular network of spindle-shaped, crisscrossing BAEC growing either on top or beneath the monolayer, and by the assembly of elongated BAEC into well-differentiated capillary-like tubes. In contrast, senescent BAEC (passages 35-45) form perfect cobblestone monolayers that contain several, often multinucleated giant cells and a few capillary-like tubes but not the crisscrossing networks of their early-passage counterparts. The rates of BAEC migration and proliferation gradually decline during in vitro senescence. This decline is neutralized by exogenous basic fibroblast growth factor (bFGF) which elevates the migratory and proliferative capacities of early-passage and senescent BAEC to uniformly high levels. Northern blot analysis shows a gradual decline in bFGF message and an increase in laminin message during in vitro BAEC senescence. The present study supports the concept of autocrine growth regulation of BAEC and associates a decreased bFGF message with decreased rates of migration and proliferation as well as loss of the crisscrossing BAEC morphotype in senescent cultures.

Deficiency in integrin-mediated transmembrane signaling and microfilament stress fiber formation by aging dermal fibroblasts from normal and Down's syndrome patients

Previous evidence has shown a deficiency in microfilament stress fiber formation upon short-term cycloheximide treatment of cultured human dermal fibroblasts while cytoplasmic spreading appeared completely normal and other cytoskeletal networks organized normally. This deficiency applied to collagen substrata (not fibronectin substrata) and was specific for in vitro-aged normal fibroblasts and for fibroblasts from three different Down's syndrome patients at any passage level. To identify the mechanism(s) for matrix receptor deficiency in aging cells, cells were evaluated for amounts and distributions of several integrin subunits using specific monoclonal antibodies and two complementary experimental approaches. Flow cytometric analyses have shown that all these cells at all passage levels have large amounts of alpha 3 and beta 1 integrin subunits and smaller amounts of the alpha 5 subunit, directed to fibronectin, which are minimally affected in their cell surface availability by cycloheximide treatment. In contrast, cycloheximide treatment leads to the loss from surface availability of most of the alpha 2 subunit, directed to collagen, in late-passage papillary and reticular normal fibroblasts and in all three Down's patient cells at all passages. Prior growth of cells in ascorbate-supplemented medium, which overcomes the deficiency in stress fiber formation, conserves the large amounts of cell surface-available alpha 2 subunit detectable by flow cytometry. When amounts of integrin subunits were evaluated by immunoprecipitation of [35S]methionine-radiolabeled cells, there was no diminution of the alpha 2 subunit or any other subunit for any cells upon cycloheximide treatment; however, there was much less alpha 2 subunit complexed with beta 1 in aging normal and Down's cells. Therefore, cycloheximide treatment does not lead to loss in the amounts of the alpha 2 subunit but rather to its masking at the cell surface and inability to transmit signals across the plasma membrane to effect stress fiber formation. This aging-related deficiency in integrin-mediated signaling can now be studied mechanistically with a variety of approaches to determine the nature of cell-surface molecules interacting with integrins (cis- and/or trans-acting molecules) that discriminate functional from nonfunctional receptors.

Similar, but not identical, modulation of expression of extracellular matrix components during in vitro and in vivo aging of human skin fibroblasts

Regulation of the synthesis of procollagen and other extracellular matrix components was examined in human skin fibroblasts obtained from donors of various ages, from fetal to 80 years old (in vivo aged), and in fetal fibroblasts at varying passage levels (in vitro aged). Growth rates and saturation densities of fibroblasts decreased with increasing age of the donor and after passage 20 of fetal fibroblasts. The rates of collagen and proteoglycan synthesis also decreased during both types of aging to about 10-25% of the rate in early passage fetal fibroblasts, whereas the synthesis of total noncollagenous proteins was not greatly affected. Decreased collagen synthesis in both types of aging was correlated with lower steady-state levels of mRNAs for the two subunits of type I procollagen mRNA, although their regulation was not coordinate. Type III collagen mRNA levels also declined in both types of aging. The concentration of fibronectin mRNA also decreased during in vitro aging but more rapidly than the collagen mRNAs, whereas in fibroblasts from 51-80-year-old donors, it was similar to or higher than in early passage fetal fibroblasts. This study suggests that the decreased synthesis of procollagen and proteoglycans in in vivo aged fibroblasts represents changes that are responsible for intrinsic degenerative changes that occur in human skin during aging. Furthermore, although in vitro and in vivo aging were similar in many respects, they were not equivalent, as evidenced by the differences in regulation of fibronectin expression.