Requirements for maintaining the embryonic state of avian tendon cells in culture

A synthetic cell density signal can drive proliferation in chick embryonic tendon cells and tendon cells from a full size rooster can produce high levels of procollagen in cell culture

Cell density signaling drives tendon morphogenesis by regulating both procollagen production and cell proliferation. The signal is composed of a small, highly conserved protein (SNZR P) tightly bound to a tissue-specific, unique lipid (SNZR L). This allows the complex (SNZR PL) to bind to the membrane of the cell and locally diffuse over a radius of ~1 mm. The cell produces low levels of this signal but the binding to the membrane increases with the number of tendon cells in the local environment. In this article SNZR P was produced in E.coli and SNZR L was chemically synthesized. The two bind together when heated to 60 °C in the presence of Ca⁺⁺ and Mg⁺⁺ and the synthesized SNZR PL at ng/ml levels can replace serum. Adding SNZR PL to the medium was also tested on primary tendon cells from adult roosters. The older cells were in a maintenance state in vivo and in cell culture they proliferate more slowly than embryonic cells. Nevertheless, after reaching a moderately high cell density, they produced high levels of procollagen similar to the embryonic cells. This data was not expected from older cells but suggests that adult tendon cells can regenerate the tissue after injury when given the correct signals.

Cells determine cell density using a small protein bound to a unique tissue-specific phospholipid

Cell density is the critical parameter controlling tendon morphogenesis. Knowing its neighbors allows a cell to regulate correctly its proliferation and collagen production. A missing link to understanding this process is a molecular description of the sensing mechanism. Previously, this mechanism was shown in cell culture to rely on a diffusible factor (SNZR [sensor]) with an affinity for the cell layer. This led to purifying conditioned medium over 4 columns and analyzing the final column fractions for band intensity on SDS gels versus biological activity - a 16 kD band strongly correlated between assays. N-terminal sequencing - EPLAVVDL - identified a large gene (424 AA), extremely conserved between chicken and human. In this paper we probe whether this is the correct gene. Can the predicted large protein be cleaved to a smaller protein? EPLAVVDL occurs towards the C-terminus and cleavage would create a small 94 AA protein. This protein would run at ∼10 kD, so what modifications or cofactor binding accounts for its running at 16 kD on SDS gels? This protein has no prominent hydrophobic regions, so can it be secreted? To validate its role, the chicken cDNA for this gene was tagged with myc and his and transfected into a human osteosarcoma cell line (U2OS). U2OS cells expressed the gene but not passively: differentiating into structures resembling spongy bone and expressing alkaline phosphatase, an early bone marker. Intracellularly, two bands were observed by Western blotting: the full length protein and a smaller form (26 kD). Outside the cell, a small band (28 kD) was detected, although it was 40% larger than expected, as well as multiple larger bands. These larger forms could be converted to the predicted smaller protein (94 AA + tags) by changing salt concentrations and ultrafiltering - releasing a cofactor to the filtrate while leaving a protein factor in the retentate. Using specific degradative enzymes and mass spectrometry, the bone cofactor was identified as a lipid containing a ceramide phosphate, a single chained glycerol lipid and a linker. Tendon uses a different cofactor made up of two fatty acid chains linked directly to the phosphate yielding a molecule about half the size. Moreover, adding the tendon factor/cofactor to osteosarcoma cells causes them to stop growing, which is opposite to its role with tendon cells. Thus, the cofactor is cell type specific both in composition and in the triggered response. Further support of its proposed role came from frozen sections from 5 week old mice where an antibody to the factor stained strongly at the growing ends of the tendon as predicted. In conclusion, the molecule needed for cell density signaling is a small protein bound to a unique, tissue-specific phospholipid yielding a membrane associated but diffusible molecule. Signal transduction is postulated to occur by an increased ordering of the plasma membrane as the concentration of this protein/lipid increases with cell density.

Ascorbate induction of collagen synthesis as a means for elucidating a mechanism of quantitative control of tissue-specific function

Ascorbic acid displays the characteristics of an ideal inducer of tissue-specific function in primary avian tendon cells in culture. It is a highly specific, potent stimulator of collagen synthesis, it demonstrates slow reversible kinetics, and it has no effect on growth rate of the cultured cells. Kinetic analysis of ascorbate induction of collagen synthesis was used to determine the critical steps in this complex biosynthetic pathway. Full hydroxylation of the proline residues in collagen, although probably a necessary step for collagen induction, was in itself not sufficient for achieving either increased secretion or increased synthesis. On the other hand, an increase in secretion rate, which required both the presence of ascorbate and a high cell density, did correlate with the later stimulation in procollagen production. The process of procollagen secretion, therefore, meets the minimal requirements for the rate-limiting step. The fact that the cells maintained a large pool of intracellular procollagen despite changes in the rates of translation or secretion led us to postulate a possible feedback between the level of the internal procollagen pool and the rate of procollagen synthesis.

Evidence supporting the role of a proteinaceous, loosely bound extracellular molecule in the cell density signaling between tendon cells

Normal cells in culture respond to cell density by altering their proliferation rates and their pattern of protein expression. Primary avian tendon (PAT) cells are a case in point where procollagen production increases approximately 10-fold at high cell density while proliferation almost ceases. In an earlier report focusing on the cell density regulation of procollagen expression, the signaling mechanism communicating the presence of other cells was shown to have the characteristics of a loosely bound component of the cell layer. Extending these studies to the cell density regulation of proliferation, the cell density signal (CDS) was again shown to be altered by medium agitation, stimulating cell division. Agitation, however, was only disruptive to cell signaling when there was a high ratio of medium to cells. When sufficient cells were present, agitation was less effective. Therefore, the CDS controlling procollagen production and the CDS controlling the inhibition of growth seemed to be linked because the signaling mechanism is disrupted in a parallel manner by agitation. However, the proliferative response of PAT cells is more complex in that there is also a positive influence at moderate cell density (> 2 x 10(4) cells/cm2) on the rate of cell division. As a consequence, PAT cells would not proliferate into an area of low cell density, but within the same dish would rapidly fill an area of moderate density. PAT cells were capable of filling a gap between high cell density areas if the gap was less than 2 mm. Medium agitation also affected cells at low cell density in a different manner. It was inhibitory if all the cells were at low cell density but it was stimulatory if the cells at low cell density were in close proximity to cells at high cell density. In addition, medium conditioned by agitation over cells at a high cell density would stimulate cells at low cell density to divide and grow out into low cell density regions. Using the growth-promoting activity of the conditioned medium as an assay, this component of the CDS was shown to have unique characteristics: heat, pH, dithiothreitol (DTT) stable; tris ion and protease sensitive. By gel exclusion chromatography it was larger than 100 kDa. But after DTT treatment its mobility shifted to < 30 kDa while retaining activity.

Wound-factor-induced and cell cycle phase-dependent expression of 9E3/CEF4, the avian gro gene

The gro genes encode for secreted proteins with sequence homologies to inflammatory mediators. Little is known about the function of these proteins or their regulation. The chicken gro (9E3/CEF4) is expressed abundantly in the cells of proliferating cultures but at very low levels in confluent cultures. In vivo, this gene is expressed in connective tissue and overexpressed at sites of injury, especially in areas of neovascularization. Here we provide a bridge between these observations by examining in culture the effect on 9E3 expression and DNA synthesis induced by cell damage and by addition of factors known to be released on wounding. We mimicked wounding by scraping swaths across confluent cultures of embryonic fibroblasts and determined the time dependence of expression of 9E3 mRNA and incorporation of 3H-thymidine. We find that 9E3 is (1) transiently expressed after "wounding" or serum-stimulation; (2) expressed in a cell cycle phase-dependent manner; it is triggered during the G0-G1 transition or early in G1 and subsides during S-phase; and (3) stimulated to high levels by a-fibroblast growth factor (aFGF), bFGF, transforming growth factor alpha (TGF alpha), and TGF beta, to intermediate levels by platelet-derived growth factor and not stimulated by epidermal growth factor. We also find that cells that are constantly cycling do not express 9E3, indicating that they skip either the portion of the cell cycle where 9E3 is induced or that they constitutively express a repressor of transcription or an RNA-degrading enzyme. Taken together, these observations suggest that the product of this gene could play more than one role in vivo. For example, in normal tissues the 9E3 protein could be involved in the exit of cells from the resting stage, whereas during wound healing the secreted protein or its cleavage products also could play a role in angiogenesis.

Cell-to-cell signaling in the regulation of procollagen expression in primary avian tendon cells

High cell density is required for high procollagen expression (50% of total protein synthesis) in primary avian tendon (PAT) cells but the signaling mechanism that triggers this response has been difficult to decipher. By using a quantitative in situ hybridization assay for procollagen mRNA, cell density dependent changes in procollagen expression can be followed at the single cell level. PAT cells can then be shown to respond to the presence of their neighbors over approximately 1-mm distance. The cell density signal remains effective independent of the medium volume to cell ratio but becomes sensitive to dispersion and dilution when the medium is agitated. PAT cells respond to a reduction in cell density, when neighboring cells are scraped away, by outgrowth (approximately 1 mm) and reestablishment of a cell density gradient in cellular procollagen mRNA levels. However, removing neighboring cells while preventing migration off of their own extracellular matrix retards the drop in procollagen mRNA levels. The evidence, taken as a whole, is consistent with a model whereby the cell density signal is a loosely bound component of the cell layer thereby restricting its diffusion to two dimensions but making it susceptible to dispersion by medium agitation.

Long-term explant culture of rabbit flexor tendon: effects of recombinant human insulin-like growth factor-I and serum on matrix metabolism

The effects of human recombinant insulin-like growth factor-I (rhIGF-I, 50 ng/ml) on matrix metabolism in the deep flexor tendon from the tendon sheath region of the rabbit were studied in explants cultured for 3 weeks. Tendon segments cultured in medium supplemented with fetal calf serum (FCS) exhibited proliferation of the superficial cell layers. Synthesis of proteoglycan and non-collagen protein (NCP) increased threefold during the first week and remained elevated during the next 2 weeks of culture in medium supplemented with rhIGF-I or FCS, but not in medium without supplements (bovine serum albumin, BSA). The estimated halflife (t1/2) for elimination of newly labeled proteoglycans from the tendon explants ranged from 5.1 to 8.5 days and from 4.9 to 6.8 days for NCP in supplemented medium. Presence of rhIGF-I or FCS did not affect degradation of matrix as compared with BSA. The total hexosamine content per tendon segment was stable during the culture period, but the non-collagen protein content decreased by 25%. Collagen synthesis decreased to 10% of the initial level after 3 weeks in supplemented medium, but to 3% in unsupplemented medium. There was no measurable turnover of collagen in explants cultured in either medium, and the collagen content remained unchanged. Our results suggest that rhIGF-I, as well as FCS, stimulates matrix synthesis but does not influence matrix turnover in rabbit flexor tendon explants in long-term culture as compared with medium without supplements. We conclude that rhIGF-I may be used as a defined growth-promoting factor in serum-free media and may be of importance in tendon healing.

Recombinant human insulin-like growth factor-I stimulates in vitro matrix synthesis and cell proliferation in rabbit flexor tendon

Flexor tendons have an intrinsic ability for repair, with a capacity to metabolize matrix components and to proliferate. To identify factors with the potential of affecting those abilities, the effects of recombinant human insulin-like growth factor (rhIGF-I), insulin and fetal calf serum (FCS) on the synthesis of proteoglycan, collagen, and non-collagen protein and cell proliferation were investigated in short-term explant cultures of the deep flexor tendon of the rabbit. Matrix synthesis and cell proliferation were stimulated dose dependently by rhIGF-I at doses between 10 and 250 and at 10-100 ng/ml, respectively, by insulin at 250-5,000 ng/ml, and by FCS at 2-15%. Estimated maximal stimulation (Emax) of up to three times the control value was observed with rhIGF-I at 250 ng/ml. Maximal stimulation was observed at 5,000 ng/ml with insulin, and FCS at 15%. rhIGF-I was more potent than insulin in stimulating protein synthesis and cell proliferation. The Emax of stimulation of proteoglycan and collagen synthesis by rhIGF-I were two times that of FCS, and the Emax of cell proliferation by FCS was twice that of rhIGF-I. Growth factors thus have the ability to stimulate matrix synthesis and cell proliferation in rabbit flexor tendon. This provides a rationale for further studies on the role of growth factors in flexor tendon healing in humans.

Relative content of isoaccepting tRNAs for glycine and proline in avian tendon cells with different rates of procollagen synthesis

The relative amounts of iso-tRNAsGly and iso-tRNAsPro existing in chick embryo tendon are indicative of a specialization of the tRNA population for collagen synthesis. These amounts are not modified (i) in primary avian tendon (PAT) cells in culture for which the procollagen production varies from about 10% of total protein synthesis to 60% and (ii) in tendons from immature chicks, which show a 3-fold decrease of procollagen production with increasing age. The characteristic tRNA pattern was not maintained in cells which had lost the ability to make high levels of collagen as observed in the cases of: (i) PAT cells reaching confluency; (ii) virus-transformed PAT cells and (iii) tendon from adult chick. Our data are consistent with the idea that tendon tRNA specialization for collagen synthesis is a differentiation feature independent of the expression level of the collagenic function but related to its maintenance.

High serum levels interfere with the normal differentiated state of avian tendon cells by altering translational regulation

In low serum (0.2%) medium, ascorbate stimulates primary avian tendon cells to increase procollagen synthesis from 12 to 50% of total protein synthesis. This is reversibly blocked by an increase of serum levels from 0.2 to 3%. Ascorbate in low serum medium has been shown previously to stimulate the procollagen pathway by sequentially increasing by sixfold the secretion rate constant, then translation rates, and finally mRNA levels. We now show that addition of ascorbate to cultures containing 3% serum induces a sixfold increase in the secretion rate constant but translation rates and mRNA levels remain unchanged. In fully induced cells, an increase in serum levels causes a down-regulation of procollagen synthesis. In this case, the translational products of the induced cell are rapidly altered (less than 1 h), with noncollagen protein synthesis being stimulated preferentially over procollagen synthesis. This change is not reflected in procollagen mRNA levels since they remain constant for at least 6 h following addition of high serum. After 48 h in high serum, the induction of procollagen synthesis by ascorbate is reversed and the level of procollagen mRNA drops to that of uninduced cells. The data are consistent with the model that serum acts primarily at the translational level. High serum levels break the coupling in the ascorbate induction process that ties the stimulation of procollagen secretion rates to the increase in procollagen translation rates, and this prevents the maintenance of the induced state.

Regulation of collagen production and collagen mRNA amounts in fibroblasts in response to culture conditions

Collagen synthesis and mRNA amounts for the alpha 1 and alpha 2 polypeptide chains of Type I collagen were measured in embryonic-chick tendons and in tendon cells both in suspension and in primary cultures. The percentage of protein production represented by collagen in suspension-cultured cells was initially the same as in the intact tendon; however, on an hourly basis, there was actually a steady decline in collagen production by suspended cells. Collagen production in primary cultures of chick tendon fibroblasts was decreased when compared with intact tendon, even though ascorbate-supplemented primary cultures were able to maintain higher rates of collagen production than were non-supplemented cultures. The amounts of mRNA for alpha 1(I) and alpha 2(I) polypeptide chains of collagen responded in similar fashions to different culture conditions and were compared with the amounts of mRNA for beta-actin. In primary cultures the available alpha 1 and alpha 2 collagen mRNAs support proportionately higher collagen production than in the intact tendon. However, the ratio of alpha 1/alpha 2 mRNA and polypeptide-chain synthesis did not remain 2:1, but increased with the concomitant production of Type I trimers composed of three alpha 1 chains. Removal of fibroblasts from their environment in vivo appears to alter the amounts of mRNA for alpha 1 and alpha 2 chains and to alter the utilization of those mRNAs for polypeptide synthesis.

A simple fractionation of chicken egg yolk yields a protein component that stimulates cell proliferation and differentiation in primary avian tendon cells

An easily prepared and stable protein fraction from chick egg yolk promotes cell division (40 h generation time) and expression of procollagen (60% of total protein synthesis) in primary avian tendon cells in a serum-free medium. The activity of this yolk fraction (YF) is proteinaceous as reflected by its sensitivity to protease treatment. Yolk fraction is resolved into four major components on sodium dodecyl sulfate polyacrylamide gel electrophoresis with apparent molecular weights of 82,70,42,35 (X 10(-3)). Under nondenaturing conditions, YF runs as a mixture of high molecular weight aggregates on Sephacryl G-200. We postulate that the active part of YF could be the in ovo growth promoter for embryonic chick tendon cells.

Prolyl hydroxylase production can be uncoupled from the regulation of procollagen synthesis

Primary avian tendon (PAT) cells increase the production of procollagen from 10-12% to 40-50% of total protein synthesis in response to the addition of ascorbate and an increasing cell density. We now show that prolyl hydroxylase (PH) also increases its activity by greater than five-fold in response to increasing cell density; but unlike procollagen production, this is independent of the presence of ascorbate. The increased activity is a result of greater enzyme production and not a shift in the ratio of inactive to active forms which remains constant at about 10% of the total enzyme proteins. We present the possibility that at low cell density the levels of PH activity could limit production of collagen.

Inability of Rous sarcoma virus to cause sarcomas in the avian embryo

The injection of Rous sarcoma virus (RSV) into the wing web of newly hatched chicks causes a rapidly growing sarcomatous tumour which is palpable within 1 week of inoculation; and cultures of fibroblasts derived from chick embryos (CEF) and infected with RSV become rapidly transformed. Genetic studies have determined that expression of a single viral gene, designated v-src, is necessary for neoplastic transformation. This gene codes for a 60,000-molecular weight phosphoprotein termed pp60SPC , which possesses a protein kinase activity that phosphorylates polypeptides on tyrosine residues and is constitutively expressed in infected CEF cells. It has been suggested that transformation, and possibly tumorigenesis, may result solely from the consequences of this increase in tyrosine phosphorylations. The pathogenicity of RSV in chick embryos in ovo is less clear. Murphy and Rous suggested that RSV may have caused tumours in "various tissues" of "some embryos", but the subsequent studies of Milford and Duran - Reynals , as well as several other laboratories, failed to find any evidence of intraembryonic tumours in RSV-infected early embryos. The findings of Duran - Reynals , if correct, cannot be explained easily in view of our present understanding of RSV tumorigenicity. Thus, we have re-examined the interaction of RSV with the avian embryo and confirm here that RSV is nontumorigenic and non-teratogenic when microinjected into day 4 chicken embryos. In addition, we found that (1) the virus not only replicates in the embryo, but it also expresses an active src-specific protein kinase and (2) once the cells from the infected limbs are disrupted and placed in culture, they are capable of expressing the transformed phenotype after a 24-h delay.

Ascorbate stimulation of PAT cells causes an increase in transcription rates and a decrease in degradation rates of procollagen mRNA

Procollagen alpha 2 (I) mRNA can be induced congruent to 6-fold in primary avian tendon (PAT) cells on addition of ascorbate to the culture medium. Previously, we have shown that the induction is linear after a 12 h lag and requires a total of 60-72 h to achieve maximum levels. We have now investigated in more detail the changes that have occurred in the metabolism of procollagen mRNA in fully induced cells to account for the observed induction. Ascorbate was found to triple the rate of procollagen gene transcription. In addition, there was a stabilization of the mRNA causing the half-life to increase from 10.5 h to 20 h. The increased stability of the procollagen mRNA, however, did not correlate with its ability to bind to oligo (dT)-cellulose. Since a 3-fold change in transcription rates and a 2-fold increase in half-life would account for the 6-fold overall increase in procollagen mRNA levels, we conclude that these are the primary alterations caused by ascorbate addition that give rise to the specific increase in procollagen mRNA.

Role of procollagen mRNA levels in controlling the rate of procollagen synthesis

Two factors must be present for primary avian tendon cells to commit 50% of their total protein production to procollagen: ascorbate and high cell density. Scorbutic primary avian tendon cells at high cell density (greater than 4 X 10(4) cells per cm2) responded to the addition of ascorbate by a sixfold increase in the rate of procollagen synthesis. The kinetics were biphasic, showing a slow increase during the first 12 h followed by a more rapid rise to a maximum after 36 to 48 h. In contrast, after ascorbate addition, the level of accumulated cytoplasmic procollagen mRNA (alpha 2) showed a 12-h lag followed by a slow linear increase requiring 60 to 72 h to reach full induction. At all stages of the induction process, the relative increase in the rate of procollagen synthesis over the uninduced state exceeded the relative increase in the accumulation of procollagen mRNA. A similar delay in mRNA induction was observed when the cells were grown in an ascorbate-containing medium but the cell density was allowed to increase. In all cases, the rate of procollagen synthesis peaked approximately 24 h before the maximum accumulation of procollagen mRNA. The kinetics for the increase in procollagen synthesis are not, therefore, in agreement with the simple model that mRNA levels are the rate-limiting factor in the collagen pathway. We propose that the primary control point is at a later step. Further support for this idea comes from inhibitor studies, using alpha, alpha'-dipyridyl to block ascorbate action. In the presence of 0.3 mM alpha, alpha'-dipyridyl there was a specific two- to threefold decrease in procollagen production after 4 h, but this was unaccompanied by a drop in procollagen mRNA levels. Therefore, inhibitor studies give further support to the idea that primary action of ascorbate is to release a post-translational block.

Attachment and extracellular matrix differences between tendon and synovial fibroblastic cells

Fibroblasts of the synovium of sheathed tendons were isolated, and their biochemical properties were compared with those of the fibroblasts of the remaining tendon. The synovial cells had a lower attachment efficiency than did the tendon cells. On the day of cell isolation the synovial cells synthesized collagen as 10% of their total protein, whereas the tendon cells synthesized 30% collagen. After growth in fetal bovine serum (FBS), the percentage of collagen synthesized by both populations decreased; however, the synovial cells still made less collagen than did the tendon cells (5 versus 11%). On the basis of cyanogen bromide peptide analysis, the synovial cells were found to synthesize Types I and III collagen in primary culture, whereas the tendon cells synthesized only Type I. The synovial cells also synthesized two to three times less sulfated glycosaminoglycans in culture than did the tendon cells. Thus, the two cell populations differed in attachment efficiency and in their biosynthesis of collagen and sulfated glycosaminoglycans. These differences reflect extracellular matrix differences that have been observed in the tendon in vivo. In addition, the results augment existing data showing that not all fibroblasts have identical phenotypes.

Role of procollagen mRNA levels in controlling the rate of procollagen synthesis

Two factors must be present for primary avian tendon cells to commit 50% of their total protein production to procollagen: ascorbate and high cell density. Scorbutic primary avian tendon cells at high cell density (greater than 4 X 10(4) cells per cm2) responded to the addition of ascorbate by a sixfold increase in the rate of procollagen synthesis. The kinetics were biphasic, showing a slow increase during the first 12 h followed by a more rapid rise to a maximum after 36 to 48 h. In contrast, after ascorbate addition, the level of accumulated cytoplasmic procollagen mRNA (alpha 2) showed a 12-h lag followed by a slow linear increase requiring 60 to 72 h to reach full induction. At all stages of the induction process, the relative increase in the rate of procollagen synthesis over the uninduced state exceeded the relative increase in the accumulation of procollagen mRNA. A similar delay in mRNA induction was observed when the cells were grown in an ascorbate-containing medium but the cell density was allowed to increase. In all cases, the rate of procollagen synthesis peaked approximately 24 h before the maximum accumulation of procollagen mRNA. The kinetics for the increase in procollagen synthesis are not, therefore, in agreement with the simple model that mRNA levels are the rate-limiting factor in the collagen pathway. We propose that the primary control point is at a later step. Further support for this idea comes from inhibitor studies, using alpha, alpha'-dipyridyl to block ascorbate action. In the presence of 0.3 mM alpha, alpha'-dipyridyl there was a specific two- to threefold decrease in procollagen production after 4 h, but this was unaccompanied by a drop in procollagen mRNA levels. Therefore, inhibitor studies give further support to the idea that primary action of ascorbate is to release a post-translational block.

Contact-stimulated proliferation of cultured mouse epidermal cells by 3T3 feeder layers: inhibition of proliferation by 12-O-tetradecanoylphorbol-13-acetate (TPA)

Mouse epidermal cells can be subcultured at 31 degrees C onto an irradiated BALB/c 3T3 clone A31 feeder layer. A31 cells (supposedly derived from embryonic fibroblasts) were found to be specifically required for the optimal production of keratinizing epidermal colonies in secondary culture. This effect was not transmitted through the medium nor by the culture surface, since A31 cells plated on one end of a flask did not stimulate epidermal cell proliferation at the other end, even if the other end had previously held A31 cells. Epidermal cell contact with metabolizing A31 cells was probably necessary for the effect; fixed or freeze-thawed A31 cells were ineffective. The tumor promoter 12-O-tetradecanoylphorbol-13-acetate, recently shown to interfere with contact-mediated transfer of label (metabolic cooperation) between Swiss 3T3 cells and cells of an established epidermal line in vitro, also blocked epidermal colony formation. The A31-epidermal cell interaction is apparently not a typical mesenchymal-epithelial interaction, since the basement membrane would prevent this contact in intact skin.

Ascorbate induction of collagen synthesis as a means for elucidating a mechanism of quantitative control of tissue-specific function

Ascorbic acid displays the characteristics of an ideal inducer of tissue-specific function in primary avian tendon cells in culture. It is a highly specific, potent stimulator of collagen synthesis, it demonstrates slow reversible kinetics, and it has no effect on growth rate of the cultured cells. Kinetic analysis of ascorbate induction of collagen synthesis was used to determine the critical steps in this complex biosynthetic pathway. Full hydroxylation of the proline residues in collagen, although probably a necessary step for collagen induction, was in itself not sufficient for achieving either increased secretion or increased synthesis. On the other hand, an increase in secretion rate, which required both the presence of ascorbate and a high cell density, did correlate with the later stimulation in procollagen production. The process of procollagen secretion, therefore, meets the minimal requirements for the rate-limiting step. The fact that the cells maintained a large pool of intracellular procollagen despite changes in the rates of translation or secretion led us to postulate a possible feedback between the level of the internal procollagen pool and the rate of procollagen synthesis.