Dissociation of proto-oncogene induction from growth response in normal human fibroblasts

Chronic wound fluid suppresses proliferation of dermal fibroblasts through a Ras-mediated signaling pathway

Wound fluid collected from chronic venous leg ulcers (chronic wound fluid (CWF)) has been shown to inhibit the growth of dermal fibroblasts by interfering with cell-cycle progression from G1 into S phase. Specifically, CWF was shown to downregulate the levels of hyperphosphorylated retinoblastoma tumor-suppressor gene (Rb) and cyclin D1, known to be critical for entering the S phase of the cell cycle. To further elucidate the effects of CWF, a Ras-mediated signaling pathway involving the mitogen-activated protein kinase kinase (MEK), known to modulate the expression of these cell-cycle-regulatory proteins, was examined. Transient transfection of dermal fibroblasts with constitutively active Ras abrogated the growth suppressive effects of CWF on hyperphosphorylated Rb (ppRb) and cyclin D1. In contrast, an MEK inhibitor PD 98059 mimicked the effects of CWF on these cell-cycle-regulatory proteins. Concurrent treatment with PD 98059 and CWF produced additive effects. Taken together, these results suggest that CWF inhibits the growth of dermal fibroblasts at least in part by decreasing the level of active Ras, resulting in decreased levels of ppRb and cyclin D1. Therefore, a Ras-dependent signaling pathway may mediate the growth inhibitory effect of CWF, and reconstitution of Ras activity may overcome this growth inhibitory effect.

COMPARISON OF NORMAL HUMAN SKIN GENE EXPRESSION USING CDNA MICROARRAYS

Perturbations in normal wound healing may be traced to perturbations in gene expression in uninjured skin. In order to decipher normal and abnormal genetic responses to cutaneous injury, baseline gene expression in uninjured skin must first be defined. There is little data on gene expression profiles of normal human skin, i.e., which genes tend to be variable in expression and which tend to remain comparable. Therefore this study was designed to determine the degree of variability in human skin mRNA expression. Samples of normal skin were obtained from 9 healthy females undergoing breast reduction surgery. RNA was extracted, reverse transcribed into radiolabeled cDNA and hybridized onto cDNA microarrays of approximately 4400 genes. Gene expression intensities from the 9 samples were normalized and compared as a ratio of highest/lowest expression intensity. Deviation greater than 2 standard deviations from the mean of each gene was used as a cut-off. Seventy-one genes (1.7%) were substantially variable in their expression. These included genes coding for transport proteins, gene transcription, cell signaling proteins, and cell surface proteins. We found minimal variability in the matrix genes, growth factor genes and other groups of genes that are the most often studied in wound healing research. A small but definite variability in gene expression across 9 samples of clinically comparable specimens of normal skin was detected. This is in keeping with clinical observations of the variability in normal skin across individuals. These data provide high-throughput comparison of normal skin gene expression and suggest new molecules that may be studied in skin biology and perhaps, wound repair.

Protein kinase C-alpha levels are inversely associated with growth rate in cultured human dermal fibroblasts

Human dermal fibroblasts are known to express the alpha, delta, epsilon, and zeta isoforms of protein kinase C (PKC). We asked whether the growth of human dermal fibroblasts correlates with expression of a particular PKC isoform. Of total PKC activity measured in the presence of calcium, a condition permissive for activation of all PKC isoforms, 75%) was contributed by PKC-alpha, suggesting that PKC-alpha is the dominant isoform in human dermal fibroblasts. We then further studied PKC-alpha under different culture conditions and in cultures derived from different aged donors. In both subconfluent and confluent cultures, total PKC activity and the level of PKC-alpha protein were consistently higher in slowly proliferating adult cells than in more rapidly proliferating newborn cells. Moreover, in newborn fibroblasts density strongly influenced these parameters. At subconfluent density, when cells were dividing exponentially, total PKC activity was 345+/-63 cpm/,ug protein; whereas at confluent density, when cells were growth arrested, it was 6-7 fold higher, 2334+/-50 cpm/ug protein. Immunoblot analysis using a specific monoclonal antibody against PKC-alpha exhibited a similar 6-7 fold increase in the level of PKC-alpha protein at confluent density. However, in adult cells, density had no influence on the already high total activity or level of PKC-alpha. To further determine whether the increases in the levels of total PKC activity and the alpha isoform correlate with the decreased growth rate, a characteristic of both adult donor-derived and confluent cells, total PKC activity and the level of PKC-alpha in subconfluent quiescent cells was compared to that in paired exponentially growing cells at the same density. Total PKC activity was 8836+/-71 cpm/microg protein in subconfluent quiescent cells versus 4415+/-175 cpm/microg protein in dividing cells. The level of PKC-alpha protein was also 2-3 fold higher in quiescent than in growing cultures. However, the amount of PKC-alpha mRNA in these two conditions was identical as determined by northern blot analysis. Taken together, these results suggest an inverse relationship between the levels of total PKC activity and PKC-alpha protein and fibroblast growth rate that is regulated at the post-transcriptional level.

Vitamin D3 inhibits proliferation and increases c-myc expression in fibroblasts from psoriatic patients

Fibroblast cultures were established from the skin of normal and psoriatic subjects. The response to 1 alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3] of each kind of cells was assessed by measuring tritiated thymidine incorporation into DNA as an index of cell proliferation. We found that both types of cells responded with a similar dose- and time-dependent inhibition of thymidine incorporation. We also studied the response of the mRNA encoding the proto-oncogene c-myc, since its level is associated to the proliferative state in many cell types. Psoriatic fibroblasts contained higher basal amounts of c-myc RNA than control fibroblasts. Addition of 1,25-(OH)2D3 to the culture medium induced a time-dependent increase of c-myc RNA in psoriatic fibroblasts but not in controls. As a control, retinoic acid had no effect in any of the two cell types. It is concluded that in primary normal human fibroblasts, c-myc RNA levels are not correlated with the proliferative state, and that there is an altered expression of this proto-oncogene in psoriasis.

The trk family of receptors mediates nerve growth factor and neurotrophin-3 effects in melanocytes

We have recently shown that (a) human melanocytes express the p75 nerve growth factor (NGF) receptor in vitro; (b) that melanocyte dendricity and migration, among other behaviors, are regulated at least in part by NGF; and (c) that cultured human epidermal keratinocytes produce NGF. We now report that melanocyte stimulation with phorbol 12-tetra decanoate 13-acetate (TPA), previously reported to induce p75 NGF receptor, also induces trk in melanocytes, and TPA effect is further potentiated by the presence of keratinocytes in culture. Moreover, trk in melanocytes becomes phosphorylated within minutes after NGF stimulation. As well, cultures of dermal fibroblasts express neurotrophin-3 (NT-3) mRNA; NT-3 mRNA levels in cultured fibroblasts are modulated by mitogenic stimulation, UV irradiation, and exposure to melanocyte-conditioned medium. Moreover, melanocytes constitutively express low levels of trk-C, and its expression is downregulated after TPA stimulation. NT-3 supplementation to cultured melanocytes maintained in Medium 199 alone prevents cell death. These combined data suggest that melanocyte behavior in human skin may be influenced by neurotrophic factors, possibly of keratinocyte and fibroblast origin, which act through high affinity receptors.

Altered gene expression in human leukemia K562 cells selected for resistance to etoposide

Sublines of K562 human leukemia cells were selected for resistance (30- to 80-fold) to etoposide by continuous exposure to 0.5 microM VP-16. Two etoposide-resistant cell lines, K/VP.5 and K/VP.5-1, showed a 5-fold reduction in levels of topoisomerase II alpha protein compared with K562 cells. Northern analysis indicated a 2.5-fold reduction in topoisomerase II alpha mRNA in etoposide-resistant cell lines, due in part to a 1.7-fold decrease in topoisomerase II mRNA stability with no change in transcription rate. Immunoblotting assays of electrophoresed cell lysates from VP-16-treated cells revealed less drug-induced covalent topoisomerase II/DNA adducts in resistant than in sensitive cells, suggesting a functional alteration in resistant cell topoisomerase II. Recent reports of specific topoisomerase II DNA binding sites near the promoter sites of growth response genes and alterations of gene expression in cells treated with topoisomerase II inhibitory drugs led to experiments to determine if the apparent functional alterations of topoisomerase II were accompanied by changes in the regulation of these genes. Therefore, the expression of several growth response genes was compared by northern analysis in parental K562 and both VP-16-resistant cell lines. Basal levels of c-myc were comparable for all three cell lines, but levels of c-jun and c-fos were elevated 2- to 4-fold in VP-16-resistant cell lines. Increased levels of c-fos and c-jun were not a result of altered rates of transcription, as determined by nuclear run-off assays. Exposure of both sensitive and resistant cells to 200 microM VP-16 for 5 hr resulted in no further changes in topoisomerase II mRNA levels but caused an additional 2- to 3-fold elevation in the level of c-jun mRNA, indicating that altered basal levels of this gene were not due to deregulation of this gene. Acquired VP-16 resistance in K/VP.5 and K/VP.5-1 cells was accompanied by reduced levels and altered activities of DNA topoisomerase II as well as changes affecting the expression of genes important for growth and differentiation.

Regulation of connective tissue growth factor gene expression in human skin fibroblasts and during wound repair

Connective tissue growth factor (CTGF) is a cysteine-rich peptide that exhibits platelet-derived growth factor (PDGF)-like biological and immunological activities. CTGF is a member of a family of peptides that include serum-induced immediate early gene products, a v-src-induced peptide, and a putative avian transforming gene, nov. In the present study, we demonstrate that human foreskin fibroblasts produce high levels of CTGF mRNA and protein after activation with transforming growth factor beta (TGF-beta) but not other growth factors including PDGF, epidermal growth factor, and basic fibroblast growth factor. Because of the high level selective induction of CTGF by TGF-beta, it appears that CTGF is a major autocrine growth factor produced by TGF-beta-treated human skin fibroblasts. Cycloheximide did not block the large TGF-beta stimulation of CTGF gene expression, indicating that it is directly regulated by TGF-beta. Similar regulatory mechanisms appear to function in vivo during wound repair where there is a coordinate expression of TGF-beta 1 before CTGF in regenerating tissue, suggesting a cascade process for control of tissue regeneration and repair.

Evidence that filopodia outgrowth is a common final pathway for fibroblast growth inhibition in vitro

To identify events associated with fibroblast growth inhibition, the effect of two known inhibitors, interferon-alpha and all-trans retinoic acid, on the growth and surface morphology of cultured fibroblasts was examined. Interferon-alpha administered at seeding reduced both growth rate and saturation density; all-trans retinoic acid reduced only saturation density. However, both negative growth modulators were associated with an increase in filopodia outgrowth and an increase in intracellular filamentous actin in a time course corresponding to onset of growth inhibition by these agents. In combination with earlier findings, these data suggested that, regardless of etiology, cultured fibroblast growth restriction is mediated in part by an actin-dependent outgrowth of filopodia that augment intercellular contact.

Ageing and photoageing of the skin: observations at the cellular and molecular level

It is now well established that ageing occurs at the level of individual cells in the skin and other organ systems. Changes in cell behaviour, protein production and gene expression in response to standardized stimuli are readily observed in cultured cells derived from young vs old donors and from photoaged vs sun-protected body sites. Whether these changes are best viewed as a cause or a consequence of ageing cannot be determined at present. Nevertheless, available data now provide cellular and molecular correlates for the well-known differences in clinical responsiveness between newborn, adult and photoaged skin. From this basis, it will hopefully be possible to develop a more comprehensive understanding of cutaneous ageing processes.

Reduction by interferon-alpha of levels of c-myc protein and DNA synthesis in a human hepatoma cell line mediated by inhibition of putrescine synthesis

Interferon-alpha inhibited increases in ornithine decarboxylase, intracellular putrescine, and DNA synthesis as human hepatoma cells were stimulated to grow. Interferon-alpha inhibited the increase in the c-myc protein level, but not its mRNA level. Added putrescine abrogated the effects on c-myc protein and DNA synthesis. Interferon-alpha seemed to inhibit the increase in the c-myc protein level post-transcriptionally by reducing the putrescine level, inhibiting DNA synthesis.