Melanocytes in regenerative medicine applications and disease modeling.

Melanocytes are dendritic cells localized in skin, eyes, hair follicles, ears, heart and central nervous system. They are characterized by the presence of melanosomes enriched in melanin which are responsible for skin, eye and hair pigmentation. They also have different functions in photoprotection, immunity and sound perception. Melanocyte dysfunction can cause pigmentary disorders, hearing and vision impairments or increased cancer susceptibility. This review focuses on the role of melanocytes in homeostasis and disease, before discussing their potential in regenerative medicine applications, such as for disease modeling, drug testing or therapy development using stem cell technologies, tissue engineering and extracellular vesicles.

Mass Spectrometry-Based Profiling of Histone Post-Translational Modifications in Uveal Melanoma Tissues, Human Melanocytes, and Uveal Melanoma Cell Lines - A Pilot Study.

Purpose: Epigenetic alterations in uveal melanoma (UM) are still neither well characterized, nor understood. In this pilot study, we sought to provide a deeper insight into the possible role of epigenetic alterations in the pathogenesis of UM and their potential prognostic relevance. To this aim, we comprehensively profiled histone post-translational modifications (PTMs), which represent epigenetic features regulating chromatin accessibility and gene transcription, in UM formalin-fixed paraffin-embedded (FFPE) tissues, control tissues, UM cell lines, and healthy melanocytes. Methods: FFPE tissues of UM (n = 24), normal choroid (n = 4), human UM cell lines (n = 7), skin melanocytes (n = 6), and uveal melanocytes (n = 2) were analyzed through a quantitative liquid chromatography-mass spectrometry (LC-MS) approach. Results: Hierarchical clustering showed a clear separation with several histone PTMs that changed significantly in a tumor compared to normal samples, in both tissues and cell lines. In addition, several acetylations and H4K20me1 showed lower levels in BAP1 mutant tumors. Some of these changes were also observed when we compared GNA11 mutant tumors with GNAQ tumors. The epigenetic profiling of cell lines revealed that the UM cell lines MP65 and UPMM1 have a histone PTM pattern closer to the primary tissues than the other cell lines analyzed. Conclusions: Our results suggest the existence of different histone PTM patterns that may be important for diagnosis and prognosis in UM. However, further analyses are needed to confirm these findings in a larger cohort. The epigenetic characterization of a panel of UM cell lines suggested which cellular models are more suitable for epigenetic investigations.

LKB1-SIK2 loss drives uveal melanoma proliferation and hypersensitivity to SLC8A1 and ROS inhibition.

Metastatic uveal melanomas are highly resistant to all existing treatments. To address this critical issue, we performed a kinome-wide CRISPR-Cas9 knockout screen, which revealed the LKB1-SIK2 module in restraining uveal melanoma tumorigenesis. Functionally, LKB1 loss enhances proliferation and survival through SIK2 inhibition and upregulation of the sodium/calcium (Na+ /Ca2+ ) exchanger SLC8A1. This signaling cascade promotes increased levels of intracellular calcium and mitochondrial reactive oxygen species, two hallmarks of cancer. We further demonstrate that combination of an SLC8A1 inhibitor and a mitochondria-targeted antioxidant promotes enhanced cell death efficacy in LKB1- and SIK2-negative uveal melanoma cells compared to control cells. Our study also identified an LKB1-loss gene signature for the survival prognostic of patients with uveal melanoma that may be also predictive of response to the therapy combination. Our data thus identify not only metabolic vulnerabilities but also new prognostic markers, thereby providing a therapeutic strategy for particular subtypes of metastatic uveal melanoma.

Tumor Shape-Specific Brachytherapy Implants by 3D-Printing, Precision Radioactivity Painting, and Biomedical Imaging.

In brachytherapy (BT), or internal radiation therapy, cancer is treated by radioactive implants. For instance, episcleral plaques (EPs) for the treatment of uveal melanoma, are designed according to generic population approximations. However, more personalized implants can enhance treatment precision through better adjustment of dose profiles to the contours of cancerous tissues. An original approach integrating biomedical imaging, 3D printing, radioactivity painting, and biomedical imaging, is developed as a workflow for the development of tumor shape-specific BT implants. First, computer-aided design plans of EP are prepared according to guidelines prescribed by the Collaborative Ocular Melanoma Study protocol. Polyetheretherketone (PEEK), a high-performance polymer suitable for permanent implants, is used to 3D-print plaques and the geometrical accuracy of the printed design is evaluated by imaging. The possibility to modulate the dose distribution in a tridimensional manner is demonstrated by painting the inner surfaces of the EPs with radioactive 103Pd, followed by dose profile measurements. The possibility to modulate dose distributions generated by these 3D-printed plaques through radioactivity painting is therefore confirmed. Ex vivo surgical tests on human eyeballs are performed as an assessment of manipulation ease. Overall, this work provides a solution for the fabrication of tumor-specific radioactive implants requiring higher dose precision.

Co-Targeting FASN and mTOR Suppresses Uveal Melanoma Growth.

Uveal melanoma (UM) displays a high frequency of metastasis; however, effective therapies for metastatic UM are limited. Identifying unique metabolic features of UM may provide a potential targeting strategy. A lipid metabolism protein expression signature was induced in a normal choroidal melanocyte (NCM) line transduced with GNAQ (Q209L), a driver in UM growth and development. Consistently, UM cells expressed elevated levels of fatty acid synthase (FASN) compared to NCMs. FASN upregulation was associated with increased mammalian target of rapamycin (mTOR) activation and sterol regulatory element-binding protein 1 (SREBP1) levels. FASN and mTOR inhibitors alone significantly reduced UM cell growth. Concurrent inhibition of FASN and mTOR further reduced UM cell growth by promoting cell cycle arrest and inhibiting glucose utilization, TCA cycle metabolism, and de novo fatty acid biosynthesis. Our findings indicate that FASN is important for UM cell growth and co-inhibition of FASN and mTOR signaling may be considered for treatment of UM.

MiR-181a-5p inhibits uveal melanoma development by targeting GNAQ and AKT3.

Uveal melanoma (UM) is the most common primary intraocular malignant tumor type in adults. Even after the treatment of the ocular tumor, the prognosis of patients with metastasis remains poor. Hence, an urgent unmet need exists to identify novel approaches to treat advanced UM. Previous studies have revealed G subunit alpha Q and alpha 11 (GNAQ/11) mutations in more than 85% of patients with UM, thus indicating the importance of GNAQ and downstream signaling pathways in UM occurrence. Here, we demonstrate that microRNA (miR)-181a-5p, a small non-coding RNA, effectively inhibited the viability, proliferation, and colony formation but induced apoptosis of UM cells. Furthermore, silencing GNAQ or AKT3 mimicked the anti-UM effects of miR-181a-5p, whereas overexpression of GNAQ or AKT3 rescued the anti-UM effects induced by miR-181a-5p. In addition, miR-181a-5p had a stronger effect in decreasing the viability of GNAQ mutant than GNAQ wild-type cells. Moreover, miR-181a-5p suppressed the total expression and phosphorylation of members of the ERK and PI3K/AKT/mTOR signaling pathways. Importantly, miR-181a-5p potently inhibited the growth of UM xenografts in nude mice. MiR-181a-5p also decreased the expression of Ki67, GNAQ, and AKT3, and induced the expression of cleaved-caspase3 in UM tumors. These results suggest that miR-181a-5p inhibits UM development by targeting GNAQ and AKT3.

Extracellular Vesicles from Ocular Melanoma Have Pro-Fibrotic and Pro-Angiogenic Properties on the Tumor Microenvironment.

Uveal melanoma (UM) is the most common primary intraocular tumor and often spreads to the liver. Intercellular communication though extracellular vesicles (EVs) plays an important role in several oncogenic processes, including metastasis, therapeutic resistance, and immune escape. This study examines how EVs released by UM cells modify stellate and endothelial cells in the tumor microenvironment. The surface markers, and the concentration and size of EVs derived from UM cells or choroidal melanocytes were characterized by high-resolution flow cytometry, electron microscopy, and Western blotting. The selective biodistribution of EVs was studied in mice by fluorescence imaging. The activation/contractility of stellate cells and the tubular organization of endothelial cells after exposure to melanomic EVs were determined by traction force microscopy, collagen gel contraction, or endothelial tube formation assays. We showed that large EVs from UM cells and healthy melanocytes are heterogenous in size, as well as their expression of phosphatidylserine, tetraspanins, and Tsg101. Melanomic EVs mainly accumulated in the liver and lungs of mice. Hepatic stellate cells with internalized melanomic EVs had increased contractility, whereas EV-treated endothelial cells developed more capillary-like networks. Our study demonstrates that the transfer of EVs from UM cells leads to a pro-fibrotic and pro-angiogenic phenotype in hepatic stellate and endothelial cells.

Experimental eye research / short communication format characterization of DNA hydroxymethylation in the ocular choroid.

DNA methylation and hydroxymethylation represent important epigenetic modifications involved in cell differentiation. DNA hydroxymethylation can be used to classify independent biological samples by tissue type. Relatively little is known regarding the genomic abundance and function of 5-hydroxymethylcytosine (5-hmC) in ocular tissues. The choroid supplies oxygen and nutrients to the outer retina through its dense network of blood vessels. This connective tissue is mainly composed of pigmented melanocytes, and stromal fibroblasts. Since DNA hydroxymethylation level is relatively high in cutaneous melanocytes, we investigated the presence of 5-hmC in choroidal melanocytes, as well as the expression of ten-eleven translocation methylcytosine dioxygenases (TETs) and isocitrate dehydrogenases (IDHs) implicated in this DNA demethylation pathway. Immunofluorescence, DNA slot blots and liquid chromatography coupled to tandem mass spectrometry performed with choroidal tissues and melanocytes within these tissues revealed that they have a relatively high level of 5-hmC. We also examined the expression of TET1/2 and IDH1/2 in choroidal melanocytes by gene expression profiling, qPCR and Western blotting. In addition, we detected decreased levels of 5-hmC when choroidal melanocytes were exposed to a lower concentration of oxygen. Our study therefore demonstrates that DNA hydroxymethylation is present in choroidal melanocytes, and that the abundance of this epigenetic mark is impacted by hypoxia.

Uveal melanoma pathobiology: Metastasis to the liver.

Uveal melanoma (UM) is a type of intraocular tumor with a propensity to disseminate to the liver. Despite the identification of the early driver mutations during the development of the pathology, the process of UM metastasis is still not fully comprehended. A better understanding of the genetic, molecular, and environmental factors participating to its spread and metastatic outgrowth could provide additional approaches for UM treatment. In this review, we will discuss the advances made towards the understanding of the pathogenesis of metastatic UM, summarize the current and prospective treatments, and introduce some of the ongoing research in this field.

Human Organ-Specific 3D Cancer Models Produced by the Stromal Self-Assembly Method of Tissue Engineering for the Study of Solid Tumors.

Cancer research has considerably progressed with the improvement of in vitro study models, helping to understand the key role of the tumor microenvironment in cancer development and progression. Over the last few years, complex 3D human cell culture systems have gained much popularity over in vivo models, as they accurately mimic the tumor microenvironment and allow high-throughput drug screening. Of particular interest, in vitrohuman 3D tissue constructs, produced by the self-assembly method of tissue engineering, have been successfully used to model the tumor microenvironment and now represent a very promising approach to further develop diverse cancer models. In this review, we describe the importance of the tumor microenvironment and present the existing in vitro cancer models generated through the self-assembly method of tissue engineering. Lastly, we highlight the relevance of this approach to mimic various and complex tumors, including basal cell carcinoma, cutaneous neurofibroma, skin melanoma, bladder cancer, and uveal melanoma.

Synergic Interactions Between Hepatic Stellate Cells and Uveal Melanoma in Metastatic Growth.

Uveal melanoma (UM) is a malignant intraocular tumor that spreads to the liver in half of the cases. Since hepatic cells could play a role in the therapeutic resistance of metastatic UM, the purpose of our study was to investigate the pro-invasive role of hepatic stellate cells (HSteCs) in metastatic UM at the micro- and macro-metastatic stages. We first performed an immunostaining with the alpha-smooth muscle actin (αSMA) to localize activated HSteCs in UM liver macro-metastases from four patients. Their accumulation of collagen was assessed with Masson's Trichrome stain. Next, we inoculated metastatic UM cells alone or with human HSteCs in triple-immunodeficient mice, in order to determine if HSteCs are recruited as early as the micro-metastatic stage. The growth of metastatic foci was imaged in the liver by ex vivo fluorescence imaging. Histological analyses were performed with Masson's Trichrome and Picrosirius Red stains, and antibodies against Melan-A and αSMA. The collagen content was measured in xenografts by quantitative polarization microscopy. In patient hepatectomy samples, activated HSteCs and their pathological matrix were localized surrounding the malignant lesions. In the mouse xenograft model, the number of hepatic metastases was increased when human HSteCs were co-inoculated. Histological analyses revealed a significant recruitment of HSteCs near the micro/macrolesions, and an increase in fibrillar collagen production. Our results show that HSteCs can provide a permissive microenvironment and might increase the therapeutic resistance of metastatic UM.

Analysis of the proteasome activity and the turnover of the serotonin receptor 2B (HTR2B) in human uveal melanoma.

Uveal melanoma (UM), although a very rare disease, remains a particularly aggressive type of cancer as near 50% of the UM presenting patients will also develop liver metastases within 15 years from the initial diagnostic. One of the most reliable predictive markers of UM at risk of evolving toward the formation of liver lesions is an abnormally elevated level of expression of the transcript encoding the 5-Hydroxytryptamine (serotonin) receptor 2B (HTR2B). In our previous study, we demonstrated that transcription of the HTR2B gene was under the regulatory influences of two transcription factors (TFs), NFI and RUNX1. However, the action of these TFs was insufficient to explain the elevated level of the HTR2B protein in metastatic UM cells or the discrepancies we observed between its expression at the transcriptional and protein levels, therefore suggesting that additional post-translational modifications may also contribute to the altered expression of HTR2B in UM cells. In the present study, we investigated whether the turnover of HTR2B by the proteasome could account at least in part for its deregulated expression. Microarray analyses performed with UM cell lines derived from both non-metastatic and metastatic UM primary tumors revealed important alterations in the expression of some of the transcripts encoding both the E3 ubiquitin ligases and the various subunits of the proteasome, and these modifications were further exacerbated by cell passaging in culture. These alterations also correlated with significant changes in the enzymatic activity of the proteasome. However, the highest proteasome activity and amount of ubiquitinated HTR2B observed in the metastatic T142 cell line, as revealed by immunoprecipitation of ubiquitinated proteins and Western blotting using the HTR2B antibody, apparently had little impact on the total content of HTR2B protein. This contrasts with the near total disappearance of this receptor in the non-metastatic T108 cell line. Our study therefore suggests that the inability of the proteasome to degrade HTR2B in metastatic UM cells might rely on an increased stability of the ubiquitinated receptor in these cells.

The bidirectional crosstalk between metastatic uveal melanoma cells and hepatic stellate cells engenders an inflammatory microenvironment.

Uveal melanoma is the most common primary ocular neoplasm in adults. It is peculiar for its hematogenous dissemination and its high propensity to spread to the liver. Current treatments rarely prolong patient survival. We hypothesized that metastatic uveal melanoma cells modulate the function of surrounding hepatic stellate cells to facilitate their own growth and survival. This study was conducted to investigate the role of the hepatic microenvironment on uveal melanoma aggressiveness. We demonstrated that the paracrine signaling of surrounding hepatic stellate cells have more transcriptional impact on metastatic uveal melanoma cells. Upregulated transcripts were linked to inflammation and included several interleukins. The uveal melanoma-stellate cell crosstalk induced as well the expression of transmembrane integrins. In addition, the interleukin-6 receptor inhibitor Tocilizumab did not reduce the growth of uveal melanoma cells. Our results provide evidence that inflammatory mediators are key players in the homing of uveal melanoma cells to the liver. The bidirectional crosstalk between uveal melanoma cells and hepatic stellate cells involved pro-fibrogenic interleukins. The inflammatory characteristics of the metastatic microenvironment might offer relevant therapeutic opportunities in uveal melanoma.

Characterization of a tissue-engineered choroid.

The choroid of the eye is a vascularized and pigmented connective tissue lying between the retina and the sclera. Increasing evidence demonstrates that, beyond supplying nutrients to the outer retina, the different choroidal cells contribute to the retina's homeostasis, especially by paracrine signaling. However, the precise role of each cell type is currently unclear. Here, we developed a choroidal substitute using the self-assembly approach of tissue engineering. Retinal pigment epithelial (RPE) cells, as well as choroidal stromal fibroblasts, vascular endothelial cells and melanocytes, were isolated from human eye bank donor eyes. Fibroblasts were cultured in a medium containing serum and ascorbic acid. After six weeks, cells formed sheets of extracellular matrix (ECM), which were stacked to produce a tissue-engineered choroidal stroma (TECS). These stromal substitutes were then characterized and compared to the native choroid. Their ECM composition (collagens and proteoglycans) and biomechanical properties (ultimate tensile strength, strain and elasticity) were similar. Furthermore, RPE cells, human umbilical vein endothelial cells and choroidal melanocytes successfully repopulated the stromas. Physiological structures were established, such as a confluent monolayer of RPE cells, vascular-like structures and a pigmentation of the stroma. Our TECS thus recaptured the biophysical environment of the native choroid, and can serve as study models to understand the normal interactions between the RPE and choroidal cells, as well as their reciprocal exchanges with the ECM. This will consequently pave the way to derive accurate insight in the pathophysiological mechanisms of diseases affecting the choroid. STATEMENT OF SIGNIFICANCE: The choroid is traditionally known for supplying blood to the avascular outer retina. There has been a renewed attention directed towards the choroid partly due to its implication in the development of age-related macular degeneration (AMD), the leading cause of blindness in industrialized countries. Since AMD involves the dysfunction of the choroid/retinal pigment epithelium (RPE) complex, a three-dimensional (3D) model of RPE comprising the choroid layer is warranted. We used human choroidal cells to engineer a choroidal substitute. Our approach takes advantage of the ability of cells to recreate their own environment, without exogenous materials. Our model could help to better understand the role of each choroidal cell type as well as to advance the development of new therapeutics for AMD.

Regression rate of choroidal melanoma following iodine-125 brachytherapy is not associated with metastatic spread.

Nearly half of choroidal melanomas progress to the metastatic stage at 15 years. The purpose of our study was to evaluate the prognostic value of tumour-height regression rate in medium-sized choroidal melanomas treated with iodine-125 brachytherapy. A retrospective cohort study was performed on 128 patients with medium-sized choroidal melanoma who were treated with iodine-125 brachytherapy. Tumour characteristics including tumour apical height at baseline and after irradiation, recurrence, metastasis and mortality were collected from patients' records. Regression rate was defined in mm/month or in percentage of baseline apical height. Patients were statistically stratified in three groups of regression rate at 6 months using the Ward's method and Euclidian distance (slow, medium and fast regression groups). Mean initial apical height was of 5.71±1.79 mm. At 6 months, the average regression rate was 0.02±0.12 mm/month in the slow group (n=60), 0.32±0.11 mm/month in the medium group (n=52) and 0.67±0.21 mm/month in the fast group (n=16). Cox regression analysis for the recurrence, metastasis and mortality rates according to the three groups did not show any statistically significant difference. Sensitivity analyses with the regression rates at 12 months showed similar associations. Exudative retinal detachment resolved with treatment at 5.9±4.0 months, and it was more common at presentation in the fast regression rate group. The regression rate at 6 and 12 months after iodine-125 brachytherapy is not associated with a higher metastatic rate in medium-sized choroidal melanoma.

Expression of the serotonin receptor 2B in uveal melanoma and effects of an antagonist on cell lines.

Uveal melanoma (UM) is the most common primary tumor in the adult, and disseminates to the liver in half of patients. A 15-gene expression profile prognostic assay allows to determine the likelihood of metastasis in patients using their ocular tumor DNA, but a cure still remains to be discovered. The serotonin receptor 2B represents the discriminant gene of this molecular signature with the greatest impact on the prognosis of UM. However, its contribution to the metastatic potential of UM remains unexplored. The purpose of this study was to investigate the effects of a selective serotonin receptor 2B antagonist on cellular and molecular behaviours of UM cells. UM cell lines expressing high level of serotonin receptor 2B proteins were selected by Western blotting. The selective serotonin receptor 2B antagonist PRX-08066 was evaluated for its impact on UM cells using viability assays, phosphorylated histone H3 immunostainings, clonogenic assays, migration assays, invasion assays and membrane-based protein kinase phosphorylation antibody arrays. The pharmacological inhibition of the serotonin receptor 2B reduced the viability of UM cells and the population in mitosis, and impaired their clonogenicity and potential of migration. It also decreased the phosphorylation of kinases from signaling pathways classically activated by the serotonin receptor 2B, as well as kinases ß-catenin, Proline-rich tyrosine kinase 2, and Signal transducer and activator of transcription 5. Our findings support a role for the serotonin receptor 2B in the proliferation and migration of UM cells, through activation of many signaling pathways such as WNT, Focal adhesion kinase and Janus kinase/STAT.

Orbital recurrence of iris melanoma 21 years after enucleation.

We present the case of a 56-year-old man who developed a neoplasm of epithelioid histology in his anophthalmic left orbit 21 years after he underwent enucleation for a spindle cell iris melanoma. The recurrent tumour was managed by orbital exenteration. Neither further recurrence nor metastasis was diagnosed over a 5-year follow-up period. This case, along with five other similar cases in the literature,1-3 emphasises the importance of long-term follow-up after treatment of iris melanoma.

Differential responses of choroidal melanocytes and uveal melanoma cells to low oxygen conditions.

PURPOSE: Tissue culture is traditionally performed at atmospheric oxygen concentration (21%), which induces hyperoxic stress, as endogenous physiologic oxygen tension found in tissues varies between 2% and 9%. This discrepancy may lead to misinterpretation of results and may explain why effects observed in vitro cannot always be reproduced in vivo and vice versa. Only a few studies have been conducted in low physiologic oxygen conditions to understand the development and differentiation of cells from the eye. METHODS: The aim of this study was to investigate the growth and gene expression profile of melanocytes from the choroid permanently exposed to 21% (hyperoxic) or 3% (physiologic) oxygen with proliferation assays and DNA microarray. The cellular behavior of the melanocytes was then compared to that of cancer cells. RESULTS: The gross morphology and melanin content of choroidal melanocytes changed slightly when they were exposed to 3% O2, and the doubling time was statistically significantly faster. There was an increase in the percentage of choroidal melanocytes in the active phases of the cell cycle as observed by using the proliferation marker Ki67. The caveolin-1 senescence marker was not increased in choroidal melanocytes or uveal melanoma cells grown in hyperoxia. In comparison, the morphology of the uveal melanoma cells was similar between the two oxygen levels, and the doubling time was slower at 3% O2. Surprisingly, gene expression profiling of the choroidal melanocytes did not reveal a large list of transcripts considerably dysregulated between the two oxygen concentrations; only the lactate transporter monocarboxylate transporter (MCT4) was statistically significantly upregulated at 3% O2. CONCLUSIONS: This study showed that the oxygen concentration must be tightly controlled in experimental settings, because it influences the subsequent cellular behavior of human choroidal melanocytes.

Effects of Long-term Serial Passaging on the Characteristics and Properties of Cell Lines Derived From Uveal Melanoma Primary Tumors.

PURPOSE: Development of liver metastasis remains the most common cause of mortality in uveal melanoma (UM). A few cell lines cultured from primary UM tumors have been used widely to investigate the pathobiology of UM. However, the translation of basic knowledge to the clinic for the treatment of the metastatic disease has remained incremental at best. In this study, we examined whether the properties of UM cell lines at various passages were similar to their corresponding primary tumors. METHODS: Gene expression profiling by microarray was performed on UM primary tumors and derived cell lines cultured at varying passages. Expression of UM protein markers was monitored by immunohistochemical analyses and Western blotting. The in vivo tumorigenic properties of UM cultures were evaluated using athymic nude mice. RESULTS: Cell passaging severely reduced the expression of genes encoding markers typical of UM, including those of the prognostic gene signature. Marked differences between gene expression profiles of primary tumors and cell lines could be linked to the infiltrating immune and stromal cells in situ. In addition, the tumorigenic properties of UM cell lines also increased with cell passaging in culture as evaluated by their subcutaneous injection into athymic mice. CONCLUSIONS: Together, these findings demonstrate that the short-term UM primary cultures exhibit molecular features that resemble the respective surgical material and, thus, represent the best model for in vitro-assessed cancer treatments.

Contribution of Sp1 to Telomerase Expression and Activity in Skin Keratinocytes Cultured With a Feeder Layer.

The growth of primary keratinocytes is improved by culturing them with a feeder layer. The aim of this study was to assess whether the feeder layer increases the lifespan of cultured epithelial cells by maintaining or improving telomerase activity and expression. The addition of an irradiated fibroblast feeder layer of either human or mouse origin (i3T3) helped maintain telomerase activity as well as expression of the transcription factor Sp1 in cultured keratinocytes. In contrast, senescence occurred earlier, together with a reduction of Sp1 expression and telomerase activity, in keratinocytes cultured without a feeder layer. Telomerase activity was consistently higher in keratinocytes grown on the three different feeder layers tested relative to cells grown without them. Suppression of Sp1 expression by RNA inhibition (RNAi) reduced both telomerase expression and activity in keratinocytes and also abolished their long-term growth capacity suggesting that Sp1 is a key regulator of both telomerase gene expression and cell cycle progression of primary cultured human skin keratinocytes. The results of the present study therefore suggest that the beneficial influence of the feeder layer relies on its ability to preserve telomerase activity in cultured human keratinocytes through the maintenance of stable levels of Sp1 expression.