Stem cell activity-coupled suppression of endogenous retrovirus governs adult tissue regeneration.

Mammalian retrotransposons constitute 40% of the genome. During tissue regeneration, adult stem cells coordinately repress retrotransposons and activate lineage genes, but how this coordination is controlled is poorly understood. Here, we observed that dynamic expression of histone methyltransferase SETDB1 (a retrotransposon repressor) closely mirrors stem cell activities in murine skin. SETDB1 ablation leads to the reactivation of endogenous retroviruses (ERVs, a type of retrotransposon) and the assembly of viral-like particles, resulting in hair loss and stem cell exhaustion that is reversible by antiviral drugs. Mechanistically, at least two molecularly and spatially distinct pathways are responsible: antiviral defense mediated by hair follicle stem cells and progenitors and antiviral-independent response due to replication stress in transient amplifying cells. ERV reactivation is promoted by DNA demethylase ten-eleven translocation (TET)-mediated hydroxymethylation and recapitulated by ablating cell fate transcription factors. Together, we demonstrated ERV silencing is coupled with stem cell activity and essential for adult hair regeneration.

KLF5 governs sphingolipid metabolism and barrier function of the skin.

Stem cells are fundamental units of tissue remodeling whose functions are dictated by lineage-specific transcription factors. Home to epidermal stem cells and their upward-stratifying progenies, skin relies on its secretory functions to form the outermost protective barrier, of which a transcriptional orchestrator has been elusive. KLF5 is a Krüppel-like transcription factor broadly involved in development and regeneration whose lineage specificity, if any, remains unclear. Here we report KLF5 specifically marks the epidermis, and its deletion leads to skin barrier dysfunction in vivo. Lipid envelopes and secretory lamellar bodies are defective in KLF5-deficient skin, accompanied by preferential loss of complex sphingolipids. KLF5 binds to and transcriptionally regulates genes encoding rate-limiting sphingolipid metabolism enzymes. Remarkably, skin barrier defects elicited by KLF5 ablation can be rescued by dietary interventions. Finally, we found that KLF5 is widely suppressed in human diseases with disrupted epidermal secretion, and its regulation of sphingolipid metabolism is conserved in human skin. Altogether, we established KLF5 as a disease-relevant transcription factor governing sphingolipid metabolism and barrier function in the skin, likely representing a long-sought secretory lineage-defining factor across tissue types.

Transcriptional and signalling regulation of skin epithelial stem cells in homeostasis, wounds and cancer.

The epidermis and skin appendages are maintained by their resident epithelial stem cells, which undergo long-term self-renewal and multilineage differentiation. Upon injury, stem cells are activated to mediate re-epithelialization and restore tissue function. During this process, they often mount lineage plasticity and expand their fates in response to damage signals. Stem cell function is tightly controlled by transcription machineries and signalling transductions, many of which derail in degenerative, inflammatory and malignant dermatologic diseases. Here, by describing both well-characterized and newly emerged pathways, we discuss the transcriptional and signalling mechanisms governing skin epithelial homeostasis, wound repair and squamous cancer. Throughout, we highlight common themes underscoring epithelial stem cell plasticity and tissue-level crosstalk in the context of skin physiology and pathology.

Intracytoplasmic epidermal growth factor receptor shows poor response to the cetuximab antitumor effect in irradiated non-small cell lung cancer cell lines.

PURPOSE: The purpose of this study was to evaluate the antitumor effect of a blocking antibody for EGFR in the cytoplasm of a lung cancer cell line. METHODS & MATERIALS: The A549 and H1299 cell lines were employed to demonstrate differential responses to cetuximab in combination with radiotherapy. Localization of EGFR was detected using confocal microscopy, and radiosensitivity was measured. RESULTS: Treatment with cetuximab inhibited colony formation in a dose-dependent manner in A549, but not H1299 cells. Confocal microscopy revealed EGFR localized in the cytosolic fraction, particularly around the golgi apparatus in H1299, in contrast to cell membrane localization in A549 cell line. After irradiation, nuclear EGFR was detected in the A549 cell line. However, EGFR did not translocate to the nucleus in H1299 cells. While EGFR expression was decreased in both A549 and H1299 cell lines upon treatment with a combination of cetuximab and radiation, radiosensitivity was increased solely in A549 cells. CONCLUSIONS: Our findings suggest that localization of EGFR is related to the sensitivity/resistance of cells to a combination of cetuximab and radiotherapy.

Recombinant human epidermal growth factor accelerates the proliferation of irradiated human fibroblasts and keratinocytes in vitro and in vivo.

Irradiation causes the impaired proliferation of cells lining mucosal membranes. Epidermal growth factor (EGF) facilitates proliferation of various skin cells; however, the wound healing effects of EGF on radiation-damaged cells is less well known. To evaluate the effects of recombinant human EGF (rhEGF) on the proliferation of cells following irradiation, we tested two types of fibroblast cell lines and one keratinocyte cell line. The viable cell numbers were significantly increased by rhEGF treatment for 24 h immediately after 8 Gy of irradiation. The most effective dose of rhEGF was 10 nM in all cell lines used in this study. The percentage of BrdU-labeled cells was also significantly increased by rhEGF treatment. To evaluate the effects of rhEGF on radiation-induced oral mucosal damage in BALB/c mice, we systematically injected 1 mg/kg/day EGF for three days after 17 Gy of irradiation. Administered rhEGF ameliorated radiation-induced mucosal damage in vivo. rhEGF significantly increased the epithelial cell layer thickness and the proliferation of basal layer cells as detected by Ki-67 staining. Our results suggest that rhEGF can be a therapeutic treatment for radiation-induced wounds by stimulating the proliferation of fibroblasts and keratinocytes following irradiation.

Neogenin regulates skeletal myofiber size and focal adhesion kinase and extracellular signal-regulated kinase activities in vivo and in vitro.

A variety of signaling pathways participate in the development of skeletal muscle, but the extracellular cues that regulate such pathways in myofiber formation are not well understood. Neogenin is a receptor for ligands of the netrin and repulsive guidance molecule (RGM) families involved in axon guidance. We reported previously that neogenin promoted myotube formation by C2C12 myoblasts in vitro and that the related protein Cdo (also Cdon) was a potential neogenin coreceptor in myoblasts. We report here that mice homozygous for a gene-trap mutation in the Neo1 locus (encoding neogenin) develop myotomes normally but have small myofibers at embryonic day 18.5 and at 3 wk of age. Similarly, cultured myoblasts derived from such animals form smaller myotubes with fewer nuclei than myoblasts from control animals. These in vivo and in vitro defects are associated with low levels of the activated forms of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK), both known to be involved in myotube formation, and inefficient expression of certain muscle-specific proteins. Recombinant netrin-2 activates FAK and ERK in cultured myoblasts in a neogenin- and Cdo-dependent manner, whereas recombinant RGMc displays lesser ability to activate these kinases. Together, netrin-neogenin signaling is an important extracellular cue in regulation of myogenic differentiation and myofiber size.

Elevated epithelial insulin-like growth factor expression is a risk factor for lung cancer development.

Insulin-like growth factor (IGF)-I receptor (IGF-IR) signaling has been implicated in several human neoplasms. However, the role of serum levels of IGFs in lung cancer risk is controversial. We assessed the role of tissue-derived IGFs in lung carcinogenesis. We found that IGF-I and IGF-II levels in bronchial tissue specimens containing high-grade dysplasia were significantly higher than in those containing normal epithelium, hyperplasia, and squamous metaplasia. Derivatives of human bronchial epithelial cell lines with activation mutation in KRAS(V12) or loss of p53 overexpressed IGF-I and IGF-II. The transformed characteristics of these cells were significantly suppressed by inactivation of IGF-IR or inhibition of IGF-I or IGF-II expression but enhanced by overexpression of IGF-IR or exposure to the tobacco carcinogens (TC) 4-(methylnitrosamino)-I-(3-pyridyl)-1-butanone and benzo(a)pyrene. We further determined the role of IGF-IR signaling in lung tumorigenesis by determining the antitumor activities of the selective IGF-IR tyrosine kinase inhibitor cis-3-[3-(4-methyl-piperazin-l-yl)-cyclobutyl]-1-(2-phenyl-quinolin-7-yl)-imidazo [1,5-a]pyrazin-8-ylamine using an in vitro progressive cell system and an in vivo mouse model with a lung-specific IGF-I transgene after exposure to TCs, including 4-(methylnitrosamino)-I-(3-pyridyl)-1-butanone plus benzo(a)pyrene. Our results show that airway epithelial cells produce IGFs in an autocrine or paracrine manner, and these IGFs act jointly with TCs to enhance lung carcinogenesis. Furthermore, the use of selective IGF-IR inhibitors may be a rational approach to controlling lung cancer.

Cdo promotes neuronal differentiation via activation of the p38 mitogen-activated protein kinase pathway.

Neural basic helix-loop-helix transcription factors (bHLHs) control many aspects of neurogenesis, such as proliferation, fate determination, and differentiation. We have previously shown that the promyogenic cell surface receptor Cdo modulates the Cdc42 and p38 mitogen-activated protein kinase (MAPK) pathways via a direct association with two scaffold-type proteins, JLP and Bnip-2, to regulate activities of myogenic bHLH factors and myogenic differentiation. We report here that Cdo uses similar regulatory mechanisms to promote neuronal differentiation. Expression of JLP, a scaffold protein for p38MAPK, and Bnip-2, a regulator of Cdc42, is increased during differentiation of C17.2 neural precursor cells and P19 embryonal carcinoma cells. These molecules regulate Cdc42 and p38MAPK activities, which increase in a Cdo-dependent manner during neuronal differentiation of C17.2 cells and retinoic acid-treated P19 cells. Furthermore, enhancement or reduction of Cdc42 and p38MAPK activities enhances or reduces, respectively, neuronal differentiation of these cell lines. Cdc42 and p38MAPK activities also promote heterodimerization of neurogenin1 and E47, suggesting that one way they promote neurogenesis is via regulation of neural bHLH factor activities. These results imply that a conserved intracellular signaling mechanism initiated by Cdo regulates the activities of tissue-specific bHLH factors and therefore functions as a key regulator of differentiation of several different cell lineages.

A Cdo-Bnip-2-Cdc42 signaling pathway regulates p38alpha/beta MAPK activity and myogenic differentiation.

The p38alpha/beta mitogen-activated protein kinase (MAPK) pathway promotes skeletal myogenesis, but the mechanisms by which it is activated during this process are unclear. During myoblast differentiation, the promyogenic cell surface receptor Cdo binds to the p38alpha/beta pathway scaffold protein JLP and, via JLP, p38alpha/beta itself. We report that Cdo also interacts with Bnip-2, a protein that binds the small guanosine triphosphatase (GTPase) Cdc42 and a negative regulator of Cdc42, Cdc42 GTPase-activating protein (GAP). Moreover, Bnip-2 and JLP are brought together through mutual interaction with Cdo. Gain- and loss-of-function experiments with myoblasts indicate that the Cdo-Bnip-2 interaction stimulates Cdc42 activity, which in turn promotes p38alpha/beta activity and cell differentiation. These results reveal a previously unknown linkage between a cell surface receptor and downstream modulation of Cdc42 activity. Furthermore, interaction with multiple scaffold-type proteins is a distinctive mode of cell surface receptor signaling and provides one mechanism for specificity of p38alpha/beta activation during cell differentiation.

Regulation of myoblast motility and fusion by the CXCR4-associated sialomucin, CD164.

Myoblast fusion is fundamental to the development and regeneration of skeletal muscle. To fuse, myoblasts undergo cell-cell recognition and adhesion and merger of membranes between apposing cells. Cell migration must occur in advance of these events to bring myoblasts into proximity, but the factors that regulate myoblast motility are not fully understood. CD164 is a cell surface sialomucin that is targeted to endosomes and lysosomes via its intracellular region. In hematopoietic progenitor cells, CD164 forms complexes with the motility-stimulating chemokine receptor, CXCR4, in response to the CXCR4 ligand, CXCL12/SDF-1 (Forde, S., Tye, B. J., Newey, S. E., Roubelakis, M., Smythe, J., McGuckin, C. P., Pettengell, R., and Watt, S. M. (2007) Blood 109, 1825-1833). We have previously shown that CD164 stimulates myotube formation in vitro. We report here that CD164 is associated with CXCR4 in C2C12 myoblasts. Cells in which CD164 levels are increased or decreased via overexpression or RNA interference-mediated knockdown, respectively, show enhanced or reduced myotube formation and cell migration, the latter both basally and in response to CXCL12/SDF-1. Furthermore, expression of CD164 cytoplasmic tail mutants that alter the endosome/lysosome targeting sequence and, consequently, the subcellular localization in myoblasts, reveals a similar correlation between cell motility and myotube formation. Finally, Cd164 mRNA is expressed in the dorsal somite (the early myogenic compartment of the mouse embryo) and in premuscle masses. Taken together, these results suggest that CD164 is a regulator of myoblast motility and that this property contributes to its ability to promote myoblast fusion into myotubes.