Radiation-induced alterations of osteogenic and chondrogenic differentiation of human mesenchymal stem cells.

While human mesenchymal stem cells (hMSCs), either in the bone marrow or in tumour microenvironment could be targeted by radiotherapy, their response is poorly understood. The oxic effects on radiosensitivity, cell cycle progression are largely unknown, and the radiation effects on hMSCs differentiation capacities remained unexplored. Here we analysed hMSCs viability and cell cycle progression in 21% O2 and 3% O2 conditions after medical X-rays irradiation. Differentiation towards osteogenesis and chondrogenesis after irradiation was evaluated through an analysis of differentiation specific genes. Finally, a 3D culture model in hypoxia was used to evaluate chondrogenesis in conditions mimicking the natural hMSCs microenvironment. The hMSCs radiosensitivity was not affected by O2 tension. A decreased number of cells in S phase and an increase in G2/M were observed in both O2 tensions after 16 hours but hMSCs released from the G2/M arrest and proliferated at day 7. Osteogenesis was increased after irradiation with an enhancement of mRNA expression of specific osteogenic genes (alkaline phosphatase, osteopontin). Osteoblastic differentiation was altered since matrix deposition was impaired with a decreased expression of collagen I, probably through an increase of its degradation by MMP-3. After induction in monolayers, chondrogenesis was altered after irradiation with an increase in COL1A1 and a decrease in both SOX9 and ACAN mRNA expression. After induction in a 3D culture in hypoxia, chondrogenesis was altered after irradiation with a decrease in COL2A1, ACAN and SOX9 mRNA amounts associated with a RUNX2 increase. Together with collagens I and II proteins decrease, associated to a MMP-13 expression increase, these data show a radiation-induced impairment of chondrogenesis. Finally, a radiation-induced impairment of both osteogenesis and chondrogenesis was characterised by a matrix composition alteration, through inhibition of synthesis and/or increased degradation. Alteration of osteogenesis and chondrogenesis in hMSCs could potentially explain bone/joints defects observed after radiotherapy.

Impact of therapeutic irradiation on healthy articular cartilage.

Radiation-induced complications in bone and cartilage are of increasing concern due to potential long-term effects in cancer survivors. Healthy articular cartilage may be exposed to radiation during either chondrosarcoma treatment or in-field radiotherapy of tumors located in close proximity to articulation. Cartilage exposed to radiation undergoes bone differentiation and senescence, which can lead to painful and disabling sequelae that can impair patient quality of life. An understanding of the biological processes involved in healthy cartilage response to radiotherapy may not only optimize the delivery of therapeutic radiation but also reduce the risk of long-term sequelae in irradiated cartilage. Over the last few decades, radiobiology studies have focused primarily on signaling and repair of DNA damage pathways induced by ionizing radiation in immortalized cells under conditions dramatically different from human homeostasis. This research needs to be continued and broadened, since the range of normal tissue responses to radiation exposure is still not fully understood, despite being recognized as the major limiting factor in the rupture of tissue homeostasis after radiotherapy. Human articular cartilage is an avascular tissue with low intracellular oxygen levels and is comprised of a single cell lineage of chondrocytes embedded in a highly dense and structured extracellular matrix. These relatively unique features may impact inherent cell radiation sensitivity and suggests that canonical cell responses to ionizing radiation may not be applicable to articular cartilage. Despite the number of studies in this field, radiation-induced modifications of chondrocyte proteome remain unclear because of the dramatic variability in reported experimental conditions. In this review, we propose to introduce cartilage tissue physiology and microenvironment concepts, and then present a comprehensive synthesis of cartilage radiation biology.

DNA polymerase η modulates replication fork progression and DNA damage responses in platinum-treated human cells.

Human cells lacking DNA polymerase η (polη) are sensitive to platinum-based cancer chemotherapeutic agents. Using DNA combing to directly investigate the role of polη in bypass of platinum-induced DNA lesions in vivo, we demonstrate that nascent DNA strands are up to 39% shorter in human cells lacking polη than in cells expressing polη. This provides the first direct evidence that polη modulates replication fork progression in vivo following cisplatin and carboplatin treatment. Severe replication inhibition in individual platinum-treated polη-deficient cells correlates with enhanced phosphorylation of the RPA2 subunit of replication protein A on serines 4 and 8, as determined using EdU labelling and immunofluorescence, consistent with formation of DNA strand breaks at arrested forks in the absence of polη. Polη-mediated bypass of platinum-induced DNA lesions may therefore represent one mechanism by which cancer cells can tolerate platinum-based chemotherapy.

Doxorubicin induces the DNA damage response in cultured human mesenchymal stem cells.

Anthracyclines, including doxorubicin, are widely used in the treatment of leukemia. While the effects of doxorubicin on hematopoietic cells have been characterized, less is known about the response of human mesenchymal stem cells (hMSCs) in the bone marrow stroma to anthracyclines. We characterized the effect of doxorubicin on key DNA damage responses in hMSCs, and compared doxorubicin sensitivity and DNA damage response activation between isolated hMSCs and the chronic myelogenous leukemia cell line, K562. Phosphorylation of H2AX, Chk1, and RPA2 was more strongly activated in K562 cells than in hMSCs, at equivalent doses of doxorubicin. hMSCs were relatively resistant to doxorubicin such that, following exposure to 15 µM doxorubicin, the level of cleaved caspase-3 detected by western blotting was lower in hMSCs compared to K562 cells. Flow cytometric analysis of cell cycle progression demonstrated that exposure to doxorubicin induced G2/M phase arrest in hMSCs, while 48 h after exposure, 15.6 % of cells were apoptotic, as determined from the percentage of cells having sub-G1 DNA content. We also show that the doxorubicin sensitivity of hMSCs isolated from a healthy donor was comparable to that of hMSCs isolated from a chronic lymphocytic leukemia patient. Overall, our results demonstrate that high doses of doxorubicin induce the DNA damage response in hMSCs, and that cultured hMSCs are relatively resistant to doxorubicin.

Activation of DNA damage response pathways in human mesenchymal stem cells exposed to cisplatin or γ-irradiation.

DNA damaging agents are widely used in treatment of hematogical malignancies and solid tumors. While effects on hematopoietic stem cells have been characterized, less is known about the DNA damage response in human mesenchymal stem cells (hMSCs) in the bone marrow stroma, progenitors of osteoblasts, chondrocytes and adipocytes. To elucidate the response of undifferentiated hMSCs to γ-irradiation and cisplatin, key DNA damage responses have been characterised in hMSCs from normal adult donors. Cisplatin and γ-irradiation activated the DNA damage response in hMSCs, including induction of p53 and p21, and activation of PI3 kinase-related protein kinase (PIKK)-dependent phosphorylation of histone H2AX on serine 139, and replication protein A2 on serine4/serine8. Chemical inhibition of ATM or DNA-PK reduced DNA damage-induced phosphorylation of H2AX, indicating a role for both PIKKs in the response of hMSCs to DNA damage. Consistent with repair of DNA strand breaks, γ-H2AX staining decreased by 24 hours following gamma-irradiation. γ-Irradiation arrested hMSCs in the G 1 phase of the cell cycle, while cisplatin induced S-phase arrest, mediated in part by the ATR/Chk1 checkpoint pathway. In hMSCs isolated from a chronic lymphocytic leukemia (CLL) patient, p53 and p21 were induced by cisplatin and γ-irradiation, while RPA2 was phosphorylated on serine4/8 in particular following cisplatin. Compared to peripheral blood lymphocytes or the leukemia cell line K562, both normal hMSCs and CLL-derived hMSCs were more resistant to cisplatin and γ-irradiation. These results provide insights into key pathways mediating the response of bone marrow-derived hMSCs to DNA damaging agents used in cancer treatment.

Analysis of protein phosphorylation in cisplatin-treated human cells following annexin V-based separation and multi-antibody screening.

Cancer chemotherapy relies heavily on DNA damaging agents such as cisplatin to induce tumour cell death. The response of cells to genotoxic insult, including cell cycle arrest, DNA repair and cell death, is mediated by the DNA damage response (DDR). To address the relationship between the DDR and the outcome of exposure, this study utilised a magnetic-activated cell sorting (MACS®)-based approach to isolate apoptotic and non-apoptotic cells from a DNA polymerase eta-deficient human cell line. The pattern of phosphorylation of the key DNA damage response protein RPA2 on serine 4/8 was altered in apoptotic cells isolated following cisplatin treatment. By combining MACS® with multi-antibody screening for phosphorylated proteins, apoptosis-associated changes were characterized in a number of key signalling pathways. Phosphorylation of Erk1 on Thr202/Tyr204, and Erk2 on Thr185/Tyr187 was increased in apoptotic cells. This approach provides novel insights into the relationship between cisplatin-induced protein phosphorylation and the cellular consequences of exposure to this chemotherapeutic agent.

Human mesenchymal stem cells (hMSCs) as targets of DNA damaging agents in cancer therapy.

Human mesenchymal stem cells (hMSCs) consist of cells that can differentiate into mesenchymal tissues, including osteoblasts, adipocytes and chondrocytes. hMSCs constitute a particular stem cell niche in the stromal compartment of the bone marrow, and also play a role in maintaining the normal function of haematopoietic stem cells. Furthermore, hMSCs localise to solid tumours, and can modulate cancer cell function through secretion of paracrine signals. While hMSCs, either in the bone marrow, or in the microenvironment of a tumour, will be targeted by DNA damaging agents used in cancer therapy, the response of the hMSC population to DNA damage is not well understood. In their role as progenitor cells, genomic DNA damage to hMSCs during cancer therapy could generate a population of surviving cells that can go on to give rise to secondary tumours. A better understanding of the response of hMSCs to DNA damage could provide new insights into the effects of cancer treatments, as well as into the development of treatment-associated secondary cancers. The article will review the relationship of hMSCs to cancer, with a focus on the response of hMSCs to DNA damaging agents.

DNA polymerase eta, a key protein in translesion synthesis in human cells.

Genomic DNA is constantly damaged by exposure to exogenous and endogenous agents. Bulky adducts such as UV-induced cyclobutane pyrimidine dimers (CPDs) in the template DNA present a barrier to DNA synthesis by the major eukaryotic replicative polymerases including DNA polymerase delta. Translesion synthesis (TLS) carried out by specialized DNA polymerases is an evolutionarily conserved mechanism of DNA damage tolerance. The Y family of DNA polymerases, including DNA polymerase eta (Pol eta), the subject of this chapter, play a key role in TLS. Mutations in the human POLH gene encoding Pol eta underlie the genetic disease xeroderma pigmentosum variant (XPV), characterized by sun sensitivity, elevated incidence of skin cancer, and at the cellular level, by delayed replication and hypermutability after UV-irradiation. Pol eta is a low fidelity enzyme when copying undamaged DNA, but can carry out error-free TLS at sites of UV-induced dithymine CPDs. The active site of Pol eta has an open conformation that can accommodate CPDs, as well as cisplatin-induced intrastrand DNA crosslinks. Pol eta is recruited to sites of replication arrest in a tightly regulated process through interaction with PCNA. Pol eta-deficient cells show strong activation of downstream DNA damage responses including ATR signaling, and accumulate strand breaks as a result of replication fork collapse. Thus, Pol eta plays an important role in preventing genome instability after UV- and cisplatin-induced DNA damage. Inhibition of DNA damage tolerance pathways in tumors might also represent an approach to potentiate the effects of DNA damaging agents such as cisplatin.

Characterization of the effects of cisplatin and carboplatin on cell cycle progression and DNA damage response activation in DNA polymerase eta-deficient human cells.

Translesion synthesis by DNA polymerase eta (poleta) is one mechanism by which cancer cells can tolerate DNA damage by platinum-based anti-cancer drugs. Cells lacking poleta are sensitive to these agents. To help define the consequences of poeta-deficiency, we characterized the effects of equitoxic doses of cisplatin and carboplatin on cell cycle progression and activation of DNA damage response pathways in a human cell line lacking poleta. We show that both cisplatin and carboplatin induce strong S-phase arrest in poleta-deficient XP30RO cells, associated with reduced expression of cyclin E and cyclin B. PIK kinase-mediated phosphorylation of Chk1, H2AX and RPA2 was strongly activated by both cisplatin and carboplatin, but phosphorylation of these proteins was induced earlier by cisplatin than by an equitoxic dose of carboplatin. Compared to Chk1 and H2AX phosphorylation, RPA2 hyperphosphorylation on serine4/serine8 is a late event in response to platinum-induced DNA damage. We directly demonstrate, using dual-labeling flow cytometry, that damage-induced phosphorylation of RPA2 on serine4/serine8 occurs primarily in the S and G(2) phases of the cell cycle, and show that the timing of RPA2 phosphorylation can be modulated by inhibition of the checkpoint kinase Chk1. Furthermore, Chk1 inhibition sensitizes poleta-deficient cells to the cytotoxic effects of carboplatin. Both hyperphosphorylated RPA2 and the homologous recombination protein Rad51 are present in nuclear foci after cisplatin treatment, but these are separable events in individual cells. These results provide insight into the relationship between cell cycle regulation and processing of platinum-induced DNA damage in human cells when poleta-mediated TLS is compromised.

Enhanced DNA-PK-mediated RPA2 hyperphosphorylation in DNA polymerase eta-deficient human cells treated with cisplatin and oxaliplatin.

The chemotherapeutic drugs cisplatin and oxaliplatin act by induction of DNA damage, including monoadducts, intrastrand and interstrand crosslinks. An increased understanding of the repair and replication of platinum-damaged DNA is required to improve the effectiveness of these drugs in killing cancer cells. We have investigated the effect of expression of DNA polymerase eta (poleta), a translesion synthesis (TLS) enzyme, on the response of human cell lines to cisplatin and oxaliplatin. Poleta-deficient cells are more sensitive to both drugs than are normal cells. In poleta-deficient cells, drug treatment leads to prolonged S-phase arrest, and increased phosphorylation of the phosphatidylinositol-3-kinase-related protein kinase (PIKK) substrates Chk1, p95/Nbs1 and RPA2, the 34kDa subunit of replication protein A. Cisplatin- and oxaliplatin-induced hyperphosphorylation of RPA2, and association of the hyperphosphorylated protein with chromatin, is elevated in poleta-deficient cells. Cisplatin-induced phosphorylation of RPA2 on serine 4/serine 8, but not on serine 33, is inhibited by the DNA-PK inhibitor, NU7441, but not by the ATM inhibitor, KU-55933. Cisplatin-induced DNA-PK-dependent hyperphosphorylation of RPA2 on serine 4/serine 8 occurs after recruitment of RPA to chromatin, as determined by immunofluorescence and by subcellular fractionation. ATR is required both for recruitment of RPA2 to chromatin and its subsequent hyperphosphorylation on serine 4/serine 8 by DNA-PK, since CGK733, an inhibitor of ATM and ATR, blocked both recruitment and hyperphosphorylation. Thus, increased sensitivity to cisplatin and oxaliplatin in DNA poleta-deficient cells is associated with prolonged S-phase arrest, and enhanced PIKK-signalling, in particular activation of DNA-PK-dependent hyperphosphorylation of RPA2 on serines 4 and 8.

UV-induced RPA phosphorylation is increased in the absence of DNA polymerase eta and requires DNA-PK.

Signaling from arrested replication forks plays a role in maintaining genome stability. We have investigated this process in xeroderma pigmentosum variant cells that carry a mutation in the POLH gene and lack functional DNA polymerase eta (poleta). Poleta is required for error-free bypass of UV-induced cyclobutane pyrimidine dimers; in the absence of poleta in XPV cells, DNA replication is arrested at sites of UV-induced DNA damage, and mutagenic bypass of lesions is ultimately carried out by other, error-prone, DNA polymerases. The present study investigates whether poleta expression influences the activation of a number of UV-induced DNA damage responses. In a stably transfected XPV cell line (TR30-9) in which active poleta can be induced by addition of tetracycline, expression of poleta determines the extent of DNA double-strand break formation following UV-irradiation. UV-induced phosphorylation of replication protein A (RPA), a key DNA-binding protein involved in DNA replication, repair and recombination, is increased in cells lacking poleta compared to when poleta is expressed in the same cell line. To identify the protein kinase responsible for increased UV-induced hyperphosphorylation of the p34 subunit of RPA, we have used NU7441, a specific small molecule inhibitor of DNA-PK. DNA-PK is necessary for RPA p34 hyperphosphorylation, but DNA-PK-mediated phosphorylation is not required for recruitment of RPA p34 into nuclear foci in response to UV-irradiation. The results demonstrate that activation of a UV-induced DNA damage response pathway, involving phosphorylation of RPA p34 by DNA-PK, is enhanced in cells lacking poleta.

Expression of alpha V-associated integrin beta subunits in epithelial ovarian cancer and its relation to prognosis in patients treated with platinum-based regimens.

The aim of the study was to investigate the relationships between the expression of alphav, beta1, beta3, beta5, and beta6, integrin subunits and clinical parameters in ovarian cancers. Ovarian surface epithelium (OSE) from five donors and tumour samples from 39 patients with an epithelial ovarian cancer (39 primary tumours and 21 associated peritoneal metastases) were analysed using immunohistochemistry on paraffin-embedded or frozen tissue sections. The alphav and beta5 integrin subunits were always present in normal OSE and in tumours. beta1 and beta3 subunit expression was significantly less frequent in grade 3 than in grade 1-2 tumours. The proportion of stage IV tumours expressing beta3 was significantly lower as compared to other stages. The beta6 subunit was undetectable in OSE but was expressed in about 40% of primary tumours. For all integrin, there was a strong relationship between the expression in primary tumours and in associated peritoneal metastases. Survival analyses restricted to patients receiving platinum-based chemotherapy did not reveal any relationship between integrin subunit expression and 3-year survival rate, in this limited series of patients. In conclusion, the expression of the various beta integrin subunits was differentially altered in ovarian carcinoma, evocative of complementary roles of alphav integrins during tumour development.

Regulation of tumor angiogenesis by integrin-linked kinase (ILK).

We show that integrin-linked kinase (ILK) stimulates the expression of VEGF by stimulating HIF-1alpha protein expression in a PKB/Akt- and mTOR/FRAP-dependent manner. In human prostate cancer cells, knockdown of ILK expression with siRNA, or inhibition of ILK activity, results in significant inhibition of HIF-1alpha and VEGF expression. In endothelial cells, VEGF stimulates ILK activity, and inhibition of ILK expression or activity results in the inhibition of VEGF-mediated endothelial cell migration, capillary formation in vitro, and angiogenesis in vivo. Inhibition of ILK activity also inhibits prostate tumor angiogenesis and suppresses tumor growth. These data demonstrate an important and essential role of ILK in two key aspects of tumor angiogenesis: VEGF expression by tumor cells and VEGF-stimulated blood vessel formation.

alpha(v) integrins regulate cell proliferation through integrin-linked kinase (ILK) in ovarian cancer cells.

Integrins regulate both adhesion and signaling processes involved in proliferation and survival. alpha(v)beta(3) and alpha(v)beta(5) integrins have been shown to mediate cell adhesion and migration. Here we used human ovarian cancer cell lines (IGROV1, SKOV-3) that express alpha(v)beta(3) and alpha(v)beta(5) to study their role in cell proliferation and the signaling pathways involved. We found that alpha(v) integrins regulate cell proliferation through activation of integrin-linked kinase (ILK). An anti-alpha(v)-blocking antibody specifically inhibits the growth of IGROV1 and SKOV-3. The inhibition of cell proliferation involves alpha(v)beta(3) in IGROV1 cells, and both alpha(v)beta(3) and alpha(v)beta(5) in SKOV-3 cells. The reduced growth rate induced by alpha(v) integrin blockade is linked in both cell lines to G1/S cell cycle arrest. alpha(v) integrin blockade by neutralizing antibody as well as cyclic-RGD peptide caused an inhibition of ILK activity and phosphorylation of PKB/Akt on serine-473 but not on threonine-308, and was accompanied by an increase in p27(Kip1) expression. Overexpression of wild-type ILK rescued the phosphorylation of PKB/Akt on serine-473 in cells treated with anti-alpha(v) antibody. Inhibition of ILK by a pharmacological inhibitor results in inhibition of cell proliferation, PKB/Akt phosphorylation and increase of p27(Kip1). These results demonstrate that alpha(v) integrins regulate ovarian cancer cell proliferation through ILK.

Altered adhesion properties and alphav integrin expression in a cisplatin-resistant human ovarian carcinoma cell line.

In order to elucidate the mechanisms underlying the development of chemoresistance in ovarian cancer, we have previously established the IGROV1-R10 cisplatin-resistant cell line by mimicking a clinical protocol of drug administration on IGROV1 human ovarian carcinoma cells. Both IGROV1 and IGROV1-R10 cells were able to grow as a monolayer and to release cell clusters into the medium. However, IGROV1-R10 cells exhibited an enhanced capacity to detach from the monolayer as compared to the parental cells. When substrate adhesion was prevented, IGROV1-R10 cells were able to survive and to proliferate as cell clusters, even at a low cell density, whereas IGROV1 cells massively died. To explore the underlying mechanisms, we have been interested in alphav integrins, which have been implicated in some aspects of ovarian cancer biology. Both IGROV1 and IGROV1-R10 adherent cells expressed alphavbeta3 integrin. During cell growth, alphavbeta5 integrin accumulated at the surface of a majority of IGROV1-R10 cells from the monolayer, whereas only a faint expression of this integrin was observed in a minority of IGROV1 cells. The growth of IGROV1-R10 cells, but not of IGROV1 cells, was partly inhibited by a specific alphavbeta5-blocking antibody suggesting that alphavbeta5 integrin contributed to IGROV1-R10 cell proliferation.