IGFBP-3 interacts with NONO and SFPQ in PARP-dependent DNA damage repair in triple-negative breast cancer.

Women with triple-negative breast cancer (TNBC) are generally treated by chemotherapy but their responsiveness may be blunted by DNA double-strand break (DSB) repair. We previously reported that IGFBP-3 forms nuclear complexes with EGFR and DNA-dependent protein kinase (DNA-PKcs) to modulate DSB repair by non-homologous end-joining (NHEJ) in TNBC cells. To discover IGFBP-3 binding partners involved in chemoresistance through stimulation of DSB repair, we analyzed the IGFBP-3 interactome by LC-MS/MS and confirmed interactions by coimmunoprecipitation and proximity ligation assay. Functional effects were demonstrated by DNA end-joining in vitro and measurement of γH2AX foci. In response to 20 µM etoposide, the DNA/RNA-binding protein, non-POU domain-containing octamer-binding protein (NONO) and its dimerization partner splicing factor, proline/glutamine-rich (SFPQ) formed complexes with IGFBP-3, demonstrated in basal-like TNBC cell lines HCC1806 and MDA-MB-468. NONO binding to IGFBP-3 was also shown in a cell-free biochemical assay. IGFBP-3 complexes with NONO and SFPQ were blocked by inhibiting EGFR with gefitinib or DNA-PKcs with NU7026, and by the PARP inhibitors veliparib and olaparib, which also reduced DNA end-joining activity and delayed the resolution of the γH2AX signal (i.e. inhibited DNA DSB repair). Downregulation of the long noncoding RNA in NHEJ pathway 1 (LINP1) by siRNA also blocked IGFBP-3 interaction with NONO-SFPQ. These findings suggest a PARP-dependent role for NONO and SFPQ in IGFBP-3-dependent DSB repair and the involvement of LINP1 in the complex formation. We propose that targeting of the DNA repair function of IGFBP-3 may enhance chemosensitivity in basal-like TNBC, thus improving patient outcomes.

Inhibition of basal-like breast cancer growth by FTY720 in combination with epidermal growth factor receptor kinase blockade.

BACKGROUND: New molecular targets are needed for women with triple-negative breast cancer (TNBC). This pre-clinical study investigated the combination of the EGFR inhibitor gefitinib with the sphingosine kinase (SphK) inhibitor FTY720 (Fingolimod), aiming to block tumorigenic signaling downstream of IGFBP-3, which is abundantly expressed in basal-like TNBC. METHODS: In studies of breast cancer cell growth in culture, proliferation was monitored by IncuCyte live-cell imaging, and protein abundance was determined by western blotting. In vivo studies of mammary tumor growth used two models: orthotopic xenograft tumors derived from three basal-like TNBC cell lines, grown in immune-deficient mice, and syngeneic murine 4T1 tumors grown in immune-competent mice. Protein abundance in tumor tissue was assessed by immunohistochemistry. RESULTS: Quantitated by live-cell imaging, the inhibitor combination showed synergistic cytostatic activity in basal-like cell lines across several TNBC molecular subtypes, the synergy being decreased by IGFBP-3 downregulation. Suppression of the tumorigenic mediator CD44 by gefitinib was potentiated by FTY720, consistent with CD44 involvement in the targeted pathway. In MDA-MB-468 and HCC1806 orthotopic TNBC xenograft tumors in nude mice, the drug combination inhibited tumor growth and prolonged mouse survival, although this effect was not significant for the gefitinib-resistant cell line HCC70. Combination treatment of murine 4T1 TNBC tumors in syngeneic BALB/c mice was more effective in immune-competent than immune-deficient (nude) mice, and a relative loss of tumor CD3 (T-cell) immunoreactivity caused by FTY720 treatment alone was alleviated by the drug combination, suggesting that, even at an FTY720 dose causing relative lymphopenia, the combination is still effective in an immune-competent setting. Immunohistochemistry of xenograft tumors showed significant enhancement of caspase-3 cleavage and suppression of Ki67 and phospho-EGFR by the drug combination, but SphK1 downregulation occurred only in MDA-MB-468 tumors, so is unlikely to be integral to treatment efficacy. CONCLUSIONS: Our data indicate that targeting IGFBP-3-dependent signaling pathways through gefitinib-FTY720 co-therapy may be effective in many basal-like breast cancers, and suggest tissue IGFBP-3 and CD44 measurement as potential biomarkers of treatment efficacy.

KCa3.1 mediates dysfunction of tubular autophagy in diabetic kidneys via PI3k/Akt/mTOR signaling pathways.

Autophagy is emerging as an important pathway in many diseases including diabetic nephropathy. It is acknowledged that oxidative stress plays a critical role in autophagy dysfunction and diabetic nephropathy, and KCa3.1 blockade ameliorates diabetic renal fibrosis through inhibiting TGF-ß1 signaling pathway. To identify the role of KCa3.1 in dysfunctional tubular autophagy in diabetic nephropathy, human proximal tubular cells (HK2) transfected with scrambled or KCa3.1 siRNAs were exposed to TGF-ß1 for 48 h, then autophagosome formation, the autophagy marker LC3, signaling molecules PI3K, Akt and mTOR, and oxidative stress marker nitrotyrosine were examined respectively. In vivo, LC3, nitrotyrosine and phosphorylated mTOR were examined in kidneys of diabetic KCa3.1+/+ and KCa3.1-/- mice. The results demonstrated that TGF-ß1 increased the formation of autophagic vacuoles, LC3 expression, and phosphorylation of PI3K, Akt and mTOR in scrambled siRNA transfected HK2 cells compared to control cells, which was reversed in KCa3.1 siRNA transfected HK2 cells. In vivo, expression of LC3 and nitrotyrosine, and phosphorylation of mTOR were significantly increased in kidneys of diabetic KCa3.1+/+ mice compared to non-diabetic mice, which were attenuated in kidneys of diabetic KCa3.1-/- mice. These results suggest that KCa3.1 activation contributes to dysfunctional tubular autophagy in diabetic nephropathy through PI3K/Akt/mTOR signaling pathways.

Involvement of p53 in insulin-like growth factor binding protein-3 regulation in the breast cancer cell response to DNA damage.

Chemotherapy drugs that induce apoptosis by causing DNA double-strand breaks, upregulate the tumor suppressor p53. This study investigated the regulation of the growth-regulatory protein insulin-like growth factor binding protein-3 (IGFBP-3), a p53 target, by DNA-damaging agents in breast cancer cells. IGFBP-3 was upregulated 1.4- to 13-fold in response to doxorubicin and etoposide in MCF-10A, Hs578T, MCF-7 and T47D cells, which express low to moderate basal levels of IGFBP-3. In contrast, IGFBP-3 was strongly downregulated by these agents in cells with high basal levels of IGFBP-3 (MDA-MB-231, MDA-MB-436 and MDA-MB-468). In MDA-MB-468 cells containing the R273H p53 mutation, reported to display gain-of-function properties, chemotherapy-induced suppression of IGFBP-3 was not reversed by the p53 reactivating drug, PRIMA-1, or by p53 silencing, suggesting that the decrease in IGFBP-3 following DNA damage is not a mutant p53 gain-of-function response. SiRNA-mediated downregulation of endogenous IGFBP-3 modestly attenuated doxorubicin-induced apoptosis in MDA-MB-468 and Hs578T cells. IGFBP-3 downregulation in some breast cancer cell lines in response to DNA-damaging chemotherapy may have clinical implications because suppression of IGFBP-3 may modulate the apoptotic response. These observations provide further evidence that endogenous IGFBP-3 plays a role in breast cancer cell responsiveness to DNA damaging therapy.

A novel truncated form of S100P predicts disease-free survival in patients with lymph node positive breast cancer.

The calcium-binding protein S100P is overexpressed in various cancers and may contribute to the oncogenic phenotype. This study used mass spectrometry to characterize a novel 9.2-kDa C-terminally truncated form of S100P (t-S100P), and to investigate its potential prognostic value in breast cancer. Univariate analysis demonstrated the association between breast tissue t-S100P levels (n = 148) and conventional pathological markers. Across all tumor samples, high t-S100P was strongly prognostic for poor disease-free survival (P = 0.005), its efficacy confined to lymph node-positive tumors (n = 74, P = 0.007). Matrix-assisted laser desorption/ionization imaging mass spectrometry confirmed differential t-S100P abundance between breast cancer and unaffected adjacent tissue. t-S100P was exclusively located in the cell nucleus of breast cancer tissue, and full-length S100P was essentially undetectable by mass spectrometry. We conclude that t-S100P is the predominant form of S100P in breast cancer tissue and is strongly prognostic for disease-free survival in women with lymph node-positive disease.

Involvement of the insulin-like growth factor binding proteins in the cancer cell response to DNA damage.

The complex mechanisms that cells have evolved to meet the challenge of constant exposure to DNA-damaging stimuli, also serve to protect cancer cells from the cytotoxic effects of chemo- and radiotherapy. IGFBPs appear to be involved, directly or indirectly, in some of these protective mechanisms. Activation of p53 is an early response to genotoxic stress, and all six human IGFBP genes have predicted p53 response elements in their promoter and/or intronic regions, at least some of which are functional. IGFBP3 has been extensively characterized as a p53-inducible gene, but in some cases it is suppressed by mutant p53 forms. DNA double-strand breaks (DSBs), induced by radiotherapy and some chemotherapies, potentially lead to apoptotic cell death, senescence, or repair and recovery. DSB damage can be repaired by homologous recombination or non-homologous end-joining (NHEJ), depending on the cell cycle stage, availability of key repair proteins, and other factors. The epidermal growth factor receptor (EGFR) has been implicated in the NHEJ pathway, and EGFR inhibition may inhibit repair, promoting apoptosis and thus improving sensitivity to chemotherapy or radiotherapy. Both IGFBP-3 and IGFBP-6 interact with components of the NHEJ pathway, and IGFBP-3 can facilitate this process through direct interaction with both EGFR and the catalytic subunit of DNA-PK. Cell fate after DNA damage may in part be regulated by the balance between the sphingolipids ceramide and sphingosine-1-phosphate, and IGFBPs can influence the production of both lipids. A better understanding of the involvement of IGFBPs in the DNA damage response in cancer cells may lead to improved methods of sensitizing cancers to DNA-damaging therapies.

Thioredoxin-interacting protein mediates dysfunction of tubular autophagy in diabetic kidneys through inhibiting autophagic flux.

Thioredoxin-interacting protein (TXNIP) expression is ubiquitous and is induced by a variety of cellular stresses, including high intracellular glucose. TXNIP is associated with activation of oxidative stress and tubulointerstitial fibrosis in diabetic nephropathy. Autophagy is a major pathway that delivers damaged proteins and organelles to lysosomes to maintain cellular homeostasis. This study aimed to investigate the dysregulation of autophagy and the regulation of TXNIP on autophagy in renal proximal tubular cells (PTCs) under diabetic conditions. The formation of autophagosomes was measured using transmission electron microscopy, and LC3-II, and the effectiveness of autophagic clearance was determined by p62 expression in diabetic kidney and in human PTCs exposed to high glucose (HG). The results collectively demonstrated increased expression of TXNIP, LC3/LC3-II and p62 in renal tubular cells of mice with diabetic nephropathy and in cultured human PTCs exposed to HG (30 mM/l) for 48 h compared with control. The formation of autophagic vacuoles was increased in HG-induced cells. Furthermore, silencing of TXNIP by siRNA transfection reduced autophagic vacuoles and the expression of LC3-II and p62 in human PTCs exposed to HG compared with control and partially reversed the accumulation of LC3-II and p62 induced by bafilomycin A1 (50 nM/l), a pharmacological inhibitor of autophagy which blocks the fusion of autophagosomes with lysosomes and impairs the degradation of LC3-II and p62. Collectively, these results suggest that hyperglycemia leads to dysfunction of autophagy in renal tubular cells and decreases autophagic clearance. HG-induced overexpression of TXNIP may contribute to the dysfunction of tubular autophagy in diabetes.

Inhibition of insulin-like growth factor-binding protein-3 signaling through sphingosine kinase-1 sensitizes triple-negative breast cancer cells to EGF receptor blockade.

The type I EGF receptor (EGFR or ErbB1) and insulin-like growth factor-binding protein-3 (IGFBP-3) are highly expressed in triple-negative breast cancer (TNBC), a particularly aggressive disease that cannot be treated with conventional therapies targeting the estrogen or progesterone receptors (ER and PR), or HER2. We have shown previously in normal breast epithelial cells that IGFBP-3 potentiates growth-stimulatory signaling transduced by EGFR, and this is mediated by the sphingosine kinase-1 (SphK1)/sphingosine 1-phosphate (S1P) system. In this study, we investigated whether cotargeting the EGFR and SphK1/S1P pathways in TNBC cells results in greater growth inhibition compared with blocking either alone, and might therefore have novel therapeutic potential in TNBC. In four TNBC cell lines, exogenous IGFBP-3 enhanced ligand-stimulated EGFR activation, associated with increased SphK1 localization to the plasma membrane. The effect of exogenous IGFBP-3 on EGFR activation was blocked by pharmacologic inhibition or siRNA-mediated silencing of SphK1, and silencing of endogenous IGFBP-3 also suppressed EGF-stimulated EGFR activation. Real-time analysis of cell proliferation revealed a combined effect of EGFR inhibition by gefitinib and SphK1 inhibition using SKi-II. Growth of MDA-MB-468 xenograft tumors in mice was significantly inhibited by SKi-II and gefitinib when used in combination, but not as single agents. We conclude that IGFBP-3 promotes growth of TNBC cells by increasing EGFR signaling, that this is mediated by SphK1, and that combined inhibition of EGFR and SphK1 has potential as an anticancer therapy in TNBC in which EGFR and IGFBP-3 expression is high.

iTRAQ-based proteomic profiling of breast cancer cell response to doxorubicin and TRAIL.

Breast cancer is a molecularly heterogeneous disease, and predicting response to chemotherapy remains a major clinical challenge. To minimize adverse side-effects or cumulative toxicity in patients unlikely to benefit from treatment, biomarkers indicating treatment efficacy are critically needed. iTRAQ labeling coupled with multidimensional LC-MS/MS of the enriched mitochondria and endoplasmic reticulum fraction, key organelles regulating apoptosis, has led to the discovery of several differentially abundant proteins in breast cancer cells treated with the chemotherapeutic agent doxorubicin followed by the death receptor ligand, TRAIL, among 571 and 801 unique proteins identified in ZR-75-1 and MDA-MB-231 breast cancer cell lines, respectively. The differentially abundant proteins represent diverse biological processes associated with cellular assembly and organization, molecular transport, oxidative stress, cell motility, cell death, and cancer. Despite many differences in molecular phenotype between the two breast cancer cell lines, a comparison of their subproteomes following drug treatment revealed three proteins displaying common regulation: PPIB, AHNAK, and SLC1A5. Changes in these proteins, detected by iTRAQ, were confirmed by immunofluorescence, visualized by confocal microscopy. These novel potential biomarkers may have clinical utility for assessing response to cancer treatment and may provide insight into new therapeutic targets for breast cancer.

Potentiation of growth factor signaling by insulin-like growth factor-binding protein-3 in breast epithelial cells requires sphingosine kinase activity.

We have investigated the mechanism underlying potentiation of epidermal growth factor receptor (EGFR) and type 1 insulin-like growth factor receptor (IGFR1) signaling by IGF-binding protein-3 (IGFBP-3) in MCF-10A breast epithelial cells, focusing on a possible involvement of the sphingosine kinase (SphK) system. IGFBP-3 potentiated EGF-stimulated EGF receptor activation and DNA synthesis, and this was blocked by inhibitors of SphK activity or small interference RNA-mediated silencing of SphK1, but not SphK2, expression. Similarly, IGFR1 phosphorylation and DNA synthesis stimulated by LR3-IGF-I (an IGF-I analog not bound by IGFBP-3), were enhanced by IGFBP-3, and this was blocked by SphK1 silencing. SphK1 expression and activity were stimulated by IGFBP-3 approximately 2-fold over 24 h. Silencing of sphingosine 1-phosphate receptor 1 (S1P1) or S1P3, but not S1P2, abolished the effect of IGFBP-3 on EGF-stimulated EGFR activation. The effects of IGFBP-3 could be reproduced with exogenous S1P or medium conditioned by cells treated with IGFBP-3, and this was also blocked by inhibition of S1P1 and S1P3. These data indicate that potentiation of growth factor signaling by IGFBP-3 in MCF-10A cells requires SphK1 activity and S1P1/S1P3, suggesting that S1P, the product of SphK activity and ligand for S1P1 and S1P3, is the "missing link" mediating IGF and EGFR transactivation and cell growth stimulation by IGFBP-3.