HtrA1, a potential predictor of response to cisplatin-based combination chemotherapy in gastric cancer

Microarray-based detection and expression analysis of drug resistance in an animal model of peritoneal metastasis from colon cancer

Chemotherapy drugs efficiently eradicate rapidly dividing differentiated cells by inducing cell death, but poorly target slowly dividing cells, including cancer stem cells and dormant cancer cells, in the later course of treatment. Prolonged exposure to chemotherapy results in a decrease in the proportion of apoptotic cells in the tumour mass. To investigate and characterize the molecular basis of this phenomenon, microarray-based expression analysis was performed to compare tHcred2-DEVD-EGFP-caspase 3-sensor transfected C-26 tumour cells that were harvested after engraftment into mice treated with or without 5-FU. Peritoneal metastasis was induced by intraperitoneal injection of C-26 cells, which were subsequently reisolated from omental metastatic tumours after the mice were sacrificed by the end of the 10th day after tumour injection. The purity of reisolated tHcred2-DEVD-EGFP-caspase 3-sensor-expressing C-26 cells was confirmed using FLIM, and total RNA was extracted for gene expression profiling. The validation of relative transcript levels was carried out via real-time semiquantitative RT‒PCR assays. Our results demonstrated that chemotherapy induced the differential expression of mediators of cancer cell dormancy and cell survival-related genes and downregulation of both intrinsic and extrinsic apoptotic signalling pathways. Despite the fact that some differentially expressed genes, such as BMP7 and Prss11, have not been thoroughly studied in the context of chemoresistance thus far, they might be potential candidates for future studies on overcoming drug resistance.

High temperature requirement A1 in cancer: biomarker and therapeutic target

As the life expectancy of the population increases worldwide, cancer is becoming a substantial public health problem. Considering its recurrence and mortality rates, most cancer cases are difficult to cure. In recent decades, a large number of studies have been carried out on different cancer types; unfortunately, tumor incidence and mortality have not been effectively improved. At present, early diagnostic biomarkers and accurate therapeutic strategies for cancer are lacking. High temperature requirement A1 (HtrA1) is a trypsin-fold serine protease that is also a chymotrypsin-like protease family member originally discovered in bacteria and later discovered in mammalian systems. HtrA1 gene expression is decreased in diverse cancers, and it may play a role as a tumor suppressor for promoting the death of tumor cells. This work aimed to examine the role of HtrA1 as a cell type-specific diagnostic biomarker or as an internal and external regulatory factor of diverse cancers. The findings of this study will facilitate the development of HtrA1 as a therapeutic target.

HtrA serine proteases in cancers: A target of interest for cancer therapy

The HtrA protein family is composed by evolutionally-conserved serine proteases, which are homologous to the HtrA protein of the model bacterium Escherichia coli. They are widely distributed in organisms including humans, prokaryotes and eukaryotes. Moreover, HtrA family proteins are important regulators of a variety of human physiological processes, which contains the maintenance of mitochondrial homeostasis, cellular signal transduction and apoptosis regulation. The HtrA family has been found to be associated with cancer and could be used as a target for future cancer treatments. The purpose of this article is to review the relationship between these HtrA and cancer and to summarize the latest researches on HtrA and cancer.

Expression of HTRA Genes and Its Association with Microsatellite Instability and Survival of Patients with Colorectal Cancer

HtrA proteases regulate cellular homeostasis and cell death. Their dysfunctions have been correlated with oncogenesis and response to therapeutic treatment. We investigated the relation between HtrA1-3 expression and clinicopathological, and survival data, as well as the microsatellite status of tumors. Sixty-five colorectal cancer patients were included in the study. The expression of HTRA1-3 was estimated at the mRNA and protein levels by quantitative PCR and immunoblotting. Microsatellite status was determined by high-resolution-melting PCR. We found that the HTRA1 mRNA level was higher in colorectal cancer tissue as compared to the unchanged mucosa, specifically in primary lesions of metastasizing cancer. The levels of HtrA1 and HtrA2 proteins were reduced in tumor tissue when compared to unchanged mucosa, specifically in primary lesions of metastasizing disease. Moreover, a decrease in HTRA1 and HTRA2 transcripts' levels in cancers with a high level of microsatellite instability compared to microsatellite stable ones has been observed. A low level of HtrA1 or/and HtrA2 in cancer tissue correlated with poorer patient survival. The expression of HTRA1 and HTRA2 changes during colorectal carcinogenesis and microsatellite instability may be, at least partially, associated with these changes. The alterations in the HTRA1/2 genes' expression are connected with metastatic potential of colorectal cancer and may affect patient survival.

HTRA1 promotes transdifferentiation of normal fibroblasts to cancer-associated fibroblasts through activation of the NF-κB/bFGF signaling pathway in gastric cancer

Cancer-associated fibroblasts comprise the major stromal cell populations in gastric cancer, which is a significant contributor to cancer-related death worldwide. As a member of the serine protease family, HTRA1 is reportedly involved in malignant transformation of various tumor types. In the present study, we observed that HTRA1 is positively correlated with α-SMA expression in gastric cancer tissues, which was also confirmed by correlation analysis and Gene Set Enrichment Analysis (GSEA) using the GEO database. Upregulation of HTRA1 in gastric cancer cell lines induces expression of α-SMA in normal fibroblasts. To explore how HTRA1 activates normal fibroblasts, an ELISA assay was performed. Secretion of bFGF/FGF2 from gastric cancer cells was significantly increased in response to HTRA1 overexpression. However, upreguation of α-SMA in normal fibroblasts induced by HTRA1 was restored by inhibiting the expression of bFGF. Furthermore, HTRA1 promotes bFGF/FGF2 expression through activation of NF-κB signaling in gastric cancer cells. Inhibition of the NF-κB signaling pathway partially restored baseline expression levels of α-SMA induced by HTRA1. In conclusion, HTRA1 promotes transdifferentiation of normal fibroblasts to cancer-associated fibroblasts by increasing bFGF/FGF2 expression, which is dependent upon activation of NF-κB signaling in gastric cancer.

HtrA1 expression and the prognosis of high-grade serous ovarian carcinoma: a cohort study using digital analysis

BACKGROUND

The expression of high temperature requirement factor A1 (Htra1) has been reported to be decreased in ovarian carcinoma, but its prognostic effect remains undetermined.

METHODS

We evaluated the impact of HtrA1 downregulation in tumoral tissues on cancer progression and death in women with serous ovarian carcinoma. HtrA1 staining was performed on tissue microarrays (TMA) comprised of tumor samples from a cohort of 106 women who were diagnosed with primary high-grade serous ovarian carcinoma and receiving standard treatment at the Québec University Hospital between 1993 and 2006. HtrA1 expression was assessed visually (percentage of positive nuclei) and by digital image analysis (percentage of positive area). Cox regression multivariate models included standard prognostic factors and were used to estimate adjusted hazard ratios (aHR) for progression or death in the cohort.

RESULTS

By visual analysis, a low percentage of HtrA1-positive nuclei (< 10% vs ≥10%) tend to be associated with a lower risk of progression (aHR = 0.71; 95% Confidence interval (CI) = 0.46-1.09; P = 0.11) and mortality (aHR = 0.65; 95% CI = 0.41-1.04; P = 0.07). Low nuclear HtrA1 expression assessed by digital image analysis (< median % vs ≥ median %) showed a significant association with lower risk of progression (aHR = 0.62; 95% CI = 0.40-0.95; p = 0.03) and death (aHR = 0.60; 95% CI = 0.38-0.95; p = 0.03).

CONCLUSION

Altogether, our results demonstrate that nuclear downregulation of HtrA1 is associated with a better prognosis in women with high grade serous ovarian carcinoma.

HtrA1 as a promising tissue marker in cancer: a meta-analysis

BACKGROUND

HtrA1 is expressed in a variety of normal human tissues and seems to be involved in numerous physiological processes as well as tumorigenesis. This study reports the results of a meta-analysis that was performed: to compare HtrA1 expression as mRNA and protein, in cancer tissue versus non-cancer tissue and to assess overall survival in relation to low or medium-high HtrA1 tissue expression.

METHODS

The PRISMA method was used for study selection. OR and HR with 95% confidence interval was used as a measure of effect size as appropriate. A random-effects model was applied to account for different sources of variation among studies. Heterogeneity across studies was assessed using Q statistic. Sensitivity analysis was conducted to check the stability of study findings. Egger's regression method was applied to test funnel plot asymmetry.

RESULTS

Sensitivity analysis indicated the stability of meta-analytic findings in each meta-analysis. The study found a significantly different HtrA1 expression in cancer and non-cancer tissue. The meta-analysis of the prognostic studies showed a different survival according to HtrA1 expression.

CONCLUSIONS

The present data may provide a contribution to future work directed at exploring the role of HtrA1 in tumor development and progression and at establishing whether it may be used as a promising tissue marker for some tumors.

Structural insights into the activation mechanisms of human HtrA serine proteases

Human HtrA1-4 proteins belong to the HtrA family of evolutionarily conserved serine proteases and function as important modulators of many physiological processes, including maintenance of mitochondrial homeostasis, cell signaling and apoptosis. Disturbances in their action are linked to severe diseases, including oncogenesis and neurodegeneration. The HtrA1-4 proteins share structural and functional features of other members of the HtrA protein family, however there are several significant differences in structural architecture and mechanisms of action which makes each of them unique. Our goal is to present recent studies regarding human HtrAs. We focus on their physiological functions, structure and regulation, and describe current models of activation mechanisms. Knowledge of molecular basis of the human HtrAs' action is a subject of great interest; it is crucial for understanding their relevance in cellular physiology and pathogenesis as well as for using them as targets in future therapies of diseases such as neurodegenerative disorders and cancer.

Molecular Dissection of Induced Platinum Resistance through Functional and Gene Expression Analysis in a Cell Culture Model of Bladder Cancer

We report herein the development, functional and molecular characterization of an isogenic, paired bladder cancer cell culture model system for studying platinum drug resistance. The 5637 human bladder cancer cell line was cultured over ten months with stepwise increases in oxaliplatin concentration to generate a drug resistant 5637R sub cell line. The MTT assay was used to measure the cytotoxicity of several bladder cancer drugs. Liquid scintillation counting allowed quantification of cellular drug uptake and efflux of radiolabeled oxaliplatin and carboplatin. The impact of intracellular drug inactivation was assessed by chemical modulation of glutathione levels. Oxaliplatin- and carboplatin-DNA adduct formation and repair was measured using accelerator mass spectrometry. Resistance factors including apoptosis, growth factor signaling and others were assessed with RNAseq of both cell lines and included confirmation of selected transcripts by RT-PCR. Oxaliplatin, carboplatin, cisplatin and gemcitabine were significantly less cytotoxic to 5637R cells compared to the 5637 cells. In contrast, doxorubicin, methotrexate and vinblastine had no cell line dependent difference in cytotoxicity. Upon exposure to therapeutically relevant doses of oxaliplatin, 5637R cells had lower drug-DNA adduct levels than 5637 cells. This difference was partially accounted for by pre-DNA damage mechanisms such as drug uptake and intracellular inactivation by glutathione, as well as faster oxaliplatin-DNA adduct repair. In contrast, both cell lines had no significant differences in carboplatin cell uptake, efflux and drug-DNA adduct formation and repair, suggesting distinct resistance mechanisms for these two closely related drugs. The functional studies were augmented by RNAseq analysis, which demonstrated a significant change in expression of 83 transcripts, including 50 known genes and 22 novel transcripts. Most of the transcripts were not previously associated with bladder cancer chemoresistance. This model system and the associated phenotypic and genotypic data has the potential to identify some novel details of resistance mechanisms of clinical importance to bladder cancer.

HtrA1: Its future potential as a novel biomarker for cancer

HtrA1 appears to be involved in several physiological processes as well as in the pathogenesis of conditions such as Alzheimer's disease and osteoarthritis. It has also been hypothesized to play a role as a tumor suppressor. This manuscript reviews the current cancer-related HtrA1 research from the methodological and clinical standpoints including studies regarding its potential role as a tumor marker and/or prognostic factor. PRISMA method was used for study selection. The articles thus collected were examined and selected by two independent reviewers; any disagreement was resolved by a methodologist. A laboratory researcher reviewed the methods and laboratory techniques. Fifteen studies met the inclusion criteria and concerned the following cancer sites: the nervous system, bladder, breast, esophagus, stomach, liver, endometrium, thyroid, ovaries, pleura, lung and skin. Most articles described in vivo studies using a morphological approach and immunohistochemistry, whereas protein expression was quantified as staining intensity scored by two raters. Often the results were not comparable due to the different rating scales and study design. Current research on HtrA1 does not conclusively support its role as a tumor suppressor.

Decrease in serine protease HtrA1 expression correlates with grade and recurrence in meningiomas

PURPOSE

HtrA1 is a serine protease which was shown to be down-regulated in a variety of human cancers. It is considered to be a tumor suppressor and suggested as a prognostic marker and a therapeutic candidate. In order to investigate any possible implication of HtrA1 in meningioma we studied 100 cases.

MATERIAL/METHODS

We used immunohistochemistry to determine HtrA1 expression in tumor tissue. Expression levels were evaluated with respect to tumor grade and recurrence.

RESULTS

Our data revealed a strong association between decrease in HtrA1 expression and increase in meningioma grade (p=0.005). Most importantly, patients with higher HtrA1 expression had a lower rate of recurrence (p<0.001).

CONCLUSIONS

According to our results HtrA1 appeared as an immunohistochemical marker to predict behaviour of the meningioma, mainly the recurrence. Although the exact mechanisms of HtrA1 are still largely unknown, we think that further in vivo and in vitro studies explaining the molecular targets of HtrA1 would have a great importance with regard to its role as a therapeutic agent for meningioma.

HtrA1 resensitizes multidrug-resistant hepatocellular carcinoma cells by targeting XIAP

The study aims to clarify the relation between chemosensitivity and HtrA1 expression, and the possible way HtrA1 works. Drug-resistant cell line HepG2/ADM was induced by increasing adriamycin (ADM), and eukaryotic expression vector pEGFP-N1-HtrA1 was constructed using BamHI and EcoRI restriction enzymes, after which, HepG2/ADM was transfected with pEGFP-N1-HtrA1. Resistance index (RI) of the hepatoma cell lines to different anti-cancer drugs (ADM, 5-Fu, MMC, L-OHP and VCR) was determined by MTT assay before and after HtrA1 high expression. After an HtrA1 inhibitor, NVP-LEB748 was adopted in the HtrA1 overexpressing cells, expression of proteins P-gp, MRP and XIAP (X-linked inhibitor of apoptosis protein) in HepG2/ADM cells were analyzed by western blot, and the activities of caspases 3, 7 and 9 were respectively measured using activity assay kits. The results showed that RI was negatively correlated with the expression of HtrA1, upregulated XIAP expression was resulted from the HtrA1 inhibitor, and variance of activities of caspases 3, 7 and 9 were remarkably descended with its increasing concentration. It was concluded that high expression of HtrA1 could significantly reverse multidrug resistance of hepatoma cells by targeting XIAP. HtrA1 is therefore expected to be an effective tool in the therapy of hepatocellular carcinoma.

[Effect of PI3K/AKT pathway on cisplatin resistance in non-small cell lung cancer]

Accumulating evidences indicate that aberrant activation of PI3K/AKT pathway in non-small cell lung cancer plays a vital role in tumor cell proliferation,apoptosis, and survival including drug resistance. Cisplatin as first-line chemotherapy are in widespread clinical use in patients with non-small cell lung cancer, however, the development of cisplatin resistance significantly impedes its clinic efficacy. Cisplatin resistance is a complicated process that various mechanisms participating in to interact, of which PI3K/AKT pathway keeping sustained activated is one of the most important reasons. This article reviewed the progress of research on the relationship between PI3K/AKT pathway and cisplatin resistance.

HTRA1 gene expression in gastric epithelial cells

OBJECTIVES

To explore HtrA1 gene expression and its regulation in human gastric cancers.

METHODS

The HtrA1 mRNA levels were examined by QPCR analysis and confirmed its expression with Northern blot analysis. The HtrA1 protein levels in all six gastric epithelial cell lines were investigated by Western blot analysis. Gene copy number was accessed and then sequenced the coding region from each mRNA in all six cell lines. The HtrA1 promoter region DNA methylation status was detected by using bisulfite sequencing analysis. Effect of decitabine and TSA on HTRA1 expression in gastric cancer cell line was determined by RTPCR.

RESULTS

HIC analysis indicated that HtrA1 was highly expressed in normal epithelium, but dramatically down-regulated in gastric carcinoma tissues and variably expressed in tumor-adjacent tissues. HtrA1 gene expression was dramatically decreased in gastric carcinoma cells compared to non-tumorigenic counterparts. The HtrA1 gene loss in any of the 4 breast cancer cell lines was not detected. Total 14 CpGs in this region were all methylated in gastric cancer cells, whereas two normal cells, GES-1 and HFI-145, were having several unmethylated cytosines in this region. HtrA1 showed as ~Mr 44,000, Expression of HtrA1 protein was not observed in any of the four gastric cancer cell lines, BGC-823, MKN-45, SGC-7901and MKN-28. HtrA1 expression was observed in the HFI-145and GES-1 cell lines.

CONCLUSIONS

The epigenetic silencing for HtrA1 gene expression could provide a possible strategy for re-activating HtrA1 gene expression in gastric cancer cells, thus facilitating further investigation of HtrA1's role in chemotherapy.

HtrA1 downregulation induces cisplatin resistance in lung adenocarcinoma by promoting cancer stem cell-like properties

Cisplatin (CDDP) resistance usually develops during lung adenocarcinoma (LAC) therapy. However, the comprehensive mechanisms remain largely unclear. In this study, we first established a CDDP-resistant LAC cell line-A549/CDDP from its parental cell line-A549. The results showed that CDDP resistance in A549/CDDP cells correlates with acquirement of cancer stem cell-like properties (increased percentage of CD133-expressing subpopulation, sphere formation and levels of some pluripotency-associated markers). HtrA1 expression at both mRNA and protein levels was reduced in CDDP-resistant A549/CDDP cells compared with that in A549 cells. Ectopic expression of HtrA1 in A549/CDDP cells reversed cancer stem cell-like properties and CDDP resistance. In A549 cells, stable knockdown of HtrA1 expression promoted cancer stem cell-like properties and CDDP insensitivity, however, these effects were blocked by inhibition of PI3K/Akt pathway using LY294002. Furthermore, HtrA1 knockdown could significantly stimulate PI3K/Akt signaling in A549 cells. In vivo studies, HtrA1 knockdown promoted tumorigenesis and conferred CDDP resistance in xenograft A549 tumors, which were reversed by intraperitoneal injection of LY294002. In conclusion, these results indicate that HtrA1 downregulation confers CDDP resistance by inducing cancer stem cell-like properties via PI3K/Akt-dependent pathway in A549 cells. Therefore, HtrA1 may be a potential target for overcoming CDDP resistance in LAC.

Multiple sporadic colorectal cancers display a unique methylation phenotype

Epigenetics are thought to play a major role in the carcinogenesis of multiple sporadic colorectal cancers (CRC). Previous studies have suggested concordant DNA hypermethylation between tumor pairs. However, only a few methylation markers have been analyzed. This study was aimed at describing the epigenetic signature of multiple CRC using a genome-scale DNA methylation profiling. We analyzed 12 patients with synchronous CRC and 29 age-, sex-, and tumor location-paired patients with solitary tumors from the EPICOLON II cohort. DNA methylation profiling was performed using the Illumina Infinium HM27 DNA methylation assay. The most significant results were validated by Methylight. Tumors samples were also analyzed for the CpG Island Methylator Phenotype (CIMP); KRAS and BRAF mutations and mismatch repair deficiency status. Functional annotation clustering was performed. We identified 102 CpG sites that showed significant DNA hypermethylation in multiple tumors with respect to the solitary counterparts (difference in β value ≥0.1). Methylight assays validated the results for 4 selected genes (p = 0.0002). Eight out of 12(66.6%) multiple tumors were classified as CIMP-high, as compared to 5 out of 29(17.2%) solitary tumors (p = 0.004). Interestingly, 76 out of the 102 (74.5%) hypermethylated CpG sites found in multiple tumors were also seen in CIMP-high tumors. Functional analysis of hypermethylated genes found in multiple tumors showed enrichment of genes involved in different tumorigenic functions. In conclusion, multiple CRC are associated with a distinct methylation phenotype, with a close association between tumor multiplicity and CIMP-high. Our results may be important to unravel the underlying mechanism of tumor multiplicity.

HtrA1 in human urothelial bladder cancer: a secreted protein and a potential novel biomarker

Our aim was to analyze the expression of the serine protease HtrA1 in human bladder tissue and urine in order to point out its possible association with the presence of urothelial bladder cancer. Bladder tissue and urine specimens from cancer patients with different tumor grades and stages (n = 68) and from individuals with cystitis (n = 16) were collected along with biopsy specimens and urine from healthy individuals (n = 68). For the first time, we demonstrated by immunohistochemistry that HtrA1 protein is produced by bladder urothelium in both physiological and inflammatory conditions, whereas it is not detectable in urothelial cancer cells regardless of tumor grade and stage. A different HtrA1 expression between normal-looking and neoplastic bladder tissue, despite similar HtrA1 mRNA levels, was also found by western blotting, which disclosed the presence of two forms of HtrA1, a native form of ∼50 kDa and an autocatalytic form of ∼38 kDa. Our investigations documented the presence of the two forms of HtrA1 also in urine. The ∼38 kDa form was significantly down-regulated in neoplastic tissue, whereas significantly higher amounts of both HtrA1 forms were found in urine from cancer patients compared with both healthy subjects and patients with cystitis. Our findings suggest that HtrA1 is a downexpressed molecule since an early stage of bladder urothelial carcinoma development and that urinary HtrA1 protein may be considered, if successfully validated, as an early and highly sensitive and specific biomarker for this neoplasia (the sensitivity and specificity of HtrA1 are 92.65% and 95.59%, respectively).

Downregulation of serine protease HTRA1 is associated with poor survival in breast cancer

HTRA1 is a highly conserved serine protease which has been implicated in suppression of epithelial-to-mesenchymal-transition (EMT) and cell motility in breast cancer. Its prognostic relevance for breast cancer is unclear so far. Therefore, we evaluated the impact of HTRA1 mRNA expression on patient outcome using a cohort of 131 breast cancer patients as well as a validation cohort including 2809 publically available data sets. Additionally, we aimed at investigating for the presence of promoter hypermethylation as a mechanism for silencing the HTRA1 gene in breast tumors. HTRA1 downregulation was detected in more than 50% of the breast cancer specimens and was associated with higher tumor stage (p = 0.025). By applying Cox proportional hazard models, we observed favorable overall (OS) and disease-free survival (DFS) related to high HTRA1 expression (HR = 0.45 [CI 0.23-0.90], p = 0.023; HR = 0.55 [CI 0.32-0.94], p = 0.028, respectively), with even more pronounced impact in node-positive patients (HR = 0.21 [CI 0.07-0.63], p = 0.006; HR = 0.29 [CI 0.13-0.65], p = 0.002, respectively). Moreover, HTRA1 remained a statistically significant factor predicting DFS among established clinical parameters in the multivariable analysis. Its impact on patient outcome was independently confirmed in the validation set (for relapse-free survival (n = 2809): HR = 0.79 [CI 0.7-0.9], log-rank p = 0.0003; for OS (n = 971): HR = 0.63 [CI 0.48-0.83], log-rank p = 0.0009). In promoter analyses, we in fact detected methylation of HTRA1 in a small subset of breast cancer specimens (two out of a series of 12), and in MCF-7 breast cancer cells which exhibited 22-fold lower HTRA1 mRNA expression levels compared to unmethylated MDA-MB-231 cells. In conclusion, we show that downregulation of HTRA1 is associated with shorter patient survival, particularly in node-positive breast cancer. Since HTRA1 loss was demonstrated to induce EMT and cancer cell invasion, these patients might benefit from demethylating agents or histone deacetylase inhibitors previously reported to lead to HTRA1 upregulation, or from novel small-molecule inhibitors targeting EMT-related processes.

Down-regulation of HtrA1 activates the epithelial-mesenchymal transition and ATM DNA damage response pathways

Expression of the serine protease HtrA1 is decreased or abrogated in a variety of human primary cancers, and higher levels of HtrA1 expression are directly related to better response to chemotherapeutics. However, the precise mechanisms leading to HtrA1 down regulation during malignant transformation are unclear. To investigate HtrA1 gene regulation in breast cancer, we characterized expression in primary breast tissues and seven human breast epithelial cell lines, including two non-tumorigenic cell lines. In human breast tissues, HtrA1 expression was prominent in normal ductal glands. In DCIS and in invasive cancers, HtrA1 expression was greatly reduced or lost entirely. HtrA1 staining was also reduced in all of the human breast cancer cell lines, compared with the normal tissue and non-tumorigenic cell line controls. Loss of HtrA1 gene expression was attributable primarily to epigenetic silencing mechanisms, with different mechanisms operative in the various cell lines. To mechanistically examine the functional consequences of HtrA1 loss, we stably reduced and/or overexpressed HtrA1 in the non-tumorigenic MCF10A cell line. Reduction of HtrA1 levels resulted in the epithelial-to-mesenchymal transition with acquisition of mesenchymal phenotypic characteristics, including increased growth rate, migration, and invasion, as well as expression of mesenchymal biomarkers. A concomitant decrease in expression of epithelial biomarkers and all microRNA 200 family members was also observed. Moreover, reduction of HtrA1 expression resulted in activation of the ATM and DNA damage response, whereas overexpression of HtrA1 prevented this activation. Collectively, these results suggest that HtrA1 may function as a tumor suppressor by controlling the epithelial-to-mesenchymal transition, and may function in chemotherapeutic responsiveness by mediating DNA damage response pathways.