alpha6 integrin cleavage: sensitizing human prostate cancer to ionizing radiation

Evaluation of Extracellular Matrix Composition to Improve Breast Cancer Modeling

The development of resistance to therapy is a significant obstacle to effective therapeutic regimens. Evaluating the effects of oncology drugs in the laboratory setting is limited by the lack of translational models that accurately recapitulate cell-microenvironment interactions present in tumors. Acquisition of resistance to therapy is facilitated, in part, by the composition of the tumor extracellular matrix (ECM), with the primary current in vitro model using collagen I (COL I). Here we seek to identify the prevalence of COL I-enhanced expression in the triple-negative breast cancer (TNBC) subtype. Furthermore, we identify if methods of response to therapy are altered depending on matrix composition. We demonstrated that collagen content varies in patient tumor samples across subtypes, with COL I expression dramatically increased in typically less aggressive estrogen receptor (ER)-positive(ER+)/progesterone receptor (PGR)-positive (PGR+) cancers irrespective of patient age or race. These findings are of significance considering how frequently COL I is implicated in tumor progression. In vitro analyses of ER+ and ER-negative (ER-) cell lines were used to determine the effects of ECM content (collagen I, collagen IV, fibronectin, and laminin) on proliferation, cellular phenotype, and survival. Neither ER+ nor ER- cells demonstrated significant increases in proliferation when cultured on these ECM substrates. ER- cells cultured on these substrates were sensitized to both chemotherapy and targeted therapy. In addition, MDA-MB-231 cells expressed different morphologies, binding affinities, and stiffness across these substrates. We also demonstrated that ECM composition significantly alters transcription of senescence-associated pathways across ER+ and ER- cell lines. Together, these results suggest that complex matrix composites should be incorporated into in vitro tumor models, especially for the drug-resistant TNBC subtype. Impact statement The importance of tumor extracellular matrix (ECM) in disease progression is often inadequately represented in models of breast cancer that rely heavily on collagen I and Matrigel. Through immunohistochemistry analysis of patient breast tumors, we show a wide variation in collagen content based on subtype, specifically a repression of fibril collagens in the receptor negative subtype, irrespective of age and race. We also demonstrated that tumor ECM composition alters cellular elasticity and oncogenic pathway activation demonstrating that physiologically relevant three-dimensional models of breast cancer should include an ECM that is subtype specific.

Nongenomic Actions of Thyroid Hormone: The Integrin Component

The extracellular domain of plasma membrane integrin αvβ3 contains a cell surface receptor for thyroid hormone analogues. The receptor is largely expressed and activated in tumor cells and rapidly dividing endothelial cells. The principal ligand for this receptor is l-thyroxine (T₄), usually regarded only as a prohormone for 3,5,3'-triiodo-l-thyronine (T₃), the hormone analogue that expresses thyroid hormone in the cell nucleus via nuclear receptors that are unrelated structurally to integrin αvβ3. At the integrin receptor for thyroid hormone, T₄ regulates cancer and endothelial cell division, tumor cell defense pathways (such as anti-apoptosis), and angiogenesis and supports metastasis, radioresistance, and chemoresistance. The molecular mechanisms involve signal transduction via mitogen-activated protein kinase and phosphatidylinositol 3-kinase, differential expression of multiple genes related to the listed cell processes, and regulation of activities of other cell surface proteins, such as vascular growth factor receptors. Tetraiodothyroacetic acid (tetrac) is derived from T₄ and competes with binding of T₄ to the integrin. In the absence of T₄, tetrac and chemically modified tetrac also have anticancer effects that culminate in altered gene transcription. Tumor xenografts are arrested by unmodified and chemically modified tetrac. The receptor requires further characterization in terms of contributions to nonmalignant cells, such as platelets and phagocytes. The integrin αvβ3 receptor for thyroid hormone offers a large panel of cellular actions that are relevant to cancer biology and that may be regulated by tetrac derivatives.

Hypoxia-induced PIM kinase and laminin-activated integrin α6 mediate resistance to PI3K inhibitors in bone-metastatic CRPC

Bone-metastatic castration-resistant prostate cancer (CRPC) is lethal due to inherent resistance to androgen deprivation therapy, chemotherapy, and targeted therapies. Despite the fact that a majority of CRPC patients (approximately 70%) harbor a constitutively active PI3K survival pathway, targeting the PI3K/mTOR pathway has failed to increase overall survival in clinical trials. Here, we identified two separate and independent survival pathways induced by the bone tumor microenvironment that are hyperactivated in CRPC and confer resistance to PI3K inhibitors. The first pathway involves integrin α6β1-mediated adhesion to laminin and the second involves hypoxia-induced expression of PIM kinases. In vitro and in vivo models demonstrate that these pathways transduce parallel but independent signals that promote survival by reducing oxidative stress and preventing cell death. We further demonstrate that both pathways drive resistance to PI3K inhibitors in PTEN-negative tumors. These results provide preclinical evidence that combined inhibition of integrin α6β1 and PIM kinase in CRPC is required to overcome tumor microenvironment-mediated resistance to PI3K inhibitors in PTEN-negative tumors within the hypoxic and laminin-rich bone microenvironment.

The Tumor Microenvironments of Lethal Prostate Cancer

Localized prostate cancer (confined to the gland) generally is considered curable, with nearly a 100% 5-year-survival rate. When the tumor escapes the prostate capsule, leading to metastasis, there is a poorer prognosis and higher mortality rate, with 5-year survival dropping to less than 30%. A major research question has been to understand the transition from indolent (low risk) disease to aggressive (high risk) disease. In this chapter, we provide details of the changing tumor microenvironments during prostate cancer invasion and their role in the progression and metastasis of lethal prostate cancer. Four microenvironments covered here include the muscle stroma, perineural invasion, hypoxia, and the role of microvesicles in altering the extracellular matrix environment. The adaptability of prostate cancer to these varied microenvironments and the cues for phenotypic changes are currently understudied areas. Model systems for understanding smooth muscle invasion both in vitro and in vivo are highlighted. Invasive human needle biopsy tissue and mouse xenograft tumors both contain smooth muscle invasion. In combination, the models can be used in an iterative process to validate molecular events for smooth muscle invasion in human tissue. Understanding the complex and interacting microenvironments in the prostate holds the key to early detection of high-risk disease and preventing tumor invasion through escape from the prostate capsule.

Integrin α6/Akt/Erk signaling is essential for human breast cancer resistance to radiotherapy

Integrin α6 (ITGA6), a transmembrane glycoprotein adhesion receptor protein, is widely upregulated in many types of tumors and promotes migration and invasion in cancer cells. However, the role that the ITGA6-associated signaling network plays in radiosensitivity in breast cancer has not been described. The expression of ITGA6 was examined in human breast cancer and normal breast cell lines using western blot analysis. We also explored the role of ITGA6 in the regulation of radiation sensitivity in breast cancer using the colony formation assays, cell cycle analyses, apoptosis assays and immunofluorescence analyses. The results showed that the protein and mRNA expression levels of ITGA6 was higher in breast cancer cells than in normal cells. ITGA6 protectived responses to radiotherapy in breast cancer cells by altering cell apoptosis, DNA damage repair and cell-cycle regulation. Furthermore, ITGA6 enhanced radiation resistance via PI3K/Akt and MEK/Erk signaling. In addition, overexpressing ITGA6 promoted radiation resistance in cells, and this effect was neutralized by the PI3K inhibitor LY294002 and MEK inhibitor U0126. Taken together, these findings indicate that ITGA6 might be involved in a mechanism that underlies radiation resistance and that ITGA6 could be a potential target for therapies aimed at overcoming radiation resistance in breast cancer.

Therapeutic Implications for Overcoming Radiation Resistance in Cancer Therapy

Ionizing radiation (IR), such as X-rays and gamma (γ)-rays, mediates various forms of cancer cell death such as apoptosis, necrosis, autophagy, mitotic catastrophe, and senescence. Among them, apoptosis and mitotic catastrophe are the main mechanisms of IR action. DNA damage and genomic instability contribute to IR-induced cancer cell death. Although IR therapy may be curative in a number of cancer types, the resistance of cancer cells to radiation remains a major therapeutic problem. In this review, we describe the morphological and molecular aspects of various IR-induced types of cell death. We also discuss cytogenetic variations representative of IR-induced DNA damage and genomic instability. Most importantly, we focus on several pathways and their associated marker proteins responsible for cancer resistance and its therapeutic implications in terms of cancer cell death of various types and characteristics. Finally, we propose radiation-sensitization strategies, such as the modification of fractionation, inflammation, and hypoxia and the combined treatment, that can counteract the resistance of tumors to IR.

The host microenvironment influences prostate cancer invasion, systemic spread, bone colonization, and osteoblastic metastasis

Prostate cancer (PCa) is the second leading cause of cancer death in men worldwide. Most PCa deaths are due to osteoblastic bone metastases. What triggers PCa metastasis to the bone and what causes osteoblastic lesions remain unanswered. A major contributor to PCa metastasis is the host microenvironment. Here, we address how the primary tumor microenvironment influences PCa metastasis via integrins, extracellular proteases, and transient epithelia-mesenchymal transition (EMT) to promote PCa progression, invasion, and metastasis. We discuss how the bone-microenvironment influences metastasis; where chemotactic cytokines favor bone homing, adhesion molecules promote colonization, and bone-derived signals induce osteoblastic lesions. Animal models that fully recapitulate human PCa progression from primary tumor to bone metastasis are needed to understand the PCa pathophysiology that leads to bone metastasis. Better delineation of the specific processes involved in PCa bone metastasize is needed to prevent or treat metastatic PCa. Therapeutic regimens that focus on the tumor microenvironment could add to the PCa pharmacopeia.

Integrin α3β1 regulates tumor cell responses to stromal cells and can function to suppress prostate cancer metastatic colonization

Integrin α3β1 promotes tumor cell adhesion, migration, and invasion on laminin isoforms, and several clinical studies have indicated a correlation between increased tumoral α3β1 integrin expression and tumor progression, metastasis, and poor patient outcomes. However, several other clinical and experimental studies have suggested that α3β1 can possess anti-metastatic activity in certain settings. To help define the range of α3β1 functions in tumor cells in vivo, we used RNAi to silence the α3 integrin subunit in an aggressive, in vivo-passaged subline of PC-3 prostate carcinoma cells. Loss of α3 integrin impaired adhesion and proliferation on the α3β1 integrin ligand, laminin-332 in vitro. Despite these deficits in vitro, the α3-silenced cells were significantly more aggressive in a lung colonization model in vivo, with a substantially increased rate of tumor growth that significantly reduced survival. In contrast, silencing the related α6 integrin subunit delayed metastatic growth in vivo. The increased colonization of α3-silenced tumor cells in vivo was recapitulated in 3D collagen co-cultures with lung fibroblasts or pre-osteoblast-like cells, where α3-silenced cells showed dramatically enhanced growth. The increased response of α3-silenced tumor cells to stromal cells in co-culture could be reproduced by fibroblast conditioned medium, which contains one or more heparin-binding factors that selectively favor the growth of α3-silenced cells. Our new data suggest a scenario in which α3β1 regulates tumor-host interactions within the metastatic tumor microenvironment to limit growth, providing some of the first direct evidence that specific loss of α3 function in tumor cells can have pro-metastatic consequences in vivo.

Impact of the adaptor protein GIPC1/Synectin on radioresistance and survival after irradiation of prostate cancer

PURPOSE

Studies have shown that GIPC1/Synectin is an essential adaptor protein of receptors that play an important role in cancer progression and therapy resistance. This is the first study to explore the role of GIPC1/Synectin in radioresistance of prostate cancer and as a possible predictive marker for outcome of primary radiation therapy.

MATERIALS AND METHODS

The effect of RNA interference-mediated GIPC1/Synectin depletion on clonogenic cell survival after irradiation with 0, 2, 4, or 6 Gy was assayed in two different GIPC1/Synectin-expressing human prostate cancer cell lines. The clinical outcome data of 358 men who underwent radiotherapy of prostate cancer with a curative intention were analyzed retrospectively. Uni- and multivariate analysis was performed of prostate-specific antigen recurrence-free survival and overall survival in correlation with protein expression in pretreatment biopsy specimens. Protein expression was evaluated by standard immunohistochemistry methods.

RESULTS

In cell culture experiments, no change was detected in radiosensitivity after depletion of GIPC1/Synectin in GIPC1/Synectin-expressing prostate cancer cell lines. Furthermore, there was no correlation between GIPC1/Synectin expression in human pretreatment biopsy samples and overall or biochemical recurrence-free survival after radiotherapy in a retrospective analysis of the study cohort.

CONCLUSION

Our results do not show a predictive or prognostic function of GIPC1/Synectin expression for the outcome of radiotherapy in prostate cancer. Furthermore, our in vitro results do not support a role of GIPC1 in the cellular radiation response. However, the role of GIPC1 in the progression of prostate cancer and its precursors should be subject to further research.

Metastasis Update: Human Prostate Carcinoma Invasion via Tubulogenesis

This paper proposes that human prostate carcinoma primarily invades as a cohesive cell collective through a mechanism similar to embryonic tubulogenesis, instead of the popular epithelial-mesenchymal transformation (EMT) model. Evidence supporting a tubulogenesis model is presented, along with suggestions for additional research. Additionally, observations documenting cell adhesion molecule changes in tissue and stromal components are reviewed, allowing for comparisons between the current branching morphogenesis models and the tubulogenesis model. Finally, the implications of this model on prevailing views of therapeutic and diagnostic strategies for aggressive prostatic disease are considered.

Integrin involvement in freeze resistance of androgen-insensitive prostate cancer

Cryoablation has emerged as a primary therapy to treat prostate cancer. While effective, the assumption that freezing serves as a ubiquitous lethal stress is challenged by clinical experience and experimental evidence demonstrating time-temperature related cell death dependence. The age-related transformation from an androgen-sensitive (AS) to an androgen-insensitive (AI) phenotype is a major challenge in the management of prostate cancer. AI cells exhibit morphological changes and treatment resistance to many therapies. Since this resistance has been linked with α6β4 integrin overexpression as a result of androgen receptor (AR) loss, we investigated whether α6β4 integrin expression, as a result AR loss, contributes to the reported increased freeze tolerance of AI prostate cancer. A series of studies using AS (LNCaP LP and PC-3 AR) and AI (LNCaP HP and PC-3) cell lines were designed to investigate the cellular mechanisms contributing to variations in freezing response. Investigation into α6β4 integrin expression revealed that AI cell lines overexpressed this protein, thereby altering morphological characteristics and increasing adhesion characteristics. Molecular investigations revealed a significant decrease in caspase 8, 9, and 3 levels AI cells following freezing. Inhibition of α6β4 integrin resulted in increased caspase activity following freezing (similar to AS cells) and enhanced cell death. These data demonstrate that AI cells show an increase in post-freeze susceptibility following inhibition of α6β4 integrin function. Further understanding the role of androgen-receptor related α6β4 integrin expression in prostate cancer cells responses to freezing might lead to novel options for neo-adjunctive treatments targeting the AR signaling pathway.

beta1 integrin targeting to enhance radiation therapy

PURPOSE

Cell adhesion to extracellular matrix (ECM) proteins is mediated by the integrin family and has been known to modify radiation sensitivity and resistance in several cell types, including cancer cells. In particular, beta1 integrin signaling has been implicated in the progression and metastasis of various cancers and has been shown to facilitate resistance to radiation therapy.

CONCLUSION

In this mini-review, we provide a brief overview of integrin targeting in radiation therapy. We specifically focus on the updated findings of beta1 integrin-mediated signaling pathways after exposure to ionising radiation (IR) using in vitro and in vivo experimental models, which could represent promising therapeutic targets for breast cancer.

Extracellular engagement of alpha6 integrin inhibited urokinase-type plasminogen activator-mediated cleavage and delayed human prostate bone metastasis

Expression of alpha(6) integrin, a laminin receptor, on tumor cell surfaces is associated with reduced patient survival and increased metastasis in a variety of tumors. In prostate cancer, tumor extracapsular escape occurs in part via laminin-coated nerves and vascular dissemination, resulting in clinically significant bone metastases. We previously identified a novel form of alpha(6) integrin, called alpha(6)p, generated by urokinase-type plasminogen activator-dependent cleavage of the laminin-binding domain from the tumor cell surface. Cleavage increased laminin-dependent migration. Currently, we used the known conformation sensitivity of integrin function to determine if engagement of the extracellular domain inhibited integrin cleavage and the extravasation step of metastasis. We show that alpha(6) integrin was present on prostate carcinoma escaping the gland via nerves. Both endogenous and inducible levels of alpha(6)p were inhibited by engaging the extracellular domain of alpha(6) with monoclonal antibody J8H. J8H inhibited tumor cell invasion through Matrigel. A severe combined immunodeficient mouse model of extravasation and bone metastasis produced detectable, progressive osteolytic lesions within 3 weeks of intracardiac injections. Injection of tumor cells, pretreated with J8H, delayed the appearance of metastases. Validation of the alpha(6) cleavage effect on extravasation was confirmed through a genetic approach using tumor cells transfected with uncleavable alpha(6) integrin. Uncleavable alpha(6) integrin significantly delayed the onset and progression of osseous metastases out to six weeks post-injection. The results suggest that alpha(6) integrin cleavage permits extravasation of human prostate cancer cells from circulation to bone and can be manipulated to prevent metastasis.

Bicalutamide inhibits androgen-mediated adhesion of prostate cancer cells exposed to ionizing radiation

BACKGROUND

Cell adhesion plays an important role in proliferation, metastasis, and tumor growth and may represent a potential vulnerability in treatment of prostate cancer patients. Bicalutamide (Casodex) has been used as an anti-androgen agent for prostate cancer patients during hormone ablation therapy. This study focuses on the effect of Bicalutamide on cell adhesion to fibronectin (FN) in prostate cancer cells.

METHODS

Androgen-dependent LNCaP prostate cancer cells were stimulated with androgen before being irradiated with doses of 0, 5, 10, or 15 Gy. Cell adhesion to fibronectin was then measured to ascertain androgen's role in integrin mediated prostate cancer cell adhesion. Flow cytometry was used to analyze surface expression of integrin subtypes in LNCaP cells.

RESULTS

LNCaP cell adhesion to FN was significantly increased by stimulation with androgen when treated with 10 or 15 Gy ionizing radiations but not at 0 or 5 Gy. This increase was inhibited by treatment with Bicalutamide. LNCaP cells exposed to high dose radiation showed an increased expression of alpha(V) and beta(1) integrins in response to androgen treatment while Bicalutamide abolished this effect.

CONCLUSIONS

Our data show that Bicalutamide inhibits the effect of androgen on cell adhesion to FN through changes of integrin subtypes in cells given high dose radiation. This suggests new molecular targets and possible treatment strategies for prostate cancer patients to improve the outcome during hormone ablation therapy and radiation therapy.

The role of alpha 6 integrin in prostate cancer migration and bone pain in a novel xenograft model

Of the estimated 565,650 people in the U.S. who will die of cancer in 2008, almost all will have metastasis. Breast, prostate, kidney, thyroid and lung cancers metastasize to the bone. Tumor cells reside within the bone using integrin type cell adhesion receptors and elicit incapacitating bone pain and fractures. In particular, metastatic human prostate tumors express and cleave the integrin A6, a receptor for extracellular matrix components of the bone, i.e., laminin 332 and laminin 511. More than 50% of all prostate cancer patients develop severe bone pain during their remaining lifetime. One major goal is to prevent or delay cancer induced bone pain. We used a novel xenograft mouse model to directly determine if bone pain could be prevented by blocking the known cleavage of the A6 integrin adhesion receptor. Human tumor cells expressing either the wildtype or mutated A6 integrin were placed within the living bone matrix and 21 days later, integrin expression was confirmed by RT-PCR, radiographs were collected and behavioral measurements of spontaneous and evoked pain performed. All animals independent of integrin status had indistinguishable tumor burden and developed bone loss 21 days after surgery. A comparison of animals containing the wild type or mutated integrin revealed that tumor cells expressing the mutated integrin resulted in a dramatic decrease in bone loss, unicortical or bicortical fractures and a decrease in the ability of tumor cells to reach the epiphyseal plate of the bone. Further, tumor cells within the bone expressing the integrin mutation prevented cancer induced spontaneous flinching, tactile allodynia, and movement evoked pain. Preventing A6 integrin cleavage on the prostate tumor cell surface decreased the migration of tumor cells within the bone and the onset and degree of bone pain and fractures. These results suggest that strategies for blocking the cleavage of the adhesion receptors on the tumor cell surface can significantly prevent cancer induced bone pain and slow disease progression within the bone. Since integrin cleavage is mediated by Urokinase-type Plasminogen Activator (uPA), further work is warranted to test the efficacy of uPA inhibitors for prevention or delay of cancer induced bone pain.

Integrins in prostate cancer progression

Integrins, which are transmembrane receptors for extracellular matrix proteins, play a key role in cell survival, proliferation, migration, gene expression, and activation of growth factor receptors. Their functions and expression are deregulated in several types of cancer, including prostate cancer. In this article, we review the role of integrins in prostate cancer progression and their potential as therapeutic targets.

Prostate carcinoma and radiation therapy: therapeutic treatment resistance and strategies for targeted therapeutic intervention

Adenocarcinoma of the prostate remains a significant public health problem and a prevalent cancer in men. Prostate-specific antigen used as a biomarker has established a clear migration of patients towards earlier-stage disease at presentation. However, in spite of process improvements in traditional therapies including surgery, radiation therapy, and hormone management, there remains a significant cohort of patients with intermediate- to high-risk features for poor outcome in spite of optimal use of traditional management. This paper focuses on future treatment strategies integrating new therapeutic options with traditional management, specifically to pinpoint new radiation therapy strategies.