Targeting Integrin α6 Stimulates Curative-Type Bone Metastasis Lesions in a Xenograft Model

Abstract

Laminin binding integrin receptors are key mediators of epithelial cell migration and tumor metastasis. Recent studies have demonstrated a role for the alpha 6 integrin (ITGA6/CD49f) in maintaining stem cell compartments within normal bone marrow and in residency of tumors metastatic to bone. In this study, we tested a function-blocking antibody specific for ITGA6, called J8H, to determine if pre-existing cancer lesions in bone could be slowed and/or animal survival improved. Human prostate tumors were established by intracardiac injection into male severe combined immunodeficient (SCID) mice and treatment with J8H antibody was initiated after one week. Tumor progression was monitored by micro computed tomography (CT) imaging of skeletal lesions. Animals that received weekly injections of the anti-ITGA6 antibody showed radiographic progression in only 40% of osseous tumors (femur or tibia), compared to control animals, where 80% of the lesions (femur or tibia) showed progression at 5 weeks. Kaplan-Meier survival analysis demonstrated a significant survival advantage for J8H-treated animals. Unexpectedly, CT image analysis revealed an increased proportion of bone lesions displaying a sclerotic rim of new bone formation, encapsulating the arrested lytic lesions in animals that received the anti-ITGA6 antibody treatment. Histopathology of the sclerotic lesions demonstrated well-circumscribed tumor within bone, surrounded by fibrosis. These data suggest that systemic targeting of the ITGA6-dependent function of established tumors in bone may offer a non-cytotoxic approach to arrest the osteolytic progression of metastatic prostate cancer, thereby providing a new therapeutic strategy for advanced disease.

The Cohesive Metastasis Phenotype in Human Prostate Cancer

“…Recent work shows the functional role of one LBI (α6β4) as a mediator of endothelial cell protection in the setting of excessive mechanical stretch relative to lung injury [63]. LBI expression patterns have clinical significance for several epithelial-derived malignancies, for example the α6β1 integrin receptor is conserved in prostate cancer [39], is expressed on prostate tumor cells undergoing PNI [13], and acts as a marker in the aggressive phenotype of tumor cells during cancer progression [14,32,60,64]. The LBIs are especially associated with the invasion and metastasis of human prostate cancer, traversing through muscle [39,65–69].…”

“…It is currently unknown how the variant participates in cohesive tumor clusters and whether the co-localization of uPAR and α6 integrin in prostate cancer tissue would reveal aggressive subclasses of Gleason grade 6 (3+3) tumors. We have shown that preventing α6p production in prostate tumor clusters within the bone arrests bone lesion progression, resulting in curative-type lesions [64]. Considering that approximately 85% of patients with advanced disease develop bone metastases, preventing α6p production may represent a novel, non-cytotoxic treatment for prostate cancer patients with advanced disease and extensive skeletal involvement; alternatively, blocking the function of the laminin-adhesion receptors can stimulate curative-type bone metastasis lesions [64].…”

“…An alternative strategy also is required to tailor therapies to prostate cancer that requires aggressive treatment—avoiding both expensive and high-morbidity approaches when possible [20]. For example, as secondary prevention steps, early blockage of skeletal-related events (SREs) and their complications includes preventing early invasion and successful colonization of bone [60,95], stimulating the host response to bone lesions [64], preventing dormant tumors from escaping the bone [21], and identifying bone health as a preemptive determinant of secondary prevention [96]. Recent results indicate that while early initiation of zoledronic acid (ZA) therapy for patients with castration-resistant prostate cancer and bone metastasis significantly reduced skeletal complications [97] and pain [98,99], it was ineffective for the prevention of bone metastases in high-risk localized prostate cancer patients [100].…”

Intracellular modifiers of integrin alpha 6p production in aggressive prostate and breast cancer cell lines

“…Previous studies by our group have identified a cleaved form of ITGA6, called ITGA6p, which has been shown to contribute to cell invasion and migration [13]. Inhibition of ITGA6 cleavage substantially delayed the onset of bone metastasis and promoted curative-type bone metastasis lesions in xenograft mouse models [14,15]. Due to the high prevalence and pro-metastatic phenotype of ITGA6p, we further investigated selected molecular candidates as modifiers of ITGA6p production.…”

“…These data suggested that tumor activated macrophages promote prometastatic integrin-dependent pericellular proteolysis and the metastatic phenotype [12]. Furthermore, ITGA6 cleavage has been shown to contribute to cell invasion and migration on laminin, and inhibition of ITGA6 cleavage was shown to substantially delay the onset of bone metastasis and promote curative-type bone metastasis lesions in xenograft mouse models [13–15]. In addition to the role of extracellular regulators in ITGA6p production, our group has shown that cleavage of ITGA6 was dependent on actin [16].…”