Characterization of Laminin Binding Integrin Internalization in Prostate Cancer Cells

Laminin binding integrins α6 (CD49f) and α3 (CD49c) are persistently but differentially expressed in prostate cancer (PCa). Integrin internalization is an important determinant of their cell surface expression and function. Using flow cytometry, and first order kinetic modeling, we quantitated the intrinsic internalization rates of integrin subunits in a single cycle of internalization. In PCa cell line DU145, α6 integrin internalized with a rate constant (kactual ) of 3.25 min-1 , threefold faster than α3 integrin (1.0 min-1 ), 1.5-fold faster than the vitronectin binding αv integrin (CD51) (2.2 min-1 ), and significantly slower than the unrelated transferrin receptor (CD71) (15 min-1 ). Silencing of α3 integrin protein expression in DU145, PC3, and PC3B1 cells resulted in up to a 1.71-fold increase in kactual for α6 integrin. The internalized α6 integrin was targeted to early endosomes but not to lamp1 vesicles. Depletion of α3 integrin expression resulted in redistribution of α6β4 integrin to an observed cell-cell staining pattern that is consistent with a suprabasal distribution observed in epidermis and early PIN lesions in PCa. Depletion of α3 integrin increased cell migration by 1.8-fold, which was dependent on α6β1 integrin. Silencing of α6 integrin expression however, had no significant effect on the kactual of α3 integrin or its distribution in early endosomes. These results indicate that α3 and α6 integrins have significantly different internalization kinetics and that coordination exists between them for internalization. J. Cell. Biochem. 118: 1038-1049, 2017. © 2016 Wiley Periodicals, Inc.

Abstract 2457: Laminin receptor cross-talk stimulates receptor internalization and migration of human prostate cancer cells

Integrin cell adhesion receptors are internalized and recycled to the plasma membrane during cellular migration. The laminin binding A3B1 and A6B1 integrin receptors are differentially expressed on prostate tumor cells in Gleason grades 3-5 as detected in biopsy and prostatectomy specimens. The A6 integrin exists as a full length form or as a pro-metastatic variant, A6pB1, a receptor form missing the extracellular laminin binding domain via the action of the serine protease, uPA. The objective of this study is to determine whether internalization of the A6 integrin occurs in human bone metastatic lesions and whether cross-talk between the laminin receptors A6B1, A6pB1 and A3B1 modifies their respective internalization rates. In a tissue microarray of 185 metastatic prostate cancers from 45 patients, approximately 70% of bone metastases expressed A6B1 on tumor plasma membrane and/or within the cytoplasm. The A6 and A6p integrins in DU145 cells are constitutively internalized with a biological half life of approximately 30 minutes as compared to 60 minutes for A3 or 20 minutes for the unrelated transferrin receptor. Increasing production of A6pB1 by supplying exogenous uPA stimulated A3 internalization. Conversely, blockage of A6pB1 production using an A6 specific antibody (J8H) decreased A3 internalization. Targeted depletion of the A3 integrin increased DU145 cell migration that was dependent upon A6 integrin as detected by antibody blocking experiments using J8H (A6 specific) and AIIB2 (B1 specific). Analysis of transcriptomic data from prostate cancer xenografts, revealed three different candidate receptor recycling genes that significantly correlated with the loss of membrane and increase in cytoplasmic expression. Taken together, these data indicate that cross-talk between A6B1 and A3B1 modifies their internalization rates and prostate tumor cell migration. This suggests that successful therapeutic targeting of metastasizing prostate cancer cells may depend upon coordinated targeting of both receptors and the recycling machinery. Current work is to determine the involvement of candidate recycling genes in the biological control of laminin binding integrins in human prostate cancer. (Supported in part by NIH grants P30CA023074, 1RO1 CA159406 and the TACMASS core service of the University of Arizona Cancer Center).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2457. doi:1538-7445.AM2012-2457

Abstract 4979: A6B1 integrin internalization is increased by loss of A3B1 expression and promotes migration of human prostate cancer cells

Laminin binding integrins, A3B1 and A6B1 drive in part, cancer cell migration and metastasis. Cell migration requires dynamic internalization and recycling of surface integrins. However, whether laminin binding integrin internalization is coordinately or independently controlled is not known. In this study, we quantitated the internalization of integrin A3 and A6 and whether crosstalk influences their respective internalization and subsequent cancer cell migration.

We quantitated the internalization of A6 and A3 integrins, using fluorophore conjugated antibody based internalization assays and flow cytometry. In the prostate cancer cell line DU145, we observed constitutive internalization of integrin A6 with a surface half-life of approximately 30 minutes as compared to >70 minutes for integrin A3, 45 minutes for unrelated integrin Av or 10 minutes for the unrelated transferrin receptor. Silencing A3 integrin expression resulted in a 10 percent increase in the total amount of internalized A6 integrin, with a decreased surface half-life of 20 minutes. The internalized A6 integrin showed intracellular punctate staining colocalized with the early endosome marker EEA1 and Rab4 containing vesicles. Increased integrin A6 internalization led to 40% increase in migration which was dependent on A6B1 integrin. Silencing integrin A6 expression, however, did not affect the internalization of A3 integrin indicating a unidirectional regulation of integrin internalization. The changes in the integrin internalization could be clearly ascribed to alpha subunits, as the obtained half-life of A6 and A3 integrins were independent of antibody used or integrin engagement by ligand mimetic peptides. Taken together, these data indicate that A6B1 and A3B1 integrins have different internalization kinetics and the presence of integrin A3 decreased integrin A6 internalization and ensuing cell migration. These data are consistent with previous observations in normal systems suggesting that A3B1 provides a provisional matrix during early stages of wound healing, requiring subsequent stable adhesion complexes and integrin A6 function. In cancer, the loss of A3 integrin and the increased internalization of A6 integrin may account for the “wounds that won't heal” phenotype and provide a strategy for biological intervention to block tumor progression.

(Supported in part by NIH Grants CA23074 and CA159406 and the TACMASS core service of the Arizona Cancer Center)

Citation Format: Lipsa Das, Todd A. Anderson, Jaime M.c. Gard, Isis C. Sroka, Stephanie R. Strautman, Raymond B. Nagle, Beatrice S. Knudsen, Anne Cress. A6B1 integrin internalization is increased by loss of A3B1 expression and promotes migration of human prostate cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4979. doi:10.1158/1538-7445.AM2014-4979