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Integrin Agonists as Adjuvants in Chemotherapy for Melanoma

Authors' Affiliations: ¹ Department of Microbiology, ² Robert M. Berne Cardiovascular Research Center, ³ Departments of Biomedical Engineering and Cell Biology, ⁴ Department of Pathology, ⁵ Department of Biochemistry and Molecular Biology, ⁶ Department of Public Health Sciences (Biostatistics Division), ⁷ Mellon Prostate Cancer Research Institute University of Virginia, Charlottesville, Virginia; and ⁸ Department of Biochemistry and Molecular Biology and Norris Cancer Center, University of Southern California Keck School of Medicine, Los Angeles, California

Requests for reprints: Martin A. Schwartz, Robert M. Berne Cardiovascular Research Center, University of Virginia, 415 Lane Road, Charlottesville VA 22908. Phone: 434-243-4813; Fax: 434-924-2828; E-mail: maschwartz{at}virginia.edu.

Purpose: Metastatic melanomas are generally resistant to chemotherapy and radiation, even when wild-type for p53. These tumors often grow in small nests where many of the cells have little contact with extracellular matrix (ECM). Previous work showed that M21 melanomas undergo apoptosis in response to chemotherapy when cells are adherent to ECM but not in suspension. Thus, reduced integrin-dependent adhesion to ECM could mediate therapy resistance. The goal of this study was to test whether stimulation of integrin signaling could increase chemotherapeutic efficacy.

Experimental Design: Colony forming assays and survival assays were used to test the responses of melanoma lines in vitro. Severe combined immunodeficient mice with subcutaneous human melanomas received chemotherapy with or without reagents that stimulate integrin signaling; tumor volume was then monitored over time.

Results: Clonal growth assays confirmed that M21 cells showed reduced sensitivity to the chemotherapeutic drug 1-β-D-arabinofuranosylcytosine (araC). When five additional primary melanoma lines were screened, 80% showed higher sensitivity when adherent compared with suspended. Subcutaneous M21 tumors in vivo showed minimal ECM between tumor cells. To evaluate the importance of integrin signaling in chemoresistance in this model, mice were treated with araC, with or without the multivalent snake venom disintegrin contortrostatin or the activating anti-β1 integrin antibody TS2/16. Although araC, TS2/16, or contortrostatin alone had little effect on M21 tumor growth, combining araC with either integrin signaling reagents strongly reduced growth (P = 0001).

Conclusions: Loss of integrin-mediated adhesion is rate limiting for therapeutic response in this model. Combining chemotherapy with reagents that stimulate integrin signaling may therefore provide a new approach to therapy.