A common characteristic of integrins, the heterodimeric transmembrane receptor proteins that mediate cell attachment to the extracellular matrix (ECM), is their absolute requirement for divalent cations to function. Previous studies in vivo indicate that shifts in the concentrations of Mg2+ and Ca2+ occur early during wound repair, activating the α2β1 integrin-mediated migration of wound healing cell types on type I collagen, an ECM protein shown to be highly up-regulated in pancreatic cancer and to promote the malignant phenotype both in vitro and in vivo. As pancreas juice contains Mg2+ concentrations over 1200-fold greater than Ca2+, shifts in the concentrations of divalent cations probably occur in pancreatic cancer as well, the result of pancreas juice leaking into the tumor microenvironment from compromised ducts. We have previously demonstrated that pancreatic cancer cells adhere to fibronectin (Fn), laminin-1 (Ln-1), and type I collagen in Mg2+ but not in Ca2+, while cell attachment to vitronectin (Vn) occurs in both Mg2+ and Ca2+. We also demonstrated that lowering extracellular Ca2+ down-regulates E-cadherin-mediated cell-cell adhesion and increases α5β1 integrin-mediated pancreatic cancer cell migration on Fn. Additionally, we demonstrated by affinity chromatography that Ca2+ can specifically elute pancreatic cancer cell α1β1 and α2β1 integrins bound to types I and IV collagen in Mg2+. Our present results indicate that the effects of Mg2+ and Ca2+ on pancreatic cancer cell adhesion on type I collagen are reversible, in that cells attached in Mg2+ can be detached by the subsequent addition of Ca2+, and cells inhibited from attachment in Ca2+ can be induced to attach by the subsequent addition of Mg2+. We demonstrate further that pancreatic cancer cell migration on type I collagen is maximal when the Mg2+ to Ca2+ ratio is greater than 1. Because other ECM proteins are also expressed in pancreatic cancer, we examined the divalent cation dependency of these integrin-mediated interactions as well. We demonstrate in migration assays that, like type I collagen, migration of pancreatic cancer cells on Fn, Ln-1, Vn, and type IV collagen, is maximal when the Mg2+ to Ca2+ ratio is greater than 1. Additionally, we examined the effects of shifted divalent cation concentrations on the proliferation of pancreatic cancer cells on types I and IV collagen, Fn, Ln-1, and Vn. Our results indicate that concentrations of divalent cations that support maximal cell migration (Mg2+ to Ca2+ ratio>1) also promote maximal cell proliferation on each ECM protein examined. Taken together, these data indicate that shifts in the ratio between extracellular Mg2+ and Ca2+, which probably occur in the local tumor microenvironment in vivo, may be a critical regulatory component activating the integrin-mediated malignant phenotype in pancreatic cancer.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA