α₄β₁- and α₆β ₁-integrins are functional receptors for midkine, a heparin-binding growth factor

Midkine is a heparin-binding growth factor that promotes the growth, survival, migration and differentiation of various target cells. So far, receptor-type protein tyrosine phosphatase ζ, low-density-lipoprotein-receptor-related protein and anaplastic lymphoma kinase have been identified as receptors for midkine. We found β ₁ integrin in midkine-binding proteins from 13-day-old mouse embryos. β₁-Integrin bound to a midkine-agarose column and was eluted mostly with EDTA. Further study revealed that the α-subunits capable of binding to midkine were α₄ and α₆. Purified α₄β₁- and α₆β₁-integrins bound midkine. Anti-α₄ antibody inhibited the midkine-dependent migration of osteoblastic cells, and anti-α₆ antibody inhibited the midkine-dependent neurite outgrowth of embryonic neurons. After midkine treatment, tyrosine phosphorylation of paxillin, an integrin-associated molecule, was transiently increased in osteoblastic cells. Therefore, we concluded that α₄β ₁- and α₆β₁-integrins are functional receptors for midkine. We observed that the low-density-lipoprotein-receptor-related-protein-6 ectodomain was immunoprecipitated with α₆β₁-integrin and α₄β₁-integrin. The low-density-lipoprotein-receptor-related-protein-6 ectodomain was also immunoprecipitated with receptor-type protein tyrosine phosphatase ζ. α₄β₁- and α₆β ₁-Integrins are expected to co-operate with other midkine receptors, possibly in a multimolecular complex that contains other midkine receptors.