α4β1- and α6β1-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 xi, low-density-lipoprotein-receptor-related protein and anaplastic lymphoma kinase have been identified as receptors for midkine. We found beta(1) integrin in midkine-binding proteins from 13-day-old mouse embryos. beta(1)-Integrin bound to a midkine-agarose column and was eluted mostly with EDTA. Further study revealed that the alpha-subunits capable of binding to midkine were alpha(4) and alpha(6). Purified alpha(4)beta(1)- and alpha(6)beta(1)-integrins bound midkine. Anti-alpha(4) antibody inhibited the midkine-dependent migration of osteoblastic cells, and anti-alpha(6) 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 alpha(4)beta(1)- and alpha(6)beta(1)-integrins are functional receptors for midkine. We observed that the lowdensity-lipoprotein-receptor-related-protein-6 ectodomain was immunoprecipitated with alpha(6)beta(1)-integrin and alpha(4)beta(1)-integrin. The low-density-lipoprotein-receptor-relatedprotein-6 ectodomain was also immunoprecipitated with receptor-type protein tyrosine phosphatase xi. alpha(4)beta(1)- and alpha(4)beta(1)-Integrins are expected to co-operate with other midkine receptors, possibly in a multimolecular complex that contains other midkine receptors.