LIGATION OF INTEGRIN ALPHA(5)BETA(1) IS REQUIRED FOR INTERNALIZATION OF VITRONECTIN BY INTEGRIN ALPHA(V)BETA(3)

Remodeling of the matrix by tumor cells is necessary for tumor invasio n. We have shown previously that malignant astrocytomas, in contrast t o normal astrocytes, synthesize vitronectin and express integrins alph a(v) beta(3) and alpha(v) beta(5). The activity states of these two in tegrins are differentially controlled. Thus, we investigated the regul ation of the activity of integrins alpha(v) beta(3) and alpha(v) beta( 5) with regard to their role in vitronectin internalization in U-251MG astrocytoma cell monolayers adherent to fibronectin, collagen, or lam inin in serum-free conditions. Binding of [I-125]vitronectin occurred in a specific, saturable manner that was partially inhibitable by mono clonal antibodies (mAbs) specific for integrins alpha(v) beta(3) or al pha(v) beta(5). Specific, lysosomally-mediated degradation of [I-125]v itronectin was detectable at 1 h and increased over the 24 h assay per iod. The cell substrate affected the rate of turnover of [I-125]vitron ectin, which was 3.0 ng/min for cells plated on fibronectin but 0.35 n g/min for cells plated on collagen. Furthermore, although mAbs specifi c for either integrin alpha(v) beta(3) or alpha(v) beta(5) inhibited d egradation (30%; combined effect 70%) of [I-125]vitronectin by cells p lated on fibronectin, only mAb anti-alpha(v) beta(5) inhibited degrada tion (70-90%) by cells plated on collagen or laminin. To determine the requirement for integrin alpha(5) beta(1) ligation in order for integ rin alpha(v) beta(3) to internalize its ligand, cells were plated on m Abs anti-integrin alpha(5) or anti-integrin alpha(3). When plated on m Ab anti-alpha(5), mAbs anti-alpha(v) beta(3), and anti-alpha(v) beta(5 ) both inhibited degradation. However, when plated on mAb anti-alpha(3 ), mAb anti-alpha(v) beta(3) had no effect whereas mAb anti-alpha(v) b eta(5) inhibited degradation. These data indicate that a signal from i ntegrin alpha(5) beta(1) is necessary for integrain alpha(v) beta(3) t o internalize vitronectin, whereas integrin alpha(v) beta(5) constitut ively internalizes vitronectin.