Integrins play a major role in cell adhesion and migration. Previous work reported that a cleaved form of
integrin alpha6 (alpha6p) was detected in invasive human
prostate cancer tissue, absent in normal prostate tissue and was produced by
urokinase-type Plasminogen Activator (uPA) in a
plasmin-independent manner. Using site-directed mutagenesis we identified
amino acid residues R594 and R595, located in the "stalk" region of
integrin alpha6, as essential for cleavage. The cleavage site is located on the extracellular region of the
protein between the beta-barrel domain and the thigh domain.
Prostate cancer cells (PC3N) were stably transfected to overexpress the cleavable, wild-type (PC3N-alpha6-WT) or the non-cleavable form of
integrin alpha6 (PC3N-alpha6-RR). The number of cells invading
laminin 111- and
laminin 332-coated filters by PC3N-alpha6-WT cells increased by threefold as compared to PC3N-alpha6-RR cells.
Plasminogen activator inhibitor-1 (PAI-1) reduced the invasion of PC3N-alpha6-WT cells by approximately 42% through
laminin 332-coated filters and
plasmin inhibitor aprotinin had no significant effect. Linear cell migration increased production of
integrin alpha6p in the PC3N-alpha6-WT cells and not in the PC3N-alpha6-RR cells and 32% of the PC3N-alpha6-WT cells migrated on
laminin 111 in the linear migration assay as compared to the 5% PC3N-alpha6-RR cells. These data taken together suggest that the uPA-mediated cell surface cleavage of the
alpha6 integrin extracellular domain is involved in
tumor cell invasion and migration on
laminin.