Prosaposin down-modulation decreases metastatic prostate cancer cell adhesion, migration, and invasion

Our current data show that, in metastatic PCa cells, stable down-modulation of PSAP by RNA-interference via a lysosomal proteolysis-dependent pathway decreased β1A-integrin expression, its cell-surface clustering, and adhesion to basement membrane proteins; led to disassembly of focal adhesion complex; and decreased phosphorylative activity of focal adhesion kinase and its downstream adaptor molecule, paxillin. Cathepsin D (CathD) expression and proteolytic activity, migration, and invasion were also significantly decreased in PSAP knock-down cells. Transient-transfection studies with β1A integrin- or CathD-siRNA oligos confirmed the cause and effect relationship between PSAP and CathD or PSAP and Cer-β1A integrin, regulating PCa cell migration and invasion.Our findings suggest that by a coordinated regulation of Cer levels, CathD and β1A-integrin expression, and attenuation of "inside-out" integrin-signaling pathway, PSAP is involved in PCa invasion and therefore might be used as a molecular target for PCa therapy.Prosaposin (PSAP) is a dual-function highly conserved glycoprotein that exists as the lysosomal precursor of four small sphingolipid activator proteins, known as saposins A, B, C and D [1-3]. Saposins are generated by proteolytic cleavage of another lysosomal protease, cathepsin D (CathD) [4-6]. In lysosomes, mature saposins are intensively involved in metabolism of sphingolipids and ceramide (Cer), functioning either as essential co-factors for sphingolipid hydrolases and/or destabilizing the complex of lipids and membranes [3]. PSAP also exists as a secreted protein, which has been found in various body fluids such as milk, serum, and seminal fluid [2]. Secreted PSAP is a well-known potent neurotrophic factor [7,8]. Total PSAP deficiency is lethal in both man and mice [2]. However, deficiency of individual saposin proteins is responsible for a number of lipid storage diseases [9-11].Homozygous inactivation of PSAP gene in mice led to shrinkage and atrop