Cellular glycosylphosphatidylinositol-specific phospholipase D regulates urokinase receptor shedding and cell surface expression.

The glycosylphosphatidylinositol (GPI)-anchored, multifunctional receptor for the serine proteinase, urokinase plasminogen activator (uPAR, CD87), regulates plasminogen activation and cell migration, adhesion, and proliferation. uPAR occurs in functionally distinct, membrane-anchored and soluble isoforms (s-uPAR) in vitro and in vivo. Recent evidence indicates that s-uPAR present in the circulation of cancer patients correlates with tumor malignancy and represents a valuable prognostic marker in certain types of cancer. We have therefore analyzed the mechanism of uPAR shedding in vitro. We present evidence that uPAR is actively released from ovarian cancer cells since the rate of receptor shedding did not correlate with uPAR expression. While s-uPAR was derived from the cell surface, it lacked the hydrophobic portion of the GPI moiety indicating anchor cleavage. We show that uPAR release is catalyzed by cellular GPI-specific phospholipase D (GPI-PLD), an enzyme cleaving the GPI anchor of the receptor. Thus, recombinant GPI-PLD expression increased receptor release up to fourfold. Conversely, a 40% reduction in GPI-PLD activity by GPI-PLD antisense mRNA expression inhibited uPAR release by more than 60%. We found that GPI-PLD also regulated uPAR expression, possibly by releasing a GPI-anchored growth factor. Our data suggest that cellular GPI-PLD might be involved in the generation of circulating prognostic markers in cancer and possibly regulate the function of GPI-anchored proteins by generating functionally distinct, soluble counterparts.

Complex chitinolytic system of Aspergillus fumigatus.

Native polyacrylamide gels incorporating a glycol chitin substrate were used to detect several chitinolytic enzymes in the culture filtrate and cell surface, wall and mixed membrane fractions of Aspergillus fumigatus during the exponential phase of growth. Much of the cellular chitinase activity did not bind to concanavalin A (Con A) matrix and was heat-sensitive. In contrast, almost all chitinases secreted appeared to be heat-stable glycoproteins. The heavily glycosylated molecules, in a Con A-binding fraction, were the most immunologically-reactive components, as judged by their binding to anti-Aspergillus antibodies, present in the serum of patients with aspergillosis. Most of the cellular chitinases of A. fumigatus mycelium bound to an insoluble chitin matrix while most of the secreted chitinases did not bind to chitin.

Chitinase activity in human serum and leukocytes.

Using colloidal [3H] chitin as a substrate, we provide the first demonstration of a chitinase in human leukocytes; chitinolytic activity in whole and disrupted leukocyte preparations (approximately 0.6 and 5.5 nmol of N-acetylglucosamine [GlcNAc] released min-1 mg of protein-1, respectively) was partially inhibited by the specific chitinase inhibitor allosamidin (9 microM). Following fractionation of the leukocytes, much higher levels of chitinase activity were detected in granulocyte-rich homogenates (approximately 7.2 nmol of GlcNAc released min-1 mg of protein-1) than in lymphocyte- and monocyte-rich homogenates (approximately 0.22 and 0.26 nmol of GlcNAc released min-1 mg of protein-1, respectively). Low levels of chitinase activity were detected in human serum (approximately 4 pmol of GlcNAc released min-1 mg of protein-1). Chitinolytic activity in granulocyte-rich homogenates and serum was partially inhibited by allosamidin (9 microM). Proteins with chitinolytic activities (approximate molecular masses, 48 and 56 kDa) distinct from lysozyme (14.3 kDa) were detected on polyacrylamide gels following the electrophoresis of human granulocyte-rich preparations. Chitinase activity, detected consistently in serum and leukocytes from all human volunteers investigated, may contribute to the protection of the host by cleaving chitin in the cell walls of fungal pathogens.