Zinc-α2-glycoprotein hinders cell proliferation and reduces cdc2 expression

Zinc-α2-glycoprotein (Znα2gp) is widely distributed in body fluids and epithelia. Its expression in stratified epithelia increases with differentiation. We previously showed that Zn α2gp has ribonuclease activity, and that squamous tumor cells grown on a matrix of Znα2gp were growth-inhibited. Here we demonstrate, both by adding Znα2gp to the culture medium and, more unequivocally, by stably transfecting SiHa cells with Znα2gp cDNA, that the introduction of Znα2gp into SiHa tumor cells reduces proliferation. In response to Znα2gp, we find an accumulation of the cell population in G2/M by flow cytometry, paralleling the reduction of proliferation. In order to distinguish growth inhibition by cell cycle arrest from that produced by apoptosis or differentiation, we examine by RT-PCR how Znα2gp affects the expression of genes commonly used as markers of these properties. No changes are observed for PCNA, p53, c-myc, or bcl-2. Only cdc2 expression responds to Znα2gp, with a reduction of up to over a factor of two. Cdc2 is the only cyclin-dependent kinase regulating the G2/M transition without redundancy and is required as a rate-limiting step in the cell cycle. Its increased expression has been directly linked to increased proliferation and decreased differentiation of advanced tumors; conversely, its downregulation by Znα2gp might hinder tumor progression. © 2001 Wiley-Liss, Inc.