Zinc-alpha(2)-glycoprotein hinders cell proliferation and reduces cdc2 expression

Zinc-alpha(2)-glycoprotein (Znalpha(2)gp) is widely distributed in body fluids and epithelia. Its expression in stratified epithelia increases with differentiation. We previously showed that Zn alpha(2)gp has ribonuclease activity, and that squamous tumor cells grown on a matrix of Znalpha(2)gp were growth-inhibited. Here we demonstrate, both by adding Znalpha(2)gp to the culture medium and, more unequivocally, by stably transfecting SiHa cells with Znalpha(2)gp cDNA, that the introduction of Znalpha(2)gp into SiHa tumor cells reduces proliferation. In response to Znalpha(2)gp, we find an accumulation of the cell population in G(2)/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 Znalpha(2)gp 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 Znalpha(2)gp, with a reduction of up to over a factor of two. Cdc2 is the only cyclin-dependent kinase regulating the G(2)/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 Znalpha(2)gp might hinder tumor progression. J. Cell. Biochem. Suppl. 36: 162-169, 2001.