Zinc‐α 2 ‐glycoprotein hinders cell proliferation and reduces cdc2 expression

Zinc‐α 2 ‐glycoprotein (Znα 2 gp) is widely distributed in body fluids and epithelia. Its expression in stratified epithelia increases with differentiation. We previously showed that Zn α 2 gp has ribonuclease activity, and that squamous tumor cells grown on a matrix of Znα 2 gp were growth‐inhibited. Here we demonstrate, both by adding Znα 2 gp to the culture medium and, more unequivocally, by stably transfecting SiHa cells with Znα 2 gp cDNA, that the introduction of Znα 2 gp into SiHa tumor cells reduces proliferation. In response to Znα 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 Znα 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 Znα 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 Znα 2 gp might hinder tumor progression. J. Cell. Biochem. Suppl. 36: 162–169, 2001. © 2001 Wiley‐Liss, Inc.