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Pregnancy‐associated alpha2 glycoprotein (α2PAG) synthesis by human breast cancer tissue and cultured cell lines

Elevated α2PAG serum levels are found in pregnant women and patients with diverse types of neoplasms including breast cancer. Primary human breast cancer tissue, MCF‐7 and BT‐20 human breast cancer cultured cell lines were shown to synthesize α2PAG in vitro. BT‐20 breast cancer cells also released this pregnancy‐associated protein. Synthesis was demonstrated by l‐[14C]‐leucine incorporation into immunochemically isolated protein and confirmed by radioimmunodiffusion and radioimmuno‐electrophoresis. These data (1) provide direct evidence for α2PAG synthesis by well‐characterized human breast cancer cells per se;(2) strongly suggest that at least part of the α22PAG synthesis observed in primary human breast cancer tissue in vitro was due specifically to breast cancer cells rather than exclusively to monocytes/mac‐rophages known to infiltrate neoplastic tissues and to produce this protein during pregnancy; and (3) are consistent with the conclusion that serum α2PAG is a definite breast cancer “marker protein” which originates from and reflects breast cancer status. On the basis of the association of elevated serum α2PAG levels in breast cancer patients, its known immunosuppressive activity in vitro, and the direct evidence for α2PAG synthesis by human breast cancer cells perse, we postulate that this protein plays an important immunoregulatory role in vivo to prevent immune rejection and/or destruction of breast cancer cells.