Activated α2-macroglobulin binding to human prostate cancer cells triggers insulin-like responses

Abstract

Ligation of cell surface GRP78 by activated α 2 -macroglobulin (α 2 M) promotes cell proliferation and suppresses apoptosis. α 2 M-treated human prostate cancer cells exhibit a 2-3-fold increase in glucose uptake and lactate secretion, an effect similar to insulin treatment. In both α 2 Mand insulin-treated cells, the mRNA levels of SREBP1-c, SREBP2, fatty-acid synthase, acetyl-CoA carboxylase, ATP citrate lyase, and Glut-1 were significantly increased together with their protein levels, except for SREBP2. Pretreatment of cells with α 2 Mantagonist antibody directed against the carboxyl-terminal domain of GRP78 blocks these α 2 M-mediated effects, and silencing GRP78 expression by RNAi inhibits up-regulation of ATP citrate lyase and fattyacid synthase. α 2 Minduces a 2-3-fold increase in lipogenesis as determined by 6-[ 14 C]glucose or 1-[ 14 C]acetate incorporation into free cholesterol, cholesterol esters, triglycerides, free fatty acids, and phosphatidylcholine, which is blocked by inhibitors of fatty-acid synthase, PI 3-kinase, mTORC, or an antibody against the carboxyl-terminal domain of GRP78. We also assessed the incorporation of [ 14 CH 3 ]choline into phosphatidylcholine and observed similar effects. Lipogenesis is significantly affected by pretreatment of prostate cancer cells with fatostatin A, which blocks sterol regulatory element-binding protein proteolytic cleavage and activation. This study demonstrates that α 2 Mfunctions as a growth factor, leading to proliferation of prostate cancer cells by promoting insulin-like responses. An antibody against the carboxyl-terminal domain of GRP78 may have important applications in prostate cancer therapy.