High expression of Cyr61, an extracellular
cysteine-rich
heparin-binding protein, has been associated with a malignant cell phenotype and poor outcome in
prostate cancers. Although Cyr61 was found by us to be overproduced in
androgen-independent PC-3 cells treated with
N-acetylcysteine (NAC), its significance is still unclear. We therefore aimed to determine how and why
Cyr61 protein is overexpressed in NAC-treated cells. Here, we found that
Cyr61 protein level markedly increased in cells treated with NAC at high cell seeding density. Silencing of Cyr61 by
siRNA induced enhanced activity of
caspase-3/7, upregulation of the proapototic Bok, BimL and BimS, cleavage of apoptosis hallmarkers such as Bax, PARP and
caspase-3, and downregulation of antiapoptotic Bcl2, Bcl-xL and Mcl-1
proteins. NAC treatment caused a reduction of extracellular medium pH to acidic and an increase in Akt phosphorylation, after which the replacement with NAC-free medium returned them to control levels within 24h.
Acid stimulation increased the levels of Cyr61 and p-Akt
proteins, whereas it suppressed the induction of proapoptotic and antiapoptotic
proteins. Overall, our data indicate that PC-3 cells overproduce
Cyr61 protein via activation of the PI3K/Akt signaling as a part of the survival mechanisms under the conditions causing extracellular acidity and further cytotoxicity.