The activation of
protein kinase G (PKG) by cGMP has become of considerable interest as a novel molecular mechanism for the induction of apoptosis in
cancer cells, because
sulindac sulfone (
exisulind,
Aptosyn) and certain derivatives that inhibit cGMP-
phosphodiesterases and thereby increase cellular levels of cGMP appear to induce apoptosis via this mechanism. However, other effects of these compounds have not been excluded, and the precise mechanism by which PKG activation induces apoptosis has not been elucidated in detail. To directly examine the effects of PKG on cell growth and apoptosis, we generated a series of mutants of PKG Ialpha: PKG IalphaS65D, a constitutively activated point mutant; PKG IalphaDelta, a constitutively activated N-terminal truncated mutant; and PKG IalphaK390R, a dominant-negative point mutant. A similar series of mutants of PKG Ibeta were also constructed (Deguchi et al., Mol.
Cancer Ther., 1: 803-809, 2002). The present study demonstrates that when transiently expressed in SW480
colon cancer, the constitutively activated mutants of PKG Ibeta, and to a lesser extent PKG Ialpha, inhibit colony formation and induce apoptosis. We were not able to obtain derivatives of SW480 cells that stably expressed these constitutively activated mutants, presumably because of toxicity. However, derivatives that stably overexpressed wild-type PKG Ibeta displayed growth inhibition, whereas derivatives that stably expressed the dominant-negative mutant (KR) of PKG Ibeta grew more rapidly and were more resistant to
Aptosyn-induced growth inhibition than vector control cells. Stable overexpression of PKG Ibeta was associated with decreased cellular levels of
beta-catenin and
cyclin D1 and increased levels of p21(CIP1). Reporter assays indicated that activation of PKG Ibeta inhibits the transcriptional activity of the
cyclin D1 promoter. We also found that transient expression of the constitutively activated mutants of PKG Ibeta inhibited cell migration. Taken together, these results indicate that activation of PKG Ibeta is sufficient to inhibit growth and cell migration and induce apoptosis in human
colon cancer cells and that these effects are associated with inhibition of the transcription of
cyclin D1 and an increase in the expression of p21(CIP1).