Activation of Protein Kinase G Is Sufficient to Induce Apoptosis and Inhibit Cell Migration in Colon Cancer Cells

Abstract

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 I␣: PKG I␣S65D, a constitutively activated point mutant; PKG I␣⌬, a constitutively activated N-terminal truncated mutant; and PKG I␣K390R, a dominant-negative point mutant. A similar series of mutants of PKG I␤ 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 I␤, and to a lesser extent PKG I␣, 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 wildtype PKG I␤ displayed growth inhibition, whereas derivatives that stably expressed the dominant-negative mutant (KR) of PKG I␤ grew more rapidly and were more resistant to Aptosyn-induced growth inhibition than vector control cells. Stable overexpression of PKG I␤ was associated with decreased cellular levels of ␤-catenin and cyclin D1 and increased levels of p21 CIP1 . Reporter assays indicated that activation of PKG I␤ inhibits the transcriptional activity of the cyclin D1 promoter. We also found that transient expression of the constitutively activated mutants of PKG I␤ inhibited cell migration. Taken together, these results indicate that activation of PKG I␤ 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 .

Exisulind and guanylyl cyclase C induce distinct antineoplastic signaling mechanisms in human colon cancer cells

“…Further, exisulind was fully efficacious in suppressing tumor growth in the presence of a specific inhibitor of PKG I (24,25), eliminating the involvement of that kinase in mediating the antineoplastic effects of exisulind. Additionally, overexpression of PKG Ih did not significantly alter the antineoplastic potency of exisulind in colon cancer cells (36). Thus, the present observations support a model in which the antitumor effects of exisulind in colon cancer reflect the sequential recruitment and activation of multiple signaling components, other than exisulind-induced PKG Ih, which seems to be an epiphenomenon, rather than mechanistically related to exisulind-induced tumor suppression.…”

Overexpression of cGMP-dependent protein kinase I (PKG-I) attenuates ischemia-reperfusion-induced kidney injury

“…PKG levels/activity have been shown to be modulated in many disease conditions. For example, PKG expression is downregulated in diabetes or cancer (7,16,21). Our previous studies demonstrated that the NO and cGMP levels were decreased in kidney mesangial cells under highglucose conditions, resulting in decreased PKG kinase activity (45,48).…”

Phosphodiesterase-4 promotes proliferation and angiogenesis of lung cancer by crosstalk with HIF

“…[4][5][6][7] The impairment of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) generation by regulation of phosphodiesterases (PDEs) has been implicated in various cancer pathologies. 8,9 PDEs are enzymes that regulate cellular levels of cAMP/cGMP by controlling their degradation.…”