Interruption of Homologous Desensitization in Cyclic Guanosine 3′,5′-Monophosphate Signaling Restores Colon Cancer Cytostasis by Bacterial Enterotoxins

Abstract

Bacterial diarrheagenic heat-stable enterotoxins induce colon cancer cell cytostasis by targeting guanylyl cyclase C (GCC) signaling. Anticancer actions of these toxins are mediated by cyclic guanosine 3V,5V-monophosphate (cGMP)-dependent influx of Ca 2+ through cyclic nucleotide-gated channels. However, prolonged stimulation of GCC produces resistance in tumor cells to heat-stable enterotoxin-induced cytostasis. Resistance reflects rapid (tachyphylaxis) and slow (bradyphylaxis) mechanisms of desensitization induced by cGMP. Tachyphylaxis is mediated by cGMP-dependent protein kinase, which limits the conductance of cyclic nucleotide-gated channels, reducing the influx of Ca 2+ propagating the antiproliferative signal from the membrane to the nucleus. In contrast, bradyphylaxis is mediated by cGMP-dependent allosteric activation of phosphodiesterase 5, which shapes the amplitude and duration of heat-stable enterotoxin-dependent cyclic nucleotide accumulation required for cytostasis. Importantly, interruption of tachyphylaxis and bradyphylaxis restores cancer cell cytostasis induced by heat-stable enterotoxins. Thus, regimens that incorporate cytostatic bacterial enterotoxins and inhibitors of cGMP-mediated desensitization offer a previously unrecognized therapeutic paradigm for treatment and prevention of colorectal cancer. (Cancer Res 2005; 65(23): 11129-35)

Cross Talk between Receptor Guanylyl Cyclase C and c-src Tyrosine Kinase Regulates Colon Cancer Cell Cytostasis

“…Monitoring of cell proliferation and cytostasis following activation of GC-C. Exponentially growing T84 or T84SF cells in 96-well plates were starved in medium without glutamine for 48 h. ST (100 nM) along with PV and HgCl 2 was then applied in serum-containing medium for 24 h. [methyl- 3 H]thymidine incorporation into DNA was quantified as described previously (40).…”

Emergent Concepts from the Intestinal Guanylyl Cyclase C Pathway

“…Further, the resistance to colon cancer initiation and progression exhibited by populations living in the developing world (Pitari et al, 2003;Shailubhai et al, 2000), where enterotoxigenic infections are highest, suggests that replacement therapy with the exogenous GCC ligand ST, the enterotoxin produced by E. coli, might be an effective treatment for colorectal cancer patients . This latter consideration is supported by observations that ST is the most potent GCC agonist available (Lucas et al, 2000), and the only ligand successfully investigated to fully restore the tumor suppressor activities of the GCC pathway in colorectal cancer cells (Lubbe et al, 2009;Pitari et al, 2001Pitari et al, , 2003Pitari et al, , 2005Pitari et al, , 2007Zuzga et al,2011). Distal components of the GCC pathway also could be exploited in original clinical applications against colon cancer.…”

“…Indeed, the increased migration and proliferation induced by loss of GCC signalling in mucosal colonocytes (Li et al, 2007a) represents a significant oncogenic stress (Aoki et al, 2003;Spruck et al, 1999) that creates the pre-neoplastic intestinal crypt . Accordingly, cell cycle progression and growth of human colon cancer cells, experimental mimicry of the GCC dormancy characterizing the human disease because they express GCC but not the endogenous ligands (Lucas et al, 2000;Pitari et al, 2001), are greatly impaired upon reactivation of GCC signalling with exogenous supplementation of its specific agonists (Pitari et al, 2001(Pitari et al, , 2003(Pitari et al, , 2005. Ligand-dependent GCC activation restores lost cGMP-regulated circuits and imposes cancer cytostasis by reducing nuclear DNA synthesis and the G 1 /S transition (Lin et al, 2010;Pitari et al, 2001).…”

Can Colorectal Cancer be Prevented or Treated by Oral Hormone Replacement Therapy?

“…Thus, mechanisms underlying inhibition of tumorigenesis by uroguanylin administration remain undefined. [10,69,76] In summary, these observations suggest that re-activation of GCC signaling in human colorectal cancer cells by supplementing GCC ligands inhibits cell proliferation through …”

“…Oral supplementation of GCC ligands should restore and preserve intestinal paracrine hormone functions and defend homeostatic processes including proliferative restriction, lineage specification, metabolic programming and genomic stability that oppose intestinal neoplastic transformation. In that context, preliminary studies suggest that supplementation of GCC ligands is effective [10,11,65,69,76] and safe 10 for colon cancer prevention and therapy. However, more studies in mouse models of colon cancer and clinical trials in human need to be performed to demonstrate the safety, efficacy, specificity and possible side-effects of oral supplementation of GCC ligands for targeted colon cancer prevention and therapy.…”