Research into the interaction between the E. coli heat-stable
enterotoxin (STh) and the
guanylin receptor
guanylate cyclase C (GC-C) has generated >100 synthetic analogs of the
peptide, several of which have been investigated as imaging or therapeutic agents for
colorectal cancers. The evidence presented here suggests that in addition to STh binding to GC-C expressing cell lines derived from human colon, STh also specifically binds to an as yet unidentified receptor expressed in high densities on the surface of cell lines derived from human breast
cancers. In vitro whole-cell crosslinking studies using 125I-labeled F19-STh(1-19) demonstrate that the putative STh
binding protein migrates as an approximately 120-125 kDa species by SDS-PAGE, significantly smaller than the glycosylated GC-C molecule found in the T84 human
colon cancer cell line. RT-PCR using total
RNA isolated from breast and
colon cancer cell lines indicates that GC-C transcripts are undetectable in human
breast cancer cell lines and abundant in human
colon cancer cell lines. In vitro competitive binding studies using STh analogs and the
estrogen receptor positive (ER+) T-47D cell line demonstrated IC50 values between 2.6 and 8.5 nM. Similar studies on the
estrogen receptor negative (ER-) cell line MDA-MB-231 showed IC50's between 5.6 and 9.9 nM. Saturation binding analysis revealed receptor expression to fall between 40,000 and 120,000 sites per cell in these cell lines, receptor abundances equal to or greater than the abundance of GC-C in
colorectal cancer cell lines. STh binding to these cells, although of similar affinity to STh binding to GC-C, is distinguishable from it on the basis of its
ligand specificity. The characteristics of STh analogs as
radiopharmaceutical agents were tested in an in vivo model utilizing T-47D human
breast cancer cell xenografts in SCID mice. Clearance of STh analogs was rapid, primarily via renal excretion into the urine, with >85% ID excreted into the urine at 1 h p.i.
Tumor uptake at 1 h p.i. in T-47D
tumor cell xenografts was 0.67+/-0.23% ID/g, and was significantly decreased (p<0.05) upon co-administration of 4 mg/kg unlabeled STh. These results suggest that STh may find application for the imaging and treatment of
breast cancer.