Antagonism of TGF-alpha receptor binding and TGF-alpha induced stimulation of cell proliferation by methyl pheophorbides.
Over 4,500 natural product extracts were screened for their abilities to inhibit binding of radiolabeled TGF-alpha to A431 cells; several plant extracts were identified as potential leads with IC50 values of less than 30 micrograms/mL. The active components of one extract were purified to homogeneity and identified as the porphyrin structures, methyl pheophorbides a and b. These compounds inhibited both TGF-alpha receptor binding and the TGF-alpha induced proliferation of NRK-49F cells in soft agar. To construct a structure-function relationship, a series of commercially available porphyrin derivatives was evaluated. The most potent compound, hematoporphyrin IX, inhibited TGF-alpha functions in a dose-dependent fashion with IC50 values slightly lower than the methyl pheophorbides. Further studies revealed that inhibition of TGF-alpha binding was light dependent and that inhibition did not involve direct competition of porphyrins for the TGF-alpha binding site. To determine the specificity of inhibition, the porphyrins were tested in a number of other receptor-ligand assays. TNF-alpha and beta-adrenoceptor bindings were unaffected, whereas IL-1 beta binding to EL-4 membranes and platelet-derived growth factor induced thymidine incorporation in NIH-3T3 cells were both antagonized by the most active porphyrins. Inhibition of TGF-beta binding to NRK-49F cells and TGF-beta-induced growth of AKR-2B cells was also observed. In summary, we report that methyl pheophorbides are naturally occurring, photodynamic antagonists of TGF-alpha, and although the inhibitory properties of these molecules were not confined to TGF-alpha alone, some level of receptor selectivity was observed.
Rieman
D J
DJ
Department of Cellular Biochemistry and Immunology, SmithKline Beecham Pharmaceuticals, Research and Development, King of Prussia, PA 19406.
Anzano
M A
MA
Chan
G W
GW
Imburgia
T J
TJ
Chan
J A
JA
Johnson
R K
RK
Greig
R G
RG
D000818:Animals; D002455:Cell Division / Q000187:drug effects*; D002460:Cell Line; D002734:Chlorophyll / Q000031:analogs & derivatives* / Q000494:pharmacology; D016207:Cytokines / Q000494:pharmacology*; D004357:Drug Synergism; D066246:ErbB Receptors / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006801:Humans; D010936:Plant Extracts / Q000494:pharmacology*; D011166:Porphyrins / Q000494:pharmacology*; D011943:Receptors, Adrenergic, beta / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism / Q000494:pharmacology; D014020:Tissue Extracts / Q000494:pharmacology*; D016211:Transforming Growth Factor alpha / Q000378:metabolism* / Q000494:pharmacology*; D016212:Transforming Growth Factor beta / Q000494:pharmacology
D016207:Cytokines; D010936:Plant Extracts; D011166:Porphyrins; D011943:Receptors, Adrenergic, beta; D011994:Recombinant Proteins; D014020:Tissue Extracts; D016211:Transforming Growth Factor alpha; D016212:Transforming Growth Factor beta; D002734:Chlorophyll; C032625:methylpheophorbide; D066246:ErbB Receptors