Thyrotropin-induced mitogenesis is Ras dependent but appears to bypass the Raf-dependent cytoplasmic kinase cascade.
Cellular growth control requires the coordination and integration of multiple signaling pathways which are likely to be activated concomitantly. Mitogenic signaling initiated by thyrotropin (TSH) in thyroid cells seems to require two distinct signaling pathways, a cyclic AMP (cAMP)-dependent signaling pathway and a Ras-dependent pathway. This is a paradox, since activated cAMP-dependent protein kinase disrupts Ras-dependent signaling induced by growth factors such as epidermal growth factor and platelet-derived growth factor. This inhibition may occur by preventing Raf-1 protein kinase from binding to Ras, an event thought to be necessary for the activation of Raf-1 and the subsequent activation of the mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinases (MEKs) and MAP kinase (MAPK)/ERKs. Here we report that serum-stimulated hyperphosphorylation of Raf-1 was inhibited by TSH treatment of Wistar rat thyroid cells, indicating that in this cell line, as in other cell types, increases in intracellular cAMP levels inhibit activation of downstream kinases targeted by Ras. Ras-stimulated expression of genes containing AP-1 promoter elements was similarly inhibited by TSH. On the other hand, stimulation of thyroid cells with TSH resulted in stimulation of DNA synthesis which was Ras dependent but both Raf-1 and MEK independent. We also show that Ras-stimulated DNA synthesis required the use of this kinase cascade in untreated quiescent cells but not in TSH-treated cells. These data suggest that in TSH-treated thyroid cells, Ras might be able to signal through effectors other than the well-studied cytoplasmic kinase cascade.
al-Alawi
N
N
Department of Pharmacology, University of California at San Diego, La Jolla 92093.
Rose
D W
DW
Buckmaster
C
C
Ahn
N
N
Rapp
U
U
Meinkoth
J
J
Feramisco
J R
JR
D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D000242:Cyclic AMP / Q000502:physiology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D003593:Cytoplasm / Q000201:enzymology; D004247:DNA / Q000096:biosynthesis; D004789:Enzyme Activation; D015870:Gene Expression; D007074:Immunoglobulin G / Q000494:pharmacology; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf; D051381:Rats; D017208:Rats, Wistar; D011972:Receptor, Insulin / Q000096:biosynthesis / Q000378:metabolism*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D013961:Thyroid Gland / Q000187:drug effects / Q000378:metabolism; D013972:Thyrotropin / Q000494:pharmacology*; D014162:Transfection; D018631:ras Proteins / Q000096:biosynthesis / Q000378:metabolism*
D007074:Immunoglobulin G; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D013972:Thyrotropin; D004247:DNA; D000242:Cyclic AMP; D011494:Protein Kinases; D011972:Receptor, Insulin; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D017868:Cyclic AMP-Dependent Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D018631:ras Proteins