DUAL ACTIVITY OF HUMAN PITUITARY THYROTROPIN ISOFORMS ON THYROID-CELLGROWTH

Alkaline (pI 8.6-7.5) and neutral (pI 7.0-6.0) isoforms of human TSH h ave been isolated from a highly purified intrapituitary preparation by isoelectric focusing and compared for their respective actions on thy roid cell proliferation. Both TSH isoforms displayed the same ability to bind to porcine thyroid membranes as the original hormone preparati on, indicating a similar recognition at the receptor sites. Alkaline f orms showed a higher potency in inducing either cyclic AMP (cAMP) prod uction or [H-3]thymidine incorporation in FRTL-5 cells (half-maximal e ffective doses (ED(50) values)=0.25 and 0.29 nM respectively) compared with their neutral counterparts (ED(50) values=0.66 and 0.70 nM respe ctively). Increasing the concentration of alkaline forms in the presen ce of a half-maximal concentration of neutral TSH resulted in a profou nd inhibition of cell growth without a significant change in cAMP. Con versely, increasing the amount of neutral forms in the presence of a h alf-maximal dose of alkaline TSH resulted in an additive response for cAMP production but not in cell proliferation. To assess whether glyco sylation might be responsible for the variation in hormone action, bot h alkaline and neutral TSH isoforms were tested for recognition of the ir carbohydrate chains by concanavalin A (Con A) and ricin. No major d ifference was found in binding to Con A, indicating that the contribut ion of carbohydrates to changes in hormone pi was not related to core branching. Very few galactose residues were accessible in either hormo ne fraction since little binding to ricin was observed. Isoelectric fo cusing of TSH forms before and after neuraminidase treatment revealed that neutral forms had a higher sialic acid content than alkaline TSH. In conclusion, the current findings show that TSH isoforms differenti ally affect cAMP production and cell growth. TSH fractions with a high sialic acid content and a low mitogenic activity behave as antagonist s to the more active forms for cell proliferation. It is suggested tha t physiological control of TSH action at the thyroid gland may reside in the respective amounts of various TSH forms which, once bound to th eir receptor, can induce variable activation of post-receptor events w hile controlling cell proliferation.