Stimulatory Effects of Retinoic Acid on Tumor Growth and Serum Insulin‐like Growth Factor‐1 in Rats Bearing Estrogen‐responsive Pituitary Tumor MtT/Se

MtT/Se is one of 4 cell lines derived from an estrogen‐dependent pituitary tumor, MtT/F84. The main difference between these tumor types is that MtT/F84 secretes both growth hormone (GH) and prolactin (PRL) whereas MtT/Se secretes only GH. MtT/Se grew slowly in ovariectomized (ovex) rats, but tumor growth was much faster in estrogen‐treated ovex rats. Effects of dietary retinoic acid (RA) on tumor growth, serum GH and insulin‐like growth factor‐1 (IGF‐1) levels were examined in ovex rats. Latency of tumor growth was shortened, and tumor take and weight were promoted by all‐trans RA both in the presence and absence of exogenous estrogen. Serum GH and IGF‐1 levels became increased in tumor‐hearing rats whereas PRL levels remained unchanged. Serum IGF‐1 levels exhibited a good correlation with tumor weights (r=0.84). Our results suggest a close relationship between increase of tumor weight and stimulation of serum IGF‐1 level by RA in tumor‐bearing rats.