KRASG12D-mediated oncogenic transformation of thyroid follicular cells requires long-term TSH stimulation and is regulated by SPRY1

Abstract

KRASG12D can cause lung cancer rapidly, but is not sufficient to induce thyroid cancer. It is not clear whether long-term serum thyroid stimulating hormone (TSH) stimulation can promote KRASG12D-mediated thyroid follicular cell transformation. In the present study, we investigated the effect of long-term TSH stimulation in KRASG12D knock-in mice and the role of Sprouty1 (SPRY1) in KRASG12D-mediated signaling. We used TPO-KRASG12D mice for thyroid-specific expression of KRASG12D under the endogenous KRAS promoter. Twenty TPO-KRASG12D mice were given anti-thyroid drug propylthiouracil (PTU, 0.1% w/v) in drinking water to induce serum TSH and 20 mice were without PTU treatment. Equal number of wild-type littermates (TPO-KRASWT) was given the same treatment. The expression of SPRY1, a negative regulator of receptor tyrosine kinase (RTK) signaling, was analyzed in both KRASG12D-and BRAFV600E-induced thyroid cancers. Without PTU treatment, only mild thyroid enlargement and hyperplasia were observed in TPO-KRASG12D mice. With PTU treatment, significant thyroid enlargement and hyperplasia occurred in both TPO-KRASG12D and TPO-KRASWT littermates. Thyroids from TPO-KRASG12D mice were six times larger than TPO-KRASWT littermates. Distinct thyroid histology was found between TPO-KRASG12D and TPO-KRASWT mice: thyroid from TPO-KRASG12D mice showed hyperplasia with well-maintained follicular architecture whereas in TPO-KRASWT mice this structure was replaced by papillary hyperplasia. Among 10 TPO-KRASG12D mice monitored for 14 months, two developed follicular thyroid cancer (FTC), one with pulmonary metastasis. Differential SPRY1 expression was demonstrated: increased in FTC and reduced in papillary thyroid cancer (PTC). The increased SPRY1 expression in FTC promoted TSH-RAS signaling through PI3K/AKT pathway whereas downregulation of SPRY1 by BRAFV600E in PTC resulted in both MAPK and PI3K/AKT activation. We conclude that chronic TSH stimulation can enhance KRASG12D-mediated oncogenesis, leading to FTC. SPRY1 may function as a molecular switch to control MAPK signaling and its downregulation by BRAFV600E favors PTC development.