KRAS(G12D)-mediated oncogenic transformation of thyroid follicular cells requires long-term TSH stimulation and is regulated by SPRY1.

ABSTRACT

KRAS(G12D) 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 KRAS(G12D)-mediated thyroid follicular cell transformation. In the present study, we investigated the effect of long-term TSH stimulation in KRAS(G12D) knock-in mice and the role of Sprouty1 (SPRY1) in KRAS(G12D)-mediated signaling. We used TPO-KRAS(G12D) mice for thyroid-specific expression of KRAS(G12D) under the endogenous KRAS promoter. Twenty TPO-KRAS(G12D) 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-KRAS(WT)) was given the same treatment. The expression of SPRY1, a negative regulator of receptor tyrosine kinase (RTK) signaling, was analyzed in both KRAS(G12D)-and BRAF(V600E)-induced thyroid cancers. Without PTU treatment, only mild thyroid enlargement and hyperplasia were observed in TPO-KRAS(G12D) mice. With PTU treatment, significant thyroid enlargement and hyperplasia occurred in both TPO-KRAS(G12D) and TPO-KRAS(WT) littermates. Thyroids from TPO-KRAS(G12D) mice were six times larger than TPO-KRAS(WT) littermates. Distinct thyroid histology was found between TPO-KRAS(G12D) and TPO-KRAS(WT) mice thyroid from TPO-KRAS(G12D) mice showed hyperplasia with well-maintained follicular architecture whereas in TPO-KRAS(WT) mice this structure was replaced by papillary hyperplasia. Among 10 TPO-KRAS(G12D) 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 BRAF(V600E) in PTC resulted in both MAPK and PI3K/AKT activation. We conclude that chronic TSH stimulation can enhance KRAS(G12D)-mediated oncogenesis, leading to FTC. SPRY1 may function as a molecular switch to control MAPK signaling and its downregulation by BRAF(V600E) favors PTC development.