ablation in the thyroid gland in vivo increases EGFR and ERK activity but decreases NF-κ
B activity. These effects on EGFR/ERK and NF-κ
B are consistent with our previous findings in thyroid cancer cell lines.4
However, we previously observed increased proliferation in Mig-6
knockdown cells in vitro. In contrast, the histologic examination of Mig-6d/d
thyroids did not reveal adenomatous changes or evidence of increased proliferation by Ki-67 immunohistochemical staining, at least up to 8 weeks of age. This may be because of the young age of the mice at analysis, and more time may be necessary to observe the histologic evidence of hyperproliferation in Mig-6d/d
A previous study revealed that the thyroid-specific loss of phosphatase and tensin homolog (PTEN), a known tumor suppressor, attenuated Akt signaling activation in 10-week-old mice. However, goiters and adenomas were observed only later in 10-month-old mice.18
PTEN knockdown, in addition to overexpression of PAX8/peroxisome proliferator-activated receptor γ
fusion protein (PPFP), synergistically caused thyroid hyperplasia by 12 months of age.19
mice may also develop hyperplastic changes in later stages of development.
Alternatively, it is possible that Mig-6
loss alone is not sufficient for tumorigenesis. Others have observed that the Mig-6
and PTEN loss together cause endometrial tumors in mice as early as 2 to 4 weeks of age.2
Skin-specific p53 knockout in addition to oncogene K-RAS activation synergistically causes squamous cell carcinoma in mice as early as by 2 weeks.20
ablation alone may not be sufficient for the development of thyroid cancer; oncogenesis in Mig-6d/d
mice may require the acquisition of other genetic defects, such as the loss of an additional tumor suppressor or oncogenic activation.
EGFR internalization and trafficking toward the cell cytoplasm and nucleus is a key mechanism for amplifying downstream signaling pathways, such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK).21
However, EGFR internalization also promotes trafficking toward the endosome and finally results in EGFR degradation.22
EGFR signaling can either be terminated or amplified by EGFR internalization, and these events are critical for oncogenesis.23
Many tumor suppressors modulate EGFR downstream signaling through regulating EGFR internalization. Merlin has been shown to inhibit EGFR internalization, and it therefore attenuates downstream signaling pathway including MAPK in renal carcinoma cells.24
Our data also show that, in murine thyroid, Mig-6
blocks EGFR activation and attenuates ERK, a subunit of the MAPK signaling pathway. Mig-6
also appears to promote the distribution of EGFR to caveolin-containing membrane fractions and may thereby promote NCE leading to receptor degradation rather than activation of signaling pathways in response to ligand binding. In agreement with such a model, Mig-6
promotes EGFR internalization, results in receptor degradation, leading to attenuation of ERK activity.25
Unlike these previous studies, however, we do not observe an effect of Mig-6
on total levels of EGFR in mouse thyroid. This may reflect differences in the basal levels of EGFR receptor-mediated endocytosis in these distinct cell types or may suggest that Mig-6
attenuates EGFR signaling instead of affecting EGFR degradation in the thyroid.
In conclusion, our results show that Mig-6 loss in vivo increases EGFR and ERK activity and inactivates NF-κB in the mouse thyroid. We did not find histologic evidence of hyperproliferation in Mig-6 knockout mice; this may be related to the young age of the mice studied, or it may indicate that loss of Mig-6 alone is insufficient to cause tumorigenesis. Future studies should include more mature Mig-6d/d mice and should investigate the effects of adding other genetic defects in combination with Mig-6 loss on the thyroid.