Targeting of Thyroid Hormone Receptor Variants to Aggresomes

The thyroid hormone receptor (TR) alters gene transcription in response to thyroid hormone (T3). Our prior studies demonstrated that dominant negative variants, including the retroviral oncoprotein v-ErbA, mislocalize to the cytoplasm and sequester TR in foci suggestive of aggresomes. Formation of the aggresome is a cellular response to the accumulation of misfolded protein aggregates for turnover. Starting as mini-aggregates, they are transported along microtubule tracks to the microtubule organizing center (MTOC), where they disrupt vimentin intermediate filaments and recruit machinery for protein degradation. Viral particles also follow the same aggresomal pathway to facilitate replication and assembly. To test for association with aggresomes, HeLa cells were cotransfected with the aggresomal markers GFP-250 and GFP-170 and DsRed2-tagged v-ErbA. There was a strong colocalization between the aggresomal markers and v-ErbA, suggesting that they are both targeted to the same subcellular location. v-ErbA foci disrupt vimentin, further demonstrating their aggresome-like properties. Proteasome inhibition is known to induce aggresome formation; thus, the effect of treatment of v-ErbA-expressing cells with the proteasome inhibitor MG132 was assessed. Upon treatment, there was a significant increase in foci size. Additionally, treatment with the microtubule-disrupting drug nocodazole inhibited aggresome formation. Taken together, these studies provide evidence for targeting of the oncoprotein v-ErbA to aggresomes, which is most likely a mechanism for its turnover.