Abstract
Disruption of the microtubule cytoskeleton impairs tumor angiogenesis by inhibiting the hypoxia-inducible factor (HIF-1α) pathway. However, the signaling cascade linking microtubule disruption to HIF-1α inactivation has not been elucidated. Here, we show that microtubule-targeting drug (MTD) treatment impaired HIF-1α protein nuclear translocation, which significantly down-regulated HIF transcriptional activity. We provide strong evidence that HIF-1α protein associates with polymerized microtubules and traffics to the nucleus, with the aid of the dynein motor protein. Together, these data suggest that microtubules are critically involved in the nuclear trafficking and transcriptional activity of HIF-1α. We also show that the connection between the microtubule cytoskeleton and HIF-1α regulation is lost in renal cell carcinoma (RCC), where HIF-1α is overexpressed because of mutations in the von Hippel Lindau (VHL) tumor suppressor protein. Specifically, we show that MTD treatment of RCC cells did not impair HIF-1α nuclear accumulation or transcriptional activity, and had no effect on the polysome association profile of HIF-1α. Interestingly, we found that HIF-1α protein did not bind microtubules in RCC. Moreover, restoration of VHL function failed to restore the ability of MTDs to inhibit HIF-1α, suggesting that VHL does not contribute to this phenotype. Together, these results suggest that HIF-1α regulation is microtubule-independent, and likely contributes to the chemoresistant nature of RCCs. Further understanding of the microtubule-dependent HIF-1α regulation, and lack thereof in RCC, is essential given the importance of HIF-1α in tumor biology, and the widespread use of MTDs in clinical oncology.
Highlights
HIF-1␣ tumor pro-survival activity requires functional microtubules
We showed that once the integrity of the microtubule cytoskeleton is restored, upon repolymerization, HIF-1␣ mRNA reenters active translation, reinstating HIF-1␣ protein expression and activity. While these results indicate that microtubule dynamics tightly and reversibly regulate the fate of HIF-1␣ mRNA, the temporal relationship between microtubule disruption, HIF-1␣ translation and HIF-1␣ protein function is not well elucidated
Microtubule-targeting drugs are the most effective class of chemotherapeutics, and their success in clinical oncology argues that tubulin is one of the best drug targets identified to date [24, 25]
Summary
HIF-1␣ tumor pro-survival activity requires functional microtubules. Results: Microtubules and the dynein motor protein transport HIF-1␣ protein toward the nucleus enabling its transcriptional activity in taxane-sensitive cell lines. We showed that the inhibition of HIF-1␣ translation occurs downstream of microtubule disruption, and is reversed following microtubule repolymerization [4] These findings provide solid proof of a functional relationship between the anti-tubulin and anti-angiogenic effects of MTDs. Though MTDs are traditionally thought of as anti-mitotic agents, we and others [7, 8] have shown that these drugs affect important cellular functions during interphase. Treatment with taxanes and other MTDs inhibits HIF-1␣ nuclear accumulation and transcriptional activity as a result of microtubule disruption This new mechanism adds HIF-1␣ to the list of transcription factors that are transported on interphase microtubules, and reveals that microtubules regulate HIF-1␣ at both the level of protein synthesis and protein trafficking. Further understanding of the mechanism responsible for the uncoupling of the microtubule cytoskeleton from HIF-1␣ activity in RCC will lead to the development of more effective therapies for this disease
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