By Jamshed Arslan Pharm.D.
Parkin got its name from Parkinson’s disease (PD). Being an E3 ubiquitin ligase enables Parkin to ubiquitinate and degrade proteins involved in PD (such as CDCrel-1, α-synuclein, and synphilin-1). Now we know that mutations in the Parkin gene (PARK2) can not only lead to a hereditary form of PD, but may also make people prone to various malignancies including breast cancer. Hypoxia-inducible factor-1α (HIF-1α), which adapts cells to hypoxic conditions in certain cancers, has been suggested as a protein target behind Parkin’s tumor suppressive actions.1,2 The role of Parkin-HIF-1α interaction in cancer remained unknown until researchers from institutes in the US and China recently discovered that Parkin ubiquitinates HIF-1α inhibiting breast cancer progression.3
Parkin downregulates HIF-1α
As expected, a decline in Parkin in breast cancer samples was found in tissue microarrays, but surprisingly, the decrease in Parkin was independent of tumor subtypes in terms of ER, PR, and HER2 status. The researchers then transduced a Parkin-expressing vector in breast cancer cell lines (such as
MCF7), which resulted in diminished levels of HIF-1α protein (but not the mRNA) under both normoxic and hypoxic conditions. Intriguingly, reduced mRNA levels of HIF-target genes (
VEGFA,
CXCR4 and
LOX) were observed, which was indicative of the fact that Parkin downregulates the transcriptional activity of HIF-1α. In line with Parkin’s ubiquitin ligase activity,
in vivo ubiquitination assays and chromatography/mass spectrometry using mutated HIF-1α vectors revealed that Parkin promotes ubiquitination of HIF-1α at lysine 477, resulting in its consequent degradation.
After confirming Parkin-induced downregulation of HIF-1α, the next step was to investigate what the Parkin-HIF-1α interplay means for cancer.
VEGF was detected in immersion fixed paraffin-embedded sections of human breast cancer tissue using Human VEGF 165 Polyclonal Antibody (Catalog # AB-293-NA) at 15 µg/mL overnight at 4 °C. Tissue was stained using the Anti-Goat HRP-DAB Cell & Tissue Staining Kit (brown; Catalog # CTS008) and counterstained with hematoxylin (blue). Lower panel shows a lack of labeling
Parkin inhibits cancer cell migration, invasion, and metastasis through HIF-1α
Researchers found that transducing breast cancer cells with a Parkin vector inhibited cell migration and invasion in transwell assays, but these effects of Parkin were significantly reduced in cancer cells lacking HIF-1α. To translate these observations in a mammalian model, they implanted and injected breast tumor cells into mice, and knocked down the endogenous Parkin. This promoted metastasis, which could be inhibited by YC-1, a
small molecule HIF-1α-inhibitor. Using RNA interference to block HIF-1α also reversed the metastatic effects of Parkin knockdown. Similarly, immunohistochemical staining of different human breast tumor specimens revealed that low Parkin expression is correlated with high HIF-1α levels.
These results show that Parkin expression is inversely correlated with breast cancer metastasis because of a Parkin-induced negative impact on HIF-1α.
Lead author of the study, Zhaohui Feng, PhD, of the Rutgers Cancer Institute of New Jersey, pointed towards some HIF-1α-blockers (such as EZN-2968 and RO7070179)4,5 that have been tried in humans to treat cancer(s).
Significance
The discovery that Parkin’s tumor suppressive functions depend to a significant degree on its capacity to regulate HIF-1α paves the way for understanding tumorigenesis and cancer progression. The findings not only indicate HIF-1α as a feasible therapeutic target in breast tumors, but can also be translatable to other cancers that have low Parkin expression, such as ovarian and non-small cell lung carcinomas. Only human trials can uncover the relevance and safety of blocking HIF-1α for curing cancers.
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Jamshed Arslan, Pharm D.
University of Alabama at Birmingham, School of Medicine
Dr. Arslan studies cell signaling in mitochondrial defects in
C. elegans and transgenic mice.
References
- Sarraf, Shireen A., et al. “Landscape of the PARKIN-Dependent Ubiquitylome in Response to Mitochondrial Depolarization.” Nature, vol. 496, no. 7445, 2013, pp. 372–376. doi: 10.1038/nature12043.
- Maugeri, Grazia, et al. “Parkin Modulates Expression of HIF-1α and HIF-3α during Hypoxia in Gliobastoma-Derived Cell Lines In Vitro.” Cell and Tissue Research, vol. 364, no. 3, 2016, pp. 465–474. doi: 10.1007/s00441-015-2340-3.
- Liu, Juan, et al. “Parkin Targets HIF-1α for Ubiquitination and Degradation to Inhibit Breast Tumor Progression.” Nature Communications, vol. 8, 2017, n. pag. DOI: 10.1038/s41467-017-01947-w
- Jeong, W., et al. “Pilot Trial of EZN-2968, an Antisense Oligonucleotide Inhibitor of Hypoxia-Inducible Factor-1 Alpha (HIF-1α), in Patients with Refractory Solid Tumors.” Cancer Chemotherapy and Pharmacology, vol. 73, no. 2, 2014, pp. 343–348. doi: 10.1007/s00280-013-2362-z
- https://clinicaltrials.gov/ct2/show/NCT02564614
