Cholesterol accumulation in ccRCC: the role of ccRCC-initiating VHL-HIFα pathway

Accounting for approximately 75% of all renal cancer cases, clear cell renal cell carcinoma (ccRCC) is characterized by the bi-allelic inactivation of the VHL tumour suppressor gene, leading to the abnormal stabilization of hypoxia-inducible factor alpha (HIFα).1Hsieh J.J. Purdue M.P. Signoretti S. et al.Renal cell carcinoma.Nat Rev Dis Primers. 2017; 317009https://doi.org/10.1038/nrdp.2017.9Crossref PubMed Scopus (1588) Google Scholar Of the two major HIFα subunits, HIF2α is responsible for driving ccRCC growth while HIF1α may exhibit suppressive effects on ccRCC progression.2Shen C. Kaelin W.G. The VHL/HIF axis in clear cell renal carcinoma.Semin Cancer Biol. 2013; 23: 18-25https://doi.org/10.1016/j.semcancer.2012.06.001Crossref PubMed Scopus (288) Google Scholar Moreover, ccRCC has widespread genetic heterogeneities with VHL inactivation being the truncal event observed in ∼90% of sporadic ccRCC cases.3Turajlic S. Xu H. Litchfield K. et al.Deterministic evolutionary trajectories influence primary tumor growth: TRACERx renal.Cell. 2018; 173: 595-610.e11https://doi.org/10.1016/j.cell.2018.03.043Summary Full Text Full Text PDF PubMed Scopus (390) Google Scholar Another distinctive feature of ccRCC is the accumulation of lipid droplet (LD), predominantly in the form of cholesteryl ester (CE) and triglycerides.4Gebhard R.L. Clayman R.V. Prigge W.F. et al.Abnormal cholesterol metabolism in renal clear cell carcinoma.J Lipid Res. 1987; 28: 1177-1184Summary Full Text PDF PubMed Google Scholar,5Saito K. Arai E. Maekawa K. et al.Lipidomic signatures and associated transcriptomic profiles of clear cell renal cell carcinoma.Sci Rep. 2016; 628932https://doi.org/10.1038/srep28932Crossref Scopus (88) Google Scholar Notably, aberrant stabilization of HIFα has been implicated in driving the LD formation in ccRCC by regulating the expression of PLIN2 and CPT1A.6Qiu B. Ackerman D. Sanchez D.J. et al.HIF2α-dependent lipid storage promotes endoplasmic reticulum homeostasis in clear-cell renal cell carcinoma.Cancer Discov. 2015; 5: 652-667https://doi.org/10.1158/2159-8290.CD-14-1507Crossref PubMed Scopus (258) Google Scholar,7Du W. Zhang L. Brett-Morris A. et al.HIF drives lipid deposition and cancer in ccRCC via repression of fatty acid metabolism.Nat Commun. 2017; 8: 1769https://doi.org/10.1038/s41467-017-01965-8Crossref PubMed Scopus (272) Google Scholar Despite these findings, the pathological relevance and molecular mechanisms underpinning lipid accumulation in ccRCC, specifically with respect to CE, remains elusive. In the latest issue of eBioMedicine,8Zhang S. Fang T. He Y. et al.VHL mutation drives human clear cell renal cell carcinoma progression through PI3K/AKT-dependent cholesteryl ester accumulation.eBioMedicine. 2024; https://doi.org/10.1016/j.ebiom.2024.105070Summary Full Text Full Text PDF Google Scholar Zhang et al. leveraged label-free Raman spectromicroscopy to elucidate the mechanisms underpinning lipid accumulation in ccRCC. The utilisation of Raman spectromicroscopy enables concurrent quantitative analysis of LD distribution and composition, making it a robust approach for studying lipid metabolism. By employing this method in tandem with validation via liquid chromatography-mass spectrometry, the authors observed aberrant LD accumulation, predominantly in the form of CE, in ccRCC tissue samples as compared to their normal adjacent counterparts. This observation resonates with previous reports on the high accumulation of CE in ccRCC. Intriguingly, CE accumulation was absent in a panel of commonly used ccRCC cell lines, prompting scrutiny regarding their suitability as models for studying CE accumulation in ccRCC. Instead, primary ccRCC cells were identified as an appropriate model due to their high abundance of CE, which was similar to levels detected in the source tissues. Nonetheless, careful experimental design is essential when employing these primary ccRCC cells due to the observed reduction in LDs and CE levels following cryopreservation and prolonged culture time. Further investigation revealed that the VHL-mutated primary ccRCC cells had significantly higher CE levels than those without the VHL mutation. The reintroduction of wild-type VHL into the VHL-mutated primary ccRCC cells, which induced HIFα degradation, led to a significant reduction in both the LD and CE levels. The shRNA-mediated knockdown of either HIF1α or HIF2α reinforced the notion that the accumulation of CE in ccRCC was primarily driven by HIF2α. These findings underscore the role of the VHL-HIFα pathway in regulating CE metabolism and accumulation in ccRCC. Mechanistically, the stabilization of HIF2α in VHL-mutated ccRCC cells resulted in the upregulation of PI3K/AKT/mTOR/SREBP pathway that increased cholesterol uptake and subsequent cholesterol esterification to CE for storage into LDs. These findings echo previous study that linked HIF2α with the activation of the PI3K/AKT/mTOR/SREBP pathway. This connection was demonstrated to be mediated by the KLF6-PDGFβ axis, which is crucial for cholesterol homeostasis and supporting ccRCC growth and progression.9Syafruddin S.E. Rodrigues P. Vojtasova E. et al.A KLF6-driven transcriptional network links lipid homeostasis and tumour growth in renal carcinoma.Nat Commun. 2019; 10: 1152https://doi.org/10.1038/s41467-019-09116-xCrossref PubMed Scopus (57) Google Scholar Moreover, chemical inhibition of cholesterol esterification using Avasimibe significantly depleted LD and CE levels and importantly, impaired ccRCC growth in vivo with no detectable signs of toxicity. These observations corroborate the functional significance of CE in supporting ccRCC pathogenesis. Furthermore, whole transcriptomic analysis of the Avasimibe-treated primary ccRCC cells identified several genes whose expression were dependent on membrane cholesterol levels, namely, ITGA6, ITGB1 and CAV1. It was proposed that the downregulation of these genes, upon Avasimibe-mediated CE depletion, contributed to the impaired ccRCC growth. Collectively, Zhang et al., provided important insights into the role of the ccRCC initiating-VHL-HIFα pathway in driving the enigmatic CE accumulation in this cancer type. Remarkably, the identification of an appropriate ccRCC in-vitro model for studying CE accumulation could advance this area of research and pave the way for more significant discoveries in the future. Given the dependency of ccRCC growth on cholesterol levels and functions, targeting cholesterol metabolism is considered to be a promising therapeutic option for ccRCC. Moving forward, it is imperative for future studies to comprehensively map the molecular networks governing cholesterol metabolism in ccRCC. A deeper understanding of these underlying mechanisms would help to identify other potential ccRCC "Achilles' heels". Such insights are beneficial for the development of robust treatment strategies for ccRCC. Literature search: M.A.M., S.N.H.M.Y., and S.E.S.; Data collection: M.A.M., S.N.H.M.Y., and S.E.S.; Data interpretation: M.A.M., and S.E.S.; Writing: M.A.M., and S.E.S. All authors read and approve the final manuscript. The authors declare no conflict of interest. This study was funded by Geran Universiti Penyelidikan (GUP), Universiti Kebangsaan Malaysia, GUP-2023-005. VHL mutation drives human clear cell renal cell carcinoma progression through PI3K/AKT-dependent cholesteryl ester accumulationCollectively, our study improves current understanding of the role of CE accumulation in ccRCC and opens up new opportunities for treatment. Full-Text PDF Open Access