StarD5 Regulates Cholesterol Homeostasis to Promote Colorectal Cancer Progression and the Cancer Stem Cell Phenotype

An aberration in cellular, especially plasma membrane (PM), cholesterol level is arguably the most critical targetable hallmark of colorectal cancer and colorectal cancer stem cell (CRCSC) phenotypes. We recently identified Star-related lipid transfer protein 5 (StarD5) as an intracellular cholesterol transport protein that regulates PM cholesterol levels. In this study, we show that StarD5 protein levels are significantly elevated in most human colorectal cancer tissues compared with adjacent normal mucosa, with a preferential increase in the epithelial compartment. Additionally, allograft growth in StarD5 knockout mice was largely unaffected. StarD5 levels were particularly elevated in CRCSCs compared with non-CRCSCs in several colon cancer cell lines and primary human colorectal cancer samples. Genetic knockdown of StarD5 (shRNA; KD) inhibited the CRCSC phenotype in vitro-growth and self-renewal (1°→3° spheroid formation), CRCSC maker levels, and sensitivity to 5-fluorouracil. Also, StarD5-depleted HT-29 cells showed a robust (40-fold) reduction in tumor formation (CRCSC characteristics) in vivo and CRCSC phenotype ex vivo, including CRCSC marker expression and spheroid formation (1°→3°). StarD5 inhibition also caused a significant increase in apoptosis induction. Mechanistically, PM cholesterol levels were significantly higher in CRCSCs than in non-CRCSCs, and StarD5 depletion decreased accessible PM cholesterol, particularly in specialized lipid raft domains. Finally, cholesterol depletion in the PM is critical for StarD5 KD's inhibition of the CRCSC phenotype as supplementation with low-density lipoprotein caused a significant reversal of the StarD5 KD effects on PM/raft cholesterol levels and the CRCSC phenotype.

Given StarD5's critical role in regulation of CRCSCs, translational research targeting StarD5 will lead to improved outcomes in patients with colorectal cancer.