APP deficiency and HTRA2 modulates PrPc proteostasis in human cancer cells

• PrP c /APP imbalance affects cancer cell survival in a 3D tissue like environment. • APP gene silencing enhances cytoplasmic PrP c protein accumulation. • Impaired APP expression induces HTRA2 and destabilizes lipid raft-associated protein partners (e.g. CD24). • HTRA2 up- or down-expression dysregulates PRNP. • HTRA2 is a new modulator for PrP c proteostasis in human cancer cells. Cellular protein homeostasis (proteostasis) requires an accurate balance between protein biosynthesis, folding, and degradation, and its instability is causally related to human diseases and cancers. Here, we created numerous engineered cancer cell lines targeting APP (amyloid ß precursor protein) and/or PRNP (cellular prion) genes and we showed that APP knocking-down impaired PRNP mRNA level and vice versa, suggesting a link between their gene regulation. PRNP KD , APP KD and PRNP KD /APP KD HeLa cells encountered major difficulties to grow in a 3D tissue-like environment. Unexpectedly, we found a cytoplasmic accumulation of the PrP c protein without PRNP gene up regulation, in both APP KD and APP KO HeLa cells. Interestingly, APP and/or PRNP gene ablation enhanced the chaperone/serine protease HTRA2 gene expression, which is a protein processing quality factor involved in Alzheimer's disease. Importantly, HTRA2 gene silencing decreased PRNP mRNA level and lowered PrP c protein amounts, and conversely, HTRA2 overexpression increased PRNP gene regulation and enhanced membrane-anchored and cytoplasmic PrP c fractions. PrP c , APP and HTRA2 destabilized membrane-associated CD24 protein, suggesting changes in the lipid raft structure. Our data show for the first time that APP and the dual chaperone/serine protease HTRA2 protein could modulate PrP c proteostasis hampering cancer cell behavior.