Partial Truncation of the C-Terminal Domain of PTCH1 in Cancer Enhances Autophagy and Metabolic Adaptability

Simple Summary We have recently reported that the cytosolic C-terminal domain (CTD) of the tumour suppressor PTCH1, the Hedgehog proteins receptor, interacts with the autophagy-related protein ATG101 and impairs autophagic flux. In this study, we identified the interaction region and found that it is absent in a subset of colorectal, stomach and endometrial cancers. We demonstrated that truncation mutants lack the ability to constraint autophagy, resulting in a proliferative advantage and reduced sensitivity to autophagy inducers or glycolysis inhibitors in cell lines harbouring endogenous PTCH1 CTD mutations compared to isogenic cells expressing wild-type PTCH1. In summary, this study highlights the importance of the PTCH1-ATG101 interaction in the regulation of basal and stimulated autophagy and the metabolic flexibility in cancer cells. The Hedgehog receptor, Patched1 (PTCH1), is a well-known tumour suppressor. While the tumour suppressor's activity is mostly ascribed to its function as a repressor of the canonical Smoothened/Gli pathway, its C-terminal domain (CTD) was reported to have additional non-canonical functions. One of them is the reduction of autophagic flux through direct interaction with the Unc-51, like the autophagy activating kinase (ULK) complex subunit autophagy-related protein-101 (ATG101). With the aim of investigating whether this function of PTCH1 is important in cancer cell fitness, we first identified frameshift mutations in the CTD of PTCH1 in cancer databases. We demonstrated that those mutations disrupt PTCH1 interaction with ATG101 and increase autophagic flux. Using deletion mutants of the PTCH1 CTD in co-immunoprecipitation studies, we established that the 1309-1447 region is necessary and sufficient for interaction with ATG101. We next showed that the three most common PTCH1 CTD mutations in endometrial, stomach and colon adenocarcinomas that cause frameshifts at S1203, R1308 and Y1316 lack the ability to interact with ATG101 and limit autophagic flux, determined by bafilomycin A1-sensitive accumulation of the autophagy markers LC3BII and p62. We next engineered PTCH1 indel mutations at S1223 by CRISPR/Cas9 in SW620 colon cancer cells. Comparison of two independent clones harbouring PTCH1 S1223fs mutations to their isogenic parental cell lines expressing wild-type PTCH1 showed a significant increase in basal and rapamycin-stimulated autophagic flux, as predicted by loss of ATG101 interaction. Furthermore, the PTCH1 CTD mutant cells displayed increased proliferation in the presence of rapamycin and reduced sensitivity to glycolysis inhibitors. Our findings suggest that loss of the PTCH1-ATG101 interaction by mutations in the CTD of PTCH1 in cancer might confer a selective advantage by stimulating autophagy and facilitating adaptation to nutrient deprivation conditions.

Automated summary

We have discovered that the C-terminal domain (CTD) of the tumor suppressor PTCH1 interacts with ATG101 and affects autophagy. We found that this interaction is absent in certain types of cancer. Truncation mutants lacking this interaction show increased proliferation and reduced sensitivity to autophagy inducers or glycolysis inhibitors.