Microautophagy regulated by STK38 and GABARAPs is essential to repair lysosomes and prevent aging.

Lysosomes are degradative organelles and signaling hubs that maintain cell and tissue homeostasis, and lysosomal dysfunction is implicated in aging and reduced longevity. Lysosomes are frequently damaged, but their repair mechanisms remain unclear. Here, we demonstrate that damaged lysosomal membranes are repaired by microautophagy (a process termed "microlysophagy") and identify key regulators of the first and last steps. We reveal the AGC kinase STK38 as a novel microlysophagy regulator. Through phosphorylation of the scaffold protein DOK1, STK38 is specifically required for the lysosomal recruitment of the AAA+ ATPase VPS4, which terminates microlysophagy by promoting the disassembly of ESCRT components. By contrast, microlysophagy initiation involves non-canonical lipidation of ATG8s, especially the GABARAP subfamily, which is required for ESCRT assembly through interaction with ALIX. Depletion of STK38 and GABARAPs accelerates DNA damage-induced cellular senescence in human cells and curtails lifespan in C. elegans, respectively. Thus, microlysophagy is regulated by STK38 and GABARAPs and could be essential for maintaining lysosomal integrity and preventing aging.

Visualization of Proliferative Vascular Endothelial Cells in Tumors in Vivo by Imaging Their Partner of Sld5-1 Promoter Activity.

Vascular endothelial cells (ECs) isolated from tumors characteristically express certain genes. It has recently been suggested that tumor vessel normalization facilitates effective drug delivery into tumors; however, how tumor vessel normalization can be recognized on the basis of the molecules expressed by tumor ECs is not clearly defined. The degree of cell proliferation is an important indicator to characterize the condition of the ECs. Herein, we generated transgenic mice expressing enhanced green fluorescent protein (EGFP) under the transcriptional control of the DNA replication factor partner of Sld5-1 (PSF1; official name GINS1) promoter to assess whether active ECs can be distinguished from dormant ECs. Predictably, ECs in the adult skin exhibited no EGFP signals. However, after s.c. injection of tumor cells, some ECs shifted to EGFP positivity, enabling distinction of EGFP-positive from EGFP-negative cells. We found that only a fraction of the EGFP-negative ECs strongly expressed the glycosylphosphatidylinositol-anchor protein CD109 associated with the phosphatidylinositol 3-kinase pathway. Taken together, these data indicate that areas of vascular normalization in tumors can be detected by CD109 expression, and this provides a window of opportunity for timing chemotherapy.

Elevated expression of Tie1 is accompanied by acquisition of cancer stemness properties in colorectal cancer.

The Tie receptors 1 and 2 (Tie1/2) play crucial roles in embryonic angiogenesis. Recent studies suggest enhanced expression of Tie1 in several types of cancer and negative correlations between Tie1 levels and clinical outcome. These observations suggest important functions of Tie1 not only for vascular formation but also in tumorigenesis. Ligands for Tie2, that is angiopoietins 1-4, have been identified, but not for Tie1. To determine the molecular functions of Tie1, its detailed characterization in tumors would be helpful. Herein, we report that Tie1 is up-regulated in colorectal cancer. Detailed analysis using tumor-bearing models and immunohistochemistry combined with Flow cytometric analysis and cell sorting (FACS) revealed that Tie1 protein was expressed in a small population of malignant tumor cells. Intriguingly, Tie1 expression was observed and could be maintained only in vivo. Further analysis using sphere-formation culture revealed that Tie1-positive cells are enriched within the population of tumor cells with cancer stemness properties. Indeed, Tie1-positive tumor cells derived from a murine model overexpressed Lgr5, a typical stemness marker for colorectal cancer. Our results provide a novel insight into Tie1 function in tumorigenesis and suggest clinical applications to target cancer stem cells.