Lysosome Associated Protein Transmembrane 4B-24 Is the Predominant Protein Isoform in Human Tissues and Undergoes Rapid, Nutrient-Regulated Turnover

Abstract

Studies of lysosome associated protein transmembrane 4B (LAPTM4B) have mainly focused on the 35-kDa isoform and its association with poor prognosis in cancers. Here, by employing a novel monoclonal antibody, the authors found that the 24-kDa LAPTM4B isoform predominated in most, both healthy and malignant, human cells and tissues studied. LAPTM4B-24 lacks the extreme N-terminus and, contrary to LAPTM4B-35, failed to promote cell migration. The endogenous LAPTM4B-24 protein was subject to rapid turnover with a t 1/2 of approximately 1 hour. The protein was degraded by both lysosomal and proteasomal pathways, and its levels were increased by the availability of nutrients and lysosomal ceramide. These findings underscore the pathophysiological relevance of the LAPTM4B-24 isoform and identify it as a dynamically regulated effector in lysosomal nutrient signaling.

LAPTM4B‐35 promotes cancer cell migration via stimulating integrin beta1 recycling and focal adhesion dynamics

“…We and others have shown LAPTM4B‐35 promotes cancer cell migration in vitro 15,20,30 . Several studies show that LAPTM4B‐35 fosters cancer cell metastasis in vivo by utilizing cells overexpressing LAPTM4B‐35 in a WT background 17,31 .…”

“…We have shown LAPTM4B isoforms to display a distinct distribution, both isoforms localized on endosomes; LAPTM4B‐35, however, displays additional plasma membrane distribution and has undergone upregulation in several cancer patients' samples 20 . By utilizing the isoform‐expressing cells from a knockout (KO) background which we established in a previous study, 20 we firstly measured protein expression level and cell migration.…”

“…We have shown LAPTM4B isoforms to display a distinct distribution, both isoforms localized on endosomes; LAPTM4B‐35, however, displays additional plasma membrane distribution and has undergone upregulation in several cancer patients' samples 20 . By utilizing the isoform‐expressing cells from a knockout (KO) background which we established in a previous study, 20 we firstly measured protein expression level and cell migration. LAPTM4B‐35–expressing cells display much higher migratory ability than LAPTM4B‐24–expressing cells, even though the protein expression levels are comparable (Figure A,B), consistent with our previous finding 20 .…”

“…By utilizing the isoform‐expressing cells from a knockout (KO) background which we established in a previous study, 20 we firstly measured protein expression level and cell migration. LAPTM4B‐35–expressing cells display much higher migratory ability than LAPTM4B‐24–expressing cells, even though the protein expression levels are comparable (Figure A,B), consistent with our previous finding 20 . To further confirm the effect is not dependent on protein expression level, we generated more cell lines with higher LAPTM4B‐24 level and found LAPTM4B‐24 displays no effect on cell migration (Figure A,B).…”

LAPTM4B controls the sphingolipid and ether lipid signature of small extracellular vesicles

“…1H -I), while similar amounts of L4B and ATM3 were detected in the medium at 48 h, suggesting that SLim affects L4B secretion dynamics. The mechanism behind the diminished L4B secretion after 24 h is not known, but may be related to its high turnover and downregulation under starvation conditions [53]. In contrast, secretion of the established exosome markers CD63 and CD81 [54], was similar from L4B and ATM3 expressing cells at both time points (Fig.…”