Key Points Diffuse large B-cell lymphomas are composed of clonogenic side population (SP) cells and non-SP cells organized in a dynamic equilibrium. Exosome-mediated Wnt signaling modulates transitions of cell states and tumor progression amenable to drug targeting.

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Tightly regulated activity of the transcription factor MYC is essential for orderly cell proliferation. Upon deregulation, MYC elicits and promotes cancer progression. Proteasomal degradation is an essential element of MYC regulation, initiated by phosphorylation at Serine62 (Ser62) of the MB1 region. Here we found that Ser62 phosphorylation peaks in mitosis, but that a fraction of non-phosphorylated MYC binds to the microtubules of the mitotic spindle. Consequently, the microtubule-destabilizing drug vincristine decreases wildtype MYC stability, while phosphorylation-deficient MYC is more stable, contributing to vincristine resistance and induction of polyploidy. PI3K inhibition attenuates post-mitotic MYC formation and augments the cytotoxic effect of vincristine.Implications: The spindle´s function as a docking site for MYC during mitosis may constitute a window of specific vulnerability to be exploited for cancer treatment.

Lamp 1 and lysosomal acid phosphatase (LAP) are lysosomal membrane proteins that harbour a tyrosine-based sorting motif within their short cytoplasmic tails. Lamp 1 is delivered from the trans-Golgi network (TGN) via endosomes directly to lysosomes bypassing the plasma membrane, whereas LAP is indirectly transported to lysosomes and recycles between endosomes and the plasma membrane before being delivered to lysosomes.By analysing truncated forms of LAP and chimeras in which the cytoplasmic tail or part of the cytoplasmic tails of LAP and Lamp 1 were exchanged, we were able to show that the YRHV tyrosine motif of LAP is necessary and sufficient to mediate recycling between endosomes and the plasma membrane. When peptides corresponding to the cytoplasmic tails of LAP and Lamp 1 and chimeric or mutant forms of these tails were assayed for in vitro binding of AP1 and AP2, we found that AP2 bound to LAP- and Lamp-1-derived peptides, whereas AP1 bound only to peptides containing the YQTI tyrosine motif of Lamp 1. Residues +2 and +3 of the tyrosine motif were critical for the differential binding of adaptors. LAP in which these residues (–HV) were substituted for those of Lamp 1 (–TI) was transported directly to lysosomes, whereas a chimera carrying the Lamp 1 tail in which residues +2 and +3 were substituted for those of LAP (–HV) gained the ability to recycle. In conclusion, the residues +2 and +3 of the tyrosine motifs determine the sorting of Lamp 1 and LAP in endosomes, mediating either the direct or the indirect pathway to lysosomes.