cCalpains regulate a wide spectrum of biological functions, including migration, adhesion, apoptosis, secretion, and autophagy, through the modulating cleavage of specific substrates. Ubiquitous microcalpain (-calpain) and millicalpain (m-calpain) are heterodimers composed of catalytic subunits encoded, respectively, by CAPN1 and CAPN2 and a regulatory subunit encoded by CAPNS1. Here we show that calpain is required for the stability of the deubiquitinating enzyme USP1 in several cell lines. USP1 modulates DNA replication polymerase choice and repair by deubiquitinating PCNA. The ubiquitinated form of the USP1 substrate PCNA is stabilized in CAPNS1-depleted U2OS cells and mouse embryonic fibroblasts (MEFs), favoring polymeraseloading on chromatin and increased mutagenesis. USP1 degradation directed by the cell cycle regulator APC/C cdh1 , which marks USP1 for destruction in the G 1 phase, is upregulated in CAPNS1-depleted cells. USP1 stability can be rescued upon forced expression of calpain-activated Cdk5/p25, previously reported as a cdh1 repressor. These data suggest that calpain stabilizes USP1 by activating Cdk5, which in turn inhibits cdh1 and, consequently, USP1 degradation. Altogether these findings point to a connection between the calpain system and the ubiquitin pathway in the regulation of DNA damage response and place calpain at the interface between cell cycle modulation and DNA repair. Calpains regulate a wide spectrum of biological functions, including migration, adhesion, apoptosis, secretion, and autophagy through the modulated cleavage of specific substrates (reviewed in references 1-3). Ubiquitous microcalpain (-calpain) and millicalpain (m-calpain) are heterodimers composed of a catalytic subunit encoded, respectively, by CAPN1 and CAPN2 and a regulatory subunit encoded by CAPNS1. Both -calpain and mcalpain are negatively modulated by calpastatin. By a proteomic approach, we identified USP1 deubiquitinase as a CAPNS1-interacting protein. USP1 is a key modulator of DNA repair, partly through deubiquitination of its known targets FANCD2 (4, 5) and PCNA (6). Usp1 knockout (KO) mice have a severe phenotype and die soon after birth (7). Usp1 Ϫ/Ϫ cells are defective in FANCD2 focus formation and are hypersensitive to DNA damage (8). PCNA ubiquitination is higher in USP1-depleted cells than in control cells, thus leading to recruitment of error-prone, translesion DNA synthesis (TLS) polymerases and the consequent increase in mutation rate (6). USP1 promotes inhibitor of DNA binding (ID) protein stability and stem cell-like characteristics in osteosarcoma and is required for normal skeletogenesis (9). Interestingly, mice lacking CAPNS1 in cells of the osteoblast lineage are defective in bone development and remodeling in vivo (10).UV light irradiation activates hUSP1 autocleavage at glycines 670 and 671, inducing its subsequent proteasomal degradation (6). USP1 mutants with mutation in the catalytic cysteine 90 or in the autocleavage sites are more stable but can still be degraded in the cell (5), s...

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CAPNS1 is essential for the stability and function of ubiquitous CAPN1 and CAPN2. Calpain modulates by proteolytic cleavage many cellular substrates and its activity is often deregulated in cancer cells, therefore calpain inhibition has been proposed as a therapeutical strategy for a number of malignancies. Here we show that CAPNS1 depletion is coupled to impairment of MCF7 and MCF10AT cell lines growth on plate and defective architecture of mammary acini derived from MCF10A cells. In soft agar CAPNS1 depletion leads to cell growth increase in MCF7, and decrease in MCF10AT cells. In both MCF7 and MCF10AT, CAPNS1 depletion leads to the enlargement of the stem cell compartment, as demonstrated by mammosphere formation assays and evaluation of stem cell markers by means of FACS and western blot analysis. Accordingly, activation of calpain by thapsigargin treatment leads to a decrease in the stem cell reservoir. The expansion of the cancer stem cell population in CAPNS1 depleted cells is coupled to a defective shift from symmetric to asymmetric division during mammosphere growth coupled to a decrease in NUMB protein level.