Modest increase of KIF11 expression exposes fragilities in the mitotic spindle, causing chromosomal instability

19S proteasome loss causes monopolar spindles through ubiquitin-independent KIF11 degradation

To direct regulated protein degradation, the 26S proteasome recognizes ubiquitinated substrates through its 19S particle and then degrades them in the 20S enzymatic core. Despite this close interdependency between proteasome subunits, we demonstrate that knockouts from different proteasome subcomplexes result in distinct highly cellular phenotypes. In particular, depletion of 19S PSMD lid proteins, but not that of other proteasome subunits, prevents bipolar spindle assembly during mitosis, resulting in a mitotic arrest. We find that the monopolar spindle phenotype is caused by ubiquitin-independent proteasomal degradation of the motor protein KIF11 upon loss of 19S proteins. Thus, negative regulation of 20S-mediated proteasome degradation is essential for mitotic progression and 19S and 20S proteasome components can function independently outside of the canonical 26S structure. This work reveals a role for the proteasome in spindle formation and identifies the effects of ubiquitin-independent degradation on cell cycle control.

In vitro and in vivo induction of ochratoxin A exposure-related micronucleus formation in rat proximal tubular epithelial cells and expression profiling of chromosomal instability-related genes

Ochratoxin A (OTA) is a renal carcinogen in rats, and repeated administration induces karyomegaly in proximal tubular epithelial cells (PTECs) of the outer stripe of the outer medulla (OSOM) before inducing proliferative lesions. To investigate whether OTA induces micronuclei (MN) in PTECs, we performed an in vitro MN assay using rat renal NRK-52E PTECs after treatment for ≤21 days, and an in vivo OSOM MN assay in rats treated with OTA, other renal carcinogens, or non-carcinogenic renal toxicants for 4 or 13 weeks. The in vitro assay revealed an increased frequency of micronucleated cells from the acceptable dose level for cell viability, even after 21 days of treatment. The in vivo assay also revealed a dose- and treatment period-dependent increase in PTECs with γ-H2AX+ MN. OTA-specific gene expression profiling by OSOM RNA sequencing after week 13 revealed the altered expression of genes related to microtubule-kinetochore binding, the kinesin superfamily, centriole assembly, DNA damage repair, and cell cycle regulation. MN formation was also observed with other renal carcinogens that induce karyomegaly similarly to OTA. These results imply that γ-H2AX+ MN formation by OTA treatment is related to the induction of chromosomal instability accompanying karyomegaly formation before proliferative lesions form, providing a new insight into the carcinogenic mechanism that may be relevant to humans.