Ubiquitin-conjugating enzyme 3 delays human lens epithelial cells in metaphase

PURPOSE Ubc3/Cdc34 is a ubiquitin-conjugating enzyme (Ubc) with well established functions in the G1-to-S-phase transition. Expecting to find similar effects in human lens epithelial cells (HLECs), the authors explored roles for this ubiquitin-conjugating enzyme in regulation of the HLEC cycle.
METHODS Catalytically incompetent Ubc3 (C88S, L97S), wild-type (wt)Ubc3, and mutant (mt)Ubc2 (C93A) were expressed in HLECs, by using an adenoviral vector, and cell cycle progression was assessed.
RESULTS Expression of mt- and wtUbc3, but not empty virus or {mtUbc2,} delayed the cell cycle in metaphase, rather than the expected G1 phase. Expression of both Ubc3s also stabilized M-phase regulators, cyclin A, cyclin B, and securin. Thus, it appeared that the Ubc3 enzymes were playing roles different from canonical proteolytic functions in targeting G1/S regulators for degradation. We also directly investigated the effect of inhibiting the proteasome on the cell cycle of HLECs. When the proteasome inhibitor was added to S-phase cells, the M-phase regulators were stabilized, and the cells were arrested in the G2/M phase. In contrast, if the proteasome inhibitor was added before the cells entered the S phase, stabilization of the G1 kinase inhibitors p21WAF and p27KIP was observed and the cells were arrested in the G1 phase.
CONCLUSIONS The ubiquitin-proteasome pathway is involved in regulation of transitions between all phases of the HLEC cycle. However, in contrast with previously described roles for Ubc3 in governing G1/S transitions, expression of Ubc3 delays the HLEC cycle in metaphase. The data suggest novel roles for Ubc3 that do not involve the transfer of ubiquitin in the M phase in the HLEC cell cycle.