Regular Article
Inhibiting Proteasomes in Human HepG2 and LNCaP Cells Increases Endogenous Androgen Receptor Levels

https://doi.org/10.1006/bbrc.2000.3424Get rights and content

Abstract

Treating HepG2 cells with MG132 for 4 h to inhibit proteasomal activity increased androgen receptor immunoreactivity in two major bands with molecular weights of 102 and 110 kDa by 77% each (P < 0.05). MG132 treatment also increased the overall level of polyubiquitinated proteins between 66 and 220 kDa by 140% (P < 0.05). Antiubiquitin immunoreactivity comigrating with the androgen receptor bands was also increased by MG132 treatment. Two other proteasome inhibitors, lactacystin and epoxomycin, caused similar increases in the androgen receptor in HepG2 cells. Proteosome-inhibition studies conducted in LNCaP cells also showed that the two major androgen receptor bands with molecular weights of 102 and 110 kDa were increased by 85 and 115%, respectively (P < 0.05 for both) by MG132 treatment. Overall levels of polyubiquitinated proteins with molecular weights between 66 and 220 kDa increased 365%. Ubiquitin immunoreactivity comigrating with the androgen receptor bands was also significantly increased. Thus inhibiting proteasomes in two human androgen-responsive cell lines increases endogenous androgen receptor levels as well as androgen receptor-associated ubiquitin-modified immunoreactivity. The regulation of steady-state levels of endogenous androgen receptor by proteasomal degradation could be involved in its rapid turnover in the absence of ligand and would provide a mechanism for limiting androgen responses. A PEST sequence similar to one in the vitamin D receptor is present in the hinge region of all known mammalian androgen receptors, suggesting that it may function in proteasome-mediated androgen receptor turnover.

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      Androgen binding to the AR results in ∼6-fold increased AR half-life [34]. Direct evidence of AR degradation via the UPS was demonstrated by increased endogenous AR protein levels following treatment with the proteasome inhibitor MG-132 and detection of polyubiquitinated AR [35]. E3 ubiquitin ligases that have been reported to ubiquitinate AR proteins (Table 1), belong to two major families: the HECT (Homologous to the E6AP Carboxyl Terminus) and the RING (Really Interesting New Gene) ubiquitin E3 ligases [36,37].

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