Critical role of Ca3.2 T-type calcium channels in the peripheral neuropathy induced by bortezomib, a proteasome-inhibiting chemotherapeutic agent, in mice.

Bortezomib, a first-line agent for treatment of multiple myeloma, exhibits anticancer activity through proteasome inhibition. However, bortezomib-induced peripheral neuropathy (BIPN) is one of the most serious side effects. Since decreased proteasomal degradation of Ca3.2 T-type calcium channels in the primary afferents is involved in persistent pain, we investigated whether BIPN involves increased protein levels of Ca3.2 in mice. Six repeated i.p. administrations of bortezomib for 12 days developed persistent mechanical allodynia. Systemic administration of novel T-type calcium channel blockers, (2R/S)-6-prenylnaringenin and KTt-45, and of TTA-A2, the well-known blocker, reversed the BIPN. Ascorbic acid, known to block Ca3.2, but not Ca3.1 or 3.3, and silencing of Ca3.2 gene also suppressed BIPN. Protein levels of Ca3.2 in the dorsal root ganglion (DRG) at L4-L6 levels increased throughout days 1-21 after the onset of bortezomib treatment. Protein levels of USP5, a deubiquitinating enzyme that specifically inhibits proteasomal degradation of Ca3.2, increased in DRG on days 3-21, but not day 1, in bortezomib-treated mice. In DRG-derived ND7/23 cells, bortezomib increased protein levels of Ca3.2 and T-channel-dependent currents, as assessed by a patch-clamp method, but did not upregulate expression of Ca3.2 mRNA or USP5 protein. MG-132, another proteasome inhibitor, also increased Ca3.2 protein levels in the cultured cells. Given the previous evidence for USP5 induction following nociceptor excitation, our data suggest that BIPN involves the increased protein levels of Ca3.2 in nociceptors through inhibition of proteasomal degradation of Ca3.2 by bortezomib itself and then by USP5 that is upregulated probably in an activity-dependent manner.