The role of thioredoxin and T-type Ca2+ channels in vascular smooth muscle cell proliferation

Elevated vascular smooth muscle cell (VSMC) proliferation is a feature of various cardiovascular conditions including restenosis, abdominal aortic aneurysm (AAA) and atherosclerosis. Voltage-gated T-type Ca2+ channels are implicated in VSMC proliferation as their expression is markedly up-regulated in proliferative phases of the VSMC cell-cycle (Kuga et al., 1996). The Thioredoxin (Trx) system is also associated with proliferative disorders of the heart and vasculature, e.g. Trx concentrations are elevated in AAA (Martinez-Pinna et al., 2010) and atherosclerosis (Okuda et al., 2001). Trx has recently been shown to regulate T-type Ca2+channels (Boycott et al., 2013). This PhD has investigated the hypothesis that VSMC proliferation is modulated by interactions between Trx and T-type Ca2+ channels.
Proliferation assays revealed that the T-type Ca2+ channel inhibitor NNC55-0396 (NNC, 1-3μM) decreased A7r5, HEK293/CaV3.1 and HEK293/CaV3.2, but not wt HEK293, cell proliferation. In contrast the L-type Ca2+ channel inhibitor nifedipine (2μM) was without effect. The Trx inhibitors PX-12 (1μM) and auranofin (AuF, 300nM) preferentially inhibited the proliferation of CaV3.2-expressing cells, i.e. A7r5 and HEK293/CaV3.2 cells. Basal Ca2+ influx in A7r5 cells was also significantly reduced by NNC (3μM) and AuF (3μM). Whole-cell patch-clamp recordings in recombinant cells revealed that PX-12 (1-300μM) inhibited CaV3.1 and CaV3.2 currents with similar sensitivities. In contrast, Trx (4μg.ml-1) enhanced CaV3.2, but not CaV3.1, peak current amplitude. Similarly, AuF (3μM) selectively reduced the current-density of HEK293/CaV3.2 cells. Data suggest that CaV3.2 channels are positively and selectively regulated by Trx, yet PX-12 could inhibit T-type Ca2+ channels independently of Trx. The sensitivity of CaV3.2 channels to Trx was found to be dependent on an extracellular histidine residue at position 191 (H191), especially as mutation to a glutamine (Q) residue (H191Q) abolished Trx-sensitivity. In summary, these data indicate that interactions between Trx and CaV3.2 channels can regulate the proliferation of CaV3.2–expressing cells.