The vascular smooth muscle T-type Ca2+ channel: An anti-proliferative target for heme oxygenase-1

Pathological proliferation of vascular smooth muscle cells (VSMC) is a central feature of vascular disorders such as atherosclerosis and restenosis. During such proliferative conditions the expression of the T-type Ca2+ channel is increased, providing an important route for Ca2+ entry. The inducible stress-response protein, heme oxygenase-1 (HO-1), is also up-regulated during vascular disorders. This enzyme confers cytoprotective effects via the breakdown of free heme to produce iron, biliverdin, and carbon monoxide (CO). CO has been shown to be anti-inflammatory, anti-apoptotic, and anti-proliferative at low concentrations. Furthermore, CO is emerging as a modulator of various ion channels, and our research group has recently found that CO inhibits the T-type Ca2+ current via whole-cell patch clamp recordings. Therefore, the aim of this thesis was to investigate whether the VSMC T-type Ca2+ channel could act as an anti-proliferative target for HO-1-derived CO.
HEK293 cells over-expressing the Cav3.2 T-type Ca2+ channel produced higher basal [Ca2+]i and displayed an augmented proliferative response. [Ca2+]i and proliferation were both reduced by T-type Ca2+ channel inhibition, CO exposure, and HO-1 induction. T-type Ca2+ channel inhibition and HO-1 induction reduced [Ca2+]i and proliferation in the rat aortic VSMC line, A7r5. Exogenous CO exposure decreased [Ca2+]i in A7r5 cells, but conferred insignificant anti-proliferative effects, which correlated to a relatively low expression of the T-type Ca2+ channel. T-type Ca2+ channel inhibition, CO exposure, and HO-1 induction all have anti-proliferative effects in human VSMCs, yet simultaneous HO-1 induction and T-type Ca2+ channel inhibition do not cause additive inhibitory effects on proliferation.
These data provide evidence that CO is anti-proliferative, and that CO potentially acts via the T-type Ca2+ channel. This pathway could be a novel therapeutic target for vascular disorders involving excessive smooth muscle cell proliferation.