ZnT1 induces a crosstalk between T-type and L-type calcium channels through interactions with Raf-1 kinase and the calcium channel ß2 subunit.

ZnT1 is a major zinc transporter that regulates cellular zinc homeostasis. We have previously shown that ZnT1 has additional functions that are independent of its activity as a Zn2+ extruder. These include inhibition of the L-type calcium channel (LTCC) through interaction with the auxiliary ß-subunit of the LTCC and activation of the Raf-ERK signaling leading to augmented activity of the T-type calcium channel (TTCC). Our findings indicate that ZnT1 increases TTCC activity by enhancing the trafficking of the channel to the plasma membrane. LTCC and TTCC are co-expressed in many tissues and have different functions in a variety of tissues. In the current work, we investigated the effect of the voltage-gated calcium channel (VGCC) ß-subunit and ZnT1 on the crosstalk between LTCC and TTCC and their functions. Our results indicate that the ß-subunit inhibits the ZnT1-induced augmentation of TTCC function. This inhibition correlates with the VGCC ß-subunit-dependent reduction in ZnT1-induced activation of Ras-ERK signaling. The effect of ZnT1 is specific, as the presence of the ß-subunit did not change the effect of endothelin-1 (ET-1) on TTCC surface expression. These findings document a novel regulatory function of ZnT1 serving as a mediator in the crosstalk between TTCC and LTCC. Overall, we demonstrate that ZnT1 binds and regulates the activity of the ß-subunit of VGCC and Raf-1 kinase and modulates surface expression of the LTCC and TTCC catalytic subunits, consequently modulating the activity of these channels.

ZnT-1 enhances the activity and surface expression of T-type calcium channels through activation of Ras-ERK signaling.

Zinc transporter-1 (ZnT-1) is a putative zinc transporter that confers cellular resistance from zinc toxicity. In addition, ZnT-1 has important regulatory functions, including inhibition of L-type calcium channels and activation of Raf-1 kinase. Here we studied the effects of ZnT-1 on the expression and function of T-type calcium channels. In Xenopus oocytes expressing voltage-gated calcium channel (CaV) 3.1 or CaV3.2, ZnT-1 enhanced the low-threshold calcium currents (I(caT)) to 182 ± 15 and 167.95 ± 9.27% of control, respectively (P < 0.005 for both channels). As expected, ZnT-1 also enhanced ERK phosphorylation. Coexpression of ZnT-1 and nonactive Raf-1 blocked the ZnT-1-mediated ERK phosphorylation and abolished the ZnT-1-induced augmentation of I(caT). In mammalian cells (Chinese hamster ovary), coexpression of CaV3.1 and ZnT-1 increased the I(caT) to 166.37 ± 6.37% compared with cells expressing CaV3.1 alone (P < 0.01). Interestingly, surface expression measurements using biotinylation or total internal reflection fluorescence microscopy indicated marked ZnT-1-induced enhancement of CaV3.1 surface expression. The MEK inhibitor PD-98059 abolished the ZnT-1-induced augmentation of surface expression of CaV3.1. In cultured murine cardiomyocytes (HL-1 cells), transient exposure to zinc, leading to enhanced ZnT-1 expression, also enhanced the surface expression of endogenous CaV3.1 channels. Consistently, in these cells, endothelin-1, a potent activator of Ras-ERK signaling, enhanced the surface expression of CaV3.1 channels in a PD-98059-sensitive manner. Our findings indicate that ZnT-1 enhances the activity of CaV3.1 and CaV3.2 through activation of Ras-ERK signaling. The augmentation of CaV3.1 currents by Ras-ERK activation is associated with enhanced trafficking of the channel to the plasma membrane.

ZnT-1 protects HL-1 cells from simulated ischemia-reperfusion through activation of Ras-ERK signaling.

Activation of ERK signaling may promote cardioprotection from ischemia-reperfusion (I/R) injury. ZnT-1, a protein that confers resistance from zinc toxicity, was found to interact with Raf-1 kinase through its C-terminal domain, leading to downstream activation of ERK. In the present study, we evaluated the effects of ZnT-1 in cultured murine cardiomyocytes (HL-1 cells) that were exposed to simulated-I/R. Cellular injury was evaluated by lactate dehydrogenase (LDH) release and by staining for pro-apoptotic caspase activation. Overexpression of ZnT-1 markedly reduced LDH release and caspase activation following I/R. Knockdown of endogenous ZnT-1 augmented the I/R-induced release of LDH and increased caspase activation following I/R. Phospho-ERK levels were significantly increased following I/R in cells overexpressing ZnT-1, while knockdown of ZnT-1 reduced phospho-ERK levels. Pretreatment of cells with the MEK inhibitor PD98059 abolished the protective effect of ZnT-1 following I/R. Accordingly, a truncated form of ZnT-1 lacking the C-terminal domain failed to induce ERK activation and did not protect the cells from I/R injury. In contrast, expression of the C-terminal domain by itself was sufficient to induce ERK activation and I/R protection. Interestingly, the C-terminal of the ZnT-1 did not have protective effect against the toxicity of zinc. In the isolated rat heart, global ischemic injury rapidly increased the endogenous levels of ZnT-1. However, following reperfusion ZnT-1 levels were found to be decreased. Our findings indicate that ZnT-1 may have important role in the ischemic myocardium through its ability to interact with Raf-1 kinase.