Downregulated expression of K⁺ channels and decreased K⁺ currents in pulmonary artery smooth muscle cells (PASMC) have been implicated in the development of sustained pulmonary vasoconstriction and vascular remodeling in patients with idiopathic pulmonary arterial hypertension (IPAH). However, it is unclear exactly how K⁺ channels are downregulated in IPAH-PASMC. MicroRNAs (miRNAs) are small non-coding RNAs that are capable of posttranscriptionally regulating gene expression by binding to the 3'-untranslated regions of their targeted mRNAs. Here, we report that specific miRNAs are responsible for the decreased K⁺ channel expression and function in IPAH-PASMC. We identified 3 miRNAs (miR-29b, miR-138, and miR-222) that were highly expressed in IPAH-PASMC in comparison to normal PASMC (>2.5-fold difference). Selectively upregulated miRNAs are correlated with the decreased expression and attenuated activity of K⁺ channels. Overexpression of miR-29b, miR-138, or miR-222 in normal PASMC significantly decreased whole cell K⁺ currents and downregulated voltage-gated K⁺ channel 1.5 (K_V1.5/KCNA5) in normal PASMC. Inhibition of miR-29b in IPAH-PASMC completely recovered K⁺ channel function and K_V1.5 expression, while miR-138 and miR-222 had a partial or no effect. Luciferase assays further revealed that K_V1.5 is a direct target of miR-29b. Additionally, overexpression of miR-29b in normal PASMC decreased large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel currents and downregulated BK_Ca channel β1 subunit (BK_Caβ1 or KCNMB1) expression, while inhibition of miR-29b in IPAH-PASMC increased BK_Ca channel activity and BK_Caβ1 levels. These data indicate upregulated miR-29b contributes at least partially to the attenuated function and expression of K_V and BK_Ca channels in PASMC from patients with IPAH.

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