PKGs are serine/threonine kinases. PKG1 has two isoforms—PKG1α and β. Inositol trisphosphate receptor (IP3R)-associated cGMP-kinase substrate 1 (IRAG1) is a substrate for PKG1β. IRAG1 is also known to further interact with IP3RI, which mediates intracellular Ca2+ release. However, the role of IRAG1 in PH is not known. Herein, WT and IRAG1 KO mice were kept under normoxic or hypoxic (10% O2) conditions for five weeks. Animals were evaluated for echocardiographic variables and went through right heart catheterization. Animals were further sacrificed to prepare lungs and right ventricular (RV) for immunostaining, western blotting, and pulmonary artery smooth muscle cell (PASMC) isolation. IRAG1 is expressed in PASMCs and downregulated under hypoxic conditions. Genetic deletion of IRAG1 leads to RV hypertrophy, increase in RV systolic pressure, and RV dysfunction in mice. Absence of IRAG1 in lung and RV have direct impacts on PKG1β expression. Attenuated PKG1β expression in IRAG1 KO mice further dysregulates other downstream candidates of PKG1β in RV. IRAG1 KO mice develop PH spontaneously. Our results indicate that PKG1β signaling via IRAG1 is essential for the homeostasis of PASMCs and RV. Disturbing this signaling complex by deleting IRAG1 can lead to RV dysfunction and development of PH in mice.
The scaffold protein alpha-syntrophin (SNTA) is a component of the dystrophin glycoprotein complex and has been comprehensively studied in skeletal muscle and adipocytes. SNTA is further expressed in the liver where its biological role remains unclear. Unpublished data from our group suggested that SNTA deficiency is associated with altered tubulin alpha 8 (TUBA8) levels in fat. TUBA8 is highly expressed in different cell lines including hepatoma cells, and here we analyzed whether SNTA has a role herein. In Hepa1-6 cells, TUBA8 protein levels were increased upon SNTA knock down and were reduced upon overexpression of SNTA. This regulation was not identified when analyzing mRNA expression. In the liver of SNTA-deficient mice, TUBA8 protein was higher compared to the respective wild-type controls while RNA expression was even suppressed. Using the HaloTag platform, TUBA8 was found to form a complex with SNTA in Hepa1-6 cells. In the hepatic stellate cell line LX-2, the lack or overexpression of SNTA did, however, not change TUBA8 protein expression. SNTA and TUBA8 are described to regulate cell proliferation. Yet, knock down of SNTA did neither affect proliferation nor viability of Hepa1-6 cells. The present study shows that SNTA protein levels are inversely related to TUBA8 protein expression in the hepatocyte cell line Hepa1-6.
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