Sucrose non-fermenting related kinase enzyme is essential for cardiac metabolism

In this study, we have identified a novel member of the AMPK family, namely Sucrose non-fermenting related kinase (Snrk), that is responsible for maintaining cardiac metabolism in mammals. SNRK is expressed in the heart, and brain, and in cell types such as endothelial cells, smooth muscle cells and cardiomyocytes (CMs). Snrk knockout (KO) mice display enlarged hearts, and die at postnatal day 0. Microarray analysis of embryonic day 17.5 Snrk hearts, and blood profile of neonates display defect in lipid metabolic pathways. SNRK knockdown CMs showed altered phospho-acetyl-coA carboxylase and phospho-AMPK levels similar to global and endothelial conditional KO mouse. Finally, adult cardiac conditional KO mouse displays severe cardiac functional defects and lethality. Our results suggest that Snrk is essential for maintaining cardiac metabolic homeostasis, and shows an autonomous role for SNRK during mammalian development.

SmgGDS-558 regulates the cell cycle in pancreatic, non-small cell lung, and breast cancers

Oncogenic mutation or misregulation of small GTPases in the Ras and Rho families can promote unregulated cell cycle progression in cancer. Post-translational modification by prenylation of these GTPases allows them to signal at the cell membrane. Splice variants of SmgGDS, named SmgGDS-607 and SmgGDS-558, promote the prenylation and membrane trafficking of multiple Ras and Rho family members, which makes SmgGDS a potentially important regulator of the cell cycle. Surprisingly little is known about how SmgGDS-607 and SmgGDS-558 affect cell cycle-regulatory proteins in cancer, even though SmgGDS is overexpressed in multiple types of cancer. To examine the roles of SmgGDS splice variants in the cell cycle, we compared the effects of the RNAi-mediated depletion of SmgGDS-558 vs. SmgGDS-607 on cell cycle progression and the expression of cyclin D1, p27, and p21 in pancreatic, lung, and breast cancer cell lines. We show for the first time that SmgGDS promotes proliferation of pancreatic cancer cells, and we demonstrate that SmgGDS-558 plays a greater role than SmgGDS-607 in cell cycle progression as well as promoting cyclin D1 and suppressing p27 expression in multiple types of cancer. Silencing both splice variants of SmgGDS in the cancer cell lines produces an alternative signaling profile compared with silencing SmgGDS-558 alone. We also show that loss of both SmgGDS-607 and SmgGDS-558 simultaneously decreases tumorigenesis of NCI-H1703 non-small cell lung carcinoma (NSCLC) xenografts in mice. These findings indicate that SmgGDS promotes cell cycle progression in multiple types of cancer, making SmgGDS a valuable target for cancer therapeutics.

Characterization of the threshold for NAD(P)H:quinone oxidoreductase activity in intact sulforaphane-treated pulmonary arterial endothelial cells

Treatment of bovine pulmonary arterial endothelial cells in culture with the phase II enzyme inducer sulforaphane (5μM, 24h; sulf-treated) increased cell-lysate NAD(P)H:quinone oxidoreductase (NQO1) activity by 5.7 ± 0.6 (mean ± SEM)-fold, but intact-cell NQO1 activity by only 2.8 ± 0.1-fold compared to control cells. To evaluate the hypothesis that the threshold for sulforaphane-induced intact-cell NQO1 activity reflects a limitation in the capacity to supply NADPH at a sufficient rate to drive all the induced NQO1 to its maximum activity, total KOH-extractable pyridine nucleotides were measured in cells treated with duroquinone to stimulate maximal NQO1 activity. NQO1 activation increased NADP(+) in control and sulf-treated cells, with the effect more pronounced in the sulf-treated cells, in which the NADPH was also decreased. Glucose-6-phosphate dehydrogenase (G-6-PDH) inhibition partially blocked NQO1 activity in control and sulf-treated cells, but G-6-PDH overexpression via transient transfection with the human cDNA alleviated neither the restriction on intact sulf-treated cell NQO1 activity nor the impact on the NADPH/NADP(+) ratios. Intracellular ATP levels were not affected by NQO1 activation in control or sulf-treated cells. An increased dependence on extracellular glucose and a rightward shift in the K(m) for extracellular glucose were observed in NQO1-stimulated sulf-treated vs control cells. The data suggest that glucose transport in the sulf-treated cells may be insufficient to support the increased metabolic demand for pentose phosphate pathway-generated NADPH as an explanation for the NQO1 threshold.

Splice variants of SmgGDS control small GTPase prenylation and membrane localization

Ras and Rho small GTPases possessing a C-terminal polybasic region (PBR) are vital signaling proteins whose misregulation can lead to cancer. Signaling by these proteins depends on their ability to bind guanine nucleotides and their prenylation with a geranylgeranyl or farnesyl isoprenoid moiety and subsequent trafficking to cellular membranes. There is little previous evidence that cellular signals can restrain nonprenylated GTPases from entering the prenylation pathway, leading to the general belief that PBR-possessing GTPases are prenylated as soon as they are synthesized. Here, we present evidence that challenges this belief. We demonstrate that insertion of the dominant negative mutation to inhibit GDP/GTP exchange diminishes prenylation of Rap1A and RhoA, enhances prenylation of Rac1, and does not detectably alter prenylation of K-Ras. Our results indicate that the entrance and passage of these small GTPases through the prenylation pathway is regulated by two splice variants of SmgGDS, a protein that has been reported to promote GDP/GTP exchange by PBR-possessing GTPases and to be up-regulated in several forms of cancer. We show that the previously characterized 558-residue SmgGDS splice variant (SmgGDS-558) selectively associates with prenylated small GTPases and facilitates trafficking of Rap1A to the plasma membrane, whereas the less well characterized 607-residue SmgGDS splice variant (SmgGDS-607) associates with nonprenylated GTPases and regulates the entry of Rap1A, RhoA, and Rac1 into the prenylation pathway. These results indicate that guanine nucleotide exchange and interactions with SmgGDS splice variants can regulate the entrance and passage of PBR-possessing small GTPases through the prenylation pathway.