Soluble α-klotho and heparin modulate the pathologic cardiac actions of fibroblast growth factor 23 in chronic kidney disease

Fibroblast growth factor (FGF) 23 is a phosphate-regulating hormone that is elevated in patients with chronic kidney disease and associated with cardiovascular mortality. Experimental studies showed that elevated FGF23 levels induce cardiac hypertrophy by targeting cardiac myocytes via FGF receptor isoform 4 (FGFR4). A recent structural analysis revealed that the complex of FGF23 and FGFR1, the physiologic FGF23 receptor in the kidney, includes soluble α-klotho (klotho) and heparin, which both act as co-factors for FGF23/FGFR1 signaling. Here, we investigated whether soluble klotho, a circulating protein with cardio-protective properties, and heparin, a factor that is routinely infused into patients with kidney failure during the hemodialysis procedure, regulate FGF23/FGFR4 signaling and effects in cardiac myocytes. We developed a plate-based binding assay to quantify affinities of specific FGF23/FGFR interactions and found that soluble klotho and heparin mediate FGF23 binding to distinct FGFR isoforms. Heparin specifically mediated FGF23 binding to FGFR4 and increased FGF23 stimulatory effects on hypertrophic growth and contractility in isolated cardiac myocytes. When repetitively injected into two different mouse models with elevated serum FGF23 levels, heparin aggravated cardiac hypertrophy. We also developed a novel procedure for the synthesis and purification of recombinant soluble klotho, which showed anti-hypertrophic effects in FGF23-treated cardiac myocytes. Thus, soluble klotho and heparin act as independent FGF23 co-receptors with opposite effects on the pathologic actions of FGF23, with soluble klotho reducing and heparin increasing FGF23-induced cardiac hypertrophy. Hence, whether heparin injections during hemodialysis in patients with extremely high serum FGF23 levels contribute to their high rates of cardiovascular events and mortality remains to be studied.

An AKAP-Lbc-RhoA interaction inhibitor promotes the translocation of aquaporin-2 to the plasma membrane of renal collecting duct principal cells

Stimulation of renal collecting duct principal cells with antidiuretic hormone (arginine-vasopressin, AVP) results in inhibition of the small GTPase RhoA and the enrichment of the water channel aquaporin-2 (AQP2) in the plasma membrane. The membrane insertion facilitates water reabsorption from primary urine and fine-tuning of body water homeostasis. Rho guanine nucleotide exchange factors (GEFs) interact with RhoA, catalyze the exchange of GDP for GTP and thereby activate the GTPase. However, GEFs involved in the control of AQP2 in renal principal cells are unknown. The A-kinase anchoring protein, AKAP-Lbc, possesses GEF activity, specifically activates RhoA, and is expressed in primary renal inner medullary collecting duct principal (IMCD) cells. Through screening of 18,431 small molecules and synthesis of a focused library around one of the hits, we identified an inhibitor of the interaction of AKAP-Lbc and RhoA. This molecule, Scaff10-8, bound to RhoA, inhibited the AKAP-Lbc-mediated RhoA activation but did not interfere with RhoA activation through other GEFs or activities of other members of the Rho family of small GTPases, Rac1 and Cdc42. Scaff10-8 promoted the redistribution of AQP2 from intracellular vesicles to the periphery of IMCD cells. Thus, our data demonstrate an involvement of AKAP-Lbc-mediated RhoA activation in the control of AQP2 trafficking.

Abstract 127: Phosphorylation of Serum Responsive Factor by p90 Ribosomal S6 Kinase 3 Promotes Concentric Growth of Cardiac Myocytes

p90 Ribosomal S6 Kinase 3 (RSK3) is required for the induction of concentric hypertrophy in hearts subjected to pressure overload and catecholamine infusion, as well as in a model for Noonan Syndrome-associated hypertrophic cardiomyopathy. Serving important roles in both cardiac development and adult function, the transcription factor Serum Responsive Factor (SRF) regulates genes expression involved in myocyte growth and sarcomeric assembly. It has been reported that SRF can be phosphorylated by RSK protein kinases on residue Ser-103, but the function of that post-translational modification has remained uncertain. We now show that SRF is a substrate for RSK3 associated with muscle A-Kinase-Anchoring-Protein (mAKAP) scaffold in cardiac myocytes, contributing to the regulation of concentric myocyte growth. By co-immunoprecipitation assay, SRF and RSK3 were both associated with the mAKAP scaffold in heart extracts, such that a ternary complex could be detected when recombinant proteins were expressed in cells. Silencing RSK3 or mAKAP expression by siRNA transfection reduced phenylephrine-induced SRF(S103) phosphorylation in neonatal cardiac myocytes. Similar results were acquired following expression of a peptide comprising the mAKAP RSK3-binding domain (RBD) that competed the binding of the kinase to the scaffold. Overexpression of either a SRF S103A phosphoablative mutant or the RBD peptide diminished the increase in width of adult cardiac myocytes stimulated by phenylephrine. In contrast, overexpression of RSK3 or a SRF S103D phosphomimetic mutant promoted adult cardiac myocyte concentric hypertorphy in vitro. While baseline SRF S103 phosphorylation in the heart was inhibited by mAKAP or RSK3 gene knock-out in mice, acute transverse aortic constriction significantly increased SRF S103 phosphorylation in the heart. Based upon these results, we porpose that RSK3 phosphorylation of SRF at mAKAP signalosomes is an important regulator of concentric cardiac myocyte growth. These results are consistent with additional findings that an adeno-associated virus gene therapy vector that expresses RBD in the cardiac mycoyte attenuates pathological hypertrophy and prevents heart failure in response to pressure overload.

A pituitary POU domain protein, Pit-1, activates both growth hormone and prolactin promoters transcriptionally

The anterior pituitary gland provides a model for investigating the molecular basis for the appearance of phenotypically distinct cell types within an organ, a central question in development. The rat prolactin and growth hormone genes are expressed selectively in distinct cell types (lactotrophs and somatotrophs, respectively) of the anterior pituitary gland, reflecting differential mechanisms of gene activation or restriction, as a result of the interactions of multiple factors binding to these genes. We find that when the pituitary-specific 33-kD transcription factor Pit-1, expressed normally in both lactotrophs and somatotrophs, is expressed in either the heterologous HeLa cell line or in bacteria, it binds to and activates transcription from both growth hormone and prolactin promoters in vitro at levels even 10-fold lower than those normally present in pituitary cells. This suggests that a single factor, Pit-1, may be capable of activating the expression of two genes that define different anterior pituitary cell phenotypes. Because a putative lactotroph cell line (235-1) that does not express the growth hormone gene, but only the prolactin gene, appears to contain high levels of functional Pit-1, a mechanism selectively preventing growth hormone gene expression may, in part, account for the lactotroph phenotype.