Tyrosine phosphorylation of RACK1 triggers cardiomyocyte hypertrophy by regulating the interaction between p300 and GATA4.

The zinc finger protein GATA4 is a transcription factor involved in cardiomyocyte hypertrophy. It forms a functional complex with the intrinsic histone acetyltransferase (HAT) p300. The HAT activity of p300 is required for the acetylation and transcriptional activity of GATA4, as well as for cardiomyocyte hypertrophy and the development of heart failure. In the present study, we have identified Receptor for Activated Protein Kinase C1 (RACK1) as a novel GATA4-binding protein using tandem affinity purification and mass spectrometry analyses. We found that exogenous RACK1 repressed phenylephrine (PE)-induced hypertrophic responses, such as myofibrillar organization, increased cell size, and hypertrophy-associated gene transcription, in cultured cardiomyocytes. RACK1 physically interacted with GATA4 and the overexpression of RACK1 reduced PE-induced formation of the p300/GATA4 complex and the acetylation and DNA binding activity of GATA4. In response to hypertrophic stimulation in cultured cardiomyocytes and in the hearts of hypertensive heart disease model rats, the tyrosine phosphorylation of RACK1 was increased, and the binding between GATA4 and RACK1 was reduced. In addition, the tyrosine phosphorylation of RACK1 was required for the disruption of the RACK1/GATA4 complex and for the formation of the p300/GATA4 complex. These findings demonstrate that RACK1 is involved in p300/GATA4-dependent hypertrophic responses in cardiomyocytes and is a promising therapeutic target for heart failure.

A novel UBE2T inhibitor suppresses Wnt/ß-catenin signaling hyperactivation and gastric cancer progression by blocking RACK1 ubiquitination.

Dysregulation of the Wnt/ß-catenin signaling pathway is critically involved in gastric cancer (GC) progression. However, current Wnt pathway inhibitors being studied in preclinical or clinical settings for other cancers such as colorectal and pancreatic cancers are either too cytotoxic or insufficiently efficacious for GC. Thus, we screened new potent targets from ß-catenin destruction complex associated with GC progression from clinical samples, and found that scaffolding protein RACK1 deficiency plays a significant role in GC progression, but not APC, AXIN, and GSK3ß. Then, we identified its upstream regulator UBE2T which promotes GC progression via hyperactivating the Wnt/ß-catenin signaling pathway through the ubiquitination and degradation of RACK1 at the lysine K172, K225, and K257 residues independent of an E3 ligase. Indeed, UBE2T protein level is negatively associated with prognosis in GC patients, suggesting that UBE2T is a promising target for GC therapy. Furthermore, we identified a novel UBE2T inhibitor, M435-1279, and suggested that M435-1279 acts inhibit the Wnt/ß-catenin signaling pathway hyperactivation through blocking UBE2T-mediated degradation of RACK1, resulting in suppression of GC progression with lower cytotoxicity in the meantime. Overall, we found that increased UBE2T levels promote GC progression via the ubiquitination of RACK1 and identified a novel potent inhibitor providing a balance between growth inhibition and cytotoxicity as well, which offer a new opportunity for the specific GC patients with aberrant Wnt/ß-catenin signaling.

Rack1 mediates Src binding to drug transporter P-glycoprotein and modulates its activity through regulating Caveolin-1 phosphorylation in breast cancer cells.

The failure of chemotherapy and the emergence of multidrug resistance (MDR) are the major obstacles for effective therapy in locally advanced and metastatic breast cancer. Overexpression of the drug transporter P-glycoprotein (P-gp) in cancer cells is one of the main causes of MDR due to its ability to efflux anticancer drugs out of cells. Although the signaling node that regulates the expression of P-gp has been intensively investigated; the regulatory mechanism underlying P-gp transport activity remains obscure. Herein, we reported that Rack1 and tyrosine kinase Src confer drug resistance through modulating the transport function of P-gp without altering its protein level. We provide evidences that Rack1 and Src regulate P-gp activity by modulating caveolin-1 (Cav1) phosphorylation. Importantly, Rack1 acts as a signaling hub and mediates Src binding to P-gp, thereby facilitating the phosphorylation of Cav1 by Src and abolishing the inhibitory effect of Cav1 on P-gp. Taken together, our results demonstrate the pivotal roles of Rack1 and Src in modulating P-gp activity in drug-resistant cells. Our findings also provide novel insights into the mechanism regulating P-gp transport activity. Rack1 may represent a new target for the development of effective therapies for reversing drug resistance.

RACK1 Silencing Induces Cell Apoptosis and Inhibits Cell Proliferation in Hepatocellular Carcinoma MHCC97-H Cells.

This study aimed to explore the effects of RACK1 gene silencing on the apoptosis and proliferation of hepatocellular carcinoma (HCC) MHCC97-H cells. After transfecting MHCC97-H cells with siRNA, RACK1 gene silencing model was established. The cells were divided into blank group, siRNA group and empty plasmid group, respectively. The mRNA and protein expressions of RACK1, cyclin D1 and BAX were determined by qRT-PCR and Western blotting. CCK-8 assay, flow cytometry and FITC-Annexin V/PI staining were used to determine cell viability, cell cycle and cell apoptosis, respectively. The results of qRT-PCR and Western blotting suggested that when compared with the blank group and the empty plasmid group, the mRNA and protein expressions of RACK1 and Cyclin D1 decreased significantly while the mRNA and protein BAX expressions increased substantially in the siRNA group (all P < 0.05). The results of CCK-8 assay revealed that the siRNA group exhibited significantly lower cell viability when compared with the blank group and the empty plasmid group (both P < 0.05); and the cell viability in the siRNA group decreased gradually with the increase of time. The results of flow cytometry and FITC-Annexin V/PI staining indicated that when compared with the blank group and the empty plasmid group, the proportion of cells in S phase was markedly lower and the apoptosis rate was significantly higher in the siRNA group (both P < 0.05). Our study suggests that inhibition of RACK1 could suppress cell proliferation and induce apoptosis in HCC MHCC97-H cells.