Rho-family GTPase 1 (Rnd1) is a biomechanical stress-sensitive activator of cardiomyocyte hypertrophy.

Cardiac remodeling is induced by mechanical or humoral stress causing pathological changes to the heart. Here, we aimed at identifying the role of differentially regulated genes upon dynamic mechanical stretch. Microarray of dynamic stretch induced neonatal rat ventricular cardiomyocytes (NRVCMs) discovered Rho family GTPase 1 (Rnd1) as one of the significantly upregulated genes, a cardiac role of which is not known yet. Rnd1 was consistently upregulated in NRVCMs after dynamic stretch or phenylephrine (PE) stimulation, and in a mouse model of pressure overload. Overexpression of Rnd1 in NRVCMs activated the fetal gene program (including nppa and nppb) effected into a significant increase in cell surface area in untreated, stretched or PE-treated cells. Furthermore, Rnd1 overexpression showed a positive effect on cell proliferation as detected by significant increase in Ki67, Phosphohistone H3, and EdU positive NRVCMs. Through a Yeast two-hybrid screen and immunoprecipitation analysis, we identified Myozap, an intercalated disc protein, as novel interaction partner of Rnd1. Importantly, functional analysis of this interaction revealed the importance of RND1 in the RhoA and Myozap protein network that activates serum-response factor (SRF) signaling. In summary, we identified Rnd1 as a novel stretch-sensitive gene which influences cell proliferation and cellular hypertrophy via activation of RhoA-mediated SRF dependent and independent signaling pathways.

Myeloid leukemia factor-1 is a novel modulator of neonatal rat cardiomyocyte proliferation.

The present study focuses on the identification of the gene expression profile of neonatal rat cardiomyocytes (NRVCMs) after dynamic mechanical stretch through microarrays of RNA isolated from cells stretched for 2, 6 or 24h. In this analysis, myeloid leukemia factor-1 (MLF1) was found to be significantly downregulated during the course of stretch. We found that MLF1 is highly expressed in the heart, however, its cardiac function is unknown yet. In line with microarray data, MLF1 was profoundly downregulated in in vivo mouse models of cardiomyopathy, and also significantly reduced in the hearts of human patients with dilated cardiomyopathy. Our data indicates that the overexpression of MLF1 in NRVCMs inhibited cell proliferation while augmenting apoptosis. Conversely, knockdown of MLF1 protected NRVCMs from apoptosis and promoted cell proliferation. Moreover, we found that knockdown of MLF1 protected NRVCMs from hypoxia-induced cell death. The observed accelerated apoptosis is attributed to the activation of caspase-3/-7/PARP-dependent apoptotic signaling and upregulation of p53. Most interestingly, MLF1 knockdown significantly upregulated the expression of D cyclins suggesting its possible role in cyclin-dependent cell proliferation. Taken together, we, for the first time, identified an important role for MLF1 in NRVCM proliferation.