We appreciate the comments of Dr. Ceccarelli and colleagues1 regarding our article “Critical role of tumor necrosis factor receptor 1, but not 2, in hepatic stellate cell proliferation, extracellular matrix remodeling, and liver fibrogenesis,”2 and its suggestion of the potential participation of protein kinase R (PKR) at the cross-talk between tumor necrosis factor receptor 1 (TNFR1) and platelet-derived growth factor-β (PDGF-β) receptor in hepatic stellate cells (HSCs). In fact, in light of the background information provided and their observation that PKR expression is boosted by TNF in LX2 cells, we have decided to pursue this line of investigation by analyzing the basal protein expression of PKR in our TNFR1 knockout (TNFR1-KO) HSCs, TNFR1/TNFR2 double-knockout (TNFR-DKO) HSCs, and wild-type HSCs to validate if differences in PKR expression could account for the lack of AKT activation and proliferation observed after PDGF challenge in TNFR1-KO and TNFR-DKO HSCs.2 We did not observe differential expression in PKR protein expression between wild-type, TNFR1-KO, and TNFR-DKO HSCs (Fig. 1). Although these initial observations do not support a critical role for PKR in the proliferative effects of TNF in murine HSCs, recent observations indicate that PKR undergoes rapid phosphorylation following the engagement of TNF receptors by TNF.3 Therefore, future analyses of PKR phosphorylation, rather than its total expression, is warranted to critically examine whether PKR participates in the proliferation of HSCs by TNF. In addition, whether PKR activation is differentially involved in human LX2 cells, as described by Ceccarelli et al.,1 versus murine HSCs (our present observations) deserves further investigation. Alternatively, besides PKR, other unknown intermediates involved in nuclear factor-κB activation may also participate in the cross-talk between TNF and PDGF signaling. Although described in a different context, recent observations have uncovered a new role for Sam68, a member of the signal transducers and activators of RNA (STAR) family of proteins that regulate RNA processing, in the TNFR signaling complexes.4 Nevertheless, we fully agree with Ceccarelli et al. that further investigations to identify intermediates controlling HSC proliferation and/or activation in response to TNF and PDGF may be of significant value in the treatment of liver fibrosis.

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Figure 1. Protein kinase R (PKR) expression in 7-day-old wild-type, TNFR1 knockout (TNFR1-KO) and TNFR1/TNFR2 double-knockout (TNFR-DKO) HSCs. Samples were run in triplicate. Antibodies used were monoclonal anti-PKR (clone B-10), anti–mouse–horseradish peroxidase (HRP) (Santa Cruz Biotechnology), anti–β-actin–HRP (Sigma-Aldrich).

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  • 1
    Ceccarelli S, Panera N, Alisi A, Nobili V. Hepatic stellate cell proliferation: a potential role of PKR. HEPATOLOGY 2011; doi:10.1002/hep. 24579.
  • 2
    Tarrats N, Moles A, Morales A, García-Ruiz C, Fernández-Checa JC, Marí M. Critical role of tumor necrosis factor receptor 1, but not 2, in hepatic stellate cell proliferation, extracellular matrix remodeling, and liver fibrogenesis. HEPATOLOGY 2011; 54: 319-327.
  • 3
    Sharma B, Altman JK, Goussetis DJ, Verma AK, Platanias LC. Protein kinase R (PKR) as mediator of the effects of interferon (IFN) gamma and tumor necrosis factor (TNF) alpha on normal and dysplastic hematopoiesis. J Biol Chem 2011; 286: 27506-27518.
  • 4
    Ramakrishnan P, Baltimore D. Sam68 is required for both NF-kB activation and apoptosis signaling by the TNF receptor. Mol Cell 2011; 43: 167-179.

Núria Tarrats Ph.D.*, Anna Moles Ph.D.*, Albert Morales Ph.D.*, Carmen García-Ruiz Ph.D.*, José C. Fernández-Checa Ph.D.* †, Montserrat Marí Ph.D.*, * IDIBAPS, Liver Unit-Hospital Clínic, CIBEREHD, and Department of Cell Death and Proliferation, IIBB-CSIC, Barcelona, Spain, † Research Center for Alcoholic Liver and Pancreatic Diseases, Keck School of Medicine of the University of Southern California, Los Angeles, CA.