Horizontal RNA transfer mediates platelet-induced hepatocyte proliferation

Abstract

Key Points• Platelets stimulate proliferation of HepG2 cells, which requires uptake of platelets by the HepG2 cell.• Platelets stimulate HepG2 cell proliferation in part by transfer of RNA from the anucleate platelet to the nucleated HepG2 cell.Liver regeneration is stimulated by blood platelets, but the molecular mechanisms involved are largely unexplored. Although platelets are anucleate, they do contain coding or regulatory RNAs that can be functional within the platelet or, after transfer, in other cell types. Here, we show that platelets and platelet-like particles (PLPs) derived from the megakaryoblastic cell line MEG-01 stimulate proliferation of HepG2 cells. Platelets or PLPs were internalized within 1 hour by HepG2 cells and accumulated in the perinuclear region of the hepatocyte. Platelet internalization also occurred following a partial hepatectomy in mice. Annexin A5 blocked platelet internalization and HepG2 proliferation. We labeled total RNA of MEG-01 cells by incorporation of 5-ethynyluridine (EU) and added EU-labeled PLPs to HepG2 cells. PLP-derived RNA was detected in the cytoplasm of the HepG2 cell. We next generated PLPs containing green fluorescent protein (GFP)-tagged actin messenger RNA. PLPs did not synthesize GFP, but in coculture with HepG2 cells, significant GFP protein synthesis was demonstrated. RNA-degrading enzymes partly blocked the stimulating effect of platelets on hepatocyte proliferation. Thus, platelets stimulate hepatocyte proliferation via a mechanism that is dependent on platelet internalization by hepatocytes followed by functional transfer of RNA stored in the anucleate platelet. This mechanism may contribute to platelet-mediated liver regeneration. (Blood. 2015;126(6):798-806) IntroductionBlood platelets have essential roles in hemostasis and thrombosis, inflammation, host defense, and wound healing. [1][2][3][4] Emerging evidence from recent in vitro and in vivo studies suggests that platelets have a pivotal role in liver regeneration. [5][6][7][8] In experimental animal models in which platelets were depleted or functionally impaired, liver regeneration after a partial liver resection was substantially delayed. 6 Conversely, following a partial liver resection in animals with a drug-induced thrombocytosis, liver regeneration was accelerated. 9,10 In a clinical study, we showed that a low platelet count is an independent predictor of delayed postoperative liver function recovery following a partial liver resection, suggesting that platelets stimulate liver regeneration also in humans. 11The molecular mechanisms of platelet-mediated stimulation of liver regeneration are largely unexplored. Platelets contain 2 distinct types of storage organelles: a granules and dense granules. The a granules contain, among many proteins, a number of growth factors that have an established role in liver regeneration (platelet-derived growth factor [PDGF], hepatocyte growth factor [HGF], insulin-like growth factor [IGF], and vascular endothelial growth factor [VEGF]). 8,12 In addition...

Importance of platelet-derived growth factors in liver regeneration

“…Furthermore, it was reported recently that platelet RNAs are also involved in hepatocyte proliferation processes [26]. While we were able to document the in vivo relevance of 5-HT within the human setting, the relevance of platelet-derived RNAs during liver regeneration in humans remains to be elucidated.…”

Platelets in liver disease, cancer and regeneration

“…Thus, Kirschbaum et al[78] showed that transfer of coding or regulatory RNA could occur between platelets and hepatocytes, promoting hepatocyte proliferation. However, the role of both mechanisms, i.e .…”

Novel functions of platelets in the liver

“…Recently, it has been reported that RNA transfer mechanism as an additional contributor to platelet-mediated liver regeneration. 57 The collaborative effect with Kupffer cells. Kupffer cells play a role in the liver as macrophages that act against bacteria, endotoxins, and microbial debris derived from the gastrointestinal tract.…”