Thrombopoietin Stimulation Leads to Enhanced Polyploidization in p53-/- Bone Marrow Megakaryocytes: In Vivo and in Vitro Studies.
Authors: Apostolidis Pani A. , Lindsey Stephan , Miller William M. , Papoutsakis Eleftherios T. 
Published: October 2019

Abstract
Abstract 2531
Poster Board II-508

BACKGROUND AND HYPOTHESIS.
We have previously shown that tumor suppressor p53 is activated in differentiating megakaryocytic (Mk) cells and its knock-down (KD) leads to increased polyploidization and delayed apoptosis in CHRF, a human Mk cell line. Furthermore, bone marrow (BM)-derived Mks from p53−/− mice reach higher ploidy classes in culture. Accordingly, we hypothesized that the role of p53 during megakaryopoiesis is to delimit polyploidization and control the transition from endomitosis by inhibiting DNA synthesis and promoting apoptosis. Here, we test this hypothesis by examining the differential effect of mouse thrombopoietin (rmTpo) on the ploidy of p53−/− and p53+/+ mouse Mk cells.

METHODS.
8–10 week-old, male p53−/− mice and p53+/+ littermates were injected once with 1.2 μg rmTpo or saline. On days 2 and 5 after Tpo/saline treatment, tail-bleeding assays were performed to measure bleeding times/volumes, mice were bled for platelet counts and sacrificed to harvest BM. We employed flow cytometry to examine baseline ploidy in BM-resident Mks in p53−/− and p53+/+ mice as well as Mk cells generated from BM progenitors after 4 and 6 days of culture with rmTpo.

RESULTS.
At steady state, ploidy in BM-resident CD41+ Mk cells was similar in p53−/− and p53+/+ mice: 11.8±2.3% and 10.7±1.3% of p53−/− and p53+/+ Mks, respectively, reaching a ploidy of ≥32N (n=3-4). Platelet counts were 1.3×106±1×105/μl (12.5±1.0% reticulated) and 1.1×106±5×104/μl (12.4±1.3% reticulated) in p53−/− and p53+/+ mice, respectively (n=8). Two days following Tpo treatment of the mice, we did not observe significantly increased platelet levels, while ploidy was marginally affected. However, 5 days following Tpo treatment, we found greater ploidy in the BM in the absence of p53: 22±1.6% 16N and 10.1±0.8% ≥32N Mks in the p53−/− versus 18.6±3.3% 16N and 7.1±1.4% ≥32N Mks in the p53+/+ (n=2). This was accompanied by increased platelet formation: 23.6±8.3% reticulated platelets in the p53−/− versus 17.8±2.6% in the p53+/+ (n=2). Culture of BM cells from non-Tpo treated mice with 50ng/ml rmTpo resulted in a 50% increase in total Mks and increased polyploidy by day 6 of culture: 38.6±4.6% of p53−/− versus 19.2±2.3% of p53+/+ Mks reached ploidy classes of ≥32N (n=3-4, p < 0.01). Lack of p53 led to hyperploid Mk cells; by day 6 of culture 10.3±2.2% of p53−/− Mks were in ploidy classes of 128N and higher, while only 0.6±0.1% p53+/+ Mks achieved such high ploidy (n=3-4). In addition, a 6 day culture with Tpo of BM cells derived from p53−/− and p53+/+ mice pre-treated with Tpo 5 days prior to sacrifice led to more profound polyploidization compared to Mks generated from the non-Tpo treated mice but only in the p53−/− Mks: 48.8±1.1% of p53−/− versus only 17.6±0.2% of p53+/+ Mks reached ploidy ≥32N (n=2). Microarray analysis comparing p53KD to control CHRF cells undergoing Mk differentiation revealed down-regulation of genes coding for platelet surface complex CD41/CD61 and CD62P in the p53KD cells. To examine the possibility of altered functionality of platelets in p53−/− mice, we performed tail-bleeding assays on the mice that did not receive Tpo. Bleeding times and volumes were generally prolonged in the absence of p53 (all p53−/− mice exceeded the 10 min duration of the assay; mean p53−/− and p53+/+ blood loss was 17μl and 10μl, respectively, n=3-4).

CONCLUSIONS.
Our data indicate that in vivo polyploidization and platelet formation from Mks is increased in the p53−/− relative to p53+/+ mice after Tpo administration. These data are in line with our hypothesis that p53 activation decreases the ability of Mks to respond to Tpo and undergo polyploidization. Additionally, our preliminary data on platelet functionality suggest that p53 may have a role in hemostasis.

Disclosures:
No relevant conflicts of interest to declare.