Platelets Promote Ovarian Cancer Growth: New Insights On Proliferation
Authors: Cho Minsoon , Bottsford-Miller Justin , Zand Behrouz , Vasquez Hernan G. , Stone Rebecca L. , Kroll Michael H. , Sood Anil K. , Afshar-Kharghan Vahid 
Published: October 2019

Abstract
Abstract 96

Introduction.
Platelets can promote metastasis by a multitude of effects. We have recently shown that reducing platelet counts decreased the size and number of tumor nodules in a murine model of orthotopic ovarian cancer. Here, we report a previously unrecognized pro-proliferative effects of platelets on ovarian cancer cells, using in vivo and in vitro assays.

Methods and Results.
The proliferation rate of human and murine ovarian cancer cells increased significantly after coincubation with platelets (Figure 1A). This effect was platelet-specific, as red blood cells did not alter the proliferation rate. Direct contact between platelets and cancer cells or intact platelets was not required for the proliferative response. Furthermore, platelets isolated from tumor-bearing mice were similar to platelets from control mice in their proliferative effect (Figure 1B). Blocking platelet adhesive surface proteins (GPIba, GPIIbb3, and P-selectin) did not diminish the proliferative effect of platelets (Figure 1C) and aspirin only partially inhibited it (Figure 1D). Fixation of platelets with paraformaldehye (2%) completely eliminated the effect of platelets on proliferation of cancer cells (Figure 1D). To determine mechanism, we examined the effect of blocking antibody to TGFb1 and identified a dose-dependent inhibition of the proliferative effect of platelets on ovarian cancer cells by the anti-TGF b1 antibody. To confirm results obtained with the TGF b1-blocking antibody, we reduced expression of TGFbR1 receptors on ovarian cancer cells with siRNA prior to their incubation with platelets (Figure 1E). Consistent with results observed following treatment with the TGFb1-blocking antibody, TGFbR1 siRNA reduced proliferation of ovarian cancer cells (Figure 1F). To study the effect of platelets in vivo, we injected syngeneic platelets into mice with orthotopic ovarian cancer on a weekly basis for four weeks, and measured the proliferation index in the resected tumors using Ki67 staining. We found that mice receiving platelet infusions had a significantly higher percentage of Ki67 positivity compared to control tumor-bearing mice (83.5% vs 66.3%, respectively; p<0.0005). To extend these findings to human disease, we next measured proliferation indices in 20 tumor specimens collected from patients with ovarian cancer. Ten of these patients had thrombocytosis (> 450,000/ml) and 10 had normal platelet counts with an average of 284,000/ml. Thrombocytosis was associated with a higher percentage of Ki67 positivity in tumors (68% vs, 57%, respectively; p=0.03).

Summary and conclusion.
In summary, platelets significantly increase the proliferation of ovarian cancer cells. These results provide a new understanding of mechanisms by which platelets contribute to tumor progression.

Disclosures:
Kroll: Aplagon: Membership on an entity's Board of Directors or advisory committees; Optimer: Consultancy; Leo: Honoraria, Travel Expenses, Travel Expenses Other.