Platelet Hsp70 Promotes Platelet Spreading and Integrin aIIbb3 Activation

The Heat Shock Protein 70 (Hsp70) family is responsible for the folding of newly synthesized or misfolded proteins and is also involved in various regulatory processes, including the regulation of apoptosis (cell death) by protecting the cell from apoptosis-inducing factors. Because of this, Hsp70 proteins have been linked to diseases such as Alzheimer’s, Huntington’s, and several cancers. Cancerous tumor cells depend on high levels of Hsp70 to buffer the effect of negative mutations during cell immortalization, and to counter the stressful conditions of the nutrient-depleted, hypoxic environment of the tumor. Because of its connection to tumor survival, the inhibition of Hsp70s has become a promising target for novel anti-cancer therapies. However, Hsp70 proteins are critical in platelet survival and activation as well, so the question arises: if Hsp70 inhibition is used to treat cancerous tumors, what effect will that have on platelet activity?

This particular study focuses on the Hsp70 inhibitors VER 155008 and MKT-077 and their roles in hindering platelet activity. The goal of the research was to discover what effect Hsp70 inhibition by each of the two drugs would have on platelet activation, and what those findings would demonstrate about the mechanism of Hsp70 in the platelet. Although it is known that Hsp70 plays a role in platelet activity, there is little known for certain about the mechanism by which the protein has an effect. In running one part of the experiment, purified human platelets from healthy donors were treated with either VER 155008, MKT-077, or with an Src Kinase (PP2) positive control and then allowed to spread on surfaces coated in either fibrinogen or collagen in a process called static adhesion. In a separate experiment, the platelets were treated with the drugs and labeled with phalloidin, then analyzed by way of flow cytometry for the presence of F-actin, a sign of shape change and activation. The data from the static adhesion experiment show that platelet activity is inhibited when the protein coating is the integrin αIIbβ3 substrate fibrinogen. This in turn demonstrates that Hsp70 works to regulate platelet function by controlling the activation of platelet integrin αIIbβ3. From the flow cytometry experiment with phalloidin to label F-actin, we found that Hsp70 does not play a significant role in the cytosolic αIIbβ3 signaling that leads to actin polymerization and platelet shape change, and must have its effect by another mechanism.