Proto-oncogene tyrosine-protein kinase SRC (SRC), as other members of the SRC family kinases (SFK), plays an important role in regulating signal transduction by different cell surface receptors after changes in the cellular environment. Here, we reviewed the role of SRC in platelets and megakaryocytes (MK). In platelets, inactive closed SRC is coupled to the β subunit of integrin αIIbβ3 while upon fibrinogen binding during platelet activation, αIIbβ3-mediated outside-in signaling is initiated by activation of SRC. Active open SRC now further stimulates many downstream effectors via tyrosine phosphorylation of enzymes, adaptors, and especially cytoskeletal components. Functional platelet studies using SRC knockout mice or broad spectrum SFK inhibitors pointed out that SRC mediates their spreading on fibrinogen. On the other hand, an activating pathological SRC missense variant E527K in humans that causes bleeding inhibits collagen-induced platelet activation while stimulating platelet spreading. The role of SRC in megakaryopoiesis is much less studied. SRC knockout mice have a normal platelet count though studies with SFK inhibitors point out that SRC could interfere with MK polyploidization and proplatelet formation but these inhibitors are not specific. Patients with the SRC E527K variant have thrombocytopenia due to hyperactive SRC that inhibits proplatelet formation after increased spreading of MK on fibrinogen and enhanced formation of podosomes. Studies in humans have contributed significantly to our understanding of SRC signaling in platelets and MK.

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Background: Soft tissue sarcoma (STS) is a complex family of rare malignancies with high unmet medical need. uPARAP is an endocytic receptor expressed at low levels on selected mesenchymal cell types, which internalizes fragments of collagen and transfers them to the lysosome for degradation. Besides its physiological role, uPARAP is considered a critical modulator of the tumor microenvironment. Here, for the first time, we explored the molecular epidemiology of uPARAP in sarcoma tissue to validate this receptor as target for the development of ADCs in this indication. Methods: uPARAP expression was assessed in STS-specific tissue microarrays by immunohistochemistry in tissue samples from more than three hundred individual donors. Stainings were assessed by histopathologic scoring and grouped into high, medium and low expressing subgroups. Results: uPARAP was found to be strongly over-expressed in a high percentage of cases of common STS, with variations in terms of incidence and level of expression between histological subtypes. High uPARAP expression was found in fibro- (86% of cases), synovial (84%), dedifferentiated lipo- (68%), pleomorphic lipo- (65%), myxofibro- (59%), leiomyo- (49%), and myxoid liposarcoma (24%). Work is ongoing in additional sarcoma subtypes. Conclusions: Based on protein expression uPARAP is an attractive emerging therapeutic target for the development of uPARAP-binding ADCs in a broad range of mesenchymal malignancies. In addition, expression of uPARAP in STS may serve as a potential marker for patient selection in early clinical studies with uPARAP-targeting ADCs. Citation Format: Pernille Barkholt, Agnieszka Wozniak, Chao-Chi Wang, Che-Jui Lee, Lore De Kock, Lars H. Engelholm, Carmel Lynch, Dominik Mumberg, Patrick Schöffski. The urokinase plasminogen activator receptor-associated protein (uPARAP) is an attractive target for the development of antibody-drug conjugates (ADCs) for treatment of mesenchymal malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1547.