Spleen Tyrosine Kinase–Mediated Autophagy Is Required for Epithelial–Mesenchymal Plasticity and Metastasis in Breast Cancer

Abstract

The ability of breast cancer cells to transiently transition between epithelial and mesenchymal states contributes to their metastatic potential. Therefore, driving tumor cells into a stable mesenchymal state, as opposed to complete tumor cell eradication, presents an opportunity to pharmacologically limit disease progression by promoting an asymptomatic state of dormancy. Here, we compare a reversible model of epithelial-mesenchymal transition (EMT) induced by TGFb to a stable mesenchymal phenotype induced by chronic exposure to the ErbB kinase inhibitor lapatinib. Only cells capable of returning to an epithelial phenotype resulted in skeletal metastasis. Gene expression analyses of the two mesenchymal states indicated similar transition expression profiles. A potently downregulated gene in both datasets was spleen tyrosine kinase (SYK). In contrast to this similar diminution in mRNA, kinome analyses using a peptide array and DNA-conjugated peptide substrates showed a robust increase in SYK activity upon TGFb-induced EMT only. SYK was present in cytoplasmic RNA processing depots known as P-bodies formed during the onset of EMT, and SYK activity was required for autophagy-mediated clearance of P-bodies during mesenchymal-epithelial transition (MET). Genetic knockout of autophagy-related 7 (ATG7) or pharmacologic inhibition of SYK activity with fostamatinib, a clinically approved inhibitor of SYK, prevented P-body clearance and MET, inhibiting metastatic tumor outgrowth. Overall, this study suggests assessment of SYK activity as a biomarker for metastatic disease and the use of fostamatinib as a means to stabilize the latency of disseminated tumor cells.

Transglutaminase-2 facilitates extracellular vesicle-mediated establishment of the metastatic niche

“…However, recent efforts from our laboratory and others clearly indicate that molecular events take place very early in disease progression that can influence the success and failure of disseminated cells to proliferate in secondary tissues and establish metastatic disease 2 . Among these, the molecular and phenotypic aspects of epithelial-mesenchymal transition (EMT) play a key role in maximizing the metastatic potential of mammary tumors through several mechanisms 3 . Clearly, the ability of tumor cells to transition to a mesenchymal state contributes to cell invasion, drug resistance, and cell survival in response to the stresses of the primary tumor environment and upon systemic dissemination [4][5][6] .…”

Epigenetic feedback and stochastic partitioning during cell division can drive resistance to EMT

“…However, some cells exposed to EMT-inducing signals for longer durations (~10 days or more) may get 'locked' in a mesenchymal state, making EMT largely irreversible, at least for the timescale observed experimentally [11][12][13][14]. The possibility of an irreversible EMT is also supported by multiple phenomenological observations [15][16][17]. Multiple mechanisms have been proposed to explain the existence of a 'tipping point' -a time point beyond which continued treatment with EMT inducing signals can drive an irreversible EMT.…”

Mechanisms of Action of Autophagy Modulators Dissected by Quantitative Systems Pharmacology Analysis

“…Fostamatinib is an FDA-approved drug for the treatment of chronic immune thrombocytopenia. It has been reported that fostamatinib suppresses autophagy by inhibiting the spleen tyrosine kinase (STK) [45]. Fostamatinib also inhibits critical autophagy regulators including ULK1/2, serine/threonine-protein kinase (TBK1) [46], death-associated protein kinase 1 and 2 (DAPK1/2) [47,48], and leucine-rich repeat serine/threonine-protein kinase 2 (LRRK2) [49].…”