| Summary: | The role of Lemur tyrosine kinase 3 (LMTK3) and its association with cell proliferation and endocrine resistance in breast cancer due to its ability of regulating estrogen receptor α (ERα) has been previously addressed in our laboratory. However, the ER-independent function of LMTK3 has not been studied yet. We found that LMTK3 promotes the development of a metastatic phenotype by inducing the expression of genes encoding integrin subunits. Invasive behaviour such as actin cytoskeleton remodelling and focal adhesion were positively correlated with the abundance of LMTK3 formation in various breast cancer cell lines. Using SILAC (stable isotope labelling by amino acids in cell culture) proteomic analysis, we found that LMTK3 increases the protein levels of integrin subunits α5 and β1 through activating the CDC42 GTPase, which promotes integrin α5 and β1 expression via the transcription factor serum response factor (SRF). Furthermore, abundance of LMTK3 was positively correlated with that of integrin β1 in breast cancer patients' tumours. As LMTK3 is also localised in the nucleus, we then investigated its nuclear function. We mapped LMTK3 binding across the genome using ChIP-seq and found that LMTK3 binding events are correlated with repressive chromatin markers. We further identified KRAB-associated protein 1 (KAP1) as a binding partner of LMTK3. The LMTK3/KAP1 interaction is stabilized by PP1α, which suppresses KAP1 phosphorylation specifically at LMTK3-associated chromatin regions, inducing chromatin condensation and resulting in transcriptional repression of LMTK3-bound tumour suppressor-like genes. Furthermore, LMTK3 functions at distal regions in tethering the chromatin to the nuclear periphery, resulting in H3K9me3 modification and gene silencing. In summary, we propose a model where a scaffolding function of nuclear LMTK3 promotes cancer progression through chromatin remodelling in an ERα-independent and kinase-independent manner.
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