Expression of a kinase inactive SLK is embryonic lethal and impairs cell migration in fibroblasts.

Kinases are known to have kinase activity independent functions. To gain further insights into potential kinase-independent functions of SLK/STK2, we have developed a kinase-dead allele, SLKK63R using in vivo CRISPR/Cas technology. Our studies show that blastocysts homozygote for SLKK63R do not develop into viable mice. However, heterozygotes are viable and fertile with no overt phenotypes. Analyses of mouse embryonic fibroblasts show that expression of SLKK63R results in a 50% decrease in kinase activity in heterozygotes. In contrast to previous studies, our data show that SLK does not form homodimers and that the kinase defective allele does not act in a dominant negative fashion. Expression of SLKK63R leads to altered Rac1 and RhoA activity, increased stress fiber formation and delayed focal adhesion turnover. Our data support a previously observed role for SLK in cell migration and suggest that at least 50% kinase activity is sufficient for embryonic development.

CEACAM1 is a direct SOX10 target and inhibits melanoma immune infiltration and stemness.

SOX10 is a key regulator of melanoma progression and promotes a melanocytic/differentiated state. Here we identified melanoma cell lines lacking SOX10 expression which retain their in vivo growth capabilities. More importantly, we find that SOX10 can regulate T-cell infiltration in melanoma while also decreasing common cancer stem cell (CSC) properties. We show that SOX10 regulates CEACAM1, a surface protein with immunomodulatory properties. SOX10 directly binds to a distal CEACAM1 promoter region approximately 3-4kbps from the CEACAM1 transcriptional start site. Furthermore, we show that a SOX10-CEACAM1 axis can suppress CD8+ T-cell infiltration as well as reduce CSC pool within tumors, leading to reduced tumor growth. Overall, these results identify SOX10 as a direct regulator of CEACAM1, and uncover both a pro- and anti-tumorigenic roles for SOX10 in melanoma.

The Ste20-like kinase - a Jack of all trades?

Over the past 20 years, the Ste20-like kinase (SLK; also known as STK2) has emerged as a central regulator of cytoskeletal dynamics. Reorganization of the cytoskeleton is necessary for a plethora of biological processes including apoptosis, proliferation, migration, tissue repair and signaling. Several studies have also uncovered a role for SLK in disease progression and cancer. Here, we review the recent findings in the SLK field and summarize the various roles of SLK in different animal models and discuss the biochemical mechanisms regulating SLK activity. Together, these studies have revealed multiple roles for SLK in coupling cytoskeletal dynamics to cell growth, in muscle repair and in negative-feedback loops critical for cancer progression. Furthermore, the ability of SLK to regulate some systems appears to be kinase activity independent, suggesting that it may be an important scaffold for signal transduction pathways. These various findings reveal highly complex functions and regulation patterns of SLK in development and disease, making it a potential therapeutic target.

AKT-mediated phosphorylation of Sox9 induces Sox10 transcription in a murine model of HER2-positive breast cancer.

BACKGROUND: Approximately 5-10% of HER2-positive breast cancers can be defined by low expression of the Ste20-like kinase, SLK, and high expression of SOX10. Our lab has observed that genetic deletion of SLK results in the induction of Sox10 and significantly accelerates tumor initiation in a HER2-induced mammary tumor model. However, the mechanism responsible for the induction of SOX10 gene expression in this context remains unknown. METHODS: Using tumor-derived cell lines from MMTV-Neu mice lacking SLK and biochemical approaches, we have characterized the signaling mechanisms and relevant DNA elements driving Sox10 expression. RESULTS: Biochemical and genetic analyses of the SOX10 regulatory region in SLK-deficient mammary tumor cells show that Sox10 expression is dependent on a novel -7kb enhancer that harbors three SoxE binding sites. ChIP analyses demonstrate that Sox9 is bound to those elements in vivo. Our data show that AKT can directly phosphorylate Sox9 in vitro at serine 181 and that AKT inhibition blocks Sox9 phosphorylation and Sox10 expression in SLK(-/-) tumor cells. AKT-mediated Sox9 phosphorylation increases its transcriptional activity on the Sox10 -7kb enhancer without altering its DNA-binding activity. Interestingly, analysis of murine and human mammary tumors reveals a direct correlation between the levels of active phospho-Sox9 S181 and Sox10 expression. CONCLUSIONS: Our results have identified a novel Sox10 enhancer and validated Sox9 as a direct target for AKT. As Sox10 is a biomarker for triple-negative breast cancers (TNBC), these findings might have major implications in the targeting and treatment of those cancers.