The myocardin-related transcription factor MKL co-regulates the cellular levels of two profilin isoforms

Abstract

Megakaryoblastic leukemia (MKL)/serum-response factor (SRF)-mediated gene transcription is a highly conserved mechanism that connects dynamic reorganization of the actin cytoskeleton to regulation of expression of a wide range of genes, including SRF itself and many important structural and regulatory components of the actin cytoskeleton. In this study, we examined the possible role of MKL/SRF in the context of regulation of profilin (Pfn), a major controller of actin dynamics and actin cytoskeletal remodeling in cells. We demonstrated that despite being located on different genomic loci, two major isoforms of Pfn (Pfn1 and Pfn2) are co-regulated by a common mechanism involving the action of MKL that is independent of its SRF-related activity. We found that MKL co-regulates the expression of Pfn isoforms indirectly by modulating signal transducer and activator of transcription 1 (STAT1) and utilizing its SAP-domain function. Unexpectedly, our studies revealed that cellular externalization, rather than transcription of Pfn1, is affected by the perturbations of MKL. We further demonstrated that MKL can influence cell migration by modulating Pfn1 expression, indicating a functional connection between MKL and Pfn1 in actin-dependent cellular processes. Finally, we provide initial evidence supporting the ability of Pfn to influence MKL and SRF expression. Collectively, these findings suggest that Pfn may play a role in a possible feedback loop of the actin/MKL/SRF signaling circuit.

SRF'ing and SAP'ing – the role of MRTF proteins in cell migration

“…As outlined above, a control of cellular retention of selective actin-binding proteins (e.g. profilin) downstream of MRTF (Joy et al, 2017) would offer a new means of how MRTF might regulate cell migration; nevertheless, how exactly MRTF might mediate this control is unclear and warrants further investigation. Moreover, several other molecular pathways that, potentially, link MRTF with cell migration are also worth investigating.…”

“…Interestingly, MRTF activity promotes the cellular retention of profilin and, so, inhibits its release into the extracellular environment, rather than inducing its transcription, through an intermediate step that involved the regulation of the expressions of certain STAT isoforms. We have shown that overexpression of PFN1 can partly reverse the effect of MRTF knockdown on breast cancer cell motility (Joy et al, 2017); therefore, control of the externalization of selective actin-binding proteins is a potential novel mechanism for regulation of cell motility that is dependent of SAP-domain function of MRTF.…”

“…Although several candidate target genes that are dependent on SAP-domain activity of MRTFA are associated with cell motility, such as for example TNC ( promoting cell adhesion and pro-migratory EGF signaling), keratin 5 (Krt5; controlling cell deformability) Kif26B (controlling cell polarity and adhesion) and Adamts26 (involved in ECM proteolysis) (Gurbuz et al, 2014), it is not known to what extent these proteins actually contribute to MRTF-dependent modulation of cell migration. We have recently reported that MRTFs play a crucial role in co-regulating the cellular abundance of PFN1 and PFN2the two main isoforms of the actin-binding protein profilin, a main regulator of actin dynamics and cell migrationby utilizing their SAPdomain-mediated transcriptional activity in an SRF-independent manner (Joy et al, 2017). Interestingly, MRTF activity promotes the cellular retention of profilin and, so, inhibits its release into the extracellular environment, rather than inducing its transcription, through an intermediate step that involved the regulation of the expressions of certain STAT isoforms.…”

Primary immunodeficiencies caused by mutations in actin regulatory proteins

“…34 WD-Repeat domain 1 (WDR1) functions to promote Cofilin's actin severing ability, and actin disassembly into G-actin monomers 35,36 Megakaryoblastic leukemia (MKL1) is a member of the MTRF family that regulates the transcription of actin related proteins including serum response factor (SRF) and Profilin. 37,38 Mutations in the actin regulatory genes described above have been seen in human immune dysregulation and immunodeficiency syndromes ( Figure 1). By studying these diseases and their murine counterparts, much has been learned about the regulation of the F I G U R E 1 Actin regulatory proteins involved in human primary immunodeficiencies.…”

“…Coronin1A binds to mature F‐actin filaments and enhances their severing by Cofilin . WD‐Repeat domain 1 (WDR1) functions to promote Cofilin's actin severing ability, and actin disassembly into G‐actin monomers Megakaryoblastic leukemia (MKL1) is a member of the MTRF family that regulates the transcription of actin related proteins including serum response factor (SRF) and Profilin …”