STAT3 stimulates adipogenic stem cell proliferation and cooperates with HMGA2 during the early stage of differentiation to promote adipogenesis

Highlights

• We discovered an activation of STAT3 during the early differentiation stage of mouse 3T3-L1 preadipocytes.

• Stat3 knockdown using siRNA blocked cell cycle progression of both preadipoctes and early differentiating cells.

• Accumulation of lipid droplets was inhibited by Stat3 knockdown.

• STAT3 protein co-localized with HMGA2, which was reported to promote adipogenesis in a previous study.

Abstract

Signal transducer and activator of transcription 3 (STAT3) is abundantly expressed in the adipose tissue of obese mice and humans, but the role of STAT3 in adipogenesis is still not fully understood. In the present study, we discovered an activation of STAT3 during the early differentiation stage of mouse 3T3-L1 preadipocytes. Stat3 knockdown using siRNA blocked cell cycle progression of both preadipoctes and early differentiating cells. Moreover, accumulation of lipid droplets was inhibited by Stat3 knockdown. Importantly, in the nucleus of early differentiating cells, we demonstrated that STAT3 protein co-localized with high-mobility-group protein AT-hook 2 (HMGA2), which was reported to promote adipogenesis in a previous study. Taken together, our data indicate that STAT3 and HMGA2 cooperatively promote adipogenesis which highlight a more detail understanding of STAT3 related transcription factor network during adipogenesis.

Introduction

Obesity is a chronic, low-grade inflammatory disease that is associated with various diseases, including type 2 diabetes, hypertension, cardiovascular disease, nonalcoholic fatty liver disease and cancers [1], and is becoming a worldwide public health threat. Obesity is primarily caused by abnormal lipolysis, lipogenesis, and adipocyte differentiation and proliferation [3]. Inhibiting adipocyte differentiation and proliferation can be an effective strategy for preventing obesity [4]. The mouse 3T3-L1 cell line resembles pre-adipocytes morphologically and structurally, and is widely used as an in vitro adipogenic differentiation model [5]. Adipogenic differentiation of 3T3-L1 cells consists of three distinct stages: contact inhibition, clonal expansion, and terminal adipogenic differentiation [6].

In mammals, the signal transducer and activator of transcription (STAT) family has seven members: STAT1-4, STAT5a/b, and STAT6. STAT3 is an important transcription factor involved in many biological events, such as tumorigenesis, angiogenesis [7], [8], epithelial-to-mesenchymal transition (EMT) and apoptosis [9]. Constitutive activation of STAT3 is frequently observed in colon cancer-initiating cells and is associated with invasion, survival, and growth of colorectal cancer cells in vivo and in vitro [10], [11], [12]. Inhibition of STAT3 induces apoptosis and suppresses cell proliferation in various kinds of cancers [13], [14]. In 1996, Stephens et al. first observed that the STAT3 was abundantly expressed in proliferating pre-adipocytes [15]. IL-6-type cytokines (IL-6, IL-10, IL-11, LIF, CT-1, OSM, and CNTF), predominantly IL-6 itself, phosphorylate STAT3 via binding with GP130 and activating JAKs [16]. Zhang et al. [17] confirmed that the JAK2/STAT3 pathway was involved in the early stage of differentiation through regulating C/EBPβ transcription. It is also reported that activation of STAT3 regulated adipocyte differentiation through signaling that occurred upstream of PPARγ [18]. High-mobility-group protein AT-hook 2 (HMGA2), a sequence-specific DNA-binding protein, is directly linked to hyperplasia of white adipose tissue (WAT) and obesity [19], [20]. HMGA2 is barely detectable in mature tissues, while it is highly expressed in various stem cells, suggesting that HMGA2 functions at the early developmental stage [21], [22]. Moreover, we have reported that pulsatile expression of HMGA2 at the early stage of adipogenesis drives C/EBPβ-mediated induction of PPARγ and promotes adipocyte differentiation [23]. However, the relationship between HMGA2 and STAT3 is not known.

In this study, we provide evidence that STAT3 is activated at the early stage of adipocyte differentiation and promotes adipogenesis by stimulating 3T3-L1 stem cell proliferation. Our study suggests that STAT3 plays an essential role in the induction of adipogenesis.