Research

We decipher impacts of novel proteins or pathways in maintenance of cellular homeostasis. Using in vitro cell culture and mouse models, we try to understand mechanisms of regulation and dysregulation of metabolism in healthy and diseased conditions. A major focus is to determine in vivo significance of mammalian inositol hexakisphosphate kinases (IP6Ks), which primarily generate the inositol pyrophosphate 5-IP7. Characterization of global or tissue-specific IP6K knockout mice, or mice treated with a pan inhibitor of IP6Ks, reveal impacts of this pathway in various diseases such as, obesity, type-2 diabetes (T2D), fatty liver, cardiovascular diseases, infection and immunity, inflammation, cancer metastasis, fertility, motor learning, social behavioral and lifespan.

We discovered that the major murine isoform IP6K1 promotes insulin resistance in high fat diet (HFD)-fed mice by 5-IP7 mediated inhibition of the insulin sensitizing protein kinase Akt. We also demonstrated that IP6K1 reduces AMPK mediated thermogenic energy expenditure in the adipose tissue via a distinct mechanism. Moreover, IP6K1 deletion enhances serum levels of the insulin sensitizing adipokine adiponectin (AdipoQ). Figure 1. HFD-fed AdKO mice gain less body weight. Zhu et al. J. Clin. Invest. 2016).  Figure 2. TNP promotes weight loss in diet induced obese mice (Ghoshal et al. Mol Metab. 2016). Mice fed a HFD for 8-weeks were treated with TNP for 10-weeks in presence of HFD.

As a result, both global (IP6K1-KO) and adipocyte-specific (AdKO) IP6K1 knockout mice (Figure 1) are protected from diet induced obesity (DIO) and insulin resistance (Figure 1). Our work also reveals that the pan IP6K inhibitor TNP [N2-(m-Trifluorobenzyl), N6-(p-nitrobenzyl)purine] ameliorates DIO (Figure 2) by enhancing insulin sensitivity and thermogenic energy expenditure. Therefore, IP6K1 is a novel target in obesity/T2D (Figure 3) (Chakraborty et al. Cell, 2010, 143, 897-910; Zhu et al. J. Clin. Invest. 2016, 126, 4273-42; Ghoshal et al. Mol Metab. 2016, 5, 903-1).

Figure 3. Left panel: Adipocyte-specific IP6K1 regulates energy metabolism by inhibiting the AMPK pathway. AMPK activates various proteins/pathways including the transcriptional co-activator PGC1a via diverse mechanisms which is a major regulator of adipose tissue browning and UCP1 mediated thermogenic energy expenditure. Moreover, adipocyte-IP6K1 generated 5-IP7 inhibits Akt. IP6K1 also reduces serum levels of the insulin sensitizing adipokine adiponectin (AdipoQ), which indirectly influences these pathways. Thus, adipocyte-specific IP6K1 promotes DIO and insulin resistance. Right panel: In AdKO mice, Akt, AMPK and AdipoQ signaling are upregulated, which lead to adipose tissue browning mediated thermogenic energy expenditure and global insulin sensitivity. As a result, AdKO mice are protected against high fat diet induced weight gain and insulin resistance. Striped and bold encircling arrows indicate, reduced and increased effects of AMPK and Akt on tissue metabolism, in presence and absence of IP6K1 respectively. Thin and think straight arrows indicate less and more active pathways respectively (Zhu et al. J. Clin.Invest 2016)

Ongoing research

Although the pan IP6K inhibitor TNP is a novel pharmacologic tool to study IP6K functions in vivo, it has several shortcomings, which must be addressed prior to its advancement to the next level. Therefore, a collaboration is ongoing with chemists and structural biologists to develop potent and specific IP6K1 inhibitors based on the TNP structure. Moreover, using a highthroughput activity assay in vitro, compound libraries will be screened to discover novel isoform-specific inhibitors of IP6Ks. We use structure-based design to aid SAR (modelling and crystallography) to improve physicochemical properties and pharmacokinetics of optimized IP6K inhibitors for in vivo use. Potent and selective IP6K inhibitors will significantly enhance inositol pyrophosphate and metabolism fields as pharmacologic tools. Moreover, these compounds may emerge as anti-obesity/anti-T2D drugs. (Figure 4). 

Figure 4. Development of novel IP6K1 inhibitors by generating analogs of the TNP or by screening chemical libraries. 

In addition to mature adipocytes, the adipose tissue also contains various other cells including macrophages. Diet-induced obesity leads to a shift from the anti-inflammatory M2 macrophages to pro-inflammatory M1 macrophages, which contributes to insulin resistance. Thus, maintenance of the proper adipocyte niche is essential for its normal operation. Yet, how adipocytes maintain their niche and how the homeostasis is disrupted in metabolic diseases, are not understood. Our future discoveries may unravel a possible mechanism by which the adipocyte niche is maintained and regulated.  To identify the endogenous IP6K1-interactome in the adipose tissue, we employed a proteomic approach. We discovered that IP6K1 is the component of certain protein-network in the adipose tissue, which regulate proteostasis and cell-niche (Figure 5). We are particularly interested in the cell-niche protein-network as they are major regulators of cardiomyocyte-niche. However, their role in maintaining adipocyte-niche is not known. Therefore, current research is focused to understand role of this protein-network in maintaining adipocyte-niche. Moreover, we pursue research to determine whether IP6K1 mediated regulation of known or hitherto unknown pathways modulate cardiomyocyte-niche and functions. 

Figure 5. IP6K1-interactome in the adipose tissue. Complex 1 presumably regulates proteostasis, whereas complex 2 may modulate the adipocyte niche. 

IP6K1 is major regulator of energy metabolism. Therefore, it is critical to monitor how this enzyme is regulated at the transcription or post-translation levels. Employing mass spectrometry, we identified a number of post-translational modifications in overexpressed IP6K1. We demonstrated that protein kinases PKA and PKC phosphorylates IP6K1, which modulates its interaction with the lipolytic regulator protein perilipin1 (Ghoshal et al. Int J Biochem Cell Biol., 2016, 78, 149-155). Our unpublished studies have characterized various other modes of IP6K1 regulation. Current research is ongoing to determine significance of these regulations in metabolic and age-related diseases (Figure 6).

Figure 6. Regulators of IP6K1 expression, stability, activity and its interaction with protein targets in healthy and metabolically sick conditions. Red and green arrows: negative and positive regulation.

A complete understanding of the cellular processes that regulate differentiation, metabolism and survival is critical to design and develop therapeutic strategies against diseases. Published genome wide association, transcriptomic and proteomic studies together with our ongoing studies, identified that a number of proteins are altered in metabolic diseases, although their roles and regulations are unknown. The long-term goal of the laboratory is to elucidate functions of these proteins, with a hope that one/few would emerge as novel therapeutic targets