Disruption of the striated muscle glycogen-targeting subunit of protein phosphatase 1: influence of the genetic background

Functions and therapeutic potential of protein phosphatase 1: Insights from mouse genetics

Protein phosphatase 1 (PP1) catalyzes more than half of all phosphoserine/threonine dephosphorylation reactions in mammalian cells. In vivo PP1 does not exist as a free catalytic subunit but is always associated with at least one regulatory PP1-interacting protein (PIP) to generate a large set of distinct holoenzymes. Each PP1 complex controls the dephosphorylation of only a small subset of PP1 substrates. We screened the literature for genetically engineered mouse models and identified models for all PP1 isoforms and 104 PIPs. PP1 itself and at least 49 PIPs were connected to human disease-associated phenotypes. Additionally, phenotypes related to 17 PIPs were clearly linked to altered PP1 function, while such information was lacking for 32 other PIPs. We propose structural reverse genetics, which combines structural characterization of proteins with mouse genetics, to identify new PP1-related therapeutic targets. The available mouse models confirm the pleiotropic action of PP1 in health and diseases.

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Four protein phosphatases 1 (PP1) isoforms and >200 PP1-interacting proteins (PIPs)

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Genetically engineered mice of 49 PIPs display human disease-associated phenotypes.

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The use of structural reverse genetics to identify the PP1-dependent phenotypes

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Various interaction sites in PP1:PIP complexes suit for therapeutic targeting.

Loss of protein targeting to glycogen sensitizes human hepatocellular carcinoma cells towards glucose deprivation mediated oxidative stress and cell death

PTG is a protein that is critical for glycogen accumulation in various tissues such as the liver. Our present study shows that its loss sensitizes liver cancer cells towards metabolic and oxidative stress.

Protein targeting to glycogen (PTG) is a ubiquitously expressed scaffolding protein that critically regulates glycogen levels in many tissues, including the liver, muscle and brain. However, its importance in transformed cells has yet to be explored in detail. Since recent studies have demonstrated an important role for glycogen metabolism in cancer cells, we decided to assess the effect of PTG levels on the ability of human hepatocellular carcinoma (HepG2) cells to respond to metabolic stress. Although PTG expression did not significantly affect the proliferation of HepG2 cells under normal culture conditions, we determined that PTG plays an important role during glucose deprivation. Overexpression of PTG protected cells from cell death in the absence of glucose, whereas knocking down PTG further promoted cytotoxicity, as measured by the release of lactate dehydrogenase (LDH) into the media. Additionally, we demonstrated that PTG attenuates glucose deprivation induced haeme oxygenase-1 (HO-1) expression, suggesting that PTG protects against glucose deprivation-induced oxidative stress. Indeed, treating cells with the antioxidant N-acetyl cysteine (NAC) rescued cells from cytotoxicity caused by glucose deprivation. Finally, we showed that loss of PTG resulted in enhanced autophagy. In control cells, glucose deprivation suppressed autophagy as determined by the increase in the levels of p62, an autophagy substrate. However, in knockdown cells, this suppression was relieved. Blockade of autophagy also attenuated cytotoxicity from glucose deprivation in PTG knockdown cells. Taken together, our findings identify a novel role for PTG in protecting hepatocellular carcinoma cells from metabolic stress, in part by regulating oxidative stress and autophagy.

Fasting-induced protein phosphatase 1 regulatory subunit contributes to postprandial blood glucose homeostasis via regulation of hepatic glycogenesis

OBJECTIVE

Most animals experience fasting-feeding cycles throughout their lives. It is well known that the liver plays a central role in regulating glycogen metabolism. However, how hepatic glycogenesis is coordinated with the fasting-feeding cycle to control postprandial glucose homeostasis remains largely unknown. This study determines the molecular mechanism underlying the coupling of hepatic glycogenesis with the fasting-feeding cycle.

RESEARCH DESIGN AND METHODS

Through a series of molecular, cellular, and animal studies, we investigated how PPP1R3G, a glycogen-targeting regulatory subunit of protein phosphatase 1 (PP1), is implicated in regulating hepatic glycogenesis and glucose homeostasis in a manner tightly orchestrated with the fasting-feeding cycle.

RESULTS

PPP1R3G in the liver is upregulated during fasting and downregulated after feeding. PPP1R3G associates with glycogen pellet, interacts with the catalytic subunit of PP1, and regulates glycogen synthase (GS) activity. Fasting glucose level is reduced when PPP1R3G is overexpressed in the liver. Hepatic knockdown of PPP1R3G reduces postprandial elevation of GS activity, decreases postprandial accumulation of liver glycogen, and decelerates postprandial clearance of blood glucose. Other glycogen-targeting regulatory subunits of PP1, such as PPP1R3B, PPP1R3C, and PPP1R3D, are downregulated by fasting and increased by feeding in the liver.

CONCLUSIONS

We propose that the opposite expression pattern of PPP1R3G versus other PP1 regulatory subunits comprise an intricate regulatory machinery to control hepatic glycogenesis during the fasting-feeding cycle. Because of its unique expression pattern, PPP1R3G plays a major role to control postprandial glucose homeostasis during the fasting-feeding transition via its regulation on liver glycogenesis.