Regulation of retinoblastoma protein (Rb) by p21 is critical for adaptation to massive small bowel resection

Intestinal adaptation and rehabilitation

Massive intestinal resection is a regrettably necessary but life-saving intervention for progressive or fulminant necrotizing enterocolitis (NEC). However, the resultant short bowel syndrome (SBS) poses its own array of challenges and complications. Within hours of such an abrupt loss of intestinal length, the intestine begins to adapt. Our ability to understand this process of intestinal adaptation has proven critical in our ability to clinically treat the challenging problem of short bowel syndrome. This review first highlights key data relating to intestinal adaptation including structural and functional changes, biochemical regulation, and other factors affecting the magnitude of intestinal adaptation responses. We then focus on intestinal rehabilitation as it relates to strategies to enhance intestinal adaptation while meeting nutritional needs and preventing complications of parenteral nutrition.

miRNA-17-5p Target Prediction and its Role in Senescence Mechanism through p21 Interference

BACKGROUND: Cellular senescence is known to be correlated with the cessation of cell cycle. The progression of cell cycle is promoted by activities of various proteins, including cyclin-dependent kinase (CDK) and cyclin proteins, which work synergistically. CDK-cyclin complexes are influenced by other proteins, such as retinoblastoma (Rb) and E2F proteins. In cell cycle, both Rb and E2F proteins could be affected by one of the CDK inhibitors, that is, p21. MicroRNA (miRNA) is well known for its role in biological processes, including cell cycle. However, the contribution of miRNA in cell cycle is still poorly understood. Some miRNAs play a role in pro-proliferation and anti-proliferation. AIM: This study was performed an in silico study analysis to reveal the relationship between miRNA-17-5p and p21 in the process of cellular senescence. METHODS: The extensive data mining was conducted to determine the miRNA that contributes to the process of anti-aging prevention and the desired target genes through the Human Protein Atlas and cancer database. miRNA target prediction was performed using DIANA-microT-CDS. Gene function of the miRNA-17-5p target was annotated using DAVID GO. RESULTS: The sequence of hsa-miRNA-17-5p (CAAAGUGCUUACAGUGCAGGUAG) has three attachment sites with binding types of 8 mer, 6 mer, and 8 mer at the transcription sites of 447–474, 485–513, and 1132–1154, respectively. The main profile of hsa-miRNA-17-5p showed that it bound to 3’-untranslated region and the coding region (exon). CONCLUSIONS: The miRNA-17-5p was involved in cellular senescence by influencing the process of cell proliferation in the cell cycle pathway.

Mitochondrial AIF loss causes metabolic reprogramming, caspase-independent cell death blockade, embryonic lethality, and perinatal hydrocephalus

OBJECTIVES

Apoptosis-Inducing Factor (AIF) is a protein involved in mitochondrial electron transport chain assembly/stability and programmed cell death. The relevant role of this protein is underlined because mutations altering mitochondrial AIF properties result in acute pediatric mitochondriopathies and tumor metastasis. By generating an original AIF-deficient mouse strain, this study attempted to analyze, in a single paradigm, the cellular and developmental metabolic consequences of AIF loss and the subsequent oxidative phosphorylation (OXPHOS) dysfunction.

METHODS

We developed a novel AIF-deficient mouse strain and assessed, using molecular and cell biology approaches, the cellular, embryonic, and adult mice phenotypic alterations. Additionally, we conducted ex vivo assays with primary and immortalized AIF knockout mouse embryonic fibroblasts (MEFs) to establish the cell death characteristics and the metabolic adaptive responses provoked by the mitochondrial electron transport chain (ETC) breakdown.

RESULTS

AIF deficiency destabilized mitochondrial ETC and provoked supercomplex disorganization, mitochondrial transmembrane potential loss, and high generation of mitochondrial reactive oxygen species (ROS). AIF-/Y MEFs counterbalanced these OXPHOS alterations by mitochondrial network reorganization and a metabolic reprogramming toward anaerobic glycolysis illustrated by the AMPK phosphorylation at Thr172, the overexpression of the glucose transporter GLUT-4, the subsequent enhancement of glucose uptake, and the anaerobic lactate generation. A late phenotype was characterized by the activation of P53/P21-mediated senescence. Notably, approximately 2% of AIF-/Y MEFs diminished both mitochondrial mass and ROS levels and spontaneously proliferated. These cycling AIF-/Y MEFs were resistant to caspase-independent cell death inducers. The AIF-deficient mouse strain was embryonic lethal between E11.5 and E13.5 with energy loss, proliferation arrest, and increased apoptotic levels. Contrary to AIF-/Y MEFs, the AIF KO embryos were unable to reprogram their metabolism toward anaerobic glycolysis. Heterozygous AIF+/- females displayed progressive bone marrow, thymus, and spleen cellular loss. In addition, approximately 10% of AIF+/- females developed perinatal hydrocephaly characterized by brain development impairment, meningeal fibrosis, and medullar hemorrhages; those mice died 5 weeks after birth. AIF+/- with hydrocephaly exhibited loss of ciliated epithelium in the ependymal layer. This phenotype was triggered by the ROS excess. Accordingly, it was possible to diminish the occurrence of hydrocephalus AIF+/- females by supplying dams and newborns with an antioxidant in drinking water.

CONCLUSIONS

In a single knockout model and at 3 different levels (cell, embryo, and adult mice) we demonstrated that by controlling the mitochondrial OXPHOS/metabolism, AIF is a key factor regulating cell differentiation and fate. Additionally, by providing new insights into the pathological consequences of mitochondrial OXPHOS dysfunction, our new findings pave the way for novel pharmacological strategies.

Hepatic senescence, the good and the bad

Gradual alterations of cell’s physiology and functions due to age or exposure to various stresses lead to the conversion of normal cells to senescent cells. Once becoming senescent, the cell stops dividing permanently but remains metabolically active. Cellular senescence does not have a single marker but is characterized mainly by a combination of multiple markers, such as, morphological changes, expression of cell cycle inhibitors, senescence associated β-galactosidase activity, and changes in nuclear membrane. When cells in an organ become senescent, the entire organism can be affected. This may occur through the senescence-associated secretory phenotype (SASP). SASP may exert beneficial or harmful effects on the microenvironment of tissues. Research on senescence has become a very exciting field in cell biology since the link between age-related diseases, including cancer, and senescence has been established. The loss of regenerative and homeostatic capacity of the liver over the age is somehow connected to cellular senescence. The major contributors of senescence properties in the liver are hepatocytes and cholangiocytes. Senescent cells in the liver have been implicated in the etiology of chronic liver diseases including cirrhosis and hepatocellular carcinoma and in the interference of liver regeneration. This review summarizes recently reported findings in the understanding of the molecular mechanisms of senescence and its relationship with liver diseases.

The integrity of the FOG-2 LXCXE pRb-binding motif is required for small intestine homeostasis

NEW FINDINGS

What is the central question of this study? Do Fog2^{Rb- / Rb-} mice present a defect of small intestine homeostasis? What is the main finding and its importance? The importance of interactions between FOG-2 and pRb in adipose tissue physiology has previously been demonstrated. Here it is shown that this interaction is also intrinsic to small intestine homeostasis and exerts extrinsic control over mouse metabolism. Thus, this association is involved in maintaining small intestine morphology, and regulating crypt proliferation and lineage differentiation. It therefore affects mouse growth and adaptation to a high-fat diet.

ABSTRACT

GATA transcription factors and their FOG cofactors play a key role in tissue-specific development and differentiation, from worms to humans. We have shown that GATA-1 and FOG-2 contain an LXCXE pRb-binding motif. Interactions between retinoblastoma protein (pRb) and GATA-1 are crucial for erythroid proliferation and differentiation, whereas the LXCXE pRb-binding site of FOG-2 is involved in adipogenesis. Fog2-knock-in mice have defective pRb binding and are resistant to obesity, due to efficient white-into-brown fat conversion. Our aim was to investigate the pathophysiological impact of FOG-2-pRb interaction on the small intestine and mouse growth. Histological analysis of the small intestine revealed architectural changes in Fog2^{Rb- / Rb-} mice, including villus shortening, with crypt expansion and a change in muscularis propria thickness. These differences were more marked in the proximo-distal part of the small intestine and were associated with an increase in crypt cell proliferation and disruption of the goblet and Paneth cell lineage. The small intestine of the mutants was unable to adapt to a high-fat diet, and had significantly lower plasma lipid levels on such a diet. Fog2^{Rb- / Rb-} mice displayed higher levels of glucose-dependent insulinotropic peptide release, and lower levels of insulin-like growth factor I release on a regular diet. Their intestinal lipid absorption was impaired, resulting in restricted weight gain. In addition to the intrinsic effects of the mutation on adipose tissue, we show here an extrinsic relationship between the intestine and the effect of FOG-2 mutation on mouse metabolism. In conclusion, the interaction of FOG-2 with pRb coordinates the crypt-villus axis and controls small intestine homeostasis.

Epithelial IGF1R is dispensable for IGF2 mediated enhanced intestinal adaptation in retinoblastoma-deficient mice

PURPOSE

Previously, we demonstrated enhanced adaptation after small bowel resection (SBR) in intestinal-specific retinoblastoma (Rb)-deficient mice along with elevated levels of insulin-like growth factor 2 (IGF2) expression within the villi. The purpose of this study was to verify that the insulin-like growth factor 1 receptor (IGF1R) plays a role in this phenomenon.

METHODS

Inducible and intestinal specific Rb and IGF1R double knockout mice (iRb/IGF1R-IKO) (n=4) and Rb single knockout mice (iRb-IKO) (n=5) underwent 50% mid SBR. On post-operative day 28, mice were harvested, and structural adaptation was measured as changes in crypt depth and villus height. Rates of enterocyte proliferation were recorded. IGF2 expression within the remnant villi was measured via RT-PCR.

RESULTS

Both iRb-IKO and iRb/IGF1R-IKO mice demonstrated enhanced adaptation with at least a 45% increase in both crypt depth and villus height in the proximal and distal remnant bowel. Both groups showed elevation of IGF2 expression in the remnant villi, but there were no differences between the two groups.

CONCLUSION

Epithelial IGF1R is dispensable for IGF2-mediated enhanced intestinal adaptation in retinoblastoma-deficient mice. Our findings suggest that IGF2 signals for enhanced adaptation in cells outside of the epithelium. Further investigation is needed to study the IGF2/IGF1R signaling interaction within the mesenchyme.

LEVEL OF EVIDENCE

Animal study - not clinical.

The Pathogenesis of Resection-Associated Intestinal Adaptation

After massive small-bowel resection, the remnant bowel compensates by a process termed adaptation. Adaptation is characterized by villus elongation and crypt deepening, which increases the capacity for absorption and digestion per unit length. The mechanisms/mediators of this important response are multiple. The purpose of this review is to highlight the major basic contributions in elucidating a more comprehensive understanding of this process.

IGF-2 is necessary for retinoblastoma-mediated enhanced adaptation after small-bowel resection

Previously, we have demonstrated that genetically disrupting retinoblastoma protein (Rb) expression in enterocytes results in taller villi, mimicking resection-induced adaption responses. Rb deficiency also results in elevated insulin-like growth factor-2 (IGF-2) expression in villus enterocytes. We propose that postoperative disruption of Rb results in enhanced adaptation which is driven by IGF-2. Inducible, intestine-specific Rb-null mice (iRbIKO) and wild-type (WT) littermates underwent a 50% proximal small-bowel resection (SBR) at 7-9 weeks of age. They were then given tamoxifen on postoperative days (PODs) 4-6 and harvested on POD 28. The experiment was then repeated on double knockouts of both IGF-2 and Rb (IGF-2 null/iRbIKO). iRbIKO mice demonstrated enhanced resection-induced adaptive villus growth after SBR and increased IGF-2 messenger RNA (mRNA) in ileal villus enterocytes compared to their WT littermates. In the IGF-2 null/iRbIKO double-knockout mice, there was no additional villus growth beyond what was expected of normal resection-induced adaptation. Adult mice in which Rb is inducibly deleted from the intestinal epithelium following SBR have augmented adaptive growth. IGF-2 expression is necessary for enhanced adaptation associated with acute intestinal Rb deficiency.

Insulin-like growth factor 2 and its enterocyte receptor are not required for adaptation in response to massive small bowel resection

PURPOSE

Enhanced structural features of resection-induced intestinal adaptation have been demonstrated following the administration of multiple different growth factors and peptides. Among these, the insulin-like growth factor (IGF) system has been considered to be significant. In this study, we employ mutant mouse strains to directly test the contribution of IGF2 and its enterocyte receptor (IGF1R) toward the adaptation response to massive small bowel resection (SBR).

METHODS

IGF2-knockout (IGF2-KO) (n=8) and intestine specific IGF1R-knockout mice (IGF1R-IKO) (n=9) and their wild type (WT) littermates (n=5, n=7, respectively) underwent 50% proximal SBR. At post-operative day 7, structural adaptation was measured as crypt depth and villus height. Rates of enterocyte proliferation and apoptosis were also recorded.

RESULTS

The successful deletion of IGF2 and IGF1R expression in the enterocytes was confirmed by RT-PCR and Western blot, respectively. Normal adaptation occurred in both IGF2-KO and IGF1R-IKO mice after 50% SBR. Post-operative rates of proliferation and apoptosis in both IGF2-KO and IGF1R-IKO mice were no different than their respective controls.

CONCLUSION

IGF2 and functional IGF1R signaling in enterocytes are both dispensable for resection-induced adaptation responses. The mechanism for IGF-stimulation of intestinal adaptation may involve other ligands or cellular compartments within the intestine.

Disruption of retinoblastoma protein expression in the intestinal epithelium impairs lipid absorption

We previously demonstrated increased villus height following genetic deletion, or knockout, of retinoblastoma protein (Rb) in the intestinal epithelium (Rb-IKO). Here we determined the functional consequences of augmented mucosal growth on intestinal fat absorption and following a 50% small bowel resection (SBR). Mice with constitutively disrupted Rb expression in the intestinal epithelium (Rb-IKO) along with their floxed (wild-type, WT) littermates were placed on a high-fat diet (HFD, 42% kcal fat) for 54 wk. Mice were weighed weekly, and fat absorption, indirect calorimetry, and MRI body composition were measured. Rb-IKO mice were also subjected to a 50% SBR, followed by HFD feeding for 33 wk. In separate experiments, we examined intestinal fat absorption in mice with conditional (tamoxifen-inducible) intestinal Rb (inducible Rb-IKO) deletion. Microarray revealed that the transcriptional expression of lipid absorption/transport genes was significantly reduced in constitutive Rb-IKO mice. These mice demonstrated greater mucosal surface area yet manifested paradoxically impaired intestinal long-chain triglyceride absorption and decreased cholesterol absorption. Despite attenuated lipid absorption, there were no differences in metabolic rate, body composition, and weight gain in Rb-IKO and WT mice at baseline and following SBR. We also confirmed fat malabsorption in inducible Rb-IKO mice. We concluded that, despite an expanded mucosal surface area, Rb-IKO mice demonstrate impaired lipid absorption without compensatory alterations in energy homeostasis or body composition. These findings underscore the importance of delineating structural/functional relationships in the gut and suggest a previously unknown role for Rb in the regulation of intestinal lipid absorption.

IGF-2 mediates intestinal mucosal hyperplasia in retinoblastoma protein (Rb)-deficient mice

PURPOSE

We have previously demonstrated a hyperplastic phenotype when Rb expression was disrupted within the intestinal epithelium. These findings mimic resection-induced adaptation suggesting a possible mechanistic role for Rb during adaptation. The purpose of the present study was to elucidate a mechanism for how Rb deficiency induces intestinal hyperplasia.

METHODS

Enterocytes isolated from intestine-specific Rb knockout mice (Rb-IKO) underwent a microarray to elucidate their gene expression profile. IGF2 expression was significantly elevated, which was subsequently confirmed by RT-PCR and in situ mRNA hybridization. Mice with deficient expression of IGF2 or its receptor IGF1R were therefore crossed with Rb-IKO mice to determine the significance of IGF2 in mediating the Rb-IKO intestinal phenotype.

RESULTS

Expression of IGF2 was significantly elevated in villus enterocytes of Rb-IKO mice. The mucosal hyperplasia in Rb-IKO mice was reversed when either IGF2 or IGF1R expression was genetically disrupted in Rb-IKO mice.

CONCLUSION

IGF-2 expression is significantly elevated in villus enterocytes and is required for the hyperplastic intestinal mucosal phenotype of Rb-IKO mice. The trophic effects of IGF2 require intact IGF1R signaling within the intestinal epithelium. These findings reveal novel regulatory roles for Rb in expanding intestinal mucosal surface area.

Adaptation: paradigm for the gut and an academic career

Adaptation is an important compensatory response to environmental cues resulting in enhanced survival. In the gut, the abrupt loss of intestinal length is characterized by increased rates of enterocyte proliferation and apoptosis and culminates in adaptive villus and crypt growth. In the development of an academic pediatric surgical career, adaptation is also an important compensatory response to survive the ever changing research, clinical, and economic environment. The ability to adapt in both situations is critical for patients and a legacy of pediatric surgical contributions to advance our knowledge of multiple conditions and diseases.