Hypophosphorylated pRb knock-in mice exhibit hallmarks of aging and vitamin C-preventable diabetes.

Despite extensive analysis of pRB phosphorylation in vitro, how this modification influences development and homeostasis in vivo is unclear. Here, we show that homozygous Rb∆K4 and Rb∆K7 knock-in mice, in which either four or all seven phosphorylation sites in the C-terminal region of pRb, respectively, have been abolished by Ser/Thr-to-Ala substitutions, undergo normal embryogenesis and early development, notwithstanding suppressed phosphorylation of additional upstream sites. Whereas Rb∆K4 mice exhibit telomere attrition but no other abnormalities, Rb∆K7 mice are smaller and display additional hallmarks of premature aging including infertility, kyphosis, and diabetes, indicating an accumulative effect of blocking pRb phosphorylation. Diabetes in Rb∆K7 mice is insulin-sensitive and associated with failure of quiescent pancreatic ß-cells to re-enter the cell cycle in response to mitogens, resulting in induction of DNA damage response (DDR), senescence-associated secretory phenotype (SASP), and reduced pancreatic islet mass and circulating insulin level. Pre-treatment with the epigenetic regulator vitamin C reduces DDR, increases cell cycle re-entry, improves islet morphology, and attenuates diabetes. These results have direct implications for cell cycle regulation, CDK-inhibitor therapeutics, diabetes, and longevity.

What a pain in the back: a review of current treatment options with a focus on naproxen sodium.

Non-specific low back pain (LBP) represents a challenging and prevalent condition that is one of the most common symptoms leading to primary care physician visits. While established guidelines recommend prioritizing non-pharmacological approaches as the primary course of action, pharmacological treatments are advised when non-pharmacological approaches are ineffective or based on patient preference. These guidelines recommend non-steroidal anti-inflammatory drugs (NSAIDs) or skeletal muscle relaxers (SMRs) as the first-line pharmacological options for acute or subacute LBP, while NSAIDs are the exclusive first-line pharmacological option for chronic LBP. Although SMRs are generally effective for acute LBP, the available evidence does not support the view that they improve functional recovery, and their comparative efficacy to NSAIDs and other analgesics remains unknown, while studies have shown them to introduce adverse events without significantly reducing LBP. Moreover, opioids continue to be widely prescribed for LBP, despite limited evidence for effectiveness and known risks of addiction and overdose. Broader use of non-opioid pharmacotherapy, including the appropriate use of OTC options, is critical to addressing the opioid crisis. The balance of evidence indicates that NSAIDs have a favorable benefit-risk profile when compared to other available pharmacological treatment options for non-specific LBP, a condition that is primarily acute in nature and well-suited for self-treatment with OTC analgesics. While clinical guidelines do not differentiate between NSAIDs, evidence indicates that OTC naproxen sodium effectively relieves pain across multiple types of pain models, and furthermore, the 14-h half-life of naproxen sodium allows sustained, all day pain relief with reduced patient pill burden as compared to shorter acting options. Choosing the most appropriate approach for managing LBP, including non-pharmacological options, should be based on the patient's condition, severity of pain, potential risks, and individual patient preference and needs.

Direct and indirect effects of the pRb tumor suppressor on autophagy.

Autophagy, an intracellular degradation pathway involved in cell survival or demise, is tightly controlled by complex regulatory mechanisms. A link between the Rb tumor suppressor and autophagy is now emerging. pRb plays a critical role in cell cycle progression and survival as well as the differentiation of certain cell types. Recently, we have reported that during skeletal myogenesis, Rb-deficient myoblasts fuse to form short myotubes that quickly degenerate. Myotube degeneration was associated with increased autophagic flux and inhibition of autophagy rescued the defect leading to long, twitching myotubes. We propose that Rb-loss sensitizes cells to autophagy via direct and indirect mechanisms and we discuss how these might affect cancer progression and response to chemotherapy.

Multiple pathways counteract cell death induced by RB1 loss: implications for cancer.

Inactivation of the tumor suppressor RB1 leads to cell proliferation, cell death and abortive differentiation in certain tissues and physiological contexts. Anti-apoptotic signals are thought to be the most important mechanism by which RB1-mutant cells escape cell death. Indeed, in the course of neoplastic transformation RB1 is often inactivated in conjunction with a mutation in the pro-apoptotic tumor suppressor p53. We have previously devised a biological framework to identify factors that maintain survival of differentiating Rb-deficient muscle fibers. We showed that differentiating Rb-deficient myoblasts fuse to form short myotubes that degenerate in a process associated with enhanced autophagy, and that degeneration was rescued by antagonists of apoptosis or autophagy, induction of mitochondrial-biogenesis or hypoxia-induced glycolytic shift, leading to long, twitching myotubes. Here, we also show that lithium slows the collapse of Rb-deficient myotubes and surprisingly, this is independent of autophagy, cyclin D3 and ß-catenin. Thus, several distinct processes can suppress cell death induced by RB1 loss. We discuss these pathways and how they may cooperate with RB1 inactivation in the course of cancer initiation.

Critical role of the Rb family in myoblast survival and fusion.

The tumor suppressor Rb is thought to control cell proliferation, survival and differentiation. We recently showed that differentiating Rb-deficient mouse myoblasts can fuse to form short myotubes that quickly collapse through a mechanism involving autophagy, and that autophagy inhibitors or hypoxia could rescue the defect leading to long, twitching myotubes. Here we determined the contribution of pRb relatives, p107 and p130, to this process. We show that chronic or acute inactivation of Rb plus p107 or p130 increased myoblast cell death and reduced myotube formation relative to Rb loss alone. Treatment with autophagy antagonists or hypoxia extended survival of double-knockout myotubes, which appeared indistinguishable from control fibers. In contrast, triple mutations in Rb, p107 and p130, led to substantial increase in myoblast death and to elongated bi-nuclear myocytes, which seem to derive from nuclear duplication, as opposed to cell fusion. Under hypoxia, some rare, abnormally thin triple knockout myotubes survived and twitched. Thus, mutation of p107 or p130 reduces survival of Rb-deficient myoblasts during differentiation but does not preclude myoblast fusion or necessitate myotube degeneration, whereas combined inactivation of the entire Rb family produces a distinct phenotype, with drastically impaired myoblast fusion and survival.

Rescue of myogenic defects in Rb-deficient cells by inhibition of autophagy or by hypoxia-induced glycolytic shift.

The retinoblastoma tumor suppressor (pRb) is thought to orchestrate terminal differentiation by inhibiting cell proliferation and apoptosis and stimulating lineage-specific transcription factors. In this study, we show that in the absence of pRb, differentiating primary myoblasts fuse to form short myotubes that never twitch and degenerate via a nonapoptotic mechanism. The shortened myotubes exhibit an impaired mitochondrial network, mitochondrial perinuclear aggregation, autophagic degradation, and reduced adenosine triphosphate production. Bcl-2 and autophagy inhibitors restore mitochondrial function and rescue muscle degeneration, leading to formation of long, twitching myotubes that express normal levels of muscle-specific proteins and stably exit the cell cycle. A hypoxia-induced glycolytic switch also rescues the myogenic defect after either chronic or acute inactivation of Rb in a hypoxia-inducible factor-1 (HIF-1)-dependent manner. These results demonstrate that pRb is required to inhibit apoptosis in myoblasts and autophagy in myotubes but not to activate the differentiation program, and they also reveal a novel link between pRb and cell metabolism.