Glycogen synthase kinase 3alpha and 3beta mediate a glucose-sensitive antiapoptotic signaling pathway to stabilize Mcl-1.

Glucose uptake and utilization are growth factor-stimulated processes that are frequently upregulated in cancer cells and that correlate with enhanced cell survival. The mechanism of metabolic protection from apoptosis, however, has been unclear. Here we identify a novel signaling pathway initiated by glucose catabolism that inhibited apoptotic death of growth factor-deprived cells. We show that increased glucose metabolism protected cells against the proapoptotic Bcl-2 family protein Bim and attenuated degradation of the antiapoptotic Bcl-2 family protein Mcl-1. Maintenance of Mcl-1 was critical for this protection, as glucose metabolism failed to protect Mcl-1-deficient cells from apoptosis. Increased glucose metabolism stabilized Mcl-1 in both cell lines and primary lymphocytes via inhibitory phosphorylation of glycogen synthase kinase 3alpha and 3beta (GSK-3alpha/beta), which otherwise promoted Mcl-1 degradation. While a number of kinases can phosphorylate and inhibit GSK-3alpha/beta, we provide evidence that protein kinase C may be stimulated by glucose-induced alterations in diacylglycerol levels or distribution to phosphorylate GSK-3alpha/beta, maintain Mcl-1 levels, and inhibit cell death. These data provide a novel nutrient-sensitive mechanism linking glucose metabolism and Bcl-2 family proteins via GSK-3 that may promote survival of cells with high rates of glucose utilization, such as growth factor-stimulated or cancerous cells.

Activation of Wnt signaling in hematopoietic regeneration.

Hematopoietic stem cells (HSCs) respond to injury by rapidly proliferating and regenerating the hematopoietic system. Little is known about the intracellular programs that are activated within HSCs during this regenerative process and how this response may be influenced by alterations in signals from the injured microenvironment. Here we have examined the regenerating microenvironment and find that following injury it has an enhanced ability to support HSCs. During this regenerative phase, both hematopoietic and stromal cell elements within the bone marrow microenvironment show increased expression of Wnt10b, which can function to enhance growth of hematopoietic precursors. In addition, regenerating HSCs show increased activation of Wnt signaling, suggesting that microenvironmental changes in Wnt expression after injury may be integrated with the responses of the hematopoietic progenitors. Cumulatively, our data reveal that growth signals in the hematopoietic system are re-activated during injury, and provide novel insight into the influence of the microenvironment during regeneration.

Integration of Notch and Wnt signaling in hematopoietic stem cell maintenance.

A fundamental question in hematopoietic stem cell (HSC) biology is how self-renewal is controlled. Here we show that the molecular regulation of two critical elements of self-renewal, inhibition of differentiation and induction of proliferation, can be uncoupled, and we identify Notch signaling as a key factor in inhibiting differentiation. Using transgenic Notch reporter mice, we found that Notch signaling was active in HSCs in vivo and downregulated as HSCs differentiated. Inhibition of Notch signaling led to accelerated differentiation of HSCs in vitro and depletion of HSCs in vivo. Finally, intact Notch signaling was required for Wnt-mediated maintenance of undifferentiated HSCs but not for survival or entry into the cell cycle in vitro. These data suggest that Notch signaling has a dominant function in inhibiting differentiation and provide a model for how HSCs may integrate multiple signals to maintain the stem cell state.