Myeloma-Induced Alterations of Glutamine Metabolism Impair Bone Microenvironment Niche in Multiple Myeloma Patients

Multiple myeloma (MM) cells are characterized by tight dependence on the bone marrow (BM) microenvironment that exerts a permissive role on cell growth and survival. In turn, MM cells markedly modify their microenvironment leading, in particular, to the development of osteolytic bone lesions.

Recently, we demonstrated that metabolic alterations is a major feature of MM cells showing that BM plasma of MM patients is characterized by lower levels of Glutamine (Gln) and higher levels of Glutamate (Glu) and ammonium when compared with patients with smoldering MM (SMM) and Monoclonal Gammopathy of Uncertain Significance (MGUS). In the majority of MM patients MM cells are Gln-addicted since they strictly depend on extracellular Gln, do not express Glutamine Synthetase (GS), the enzyme that synthetizes Gln from Glu and ammonium, and are endowed with high levels of the Gln transporter ASCT2. Based on this evidence, we have hypothesized that the peculiar Gln metabolism of MM cells may have a significant impact on the relationship with the bone microenvironment and contribute to the development of osteolytic lesions.

We firstly characterized a panel of human MM cell lines (HMCLs) for their GS expression and response to decreasing levels of Gln. The majority of HMCLs, which did not express GS, consumed large amounts of extracellular Gln but secreted nearly half of the amino acid as Glu. Two HMCLs, MM1.S and U266, with a sizable GS expression, were less sensitive to Gln deprivation and secreted less Glu in the extracellular space compared with GS-negative HMCLs. Consistently, the activity of the Glu exchanger x-CT (the product of SLC7A11 gene) was lower in GS-positive than in GS-negative cells. The response to Gln starvation was then studied in mesenchymal stromal cell line (MSC), as well as in osteoblastic (HOBIT) and pre-osteocytic cells (HOB-01). HOBIT and HOB-01 were more sensitive to Gln depletion than MSC. Indeed, while MSC showed a low EC₅₀ for Gln (0.064mM), which is 10-times lower than the physiological blood Gln concentration (around 0.6 mM), the EC₅₀ values of HOBIT and HOB-01 cells were 0.250 mM and 0.297mM, respectively. Furthermore, L-methionine sulfoximine (MSO), an irreversible inhibitor of GS, emphasized the effects of Gln deprivation on all the cell lines tested. Indeed, Gln deprivation enhanced the expression of GS, suggesting that both stromal and osteoblastic cells exploit the enzyme to counteract Gln deprivation. On the basis of these data, we assessed the effects of Gln and Glu on osteogenic differentiation by incubating MSC, either immortalized or primary, with an osteogenic medium containing different concentrations of Gln and Glu. After 2 weeks, compared with cells differentiated in high Gln/high Glu conditions, MSC incubated in the presence of decreased Gln and increased Glu showed lower osteogenic ability, as assessed by real time PCR and ALP staining. Lastly, MSC co-cultured for 72 hours with GS-negative, but not with GS-positive HMCLs, showed reduced viability and increased GS expression.

Lastly, to put in a translational perspective these in vitro observations, we analyzed the BM plasma levels of Gln and Glu in a cohort of 41 patients with newly diagnosed MM, including 9 smoldering MM (SMM) and 32 active MM patients (20 of them with osteolytic bone disease, 12 of them without bone disease). All 20 osteolytic MM patients had more than three osteolytic lesions. We found that MM patients had lower Gln levels and higher Glu levels than SMM patients. Moreover, when compared with MM patients without bone disease, MM patients with bone disease showed lower levels of Gln and higher levels of Glu. The results of these analyses are being continuously updated increasing the number of samples tested.

Overall, these results indicate that MM cells are able to create a low-Gln/high-Glu bone marrow microenvironment that sustains GS expression in bone cells and impairs their differentiation and viability. Thus, the peculiar metabolic milieu in the MM bone microenvironment affects the relationship between neoplastic and bone cells and may contribute to the development of osteolytic bone disease in MM patients.