PF566 INCREASE OF MITOCHONDRIAL ACTIVITY CONTRIBUTES TO RELAPSE IN MULTIPLE MYELOMA, A NOVEL THERAPEUTIC OPPORTUNITY
Authors: Ortiz‐Ruiz A. , Ruiz‐Heredia Y. , Aguilar‐Garrido P. , Morales M.L. , Valeri A. , Barcena C. , García‐Martín R.M. , Garrido V. , Martinez‐Lopez J. , Linares M. , Gallardo M. 
Published: June 2019

Background:

Preliminary results (Ruiz‐Heredia, ASH 2017) showed an increase of the mitochondrial burden in the Multiple Myeloma (MM) and its progression, suggesting to have an important role in the pathogenesis of the disease.

Aims:

We planned to decipher the role of the mitochondria in the MM disease and its progression, and the potential exploitation of mitochondrial activity as a functional target in the MM therapy.

Methods:

In order to understand the role of mitochondria in MM and its therapeutic exploitation, firstly we studied factors involved in the mitochondrial function gene expression (c‐Myc, TFAM, EF‐Tu, NRF1 and HNRNPK RT‐PCR), protein expression (COXII IHC), and mitochondrial activity (histoenzymatic‐HE assay of COX) in 77 MM patients.

Furthermore, we have tested the effect of tigecycline, a mitochondrial inhibitor, alone or in combination with bortezomib in the MM cell lines JJN3, L363 and NCI‐H929. To characterized the molecular mechanisms underlying the cytotoxic effect of tigecycline we analysed OXPHOS expression by WB and mitochondria activity by HE assay. Finally, we followed an in vivo experiment in NSG mice (n = 40) 6–8 week‐old engrafted with the JJN3‐GFP cell line (1 x 106) via tail vein and treated by 4 weeks. Analysis of the in vivo imaging and survival curve were obtained.

Results:

Mitochondrial representative gene and protein expression and activity were found increased in MM relapse stage patients. We showed overexpression of C‐Myc, TFAM and EF‐Tu, and an increased tendency for HNRNPK on the MM relapsed group (n = 34) (Fig. 1A). Moreover, IHC reveals overexpression of mitochondrial COXII protein in relapse MM patients (n = 32; p‐value ∗∗ <0.001) (Fig. 1B). By functional assays we have demonstrated that gene and protein overexpression drives to an increase of activity (COX HE) in MM at relapse (N = 11 p‐value ∗∗∗<0.0001). (Fig. 1C). Together these results suggest elevation of mitochondrial activity plays a role in the mechanism of resistance to treatment and/or progression of MM and the consequent relapse of the patients.

In vitro studies with tigecyline and the combination with the frontline MM drug, bortezomib showed cytotoxic effects in three MM cell lines (IC50 JJN3 11,91 μM; IC50 L363 10,21 μM and NCI‐H929 26,37 μM, p‐value ∗<0.05). Moreover, bortezomib and tigecyline showed high levels of synergism (CI 0,19) (Fig. 1D). The cells treated with tigecycline and the combination reflect diminishing in the expression of the respiratory chain complexes, suggesting a reduction of mitochondrial activity, and being validated in lower activity measured by the HE assay of COX (Fig. 1E). Finally, mice groups treated with tigecycline alone or in combination with bortezomib reported a better survival and lower JJN3‐GFP infiltration (n = 40; p‐value ∗<0.05) (Fig. 1F).

Summary/Conclusion:

To sum up, we observed that mitochondria activity plays a pivotal role in MM development and specially its progression and/or potential treatment resistance, correlating the increase of activity with MM patient's poor outcomes and relapse prone.

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We proof the potential therapeutic exploitation of mitochondria in the disease, and the value of tigecycline, in monotherapy and in a synergic combination with frontline MM treatment Bortezomib. This new approach should be considered as a novel target therapy in MM, especially in relapse patients.