Decreased Thymosin Beta 4 Expression Results in Poor Prognosis and Decreased Survival in Multiple Myeloma.

Thymosin beta 4 (Tβ4) is a small cytoplasmic and nuclear protein involved in different cellular processes including differentiation, migration and proliferation. It is overexpressed in several malignant cells, resulting in an increased angiogenic respons and metastatic potential of tumor cells. In the current study we wanted to evaluate the expression of Tβ4 in primary human multiple myeloma cells (MM) and murine 5TMM cells. Microarray analysis and qRT-PCR of 171 MM patients treated with high dose therapy and autologous stem cell transplantation, showed that MM cells display a decreased Tβ4 expression compared to plasma cells from healthy individuals. We investigated whether Tβ4 expression in MM cells influences the event free (EFS) and overall survival (OAS) of these patients. As Tβ4 was expressed in all MM cells, we compared survival of patients with Tβ4 expression in MM cells in the upper thirtile (Tβ4high MMC) against the lowest thirtile (Tβ4low MMC). Patients with Tβ4high MMC had an increased EFS (P< 0.05) and also their OAS tended to be longer. A similar observation was made in the 5TMM model where qRT-PCR and ELISA revealed a decreased expression of Tβ4 in BM samples from 5T33MM and 5T2MM diseased mice compared to control mice. To study the functionality of Tβ4, we overexpressed Tβ4 in 5T33MMvitro (5T33MMvt) cells by lentiviral transduction. These 5T33MMvt-Tβ4+ cells demonstrated a significantly decreased proliferative capacitity and an increased sensitivity to different anti-myeloma agents (bortezomib, melphalan, dexamethasone) compared to control 5T33MMvt cells. Furthermore, injection of 5T33MMvt-Tβ4+ cells in C57BlKaLwRij mice resulted in a significant decreased tumor formation (paraprotein concentration was 1.92 g/dl versus 3.74 g/dl for untransduced cells, p<0.05) and a prolonged survival compared to mice injected with untransduced 5T33MMvt cells (respectively 65.9 days versus 88.9 days, p< 0.05). In conclusion, we found a decreased Tβ4 expression in human and murine MM cells compared to normal plasma cells. In our cohort of MM patients treated with high-dose chemotherapy, low expression of TB4 identifies a group of patients with adverse prognosis while in the murine 5TMM33vt model, overexpression of Tβ4 resulted in an inhibitory effect on tumor formation suggesting a role of Tβ4 as a tumor suppressor gene in MM.