Autocrine amplification of immature myeloid cells by IL-6 in multiple myeloma-infiltrated bone marrow.

Multiple myeloma (MM) invariably develops in the bone marrow (BM), indicating the strong requirement of this tumor for the peculiar BM microenvironment, rich in cytokine and hematopoietic precursor cells. Interleukin-6 (IL-6) and a proliferation inducing ligand (APRIL) are key cytokines implicated in MM development. Here, we show that MM cells changed the hematopoietic microenvironment early upon BM infiltration by strongly downregulating hematopoietic precursor cells from all lineages except myeloid precursor cells. Myeloid precursor cells constituted a major source of APRIL in MM-infiltrated BM, and their proliferative response to IL-6 upregulation explained their relative resistance to MM infiltration. The osteolytic molecule receptor activator of NF-kB ligand (RANK-L) expressed by MM cells started this myeloid proliferation by inducing in a contact-dependent manner IL-6 production by myeloid precursor cells themselves. Taken together, our data demonstrate that MM cells do not simply displace hematopoietic cells upon BM infiltration, but rather selectively modulate the BM microenvironment to preserve a pool of high APRIL-producing myeloid precursor cells. Our data also identify a positive regulation of APRIL by IL-6 in myeloid precursor cells.

Myelopoiesis dysregulation associated to sustained APRIL production in multiple myeloma-infiltrated bone marrow.

Multiple myeloma (MM) is a non-curable tumor developing in the bone marrow (BM). The BM microenvironment rich in hematopoietic precursors is suspected to have a role in MM development. Here we show that a proliferation-inducing ligand (APRIL) mediated in vivo MM promotion. In MM-infiltrated BM, APRIL originated from differentiating myeloid cells with an expression peak in precursor cells. Notably, APRIL expression stayed stable in BM despite MM infiltration. The pool of APRIL-producing cells changed upon MM infiltration. Although CD16(+) mature myeloid cells constituted about half of the APRIL-producing cells in healthy BM, CD16(-) Elastase(+) myeloid precursor cells were predominant in MM-infiltrated BM. Myeloid precursor cells secreted all the APRIL they produced, and binding of secreted APRIL to MM cells, strictly dependent of heparan sulfate carried by CD138, resulted in an in situ internalization by tumor cells. This indicated APRIL consumption by MM in BM. Taken together, our data show that myelopoiesis dysregulation characterized by an increased proportion of precursor cells occurs in MM patients. Such dysregulation correlates with a stable expression of the MM-promoting factor APRIL in infiltrated BM.

Suppression of UV-induced apoptosis by the human DNA repair protein XPG.

The severe xeroderma pigmentosum/Cockayne syndrome (XP/CS) syndrome is caused by mutations in the XPB, XPD and XPG genes that encode the helicase subunits of TFIIH and the 3' endonuclease of nucleotide excision repair (NER). Because XPB and XPD have been implicated in p53-mediated apoptosis, we examined the possible involvement of XPG in this process. After ultraviolet light (UV) irradiation, primary fibroblasts of XP complementation group G (XP-G) individuals with CS enter apoptosis more readily than other NER-deficient cells, but this is unlinked to unrepaired damage. These XP-G/CS cells accumulate p53 post-UV but they fail to accumulate the 90/92 kDa isoforms of Mdm2 and their cellular distribution of Mdm2 is impaired. Apoptosis levels revert to wild type, Mdm2 90/92 kDa isoforms accumulate, and Mdm2 regains its normal post-UV nuclear location in transduced XP-G/CS cells expressing wild-type XPG, but not an XPG catalytic site mutant. These results suggest that XPG suppresses UV-induced apoptosis and that this suppression, most simply, requires its endonuclease function.

A fission yeast homolog of Int-6, the mammalian oncoprotein and eIF3 subunit, induces drug resistance when overexpressed.

Through a screen to identify genes that induce multi-drug resistance when overexpressed, we have identified a fission yeast homolog of Int-6, a component of the human translation initiation factor eIF3. Disruption of the murine Int-6 gene by mouse mammary tumor virus (MMTV) has been implicated previously in tumorigenesis, although the underlying mechanism is not yet understood. Fission yeast Int6 was shown to interact with other presumptive components of eIF3 in vivo, and was present in size fractions consistent with its incorporation into a 43S translation preinitiation complex. Drug resistance induced by Int6 overexpression was dependent on the AP-1 transcription factor Pap1, and was associated with increased abundance of Pap1-responsive mRNAs, but not with Pap1 relocalization. Fission yeast cells lacking the int6 gene grew slowly. This growth retardation could be corrected by the expression of full length Int6 of fission yeast or human origin, or by a C-terminal fragment of the fission yeast protein that also conferred drug resistance, but not by truncated human Int-6 proteins corresponding to the predicted products of MMTV-disrupted murine alleles. Studies in fission yeast may therefore help to explain the ways in which Int-6 function can be perturbed during MMTV-induced mammary tumorigenesis.