A Mortalin/HSPA9-Mediated Switch in Tumor-Suppressive Signaling of Raf/MEK/Extracellular Signal-Regulated Kinase

Abstract

Dysregulated Raf/MEK/extracellular signal-regulated kinase (ERK) signaling, a common hallmark of tumorigenesis, can trigger innate tumor-suppressive mechanisms, which must be inactivated for carcinogenesis to occur. This innate tumor-suppressive signaling may provide a potential therapeutic target. Here we report that mortalin (HSPA9/GRP75/PBP74) is a novel negative regulator of Raf/MEK/ERK and may provide a target for the reactivation of tumor-suppressive signaling of the pathway in cancer. We found that mortalin is present in the MEK1/MEK2 proteome and is upregulated in human melanoma biopsy specimens. In different MEK/ERK-activated cancer cell lines, mortalin depletion induced cell death and growth arrest, which was accompanied by increased p21^CIP1 transcription and MEK/ERK activity. Remarkably, MEK/ERK activity was necessary for mortalin depletion to induce p21^CIP1 expression in B-Raf^V600E-transformed cancer cells regardless of their p53 status. In contrast, in cell types exhibiting normal MEK/ERK status, mortalin overexpression suppressed B-Raf^V600E- or ΔRaf-1:ER-induced MEK/ERK activation, p21^CIP1 expression, and cell cycle arrest. Other HSP70 family chaperones could not effectively replace mortalin for p21^CIP1 regulation, suggesting a unique role for mortalin. These findings reveal a novel mechanism underlying p21^CIP1 regulation in MEK/ERK-activated cancer and identify mortalin as a molecular switch that mediates the tumor-suppressive versus oncogenic result of dysregulated Raf/MEK/ERK signaling. Our study also demonstrates that p21^CIP1 has dual effects under mortalin-depleted conditions, i.e., mediating cell cycle arrest while limiting cell death.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.00021-13.

ACKNOWLEDGMENTS

We thank Bassam Wakim (Protein and Nucleic Acid Core, Medical College of Wisconsin) for mass spectrometric analysis, Daniel Eastwood (Division of Biostatistics, Medical College of Wisconsin) for statistical analysis, and Alexandra F. Lerch-Gaggl (Pediatric BioBank & Analytical Tissue Core, Medical College of Wisconsin) for imaging immunohistochemistry data. We also thank Albert Girotti, Andrew Chan, and Barry Nelkin for critical review of the manuscript.

This work was supported by the National Cancer Institute (R01CA138441), the American Cancer Society (RSGM-10-189-01-TBE), and a FAMRI Young Investigator Award (062438) to J.-I.P.

We declare no conflict of interest.