GCH1 and plasma cell myeloma: MG132 (Figure 4B), a well-known proteasome inhibitor, and Bortezomib (Figure 4C), an FDA-approved proteasome inhibitor for the treatment of multiple myeloma [27], significantly increased GCH1 protein levels in both control and shunt ECs, indicating that GCH1 undergoes rapid and regulatable proteasomal degradation.