The metabolism of proline as microenvironmental stress substrate

Abstract

Proline, a unique proteogenic secondary amino acid, has its own metabolic system with special features. Recent findings defining the regulation of this system led us to propose that proline is a stress substrate in the microenvironment of inflammation and tumorigenesis. The criteria for proline as a stress substrate are: 1) the enzymes utilizing proline respond to stress signaling; 2) there is a large, mobilizable pool of proline; and 3) the metabolism of proline serves special stress functions. Studies show that the proline-utilizing enzyme, proline oxidase (POX)/proline dehydrogenase (PRODH), responds to genotoxic, inflammatory, and nutrient stress. Proline as substrate is stored as collagen in extracellular matrix, connective tissue, and bone and it is rapidly released from this reservoir by the sequential action of matrix metalloproteinases, peptidases, and prolidase. Special functions include the use of proline by POX/PRODH to generate superoxide radicals that initiate apoptosis by intrinsic and extrinsic pathways. Under conditions of nutrient stress, proline is an energy source. It provides carbons for the tricarboxylic acid cycle and also participates in the proline cycle. The latter, catalyzed by mitochondrial POX and cytosolic pyrroline-5-carboxylate reductase, shuttles reducing potential from the pentose phosphate pathway into mitochondria to generate ATP and oxidizing potential to activate the cytosolic pentose phosphate pathway.

Figure 1

Proline metabolic pathway. Abbreviations: PRO, proline; P5C, pyrroline-5-carboxylate; X-PRO, imidodipeptide with proline as carboxyl terminus; Y-HyPro, imidodipeptide with hydroxyproline as carboxyl terminus; GLU, glutamate; GSA, glutamic-γ-semialdehyde; ORN, ornithine; ARG, arginine. Numbers designate unlabeled enzymes: 1, P5C synthase; 2, P5C dehydrogenase; 3, ornithine aminotransferase; 4, spontaneous reaction; 5, urea cycle enzymes.

Figure 2

Proline cycle (Redox shuttle). Abbreviations are as shown in Figure 1. ROS, reactive oxygen species; PPP, Pentose phosphate pathway.

Figure 3

Extracellular matrix as mobilizable proline reservoir. Abbreviations: ECM, extracellular matrix; MMPs, matrix metalloproteinases; X-PRO, imidodipeptides with proline as carboxyl terminus; Y-HyPRO, imidodipeptides with hydroxyproline as carboxyl terminus.

Figure 4

Proline oxidase produces superoxide which activates both limbs of the apoptotic pathway. Abbreviations: TZDs, thiazolidinediones; TRAIL, tumor necrosis factor related apoptosis-inducing ligand; DR5, death receptor 5, PARP, polyadenoribosyl polymerase; MEK, MAP kinase kinase; ERK, extracellular-signal regulated MAP kinase.