Cellular recycling: autophagy and sulforaphane

01

Enclose, break down, and reuse

Cells break down damaged proteins and old internal structures for reuse. Autophagy is a process responsible for this recycling. Here we focus on macroautophagy, which encloses material in a double membrane and delivers it to a lysosome.

Lysosomes are small cellular compartments containing digestive enzymes. Autophagy collects and transports materials; lysosomes break them into forms the cell can use again.

1. 01

   Enclose with a double membrane

   A membrane expands around the material. Once closed, it becomes a transport vesicle called an autophagosome.

2. 02

   Fuse with a lysosome

   The autophagosome fuses with a lysosome, whose enzymes break down its proteins and cell structures.

3. 03

   Return usable materials

   Smaller components return to the cytoplasm to build new structures or provide energy.

Sources [4]

02

mTOR: a signal regulating the start of recycling

Cells adjust synthesis and breakdown to nutrient conditions. Nutrient responsive mTORC1 supports growth and synthesis while suppressing the start of autophagy. When this signal falls, recycling can begin more readily.

In 2025, Plafker and colleagues treated cultured human retinal pigment epithelial cells (RPE-1) with 25 μM sulforaphane. Early insulin and mTORC2 signaling fell; by eight hours, phosphorylation of S6, a downstream mTORC1 marker, had decreased. Sestrin 2, which suppresses mTORC1, rose at the same time.

The researchers measured actual breakdown flow alongside signaling changes.

Sources [2]

03

TFEB: preparing the lysosomes that break material down

TFEB regulates expression of autophagy and lysosome genes in the nucleus. It coordinates genes needed to build and operate lysosomes.

A 2021 paper by Li and colleagues studied this pathway in human cervical cancer derived HeLa cells and other cells. A moderate rise in reactive oxygen species (ROS) after sulforaphane treatment was linked to calcium signaling. The calcium responsive enzyme calcineurin removed phosphate groups from TFEB, allowing TFEB to enter the nucleus and increase related gene expression.

In this experiment, TFEB activation followed a calcium signal independent of mTOR inhibition.

Sources [3]

04

ERK: another link studied in neurons

In 2014, Jo and colleagues observed activated ERK signaling and increased LC3-II, a protein associated with autophagosome membranes, after sulforaphane treatment in cultured neurons. Pretreatment with the antioxidant NAC blocked both changes, suggesting a ROS–ERK route to autophagy.

Reducing or increasing NRF2 levels did not change the autophagy response. In these cultured neurons, the ROS–ERK pathway acted independently of NRF2.

Sources [1]

05

Measure breakdown flow, not only vesicle count

More autophagosomes or LC3-II can mean that new vesicles are forming faster or that later breakdown has stalled and vesicles are accumulating. The flow from formation to breakdown is called autophagic flux.

Plafker and colleagues compared GFP-LC3 signals with and without bafilomycin A1, which blocks breakdown. This difference estimated the material cleared and showed increased flux after sulforaphane treatment. After 24 hours, lysosome abundance and related gene expression also increased.

Sources [2]